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CORNELL UNIVERSITY.
THE
Roswell P. Flower Library
THE GIFT OF

ROSWELL P. FLOWER
FOR THE USE OF

THEN. Y. STATE VETERINARY COLLEGE
1897

 

 

 
 
A

MANUAL OF ZOOLOGY

BY

RICHARD HERTWIG

Professor of Zoology in the University at Munich

FROM THE FIFTH GERMAN EDITION

TRANSLATED AND EDITED BY
J. S. KINGSLEY

Professor of Zoology in Tufts College
Zev P. £7 >
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LIBRARY.

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NEW YORK

HENRY HOLT AND COMPANY
1902
Copyright, 1902,
BY

HENRY HOLT & CO.

 

~~

ROBERT DRUMMOND, PRINTER, NEW YORK.
PREFACE.

On account of its clearness and breadth of view, its comparatively
simple character and moderate size, Professor Richard Hertwig’s
‘Lehrbuch der Zoologie’ has for ten years held the foremost place
in German schools. The first or general part of the work was
translated in 1896 by Dr. George W. Field, and the cordial recep-
tion which this has had in America has led to the present reproduc-
tion of the whole.

This American edition is not an exact translation. With the
consent of the author the whole text has been edited and modified
in places to accord with American usage. For these changes the
translator alone can be held responsible. Some of the alterations
are slight, but others are very considerable. Thus the group of
Vermes of the original has been broken up and its members dis-
tributed among several phyla; the account of the Arthropoda has
been largely rewritten and the classification materially altered ; while
the Tunicata and the Enteropneusti have been removed from their
position as appendices to the Vermes and united with the Verte-
brata to form the phylum Chordata. Other changes, like those in
the classification of the Reptilia and the nephridial system of the
vertebrates, are of less importance.

A large number of illustrations have been added, either to make
clearer points of structure or to aid in the identification of American
forms. Except in the Protozoa, American genera have in most
cases been indicated by an asterisk. Numerous genera have been
mentioned so that the student may see the relationships of forms
described in morphological literature.

In the translation the word Anlage, meaning the embryonic
material from which an organ or a part is developed, has been
transferred directly. As our language is Germanic in its genius,
there can be no valid objection to the adoption of the word.

As this work is intended for beginners, no bibliography has been
given. A list of literature to be of much value would have been so

large as to materially increase the size of the volume. Experience
iii
iv PREFACE.

has shown that beginners rarely go to the original sources. This
omission is the less important since in all schools where the book is
likely to be used other works containing good bibliographies are
accessible. Reference might here be made to those in the Anat-
omies of Lang and Wiedersheim, the Embryologies of Balfour,
Korschelt and Heider, Minot, and Hertwig, aud Wilson’s work on
The Cell.

The editor must here return his thanks to Dr. George W. Field
for his kindness in allowing the use of lis translation of the first
part of the book as the basis of the present edition.

J.S. KINGSLEY.
Turrs COLLEGE, Mass., Sept. 19, 1902.
TABLE OF CONTENTS,

EN DRO DUCTION gr udods coun s ake on teas done ies wie coe Gate neonate aaa
FEES LORY OFZ OOTOGM a tie iia ara ir at eA oneal dul naan ec cnels 7;
DEVELOPMENT OF SYSTEMATIC ZOOLOGY..........0 0.00 ccc cee eaee 8
DEVELOPMENT :OF MORPHOLOGY sx. ss ds i wa atnls 44 ae ¥ oa Lae Ss MOOR me 12
RERORM) OF THE SYSTEM As... cuis can an id weaning sae game boolean e aes 18
’ HISTORY OF THE THEORY OF EVOLUTION..........0....00 0000 ee ees 19
DaRWIN’S THEORY OF THE ORIGIN OF SPECIES..............00005 25
GENERAL MORPHOLOGY AND PHYSIOLOGY................. 57
GENERAL ANATOMY: 3 &aandten 2 adniios Vaine a A mualinged via ae Ra eas oes ot einolene 58
The Morphological Units of the Animal Body.................... 58
The Tissues of the Animal Body........... 0.0... ccc cee cee eaes 71
Epithelial iSsnes: areas a ab dane Selene cursus er achaaine wie ooo acn a aeane 73
CONNECTIVE TISSUeSti yen teharadin Pee Be ae Da wi ecia rsleoioeeats awe 83
Miiseuilar Tissuess. wes gcse ts cs ce Move Se ae wld es case aweas go
IN€rvous: LISSUCS si: o0 tad eaawnee Saale Oates erate pas 94
UTMMATY ae ded see hte: by eae os GR DEA Eee Ge eee Ke da Roatan g 97

The Combination of Tissues into Organs........... 000000000 cca 99
Veretative Organs). ow citi ve asane pad ee das eee eee eee oe 102
Organsiof Assimilation, acs cckeae deoudds cu dale de aean eee to’ 102
Digestive: Tract, sy secoutasaki ren ac aoc can cee pene Rie nests ato gmeates 103
Respiratony Organs ins csije to ihe soe Wa awe dterone tie nioar ates heen ay 107
Circulatony-Apparatuss. ccc ccd sane cdwedh wen naaisia oot gous 109
EXCretory OPeatiSive ci cc cans otedc ade dad ev bee oan ees 115

SERIA LOL FANS 7. spare g most Sala Ween alele at ane? MAUL aha Riner ave teat 117
AnimalCOKansy,. i c.z.cicn Waa Maaneins qosagalaeuies masa wae eh eben 121
QOrgansor LOcomOtlOns<:cca8 craeeade gens cans pen aN aa es 121
NERVOUS SYSEODT:.. cous Sccreuchs Crees ibis AMlod SauBeL LRG areiatdnaun sla yh acheala 122

Sense: ORGS yas ais eps carta sa atautneye cane Uecsmiee: cin Geetha sin wate Rua Ecce 125

STITT AGY ayaina eoheel eee inabe Sots each Sous avenge ke ara a dame Saco Mero rau ep onsee 131

PROM OLPHOlO Verano uate sae har eros setae seas Rrra ens aoe alia ese te 133
GENERAL, EMBRYOLOGY, gage okiacnua yee nee tees eee eee ae eves 139
Spontaneous Generationin: scusavse eos eates dice ce eed ghee ake oes 139
Generation by Parentsiics4 <4 eo: ei nesie soi Baye Gh dave yon as tees 140
Asexual Reproductlotiws cisccanasccneen iat cesee ere uee dense ae 140
Sexual Reproduction dca se taeed tvs Ghee Oke ae ees ge weenie aa 142
Combined Methods of Reproduction................... 0.0000. 143
General Phenomena of Sexual Reproduction..................... 145
Maturationvot the Be gijc coy yy. car eaa wis, ¥ gia site tancliann Soe chee sit Ales 146
HertilizatiOnin’: svt ceeg ae icin cis oust dines sale apemeaes ee ew Yee EPS 148
vi TABLE OF CONTENTS.

PAGE

Clea vale Processesi.v.5 Min aon mhn ais tian A act ma ene Dee he I51
Formation of the Germ Layers ............0.... 000 eee ees 156
Different Forms of Sexual Development....................... 160
SOUL TTA TA AEV? co3 sc tg huavcriay’, east cm ake AMA eine desea geaeenk ar aan ace bansheaahe, cA 162
RELATION OF ANIMALS TO ONE ANOTHER............0.000 000000 164
Relations between Individuals of the Same Species................ 164
Relations between Individuals of Different Species................ 167
ANIMA, CAND ® PLANT Ss 5 ccnts cchoisyd. Suing shat dhs na geese Ge ew ow Roe ee oF AE 171
GEOGRAPHICAL DISTRIBUTION OF ANIMALS............0.0000000004 174
DISTRIBUTION OF ANIMALS IN TIME...............000 0000 eevee ee 180
SPECIAL, ZOOLOGY. tkcsca wanes tains Raum wists sh BGraeee ae Ae aha oe 182
Phylum Te PROVOL ORR a8 rca he ee Peal oso ARIE RA RIS Le ial ie A a ee 183
Class: Tee Rhizopodaca.nawaisney as sures eee we eer ee ae oe ee 187
OrdercTseMoneray oe myc cake a eevee on oece aiee diac peveaees Sie oreetahees 189

Order TTT 0 DOSAR a cond Bune acts eranvaieh weeds Prgeed anaaestigy alee Ae 189

Onder: TT, UHeli0208 i o6 eter hh nee ad eeohe ee ohenes cee kes 190

Order IVi~ Radiolaria’ <2. ohnas pect neue su eaaee uncer 192

Order V;. Foraminifera: . onc. cc eet neeaeenen ueneia sca aye 196

Order VZuMy CELOZ OA Ih aca hve cp Nstaro ht fol attr ae hy Suk eta 198

Class UE. Plagella tar enn gso russ estncctae eens Seat B eet A oe eae Us Sue canoe 200
Order-l Antoflagellatass sats. co Lars euskal hea ara a 200

Order Ih. Dinoflagellata ti: Gove soe onn naearuela eee we Aas 203

Order TIY, -Cystoflagellatay .i..4 syeesce sen dea au ce wen dance 203

Class. ATT, Ciliatars, spat sae cuneate gies Bon 2 A aeanitecencugsl ane Ais ae 204
Orderly ‘Molotrichars Vwi ne jisrgs, wa dan ay Sue oes Ress oe 209

Order IT, Heterotrichar.... astscawusa does e £au Gare aebuls 209

Order TU. WPenetnehas...5c0/ oi viates ewe a Wsponis ea ais beso cee 210

Order IV. Hy potrnichar.ss Vion eek pera aeee omens ary

Opder: ViF SUCEOM Aya: gio ae, noe Geman me cuealie a ain aecde apie (oT

Classe SPOKOZOd sis ap a6 t5c Aen ce 4 ae on Seamed nee oS ee eg) 21S
Order te Gregan mid a wens. gi acei 4 tes oon ated teks ee BIAS 213

Order: [Coca digests wit e Meee nace AA Ave anenGernelers oes : 215

Order Ill. Hemosporida......0 000.05 ch ne bees ee oe eee oe 216

Order TV.. Myxosporida... 003 ccs cc ends ee eeeetiuees 217

Order V. Sarcosporiday oss iiaes page beat Aa Gans wh aduae s 218
SUMMARW: Aiacuterenaltilaletdiiehe aware e Wate At cee Mie a site on va cate 14 BIS
IM AZ OA eas tecres te is Miylecaicn Sn sau are den enue Met hm Ate va Zee 221
Phylum LE RORTRERA faa teiya5s)p weigh esl ok Ga ae tow. Gednd. Lt elahnce hemor a eee 221
Ordéerd, (Caleispongies.< 5. cadiwe vas au ee beam teal. be pa 225

Order I, Silitispongia...... 0... wc oh ba oe ca tee mraa yee 226
DSUMMARW: cc tint ee ninatan nat Ny sas ua® She ck; Miehen eae ANS haa ae en 227
Phylum DEL COVENT BRAT A sec gale top oe 4c as hoa Gath EEA EA ecg ws 228
CLASS ELV OT O7Z Oa cia nafs. tis cai cch meee avin sels weteacyn Sera teen nae 230
Order Ty EL yd tal ais ne 8 eles act tonsa gisten & Suasirseners meen Seber 240:

Order IT. Hydrocoralline ...........00..0........... 241

Order IIT. Tubularia = Anthomeduse................. 241

Order IV. Campanularia = Leptomeduse................. 242

Order Ve clrachomeéd uses ge Scone fs tenses es ak tine uals 242

Order VI. Narcomeduse 325 ie ccic se cacneaveuntacacacc, 242

Order VIT {Siplionophora:cccayyec ket. oy ne vea enable eke 243
TABLE OF CONTENTS. Vii

PAGE

Class [De Sey plhoz0a rac. cusialon a Gian So ot cain suena aoe abn deen 245

Orderl.. Stautomed usec scias Ao ee du bu pe oaceedaan wwe oe 250

Order I. Reromeduser neon wiccnsecucaioaie a yeehoae Re eee ee 250

Order III. Cubomeduse...........0000. 0000s cee eee eee 250

OrderlV.: Diseomed tse. Se yicsae sedans al sawn aw Aang 250

Glass TTL AMEnOZO8 cies vena re at baie deren has wastetle de eae p eat 251

Order. “Vetracorallas. savataine eae eee ccs wu aa wer ae Baw 258

Order Tl.’ Octocoralla: soja evade siedcee as aalacaee andes 258

Order NTL. Hexacoralla ie: de guste ed ans yok ada hale tne a 259

ClassLV, ‘Clenophora: .. osns ans vadgeeeth ne eaters wane, ae eens 261

DUMMARY stave yg cnn sist penas teh We War? Oar aad en aeeu ace Maan haha a! Stacie eusiavae 265

Phylum LV. PLATHELMINTHES cao 6056 vas SARI Ae eis Seas Sacaal 267

Classi “Turbella tia). caiceiiat eatinte aon, wandecav ainsi tuca.cs Cee mets 268
Order Tl, Polycladideass. cc.guic sas pee Be Yad Se Eee Cees 271
Orderly Cricladideays: syz..a seals eaweae peas nen as susie s 271
Order: Rhabdocodlidae a4... canes ie eee dere ee Oa 271

Classibl sir ema todaime.senscatnwns cpa terakars anna one enters Pees DTI

Orders Polystomiaeiss We c.isocaneeel sigs mighesuce he HORE e 273

Order TE i Distomiaess< 5. ata sida ce a, isalabins wnate a abel ourenee 274

Cassel TIC eStOd ag asia tices neve ar suc ears ue oasyereeetinp in sl ada era ee ee eee 278

Class:LV-s Ne@mertini 4 ¢ tis wane aous maces pause Suny pure baa 289

SUMMARY eo. cairns asa eth es maeeene Saw anes p ie aun genes a aaah) pen 202

Pipline Vc PROTIBERA’ 5 alati Asctae hh ate SRA Meese et ceaeas pe oe ar Acces 293

Phyline V1. CORMARLMINTHES) 034 ou pyle atlas $en88 asedew Gash Sohne dae Sine 2 295

Class PaChaetognatiiien tacnic aiccdon Serta t ak Cin at eee a, 296

Class IT, Nemathelminthes. 2.5 c30.0.h044caerae ste wnseeed ataes 298

Orders Nematoda g..cjistn gin seo te wae AG RPAAY seen ooo 298

Orderly Cordiaceay cs sy. across saya hvindoh Sea ees case 304,

Order ILL. Acanthocephala isis yivcaa chee geen dena end aes 304

Class-ILT:- Annelida, cs uy oh a ele ae earn h hela ces dell us aae a Ne 305

Sub: Class 1. Chaetopoda: 4.2205 oi¥aco vay ogateeded asain aaa wee 306

Order dy “Poly cheeter.js on ae te mace egiey wah tek ok ees 311

Orderdl.-Oligochiaetae 4c seas aioe neues eee Se See tae sald 314

Sub Classis Gephyrea’c sis ocala sue cas sean sue eee as 316

Orders, (Ghaetiferie 5th oie Sis aide aces ya oe ena ana ean wae 317

Order I< Ine rmes eso. sanessrs iiss ests etsy patie dali neue ah wa ee yes aes 317

Order TIl= Priapuloided. «ce son aks oars mvtis.ans SE oe ee Se 317

Sub 'Class:1TL. “Hirwdinel +. 2.5 2%an av one peeve omissions Meme He 318

Order I. Gnathobdellide..............00.00.00.00.00000. 321

Order II. Rhynchobdellide ................0...0..0.00002. 32%
GClassEVi. SPOly 20a0's ssc totic he ae Siete cist nite Seca Gone hata nels 321
Sub: Classil.c Pntoproctastccc4440 sao ahs naoee canon Bho oe 321

Sub: Class I sR ctoproctayga ec suce nad oath SA als 8 aneansrewousan 322

Class-V- PH OGOnidal cic ciation he 64d vt lea Sey tener a eA ty tan at ae Ae 325
Class Vik“ Brachiopoda: 15. piace eadiagiatesus Lame oR aa atnaane 32

Order ls sBeardines ia: 2) siecle tate ouriie vier aoe Dene Se aE 328

Order Tl. Vesticardines<.5.i5, 1a wa ce cela an tat usa Ge na thw 328

NSUMNGAR Ya sss cca! croetis ade sracn ond tanaese ass ther ace tir oae a peacne ase aIO) Sus tai eA Ae 328
Piylane VAL. OK CHINODERMA: 5.200 an parva Fat hol ie Wig eS AMN NE Hee Re 32

Class Ts ASEEr OLACA ghire wu tre co nemtass Set sacra Meaaechone eee peas area ME ane 333
vill TABLE OF CONTENTS.

PAGE

ClassIL. Ophittoidéd... nic hacen aeeeteee yates dG aeeeenen B37
Classi Crimotdea's.<.0 5 ho Sapuscn aries See 2 Aaah oy etm, wipe g REG 338
Sub: Classi (Buctinoidéas .:a2 aesomeedsnalsa Haha oe te ene Rae 342
Sub: Class]... Bdrioasterordea 22444 sie ae actos dae cee eae ee 342
SitbiClassel Mh. tCystid Cae sche. & 1 ka.co Sassseh Gay srans-guocearane See eae 342
Sub:Clacsl Vi. -Blastordeals hav cue.e uta ninonre siete, a qostysi gies aver a sshd 342
Classi Vir chimotdea crn as id-2snen eeteetuaaiars Gra conan! ies actnanenae Sonky: 343
OrderT,. Palechinoideay sais s Se ede seersed oak eee Me, jae a Sk 345
Order Wi Cidaiidee ss Aoh-sn te sea Bee eR Oe EE Se 345
Order ILI: Cly peastroidéa.e: tere ted wate eee e tee eee 346
Order [V. Spatangoideaeus cicides gia dng ches oad BS Pes 346
Glass'\V- Holothuroidea:, 4/2. crease ags ecsy ad euheeeaee ns wee B40
Order E. AGtinto pod ass ni waters euismod, Bie ae ee eames 349
Order Ue: Paractinopodas. mivsiyare ane Gee aid speed ev eiege o eha, weal 349
PSUs wk ch paws» wa we Rw wis aLM enhleTe alate Bsn he imst ene mame Motes a ern eee 350
Phat VAT Ts > MOTA USC Biers orca cies esc a te  aette a va pean Beat ehen ava ccte rece Sara abe 3 351
GlasssIscAmphinetitads. sac cccsemcadiy a mckoohe paw aire Gah te erg aoe eae aude 356
SubClass. Placophorae. nuit e anid aus Aa DAS Means Reta eRe nt 356
Sub Class.IT.. Solénogastres:.....cnece Reman iaas bene ptees anes 358
G@lass TI) Acépialas sy iectcha-wt 2 picniue aistdun Manel aautisce mee Om nee 358
Ordér Ts Protoconehige ts > ecg, aneenscdsnaninewerehusa' ete ae ae Awe: 365
Order Il. Heteroconchigz® 4. 4.c0s.d4 ance oe bailey vane debe 367
Glass TIT) Seaphopodaw so. cot Be eg ae awe Sa Awa 369
Class'LV) Gasteropodavs ai 'ncy a mes Gtiow. Dae Hee nese OO che tale 369
Order.L.. Prosobranchiata’ , 24 4 wy sie cae eae HES RRO 378
Order II. Opisthobranchiata ...........0...... 0.02.02... 381
Order ITD sPulmiona tay swe aks tag oaty actu en eaten ice meat en oes 383
Class'V. Cephalopoda. soca. casas pnacuau iden sns ane wea alge ane 384
Order: Vetrabranchias is s.44.444.n12caeun Ge oa aay eclease scat 394
Order Il, Dibranthiain. csi te shai mean ne morn Ode oes 394

SD UIMIMAIR Vico. ois hc Sly eine RNS e ait Sele NN NO Aon en Beye rec dive ati acres lal ae a 395
Phyl TX ARTAROPODE 2.340. sik: scape Sale lee RSMO ANALY Bm ri eater are 398
Class Tt Cru St aCe a 0 5 es eee ees shea can tense eather sah aan Chee 408
Sub Classy Unilobites cory eteeas Soc deal aeaee nan eben be ors 414
Sub Class II. Phyllopodas isc ccc sce hee ceca cached ve bees 415
Orderl, -Branehiopoda yy. pat yates cgay 5 Soccer wlan hemes 436
Order Tl Cladocera 1.0. cc ius aye ahs nan A cece aie Elena 417
Sub:Class TIT Copepoda. 0a. Sons aah pee ee ea Sede Sates 417
Order lh: Hueco pe pod an. ese Gia kiero ha wana hos Bae ote oe 421
Order ll: Siphonostomatay 2.0 fo sued casa hss opted even os 422
SubClass iV. Ostracodays, «i284 cn ac tan0 Goan puaameks oe oe eee 422
SubClass V2 Cirripediaic ) imi va myst s vakten oes eae ee ee 423
Orderl sepadidac sa cere arden nares ae eae ee ee elon 425
Order Thy Balamidaas: tion. 2.4 nau sie hen Rath Rawiaud oedareaeda Honedie ee 425
OrderIIT.“Rhizoce phal area oa: tact ars.cup deaceace ee ee on ve ees 426

Sub Class Vi; ‘“Malacostraca sosccicsdee coos, wenn bed ow ad cadn 426
Tepionil, Deptostiaca, yay Site sees. gach hades tee oon nae 427
Legion Il. Thoracostraca.... 6.46 cccc eee Sana ecesseacsuavs 427
Order Schizopodais,. a2.G-i.c ges uh ontog ners ee wut ho 428

Order II; Stomatopoday.< oases oe c4nh es Cee en eee belcen 429
TABLE OF CONTENTS. 1x

PAGE

Orderdil Decapoda: 2 sataq, cae die as Oars Saeed ae 429

OrderIVi Cuma cla: ee oi oe got 22.8 ema asneere eR ao 437

Legion Ll. Arthrocostracds 23 44 se nke tadione nec amare, ose 8 438

Order tl; Am plnpodari.s ions eehe sie ri vac albrahe ee oe ee 438

Order Te [sopoday oc h5 oe 8 a beac ara det neuaeanonn ae seule ees 440

Class TE ACerata nas Gata noun mee cecat eh oe aoe ene 442
sub: Class I.-Gigantostrata ass. Go adel ic keke at yak bent yous 443
Order]. “Xaphosura ss cdc. t esac tree eee eeu oe Eee he 444
Ordetll. Hury pteridacc.« occstees ee derde ne eeg ae esd eed 444
SiibClass tl; Arachmid ars «nas aauncbeeyaa aadet yuan & Ree eGend ee 444
Lesion |; Arthrogastridas sccvuonaesied G Galaatoe sasaut wepeeuaer: 447

Order. VSCOrpionsd ates re asa ere aerate « aoese a eet Led A oee tena 447

OrderLh. Phrynoideas2. ik ee ate ks oboe a teccioam se get 448

Order III. Microthelyphorida ......................... 448

Ordér LV... Solpigida. 5.502 cethay ceean gas erus peewee ed 449

Order V2 Psendoscorpil. hice tebe pane ees nar esas ences 450

Order Vi; Phalangiday. «200s <5 04 Hess Beis eee y O8 450

Lesion Ll. Spherorastrida, . c.ccihasots Aen nwnad sGaN ane 451

Order: Amaneing: on ic act macne etvae diene aiid leasen Am ames 451

Order Tig Acarifia: 64.5.cc0hs 25 ms abies ee eects BH Oe tins 453

Order lil: Wangaatulida. << .:03 ds aed eee ye hw bad ee eee 454
Tardigrada:<.s¢ aniseehnanachs menses Goda manus oe 455

PY CNOCOMGA 6) Apmis bu tee es RE A Reka een eae 456

ClasseELL Malacopodast s: suc Astvnpusiy encanta aan e ead RN ayaa 456

Gla SoH Ve CITI SEG tas coee iets fess Crean ences stats Aisa ees ha ie Rial Site ma a eee 458
Sul Class Chilopodaccssyedanenac ee ce eeu Maresh oneeretets 460

Sib Classi Hexapoda acs eases aia stomcne watah tonite aed s 461
Order. Apteryeola: oun ieee te sae tee Ones ee eves ead 477

Order 11s. Axchipterars) x pat onde nba es eacd ay wares oe Re ee 477

Order IIb, \Orthoptera., as ce sak ne Sees See odes hE et ss 480

Order TV.Neuropteran wis come cae acai, oa ee eects oe ee 481

Order Ve iStrepsipteray ion sisi ote pastas aaathe a aaihven ww mae g 483

Order Viz ‘Coleoptera: 2x sja.aus stunacuans ve Mace via hee g-heen s 483

Order VII) “Hymenoptera: as. oi0 ct a iene pees 485

Order VITI2RDYyUChOla ts ake eA ieee cd aoe cscs h-aieebh: acc avedte leet 489

Order Tes UDiptera®, cn. cin sass ancarens we ceee tem eure: He awieatcnlert 491

Order X, Aphatiptetan cccrsus acd ceae ee uaaed eae ew haan 493

Order Xi. Lepidoptera... bec orcaG sé anges eae tae Gece ue 494

Class'Ve MDiplopodarys.20t 54 catia een sows ay suena ee Goes 496
SUMMARY 0c ities So ae tute WERE RANGE hdile HATS eb tenes sone gE A 497
Phylum X : “CHORDATA. 1% Mant isa a bese ulead hay Aad ae eae aE a ms 501
Sub Phylamil, Leptocandi.. 05s... 5 sarees cA ieee ete ids aeee 502
Sub Phylume-iL, Dumeatas sc. .06dctsacegeca neo eens Sad teneae eee aan OS
Order tl. - Copelataer) ancdusucpeac meres Ea AMER ROA Ae aN 6 506
Ordérll: ‘Tethyoideayn ss 2e:e ge Sasa ak Oa Ane meee 508

Order TITS PWalig cea. torc se seccuozend ats ee wD eee tN eh 510

Sub Phylum III. Enteropneusta ....... 0... eee eee eee 512
Subs Phylum LV. Vertebrata coasted, ere oe naires aurea hostage ERO ao 514
Series Tenth yo psi dans cea da dus ites coe: sits tescdes siciteost aur bine asta Sopa AUR 555

Class-ky Cy clostomatta ian. cvtaova cialis aie Sanus yorsen od: nea aeeee aa 555
TABLE OF CONTENTS.

PAGE

Sub: Class.T. (Myzontes i329 esd eae Se Sees habe Ses 556
Sub: Class Ll... Petromyzontes 3.4 via te sree Seren os dou: 557
Class 11 Pisces jas gate sete seven haces ME ie eee ee Mee Pe Be ES 557
Sub Class I. Elasmobranchit 2. i. jace0¢ 4. secede va vars ses aas 569
Order I. Selachits.se .uesdw a ped Sock Hed aay Seer AY ee ee 570
Order Ih. Holocephalt.+.27 os spice dds ehh e Sh ato een ae oe 572
Sub Classi] <:Ganoider )winccta tre eu et He eas ad Mewes Monel er 572
Order; Crossoptery ei ory yen os rence yeaa ne ainls sm eNG wa ules 573
Order TT Chord roster ceca es csaeviona said ene estas eheacle es sleas's sn Bee 6 573
Order Tilo Holostomt: sie eau ivie suuitoecunn mentee cal caer eros 573
Subi Glass: TIL: “Leleostets 0 yeah atquapersiavey woe sigueesastie dap gokl bee ardor Bw 574
Order: (PhySostOMiss at sau gue pacm amas ee wee oe te 575
Order Th: Pharyngopnathi . 222.5214 le dete edas eas tee 576
Order III. Acanthopteri ............... 00. e eee eee eee 577
Order IV; Anacanthinis. coc dse aces cease ea bhaae bo 577
Order Vi. Lophobranchit .snccngdcannse anne sae HRM RAUNs 578
Order Vi; Plectognathion aca cuins wy mers ane aa thse en ne 578
Sub: Class IV. DipnOk s4..ncuin eo echo AL RABE EAR eRe Re as 579
Classi IL Amiphibiay (occ kess fp amdosoniae aaalnaan nen setelaee ss 580
Order. Stegoceplalyy oo oxy ead was suaves ccausde Pavel bn nies ee 586
Order IL. Gymnophiona; cavenca cavicte te ag aa oieulea a scoot 587
Order LL. Und elas. ss ccs soe gee anes ok ae, Recados Boe arsonvaea ieeata Sars 587
Order TV.) Amutalccaly uct sect ag a ave aw Sane Bie aieiaaee Skaane zie 588
Series: Lie -AimmiO tats sees oe ens who ee hg ER See Ee Se ERA ea a 588
Class? In Reptilia: s.vsars o wineceion sn ce an ane n aides woe oe Saw eens ws 588
Order], “Cheromorphay wicccwn denen wee nak ak ne wet antes 594
Order Ii. PlesiOsauriases ic casrslew.cke Selo we oak, Same s eon 594
OrderIIl, Wehthyosatitia: .o.c.5 42 ccaiss nage vols dual en ae ome 594
Order IV. Cneloniay spc G cng Sea hk ae ain ia SIN eele nite deesabo ee 594
Order V. Rhynchocephalia ..........0.0.00000.00 000 cece 595
Order ViL2 DimOsatna: 966.5 daca caade ueao same hoe teak 595
Order VAL Sa uiamatac ois) wena pate aude eed oes ae eee 596
Order VATE CroCOG Ma x ink). Scot clad alent oe ae ae oe Gas es 601
OrderIx: Pterodactyllay ss o0 a uus hadeav ds ona weleoeeeeacece: 602
Tass A ViCStia seh narnnant ec die Sid aie eacce cyan. eh Mineman ae nears A 603,
Orderly .Sauiiti nae waren aie ae iehnasecartanin iia wis Sates las cael meek 612
OrderTT. “Odontornithes: . sen cessed s Dacd a ach oe oes 612
Orderd Le sRatitaes on santas ay ce ates Mate wide Geka cae 612
Orderly Carinae. aos saci talon Sinner tet Cee eee nena a 613
Glass Tis “Matimmalliace vette h aoe eee ems ete ne ee 617
Sub Class I. Monotremata, 220.005 564 05 bea bexaa decane. . 631
Sub Class: Ti. “Marsiupialian cy <2. ican vag dunes Re ewetee eke bed 632
Order J. ‘Roly protodontas, . «cos Cavan caus aerawas tay eee see 633
Order LL Diprotodonta wia.4..1a nes ou abies J's ewan clin oaenk 633
SubClass TIT; Placentalia ¢ s94. 64.0 sue duartueare tava male ccopieew eee 634
Order ls Hdentata iy. ae sou corn tacks aoe mnce ean 44 eee 635
Onderdls Tmsectivoral -sfa1s sates urges Gon tere ssh eee le oe es 636
OrderlM.- Ghirop tera es cic ccd 5 reals hetietuene eeannels ae ee 637
Order RV,“ Rodentias 3 ir ca.5 S hecstascannlecrarenoaecae Areal eeetan 638

Order V_ Ungulata
TABLE OF CONTENTS. X1

PAGE
Order VA, Proboscidia: 32 .cc spi cacacsioues MRS aes 643
Order: VIE. Diy racoideas sna te eh eet oe as Gee emcee 644
Order VAULT, Sikenla oh Gas a4 want ela aun aes au a oes aetna allen 644
Orden IES, = Ceta Ged: ove auatga es soontina Sain nth a te Bleue og nnedek 645
Orden2e,= Car niviorassrsg Gre ne Secs ae hee ens veh aes 646
Order DEL, PEOSIII TES ai denise. aitaus ss Son Roo hev Wop est aes Lhsa Wcatr ae aN 648
Order MIT: Primates. c):oi each sc aes saan a aeannoug aa eavaee aes: 649
GENERAL PRINCIPLES OF ZOOLOGY.

INTRODUCTION.

Man’s Relation to Other Animals.—The man who has learned
to observe nature in a disinterested manner sees himself in the
midst of a manifold variety of organisms, which in their structure,
and even more in their vital phenomena, disclose to him a simi-
larity to his own being. This similarity, with many of the
mammals, especially the anthropoid apes, has the sharpness of a
caricature. In the invertebrate animals it is softened; yet even
in the lowest organisms, for our knowledge of which we are
indebted to the microscope, it is still to be found: although here
the vital processes which have reached such an astonishing com-
plexity and perfection in ourselves can only be recognized in their
simplest outlines. Man is part of a great whole, the Animal
Kingdom, one form among the many thousand forms in which
animal organization has found expression.

Purpose of Zoological Study.—If we would, therefore, fully
understand the structure of man, we must, as it were, look at it
upon the background which is formed by the conditions of
organization of the other animals, and for this purpose we must
investigate these conditions. To such endeavors the scientific
knowledge of animal life, or Zoology, owes its origin and continued
advancement. But meanwhile the subject of zoology has widened;
for, apart from its relations to man, zoology has to explain the
organization of animals and their relations to one another. This
is a rich field for scientific activity; its enormous range is a conse-
quence, on the one hand, of the well-nigh exhaustless variety of
animal organization, and, on the other hand, of the different
points of view from which the zoologist enters upon the solution
of his problem.
2 GENERAL PRINCIPLES OF ZOOLOGY.

In the first half of the last century the conception, which is
still held by the public at large, was prevalent, if not quite uni-
versal, in scientific circles, that the aim of zoology is to furnish
every animal with a name, to characterize it according to some
easily recognizable features, and to classify it in a way to facilitate
quick identification. By Natural History was understood the
classification of animals, that is to say, only one part of zoology,
indeed a part of minor importance, which can pretend to scientific
value only when it is brought into relation with other problems
(geographical distribution, evolution). This conception has
during the past five decades become more and more subordinated.
The ambition to describe the largest possible number of new forms
and to shine by means of an extensive knowledge of species belongs
to the past. In fact there is a tendency to undue neglect of
classification. Morphology and Physiology to-day dominate the
sphere of the zoologist’s work.

Morphology, or the study of form, begins with the appearances
of animals, and has first to describe all which can be seen exter-
nally, as size, color, proportion of parts. But since the external
appearance of an animal cannot be understood without knowledge
of the internal organs which condition the external form, the
morphologist must make these accessible by the aid of dissection,
of Anatomy, and likewise describe their forms and methods of
combination. In his investigation he only stops when he has
arrived at the morphological elements of the animal body, the
cells. Everywhere the morphologist has to do with conditions of
form: the only difference lies in the instruments by means of
which he obtains his insight, according to whether he gathers his
knowledge through immediate observation, or after a previous
dissection with scalpel and scissors, or by use of the micro-
scope. Therefore we cannot contrast Morphology and Anatomy,
and ascribe to the former the description of only the external, and
to the latter of only the internal parts. The distinction is not
logically correct, since the kind of knowledge and the mental
processes are the same in both cases. The distinction, too, is
unnatural, since in many instances organs which in some cases lie
in the interior of the body, and must be dissected out, belong in
other cases to the surface of the body, and are accessible for direct
description. Further, on account of their transparency the in-
ternal parts of many animals can be studied without dissection.

Comparative Anatomy.—For morphology, as for every science,
the proposition is true that the mere accumulation of facts is not
INTRODUCTION. 3

sufficient to give the subject the character of a science; an addi-
tional mental elaboration of this material is necessary. Such a
result is reached by comparison. The morphologist compares
animals with each other according to their structure, in order to
ascertain what parts of the organization recur everywhere, what
only within narrow limits, possibly restricted to the representatives
of a single species. He thus gains a double advantage: (1) an
insight into the relationships of animals, and hence the foundation
for a Natural System; (2) the evidence of the laws which govern
organisms. Any organism is not a structure which has arisen
independently and which is hence intelligible by itself: it stands
rather in a regular dependent relation to the other members of the
animal kingdom. We can only understand its structure when we
compare it with the closely and the more distantly related animals,
e.g., when we compare man with the other vertebrates and with
many lower invertebrate forms. Here we have to consider one of
the most mysterious phenomena of the organic world, the path to
the full explanation of which was first broken by the Theory of
Evolution, as will be shown in another chapter.

Ontogeny.—To morphology belongs, as an important integral
part, Ontogeny or Embryology. Only a few animals are com-
pletely formed in all their parts at the beginning of their individual
existence; most of them arise from the egg, a relatively simple
body, and then step by step attain their permanent form by com-
plicated changes. The morphologist must, with the completest
possible series, determine by observation the different stages, com-
pare them with the mature animals, and with the structure and
developmental stages of other animals. Here is revealed to him
the same conformity to law which dominates the mature animals,
and a knowledge of this conformity is of fundamental importance
as well for classification as for the causal explanation of the animal
form. The developmental stages of man show definite regular
agreements, not only with the structure of the adult human being,
which in and of itself would be intelligible, but also with the
structure of lower vertebrates, like the fishes, and even with many
of the still lower animals of the invertebrate groups.

Physiology.—In the same way as the morphologist studies the
structure, the physiologist studies the vital phenomena of animals
and the functions of their organs. Formerly life was regarded as
the expression of a special vital force peculiar to organisms, and
any attempt at a logical explanation of the vital processes was
thereby renounced. Modern physiology has abandoned this theory

 

 
4 GENERAL PRINCIPLES OF ZOOLOGY.

of vital force; it has begun the attempt to explain life as the
summation of extremely complicated chemico-physical processes,
and thus to apply to the organic world those explanatory princi-
ples which prevail in the inorganic realm. The results obtained
show that it is the correct method.

Since each organic form is the product of its development,
since, further, the development represents to us the summation of
most complicated vital processes, the explanation of the organic
bodily form is, therefore, in ultimate analysis a physiological
problem; though of course a problem whose solution lies still in
the indefinitely distant future. What has been actually accom-
plished in this direction is only the smallest beginning, even in
comparison with that which many falsely regard as already attained.

Biology.— According as the relations of each organism to the
external world are brought about through its vital phenomena,
there belongs to physiology, or at least 1s connected with it, the
study of the conditions of animal existence, (Ecology or Biology.
This branch of the science has of late attained a very considerable
importance. How animals are distributed over the globe, how
climate and conditions influence their distribution, how by known
factors the structure and the mode of life become changed, are
questions which are to-day discussed more than ever before.

Paleontology.—Finally in the realm of zoology belongs also
Paleozoology or Paleontology, the study of the extinct animals.
For between the extinct and the living animals there exists a
genetic relationship: the former are the precursors of the latter,
and their fossil remains are the most trustworthy records of the
history of the race, or Phylogeny. As in human affairs the
present conditions can only be completely understood by the aid
of history, so in many cases the zoologist must draw upon the
results of paleontology for an explanation of the living animal
world.

The science of zoology would be subdivided in the aboyre-men-
tioned manner if we wished to proceed entirely on a scientific basis.
Yet practical considerations have made many modifications neces-
sary. On account of their paramount importance to the medical
profession human anatomy and embryology have been raised to
independent branches of science. In comparative physiology only
the most general foundations have been laid; a more special
physiology exists only for man and the higher vertebrates: this,
too, for the above-named reasons has been made a special branch
of science. Paleontology also has, in addition to its specific

 
INTRODUCTION. 5

zoological tasks, attained importance as a scientific aid to geology,
since it furnishes the materials for characterizing and fixing the
various geological ages and the earth’s history during those ages.
When, therefore, at the present day we speak of zoology, we
usually refer to morphology and classification of living animals
with consideration of their general vital phenomena.

The views here given of the character of zoology have not been
the same in all time. Like every science zoology has developed
gradually; it has varied with each epoch and tendency, according
as the systematic or the morphological or the physiological point
of view was the prevailing one. It will now be interesting to take
a hasty glance at the most important phases in the development
of zoology. The reader will better understand the questions which
now dominate zoological inquiry, if he know how these have arisen
historically.
HISTORY OF ZOOLOGY.

 

Methods of Zoological Study.—In the history of zoology we
can distinguish two great currents, which have been united in a
few men, but which on the whole have developed independently,
nay, more often in pronounced opposition to each other; these are
on the one side the systematic, on the other the morphologico-
physiological mode of studying animals. In this brief historical
summary they will be kept distinct from one another, although in
the commencement of zoological investigation there was no oppo-
sition between the two points of view, and even later this has in
many instances disappeared.

Aristotle, the great Greek philosopher, has been distinguished
as the Father of Natural History, which means that his predeces-
sors’ fragmentary knowledge of zoology could not be compared
with the well-arranged order in which Aristotle had brought
together his own and the previously existing knowledge of the
nature of animals. In Aristotle favorable external conditions were
united with more ‘favorable mental ability. Equipped with the
literary aid of an extensive library, and the pecuniary means then
more indispensable than now for natural-history investigation, he
pursued the inductive method, the only one which is capable of
furnishing secure foundations in the realm of natural science. It
is a matter for great regret that there have been preserved only
parts of his three most important zoological works, ‘ Historia
animalium,” ‘*De partibus,” and ‘‘De generatione,” works in
which zoology is founded as a universal science, since anatomy and
embryology, physiology and classification find equal consideration.
llow far Aristotle, notwithstanding many errors, attained to a
correct knowledge of the structure and embryology of animals, is
shown by the fact that many of his discoveries have been confirmed
only within a century. Thus it was known to Aristotle, though
only lately rediscovered by Johannes Miiller, that many sharks are
not only viviparous, but that also in their case the embryo becomes

fixed to the maternal uterus and there is formed a contrivance for
vi
8 GENERAL PRINCIPLES OF ZOOLOGY.

nutrition resembling the mammalian and even the human pla-
centa; he knew the difference between male and female cephalo-
pods, and that the young cuttlefish has a preoral yolk-sac.

The position which Aristotle took in reference to the classifica-
tion of animals is of great interest; he mentions in his writings the
very considerable number of about five hundred species. Since he
does not mention very well-known forms, like the badger, dragon-
fly, etc., we can assume that he knew many more, but did not
regard it necessary to give a catalogue of all the forms known to
him, and that he mentioned them only if it was necessary to refer
to certain physiological or morphological conditions found in them.

This neglect of the systematic side is further shown in the fact
that the great philosopher is satisfied with two sy stematic cate-
gories, with Eidos, species or kind, and yeévos or group. His
eight yévn ueyzora would about correspond with the Classes of
modern zoology; they have been the starting-point for all later
attempts at classification, and may therefore be enumerated here:

H

Mammals (C@oroKobvta év avrois).

2. Birds (opribes).

3. Oviparous quadrupeds (retpamoda @OTOKOOYTA).
4, Fishes (iy@ves).

5. Molluses (uadakia).

6. Crustaceans (uadaKxooTpaka).

7. Insects (evtopa).

8. Animals with shells (oorpaxodéppata).

Aristotle also noticed the close connexion of the first four
groups, since he, without indeed actually carrying out the divi-
sion, has contrasted the animals with blood, evaipa (better,
ental with red blood), with the bloodless, avaiya (better,
animals with colorless blood or with no blood at all).

DEVELOPMENT OF SYSTEMATIC ZOOLOGY.

Pliny.—It is a remarkable fact that after the writings of
Aristotle, in which classification is much subordinated and only
serves to express the anatomical relationships in animals, an
exclusively systematic direction should have been taken. This is
explicable only when we consider that the mental continuity of
investigation was completely broken on the one hand by the
decline: and ultimate complete collapse of ancient classic civiliza-
tion, and on the other by the triumphant advance of Christianity.
HISTORY OF ZOOLOGY. 9

The decay of zoological investigation, that had only just begun to
bloom, begins in the writings of Pliny. Although this Roman
general and scholar was long lauded as the foremost zoologist of
antiquity, he is now given the place of a not even fortunate com-
piler, who collected from the writings of others the accurate and
the fabulous indiscriminately, and replaced the natural classifica-
tion of animals according to structure by the unnatural, purely
arbitrary division according to their place of abode (flying animals,
land animals, water animals).

Zoology of the Middle Ages.—The rise of Christianity resulted
in the complete annihilation of natural science and investigation.
The world-renouncing character, which originally was peculiar to
the Christian conception, led naturally to a disposition hostile to
any mental occupation with natural things. Then came a time
when answers to questions capable of solution by the simplest
observation were sought by painstaking learned rummaging of the
works of standard authors. How many teeth the horse has, was
debated in many polemics, which would have led to bloodshed if
one of the authors had not taken occasion to look into a horse’s
mouth. Significant of this mental bias which prevailed through-
out the entire Middle Ages is the ‘ Physiologus’ or ‘ Bestiarius,’ a
book from which the zoological authors of the Middle Ages drew
much material. The book in its various editions names about
seventy animals, among them many creatures of fable: the dragon,
the unicorn, the phoenix, etc. Most of the accounts given of
various animals are fables, intended to illustrate religions or
ethical teachings. In a similar way the religious element played
an important réle in the many-volumed Natural History of the
Dominican Albertus Magnus, and Vincentius Bellovacensis, and
of the Augustine Thomas Cantimpratensis, although these used
as a foundation for their expositions the Latin translation of
Aristotle, the works of Pliny and other authors of antiquity.

Wotton.—Under such conditions we must regard it as an im-
portant advance that at the close of the Middle Ages, when the
interest in scientific investigation awoke anew, Aristotle’s concep-
tions were taken up and elaborated from a scientific standpoint.
In this sense we can call the Englishman Wotton the successor of
Aristotle. In 1552 he published his work ‘De differentiis
animalium,” in which he essentially copied the system of Aristotle,
except that he admitted the new group of plant-animals or
zoophytes. However, the title, ‘On the Distinguishing Characters
of Animals,’ shows that of the rich treasury of Aristotelian knowl-
10 GENERAL PRINCIPLES OF ZOOLOGY.

edge the systematic results obtained the chief recognition, and
thus Wotton’s work inaugurated the period of systematic zoology,
which in the Englishman Ray, but even more in Linneus, has
found its most brilliant exponents.

Linnzus, the descendant of a Swedish clergyman, whose
family name Ingemarsson had been changed after a linden-tree
near the parsonage, to Lindelius, was born in Rashult in 1707.
Pronounced by his teachers to be good for nothing at study, he
was saved from the fate of learning the cobbler’s trade through
the influence of a physician, who recognized the fine abilities of
the boy, and won him for medical studies. He studied at Lund
and Upsala; at the age of twenty-eight he made extended tours on
the Continent, and at that time gained recognition from the fore-
most men in his profession. In 1741 he became professor of medi-
cine in Upsala, some years later professor of natural history. He
died in 1778.

Improvement of Zoological Nomenclature by Linnzeus.—
Linneus’s most important work is his ‘‘ Systema Nature,” which,
first appearing in 1735, up to 1766-68 passed through twelve
editions; after his death there came out a thirteenth, edited by
Gmelin. This has become the foundation for systematic zoology,
since it introduces for the first time (1) a sharper division into the
system, (2) a definite scientific terminology, the binomial nomen-
clature, and (3) brief, comprehensive, clear diagnoses. In classi-
fication Linneeus employed four categories; he divided the entire
Animal Kingdom into Classes, the Classes into Orders, these into
Genera, the Genera finally into Species. The term Family was
not employed in the ‘‘Systema Nature.” Still more important
was the binomial nomenclature. Hitherto the common names were
in use in the scientific world, and led to much confusion: the same
animals had different names, and different animals had the same
names; in the naming of newly discovered animals there prevailed
no generally accepted principle. This inconvenience was entirely
obviated by Linneus in the tenth edition of his Systema by the
introduction of a scientific nomenclature. The first word, a noun,
designates the genus to which the animal belongs, the following
word, usually an adjective, the species within the genus. The
names Canis familiaris, Canis lupus, Canis vulpes, indicate that
the dog, wolf, and fox are related to one another, since they belong
to the same genus, the genus of doglike animals, of which they are
different species. Linneus’s method of naming was particularly
valuable in the description of new species, inasmuch as it at the
HISTORY OF ZOOLOGY. 11

outset informed the reader to what position of relationship the new
species was to be assigned.

In his characterization of the various systematic groups Linneus
broke completely with the hitherto-prevailing custom. His
predecessors (as Gessner, Aldrovandus) in their Natural Histories
had given a verbose and detailed description of each animal, from
which the beginner was scarcely able to see what was specially
characteristic for that animal, a matter which should have been
emphasized in the definition. Linneus, on the other hand, intro-
duced brief diagnoses, which in a few words, never in sentence
form, gave only what was necessary for recognition. Thus a way
was found which insured comprehensibility in the enormously
increasing number of known animals.

Influence of the Linnean System.—But in the great superiority
of the Linnean System lay at the same time the germ of the one-
sided development which zoology came to take under his influence.
The logical perfecting of the system, which undoubtedly had
become necessary, gave that a brilliant aspect, and hid the fact
that classification is not the ultimate purpose of investigation, but
only an important and indispensable aid to it. In the zeal for
naming and classifying animals, the higher goal of investigation,
knowledge of the nature of animals, was lost sight of, and the
interest in anatomy, physiology, and embryology flagged.

From these reproaches we can scarcely spare Linneus himself,
the father of this tendency. For while in his ‘‘ Systema Nature”
he treated of a much larger number of animals than any earlier
zoologist, he brought about no deepening of our knowledge. The
manner in which he divided the animal kingdom, in comparison
with the Aristotelian system, is rather a retrogression than an
advance. Linneus divided the animal kingdom into six classes:
Mammalia, Aves, Amphibia, Pisces, Insecta, Vermes. The first
four classes correspond to Aristotle’s four groups of animals with
blood. In the division of the invertebrated animals into Insecta
and Vermes Linneus stands undoubtedly behind Aristotle, who
attempted, and in part successfully, to set up a larger number of
groups.

But in his successors, even more than in Linnzeus himself, we
see the damage wrought by the systematic method. The diagnoses
of Linneus were for the most part models, which, mutatis
mutandis, could be employed for new species with little trouble.
There was needed only some exchanging of adjectives to express
the differences. With the hundreds of thousands of different

 
12 GENERAL PRINCIPLES OF ZOOLOGY.

species of animals there was no lack of material, and so the arena
was opened for that spiritless zoology of species-making which in
the first half of the last century brought zoology into such discredit.
Zoology would have been in danger of growing into a Tower of
Babel of species-describing had not a counterpoise been created in
the strengthening of the physiologico-anatomical side.

DEVELOPMENT OF MORPHOLOGY.

Anatomists of Classic Antiquity.—Comparative anatomy—for
this chiefly concerns us here—for a long time owed its development
to the students of human anatomy; this is due to the fact that even
up to a recent date comparative anatomy was assigned to the
medical faculty, while zoology belonged to the philosophical
faculty, as if it were an entirely separate study. The disciples of
Hippocrates had previously studied animal anatomy for the pur-
pose of obtaining an idea of human organization, from the struc-
ture of other mammals, and thus to gain a secure foundation for
the diagnosis of human diseases. The work of classical antiquity
most prominent in this respect, the celebrated Human Anatomy
by Claudius Galenus (131-201 a.p.), is based chiefly upon obser-
vations upon dogs, monkeys, etc.; for in ancient times, and even
in the Middle Ages, men showed considerable repugnance to
making the human cadaver a subject of scientific investigation.

Middle Ages.—The first thousand years in which Christianity
formed the ruling power in the mental life of the people was quite
fruitless for anatomy; in the main men held to the writings of
Galen and the works of his commentators, and seldom took occa-
sion to prove their correctness by their own observations. With
the ending of the Middle Ages the interest in independent scien-
tific research first broke its bounds.

Vesal (1514-1564), the creator of modern anatomy, had the
courage carefully to investigate the human cadaver and to point
out numerous errors in Galen’s writings which had arisen through
the unwarranted application to human anatomy of the discoveries
made upon other animals. By his corrections of Galen, Vesal was
drawn into a violent controversy with his teacher, Sylvius, an
energetic defender of Galen’s authority, and with his renowned
contemporary Eustachius, which did much for the development of
comparative anatomy. At first animals were dissected only for
the purpose of disclosing the cause of Galen’s mistakes, but later
through a zeal and love for facts. It was natural that first of all
HISTORY OF ZOOLOGY. 13

vertebrates found consideration, since they stand next to man in
structure. Thus there appeared in the same century with Vesal’s
Human Anatomy drawings of skeletons of vertebrates by the
Nuremberg physician Coiter; the anatomical writings of Fabricius
ab Aquapendente, etc.

Beginning of Zootomy.—But later attention was turned also to
insects and molluscs, indeed even to the marine echinoderms,
celenterates, and Protozoa. Here, above all, three men who lived
at the end of the seventeenth century deserve mention, the Italian
Malpighi and the Dutchmen Swammerdam and Leeuwenhoek.
The former’s ‘‘ Dissertatio de bombyce ” was the pioneer for insect:
anatomy, since by the discovery of the vasa Malpighii, the heart,
the nervous system, the tracheex, etc., an extraordinary extension
of our knowledge was brought about. Of Swammerdam’s writings
attention should be called particularly to “The Bible of Nature,”
a work to which no other of that time is comparable, since it con-
tains discoveries of great accuracy on the structure of bees, May-
flies, snails, etc. Leeuwenhoek, finally, was a most fortunate
discoverer in the field of microscopic research, by him introduced
into science. Besides other things he studied especially the
minute inhabitants of the fresh waters, the ‘ infusion-animalcules,’
a more careful investigation of which has led to a complete reversal
of our conception of the essentials of animal organization.

The Dawn of Independent Observation.—The great service of
the men named above consists chiefly in that they broke away from
the thraldom of book-learning and, relying alone upon their own
eyes and their own judgment, regained what had been lost, the
blessing of independent and-unbiassed observation. They spread
the interest in observation of nature over a wide circle so that in
the eighteenth century the number of independent natural-history
writings had increased enormously. There were busy with the
study of insect structure and development, de Geer in Sweden,
Réaumur in France, Lyonet in Belgium, Résel von Rosenhof in
Germany; the latter besides wrote a monograph on the indigenous
batrachia, which is still worth reading. The investigation of the
infusoria formed a favorite occupation for the learned and the laity,
as Wrisberg, von Gleichen-Russwurm, Schaffer, Eichhorn, and
O. F. Miller. In most of the writings the religious character of
the contemplations of nature are extraordinarily emphasized, and
since we find that among these writers numerous clergymen
(Eichhorn in Danzig, Gaze in Quedlinburg, Schiffer in Regens-
burg) attained distinction, we have a sign that a reconciliation
14 GENERAL PRINCIPLES OF ZOOLOGY.

had taken place between Christianity and natural science. As a
criterion of the progress made in comparison with the earlier
centuries, a mere glance at the illustrations is sufficient. Any one
will at the first glance recognize the difference between the shabby
drawings of an Aldrovandus and the masterly figures of a Lyonet
or a Résel von Rosenhof.

Period of Comparative Anatomy.—Thus through the zeal of
numerous men filled with a love of nature a store of anatomical
facts was collected, which needed only a mental reworking; and
this mental reworking was brought about, or at least entered upon,
by the great comparative anatomists who lived at the end of the
eighteenth and the beginning of the nineteenth century. Among
these the French zoologists Lamarck, Savigny, Geoffroy St. Hilaire,
Cuvier, and the Germans Meckel and Goethe are especially to be
named.

Correlation of Parts.—When the various animals were com-
pared with one another with reference to their structure there was
obtained a series of important fundamental laws, particularly the
law of the Correlation of Parts and the law of the Homology of
Organs. The former established the fact that there exists a
dependent relation between the organs of the same animal, that
local changes in one single organ also lead to corresponding
changes at some distant part of the body, and that therefore from
the constitution of certain parts an inference can be drawn as to
the constitution of another part of the body. Cuvier particularly
made use of this principle in reconstructing the form of extinct
ammals.

Homology and Analogy.—Still more important was the theory
of the Homology of Organs. In the organs of animals a distinction
was drawn between an anatomical and a physiological character;
the anatomical character is the sum of all the anatomical features,
as found in form, structure, position, and mode of connection of
organs; the physiological character is their function. Anatomically
similar organs in closely related animals will usually have the same
functions, as, for example, the liver of all vertebrates has the
function of producing gall; here anatomical and physiological
characteristics go hand in hand. But this need not necessarily be
the case; very often it may happen that one and the same function
is possessed by organs anatomically different; as, for example, the

 

respiration of vertebrates is carried on in fishes by gills, in mammals
by lungs. Conversely, anatomically similar organs may haye
different functions, as the lungs of mammals and the swim-bladdey
HISTORY OF ZOOLOGY. 15

of fishes; similar organs may also undergo a change of function
from one group to another; the hydrostatic apparatus of fishes has
come to be the seat of respiration in the mammals. Organs with
like functions — physiologically equivalent organs—are called
‘analogous’; organs of like anatomical constitution—anatomically
equivalent organs—are called ‘homologous.’ It is the task of
comparative anatomy to discover in the various parts of animals
those which are homologous, i.e. those anatomically equivalent,
and to follow the changes in them conditioned by a change of
function.

Cuvier.—The foremost representative of comparative anatomy
was Georges Dagobert Cuvier. He was born in 1769 in the town
of Mompelgardt (Montbeillard), then belonging to Wirtemberg,
and obtained his early training in the Karlschule at Stuttgart,
where, through the influence of his teacher Kielmeyer, he was led
to the study of comparative anatomy. The opportunity of going
to the seashore which was offered to him as private instructor to
Count d’Héricy he employed for his epoch-making investigations
upon the structure of molluscs. In 1794, upon the persuasion
largely of the man who afterwards became his great opponent,
Geoffroy St. Hilaire, he moved to Paris, where he was made at
first Professor of Natural History in the central school and in the
College of France, later Professor of Comparative Anatomy in the
Jardin des Plantes. As a sign of the great regard in which Cuvier
was held, it should be noticed that he was repeatedly intrusted with
high educational positions and was made a French peer. As such
he died in 1832.

Type Theory.—Cuvier’s investigations, apart from the mol-
luses, extended to the celenterates, arthropods, and vertebrates,
living and fossil. Tle collected his extensive observations into his
two chief works ‘Le régne animal distribué d’aprés son organiza-
tion” and ‘+ Lecons d’anatomie comparée.” Of quite epoch-making
importance was his little pamphlet ‘Sur un rapprochement a ¢tablir
entre les différentes classes des animaux,” in which he founded his
celebrated type theory, and which in 1812 introduced a complete
reform of classification. The Cuviecrian division, which has become
the starting-point for all later classifications, differed, broadly
speaking, from all the earlier systems in this, that the classes of
mammals, birds, reptiles, and fishes were brought together into a
higher grade under the name, introduced by Lamarck, of ‘ verte-
brate animals’; that further the so-called ‘invertebrate animals’
were divided into three similar grades, each equal to that of the

 

 

 
16 GENERAL PRINCIPLES OF ZOOLOGY.

vertebrate animals, viz., Mollusca, Articulata, and Radiata. Cuvier
called these grades standing above the classes, provinces or chief
branches (embranchements), for which later the name Types was
introduced by Blainville. But still more important are the differ-
ences which appear in the structural basis of the system. Instead
of, like the earlier systematists, using a few external character-
istics for the division, Cuvier built upon the totality of internal
organization, as expressed in the relative positions of the most
important organs, especially the position of the nervous system,
as determining the arrangement of the other organs. ‘The
type is the relative position of parts” (von Baer). Thus for the
first time comparative anatomy was employed in the formation of
a natural system of animals.

Lastly the type theory established an entirely new conception
of the arrangement of animals. Cuvier found prevalent the theory
that all animals formed a single connected series ascending from
the lowest infusorian to man; within this series the position of
each animal was definitely determined by the degree of its organi-
zation. On the other hand Cuvier taught that the animal kingdom
consisted of several co-ordinated unities, the types, which exist
quite independently side by side, within which again there are
higher and lower forms. The position of an animal is determined
by two factors: first, by its conformity to a type, by the structural
plan which it represents; second, by its degree of organization, by
the stage to which it attains within its type.

Comparative Embryology.—Evolution vs. Epigenesis.—The
same results which Cuvier reached by the way of comparative
anatomy were attained two decades later by C. E. von Baer by the
aid of embryology. Embryology is the youngest branch of
zoology. What Aristotle really knew, what was written by
Fabricius ab Aquapendente and Malpighi upon the embryology of
the chick, did not rise above the range of aphorisms, and were not
of sufficient value to make a science. The difficulties of observa-
tion, due to the delicacy and the minuteness of the developmental
stages, were lessened by the invention of the microscope and
microscopical technique. Further, the prevailing philosophical
conceptions placed hindrances in the way; there was no belief in
Embryology in the present sense of the word; each organism was
thought to be laid down from the first complete in all its parts,
and only needed growth. to unfold its organs (evolutio*); either the

* This original meaning of ‘evolution’ is different from that prevailing
at present.
HISTORY OF ZOOLOGY. 17

spermatozoon must be the young creature which found favorable
conditions for growth in the store of food in the egg, or the egg
represents the individual and was stimulated to the ‘ evolutio’ by
the spermatozoon. This theory led to the doctrine of inclusion,
which taught that in the ovary of Eve were included the germs of
all human beings who have lived or ever will live.

Caspar Friedrich Wolff combated this idea with his ‘‘ Theoria
generationis ” (1759); he sought to prove by observation that the
hen’s egg at the beginning is without any organization, and that
gradually the various organs appear in it. In the embryo there is
a new formation of all parts, an Hpigenesis. This first assault
upon the evolutionist school was entirely without result, chiefly
because Albrecht von Haller, the most celebrated physiologist of
the eighteenth century, by his influence suppressed the idea of
epigenesis. Wolff was not able to establish himself in scientific
circles in Germany, and was obliged to emigrate to Russia. Only
after his death did his writings find, through Oken and Meckel,
proper recognition.

Von Baer.—Thus it remained for Carl Ernst von Baer in his
classic work, ‘‘ Die Entwicklung des Hithnchens, Beobachtung
und Reflexion” (1832), to establish embryology as an independent
study. Baer confirmed Wolff’s doctrine of the appearance of
layerlike Anlagen, from which the organs arose; and on account
of the accuracy with which he proved this he is considered the
founder of the germ-layer theory. Further, he came to the con-
clusion that each type had not only its peculiar structural plan,
but also its peculiar course of development; that for vertebrates
an evolutio bigemina was characteristic, for the articulates the
evolutio gemina, for the molluscs the evolutio contorta, and for
the radiates the evolutio radiata. Here we meet for the first time
the idea that for the correct solution of the questions of relation-
ship of animals, and therefore a basis for a natural classification,
comparative embryology is indispensable; an idea which in recent
years has proved exceedingly fruitful.

Cell Theory.—Of fundamental importance for the further
growth of comparative anatomy and embryology was the proof
that all organisms, as well as their embryonic forms, were com-
posed of the same elements, the cells. This knowledge is the
quintessence of the cell theory, which during the third decade of
the last century was advanced by Schleiden and Schwann, and
which two decades later was completely remodelled by the proto-
plasm theory of Max Schultze. In the cell theory a simple prin-
18 GHNERAL PRINCIPLES OF ZOOLOGY.
ciple of organization was found for all living creatures, for highly

and for lowly organized plants and animals, and the wide realm of
histology was laid open for scientific treatment.

REFORM OF THE SYSTEM.

 

Foundation of Modern Zoology.—With the establishment of
comparative anatomy and embryology and the application of these
to classification, and with the development of the cell theory and
of histology, which is connected with it, we may say that the
foundation of zoology was laid. Wonderful advances were made
in vertebrate anatomy by the classic researches of Owen, Johannes
Miller, Rathke, Gegenbaur, and others; our conceptions of organ-
ization have been completely altered by the work of Dujardin,
Max Schultze, Haeckel, and others, who have proved the unicellu-
larity of the lowest animals. The germ-layer theory was further
elaborated by Remak and Koélliker; and applied to the invertebrate
animals by Kowalewsky, Haeckel, and Huxley. It is beyond the
limits of this brief historical summary to go into what has been
accomplished in regard to the other branches of the animal king-
dom; it must here be sufficient to mention the most important
changes which the Cuvierian system has undergone under the
influence of increasing knowledge.

The Division of the Radiata.—Of the four types of Cuvier the
branch Radiata was undoubtedly the one of whose representatives
he had the least knowledge; it was therefore the least natural,
since it comprised, besides the radially symmetrical celenterates
and echinoderms, other forms, which, like the worms, were
bilaterally symmetrical, or, like many infusorians, were asym-
metrical. Thus it came about that most reforms have here found
their point of attack.

C. Th. von Siebold was the originator of the first important
reform. He limited the type Radiata, or, as he termed them, the
Zoophytes, to those animals with radially symmetrical structure
(Echinoderms and the Plant-animals); separating all the others,
he formed of the unicellular organisms the branch of * primitive
animals’ or Protozoa; the higher organized animals he grouped
together as worms or Vermes; at the same time he transferred a
part of the Articulata, the annelids, to the worm group, and pro-
posed for the other articulates, crabs, millipedes, spiders, and
insects, the term Arthropoda.

Leuckart, about the same time (1848), divided the branch

 
HISTORY OF ZOOLOGY. 19

Radiata into two branches differing greatly in structure. The
lower forms, in which no special body-cavity is present, the
interior of the body consisting only of a system of cavities serving
for digestion, the alimentary canal, he called the Cclentera
(essentially the Zoophyta of the older zoologists); to the rest, in
which the alimentary canal and the body-cavity occur as two
separate cavities, he gave the name Echinoderma.

The Present System.—Thus there resulted seven classes:
Protozoa, Ceelentera, Echinoderma, Vermes, Arthropoda, Mol-
lusca, and Vertebrata. Still this arrangement does not meet the
requirements of a natural system and hence is ‘more or less unsat-
isfactory. Some zoologists are returning to the Cuvierian classifi-
cation to the extent of uniting the segmented worms with the
arthropods in a group Articulata. Upon the ground of important
anatomical and embryological characters the Brachiopoda, the
Bryozoa, and the Tunicata have been separated from the Mollusca;
they form the subject of diverse opinions. The relationships of
the first two groups have not yet been settled: of the Tunicata we
know indeed that they are related to the Vertebrata, but the
differences are such that they cannot be included in that group.
The only way out of the difficulty is to unite vertebrates, tunicates,
and some other forms in a larger division, Chordata. The Vermes,
too, must be divided, as will appear in the second part of this
volume.

HISTORY OF THE THEORY OF EVOLUTION.

Importance of the Subject.—Before closing the historical intro-
duction we must consider the historical development of a question
whose importance might, on a superficial examination, be under-
rated, but which from a small beginning has grown into a problem
completely dominating zoological research, and has oceupied not
only zoologists, but all interested in science generally. This is the
question of the logical value of the systematic conceptions species,
genus, family, etc.

The Nature of Species.—In nature we find only separate
animals: how comes it that we classify them into larger and smaller
groups? Are the single species, genera, and the other divisions
which the systematist distinguishes, fixed quantities, as it were
fundamental conceptions of nature, or a Creator’s thoughts, which
find expression in the single forms? Or are they abstractions
which man has brought into nature for the purpose of making it

 
20 GENERAL PRINCIPLES OF ZOOLOGY.

comprehensible to his mental capabilities? Are the specific and
generic names only expressions which have become necessary, from
the nature of our mental capacity, for the gradation of relation-
ship in nature, which in and for themselves are not immutable,
and hence can undergo a gradual change? Practically speaking,
the problem reads: are species constant or changeable? What is
true for species must necessarily be true for all other categories of
the system, all of which in the ultimate analysis rest upon the
conception of species.

Ray’s Conception of Species.—One of the first to consider the
conception of species was Linneus’s predecessor, the Englishman
John Ray. In the attempt to define what should be understood
as a species he encountered difficulties. In practice, animals which
differ little in structure and appearance from one another are
ascribed to the same species; this practical procedure cannot be
carried out theoretically; for there are males and females within
the same species which differ more from one another than do the
representatives of different species. Thus John Ray reached the
genetic definition when he said: for plants there is no more
certain criterion of specific unity than their origin from the seeds
of specifically or individually like plants; that is to say, generalized
for all organisms: to one and the same species belong individuals
which spring from similar ancestors.

The ‘Cataclysm Theory.’—With Ray’s definition an entirely
uncontrollable element was brought into the conception of species,
since no systematist usually knew anything, nor indeed could he
know anything, as to whether the representatives of the species
placed before him sprang from similar parents. It was therefore
only natural that the conception of species put on a religious garb,
since by resting upon theological ideas it found a firmer support.
Linneus said: ‘* Tot sunt species quot ab initio creavit infinitum
Ens”; with this he built up a conception of species upon the
tradition of the Mosaic history of creation, a procedure quite
unjustified upon grounds of natural science, since it drew one of
its fundamental ideas from transcendental conceptions, not from
the experience of natural science. Linneus’s definition showed
itself untenable, as soon as paleontology began to make accessible
a vast quantity of extinct animals deposited as fossils. With an
odd fancy, the fossils, being inconvenient for study, were for a
long time regarded as outside the pale of scientific research. They
might be sports of nature, it was said, or remains of the Flood, or
of the influence of the stars upon the earth, or products of an aura

 

 
HISTORY OF ZOOLOGY. 21

seminalis, a fertilizing breath, which, if it fell upon organic bodies,
led to the formation of animals and plants, but if it strayed upon
inorganic materials gave rise to fossils. The foundation of
scientific paleontology by Cuvier put an end to such empty specu-
lations. Cuvier proved beyond a doubt that these fossils were the
remains of animals of a previous time. Just as the formation of
the earth’s crust by successive overlying layers made possible the
recognition of different periods in the earth’s history, so paleon-
tology taught how to recognize also the different periods in the
vegetable and animal world of life on our globe. Each geological
age was characterized by a special world of animals quite peculiar
to it; and these animal worlds differed the more from the present,
the older the period of the earth to which they belonged. All
these generalizations led Cuvier to his cataclysm theory, that a
great revolution brought each period of the earth’s history to an
end, destroying all life, and that upon the newly formed virgin
earth a new organic world of immutable species sprang up.

Objections to the Cataclysm Theory.—By the supposition of
numerous acts of creation the Linnean conception of species
seemed to be rescued, though, to be sure, by summoning to its aid
hypotheses which had neither foundation in science nor justifica-
tion in theology. The logical results of Cuvier’s cataclysm theory
were conceptions of a Creator who built up an animal world only
for the purpose of destroying it after a time as a troublesome toy;
it has therefore at no time found warm supporters, at least among
geologists, for whom it was intended. Of the prominent zoologists
there is only to be mentioned Louis Agassiz, who till the end of
his life remained faithful to this theory.

Under these conditions it is readily understood how thinking
naturalists, who felt the necessity of explaining the character of
organic nature simply and by a natural law capable of general
application, began to doubt the fixity of species, and were led to
the theory of change of form, the Theory of Descent, or Evolution.

Darwin’s Predecessors.— Even in Cuvier’s time there prevailed
a strong current in favor of this theory. It found expression in
England in the writings of Erasmus Darwin (grandfather of the
renowned Charles Darwin); in Germany in the works of Goethe,
Oken, and the disciples of the ‘natural philosophical’ school; in
France the genealogical theory was developed particularly by
Buffon, Geoffroy St. Hilaire, and Lamarck. Its completest ex-
pression was found in Lamarck’s ‘* Philosophie zoologique ” (1809);
its arguments will be considered in the following paragraphs.
22 GENERAL PRINCIPLES OF ZOOLOGY.

Lamarck (Jean Baptiste de Monet, Chevalier de Lamarck,
born in Picardy, 1744, died, Professor at the Jardin des Plantes,
1829) taught that on the earth at first organisms of the simplest
structure arose in the natural way through spontaneous generation
from non-living matter. From these simplest living creatures have
developed, by gradual changes in the course of an immeasurably
vast space of time, the present species of plants and animals,
without any break in the continuity of life upon our globe; the
terminal point of this series is man; the other animals are the
descendants of those forms from which man has developed.
Lamarck, in accordance with the then prevailing conceptions,
regarded the animal kingdom as a single series grading from the
lowest primitive animal upto man. Among the causes which may
influence the change and perfecting of organisms, Lamarck
emphasized particularly wse and disuse; the giraffe has obtained a
long neck because by a special condition of life he was compelled
to stretch, in order to browse the leaves on high trees; conversely,
the eyes of animals which live in the dark have degenerated from
lack of use into functionless structures. The direct influence of
the external world must be unimportant; the changes in the sur-
roundings (Geoffroy St. Hilaire’s le monde ambient) must for the
most part act indirectly upon animals by altering the conditions
for the use of organs.

Evolution vs. Creation.—Lamarck’s ingenious work remained
almost unnoticed by his contemporaries. On the other hand there
arose a violent controversy between the defenders and the
opponents of the evolution theory when [1830] Geoffroy St. Hilaire
in a debate in the Academy at Paris defended against Cuvier the
thesis of a near relationship of the vertebrates and the insects, and
set up the proposition that the latter were ‘* vertebrates running
on their backs.” The conflict ended in the complete overthrow ot
the theory of evolution; the defeat was so complete that the
problem vanished for a long time from scientific discussion, and
the theory of the fixity of species again became dominant. This
error was occasioned by many causes. Above all, the theory of
Geoffroy St. Hilaire and Lamarck was rather a clever conception
than founded on abundant facts; besides, it had in it as a funda-
mental error the doctrine of the serial arrangement of the animal
world. Opposed to this stood Cuvier’s great authority and his
extensive knowledge, the latter making it easy for him to show
that the animal kingdom was made up of separate co-ordinated
groups, the types.

 
HISTORY OF ZOOLOGY. 23

Lyell.—In the same year in which Cuvier obtained his victory
over Geoffroy St. Hilaire, his theory of the succession of numerous
animal worlds upon the globe received its first destructive blow.
Cuvier’s cataclysm theory had two sides, a geological anda
biological. Cuvier denied the continuity of the various terrestrial
periods, as well as the continuity of the fauna and flora belonging
to them. In 1830-32 appeared the ‘ Principles of Geology” by
Lyell, an epoch-making work, which, in the realm of geology,
completely set aside the cataclysm theory. Lyell proved that the
supposition of violent revolutions on the earth was not necessary
in order to explain the changes of the earth’s surface and the
superposition of its strata; that rather the constantly acting
forces, elevations and depressions, the erosive action of water, be
it as ebb and flow of the tide, as rain, snow, or ice, or as the flow
of rivers and brooks rushing as torrents towards the sea, are suffi-
cient to furnish a complete explanation. Very gradually in the
course of a vast space of time the earth’s surface has changed, and
passed from one period into the next, and still at the present day
the constant process of change is going on. The continuity in the
geological history of the earth, here postulated for the first time,
has since then become one of the fundamental axioms of Geology;
on the other hand the discontinuity of living creatures, although
the geological support of this was frail, was for a long time
regarded as correct.

Darwin.—lIt is the great merit of Charles Darwin that he took
up the theory of descent anew after it had rested a decade, and
later brought it into general recognition. With this began the
most important period in the history of zoology, a period in which
the science not only made such an advance as never before, but
also began to obtain a permanent influence upon the general views
of men.

Charles Darwin was born at Shrewsbury, Eng., in 1800. After
studying at the universities of Edinburgh and Cambridge, he joined
as naturalist the English war-ship ‘* Beagle.” In its voyage from
1831 to 36 around the globe, Darwin recognized the peculiar
character of island faunas, particularly of the Galapagos Islands,
and the remarkable geological succession of edentates in South
America; these facts formed for him the germ of his epoch-making
theory. Further results of this journey were his beautiful mono-
graph on the Cirripedia, and the classic investigation of coral-reefs.
After his return to England Darwin lived, entirely devoted to
scientific work, chiefly in the hamlet of Down, county Kent, up

 
24 GHNERAL PRINCIPLES OF ZOOLOGY.

to the time of his death in 1882. He was incessantly busy in
developing his conception of the origin of species, and in collect-
ing for this a constantly increasing array of facts. The first
written notes, the fundamental ideas of which he communicated
to friends, particularly the geologist Lyell and the botanist
Hooker, were made in 1844, but the author was not persuaded to
give them publicity. Not until 1858 did Darwin decide to make
his first contribution to science. In this year he received an essay
sent by the traveller Wallace, which in its most important points
coincided with his own views. At the same time with Wallace’s
manuscript an abstract of Darwin’s theory was published. In the
next year (1859) appeared the most important of his writings,
“On the Origin of Species by means of Natural Selection,” and
in rapid succession a splendid series of works, the fruit of many
years of preparatory labors. For the history of the theory the
most important of these are: (1) ‘‘ Upon the Variation of Plants
and Animals under Domestication,” two volumes, which chiefly
contain a collection of material for proofs; (2) on ‘‘ The Descent
of Man,” a work which gives the application of the theory to man.
No scientific work of this century has attracted so much atten-
tion in the zoological, we may even say in the whole educated
world, as ‘‘The Origin of Species.” It was generally received as
something entirely new, so completely had the scientific tradition
been lost. In professional circles it was stoutly combated by one
faction, with another it found well-wishing but hesitating accept-
ance. Only a few men placed themselves from the beginning in
a decided manner on the side of the great British investigator.
There was a lively scientific battle, which ended in a brilliant vic-
tory for the theory of evolution. At the present time all our
scientific thoughts are so permeated with the idea of evolution that
we can scarcely speak of any considerable opposition to it.
Post-Darwinian Writers.—Among the men who have most
influenced this rapid advance is to be mentioned, besides A. R.
Wallace, the co-founder of Darwinism, above all Ernst Haeckel,
who in his ‘“‘ General Morphology” and his ‘‘ Natural History of
Creation” has done much towards the extension of the theory.
Among other energetic defenders of the theory in Germany should
be mentioned Fritz Miller, Carl Vogt, Weismann, Moritz Wagner,
and Nageli, even if they have taken special standpoints in refer-
ence to the causes conditioning the changes of form. Among the
English naturalists are to be named particularly Huxley, Hooker,
and Lyell. In America Gray, Cope, and Ilyatt were early sup-
HISTORY OF ZOOLOGY. 25

porters. Darwinism was long in obtaining an entrance into
France.

DARWIN'S THEORY OF THE ORIGIN OF SPECIES.

Before Darwin wrote the idea of fixity of species prevailed
among systematists. It was recognized that all the individuals of
a species are not alike, and that more or less pronounced variability
occurs, so that it was possible to distinguish races and varieties
within the species, but it was believed that the variations never
transcended specific bounds.

The Problem Stated.—Darwin begins with a criticism of the
term species. Is the conception of species on the one side and
that of race and variety on the other something entirely different ?
Are there special criteria for determining beyond the possibility
of a doubt whether in a definite case we have to do with a variety
of a species or with a different species? or do the conceptions in
nature pass into one another? Are species varieties which have
become constant, and precisely in the same manner are varieties
species in the process of formation ?

Morphological Characters.— A. Distinction between Species and
Variety. —For the settlement of this fundamental question morpho-
logical and physiological characters can be considered. In the
practice of the systematists usually the morphological characters
prevail exclusively; for that reason they will be here considered
first. If, among a great number of forms similar to one another,
two groups can be adduced which differ considerably from one
another, if the difference between them be obliterated by no inter-
mediate forms, and if in several successive generations they
remain constant, then the systematist speaks of a ‘ good species’ ;
on the other hand he speaks of varieties of the same species when
the differences are slight and inconstant, and when they lose their
importance through the existence of intermediate forms. A
definite application of this rule discloses great incongruities, many
animal and vegetable groups being regarded by one set of systema-
tists as good species, by another only as ‘sports,’ ie., as varieties
of the same species. The differences between the ‘sports’ of our
domestic animals are in many instances so considerable that
formerly they were regarded not only as sufficient for the founda-
tion of good species, but even of genera and families. In the
fantail pigeon the number of tail-feathers, formerly only 12-14,
has increased to 30-42 (fig. 1c); among the other races of pigeons
26 GENERAL PRINCIPLES OF ZOOLOGY.

enormous variations are found in the size of the beak and feet in
comparison with the rest of the body (figs. 1a, 1p); even the
skeleton itself varticipates in this variation, as is shown by the fact

 

Fig. 1A.—English carrier-pigeon. (After Darwin.)

 

Fic. 13.—English tumbler-pigeon. (After Darwin.)

that the total number of vertebre varies from 38 (in the carrier-
pigeon) to 43 (in the pouter), the number of sacral vertebra from
14 to 11.

B. Variation within the Species.—Now in respect to the occur-
rence of transitional forms and the constancy of differences, there
is within one and the same ‘ good species’ the greatest conceivable
difference. In many very variable species the extremes are united
HISTORY OF ZOOLOGY. 27

by many transitions; in other cases sharply circumscribed groups
of forms, or races, can be distinguished within the same species.
In the race, the peculiar characteristics are inherited from genera-

 

Fig. 1c.—English fantail pigeon. (After Darwin.)

tion to generation with the same constancy as in good species.
This is shown in the human races, and in many pure, cultivated
races of domesticated animals.

Physiological Characters.—A. Crossing of Species and Varie-
ties.—A critical examination leads to the conclusion that Mor-
phology is indeed useful for grouping animals into species and
varieties, but that it leaves us completely in the lurch when it is
called upon to show the distinctions between what should be called
a species and what a variety. Therefore there remains open to
the systematist only one resource, i.e., to summon Physiology
to his aid. This has been done, and it has disclosed considerable
distinctions in reproduction. We should expect a priori that the
individuals of different species would not reproduce with each
other; on the other hand under normal conditions the individuals
of one and the same species, even though they are of different
varieties or races, should be entirely fertile. One must beware of
arguing in a circle in proof of these two propositions; it would be
an argument in a circle if an experimenter should regard two
animals as representatives of one species only because they proved
to be fertile together, while under their former relations he
assigned them to different species. Rather the question for him
must read: does physiological experiment lead to the same
28 GENERAL PRINCIPLES OF ZOOLOGY.

systematic distinctions as does the common systematic experience,
viz., the depreciation of constancy and the divergence of distin-
euishing characters ?

iB. Lhe Intercrossing of Species. —This field is as yet far from
being sufficiently investigated experimentally; yet some general
propositions can be set up: (1) that not a few so-called * good
species’ can be crossed with one another; (2) that in general the
difficulty of crossing increases, the more distant the systematic
relationship of the species used; (3) that these difficulties are by
no means directly proportional to the systematic divergence of the
species. The most favorable material for research is furnished by
those animals in which artificial fertilization can be carried out,
i.e., of which one can take the eggs and spermatozoa and mix
them independently of the will of the animals. Thus hybrids
have been obtained from species which belong to quite different
genera, while very often nearly-related species will not cross.
Among fishes we know hybrids of Adramis brama and Blicca
bjirkna, of Trutta salar (salmon) and Trutta fario (trout); among
sea-urchins the spermatozoa of Strongylocentrotus lividus fertilize
with great readiness the eggs of Lchinus microtuberculatus, but
only rarely the eggs of Spherechinus granularis, which is nearer
in the system. It also happens that crossing in one direction
(male of A and female of B) is easily accomplished, but in the
other direction (male of B and female of A) it completely fails; as,
for example, the sperm of Strongylocentrotus lividus fertilizes well
the eggs of Lchinus microtuberculatus, but, conversely, the sperm
of £. microtuberculatus does not fertilize the eggs of S. lividus.
Even better known is the fact that salmon eggs are fertilized by
trout sperm but not trout eggs by salmon sperm. Eggs have been
fertilized by sperm belonging to different families, orders, and
possibly classes. Eggs of Pleuronectes platessa and Labrus rupestris
by sperm of the cod (Gadus morrhua), frogs’ eggs (Rana arvalis)
by sperm of two species of Triton, eggs of a starfish (Asferias
Forbest) by milt from a sea-urchin, Arbacia pustulosa (??). In
these extreme cases, it is true, the hybrids die during or at the
close of segmentation, before the embryo is outlined.

In the case of animals where copulation is necessary the diffi-
culties of experimentation increase, since here often between males
and females of different species there exists an aversion which
prevents any union of the sexes. Yet in this case we know crosses
of different species; among the vertebrates crossing takes place,
e.g., between the horse and the ass; our domestic cattle and the
HISTORY OF ZOOLOGY. 20

zebu; ibex (or wild buck) and she-goat; sheep and goats; dog and
jackal; dog and wolf; hare and rabbit (Lepus darwini); American
bison and domestic cattle; etc.; among birds, between different
species of finches and of grouse; mallard (Anas boschas) and the
pintail duck (Dajfila acuta); the European goose and the Chinese
goose (Anser ferus and A. cygnoides). Among the insects,
especially the Lepidoptera, the cases are many, but the resulting
eggs produce larve of slight vital force only in the case of Actias
luna and A. isabelle.

C. Fertility of Hybrids and Mongrels.—Since many hybrids,
as the mule, have been known for thousands of years, the criterion
is, as it were, pushed back one stage; if the infertility in cases of
crosses in many species is not immediately noticeable, yet it may
be apparent in the products of the cross. While the products of
the crossing of varieties, the ‘mongrels,’ always have a normal,
often an increased, fertility, the products of the crossing of
species, the hybrids, should always be sterile. But even this is
a rule, not a law. The mule (which only very rarely reproduces)
and many other hybrids are indeed sterile, but there are not a few
exceptions, although the number of experiments in reference to
this point is very small. Hybrids of hares and rabbits have con-
tinued fruitful for generations; the same is true of hybrids
obtained from the wild buck and the domesticated she-goat; from
Anser cygnoides and A. domesticus; from Salmo salvelinus and
S. fontinalis; Cyprinus carpio and Carassius vulgaris; Bombyx
cynthia and B. arrindia.

D. Inbreeding.—Even the second of the above statements, that
individuals of a species, provided they are sound, always reproduce
with one another, needs limitation. Breeders of animals have long
known the disastrous consequences of inbreeding—that the repro-
ductive power is reduced even to sterility if, for breeding,
descendants of a single pair be continually chosen. Darwin has
collected not a few cases where undoubted members of the same
species have been completely sterile with one another; as certain
forms of primrose and other di- and tri-morphic species. Exam-
ples of the sterility of mongrels are known only in botany (certain
varieties of maize and mullein).

Conditions Governing Fertility in Sexual Reproduction.—
When we look over these facts it would seem as if continued
fertility in sexual reproduction were guaranteed by a not too con-
siderable difference in the sexual products. Too great similarities,
as these exist in inbreeding, and too great differences, as in the
30 GENERAL PRINCIPLES OF ZOOLOGY.

hybridization of different species, are injurious and are abhorred
by Nature. Sexual reproduction possesses an optimum; if this be
departed from in either direction, diminution gradually follows.
But for that reason it has already been said that here gradual and
not primary differences exist, and therefore this character cannot
be employed asa primary distinction between species and varieties.

Difficulties in Classification.—The final result of all this dis-
cussion may be summed up as follows: up to the present time,
neither by physiological nor by morphological evidence has there
been successfully fixed in a clear and generally applicable way a
criterion which can guide the systematist in deciding whether
certain series of forms are to be regarded as good species or as
varieties of a species. Zoologists are guided rather in practice by
a certain tact for classification, which, however, in difficult cases
leaves them in the lurch, and thus the opinions of various investi-
gators vary.

Change of Varieties into Species.—The conditions above dis-
cussed find their natural explanation in the assumption that sharp
distinctions between species and variety do not exist; that species
are varieties which have become constant, and varieties are incipient
species. The meaning of the above can be made clear by explana-
tion of a concrete case. Individuals of a species begin to vary,
i.e., compared with one another they attain a greater or less
difference in character. So Jong as the extreme differences are
bridged by transitional forms we speak of varieties of a species;
if, on the other hand, the intermediate transitions have died out.
and the differences have in the course of a long space of time
become fixed, and so very much intensified that a sexual union of
the extreme forms results either in complete sterility or at least in
a marked tendency towards sterility, then we speak of different
species.

Species may be Related to each other in Unequal Degrees.—
In favor of this view, that varieties will in longer time become
species, is the great agreement which in the large majority of
cases exists between the two. In genera which comprise a remark-
able number of species, the species usually show also many
varieties; the species are then usually grouped in sub-genera, i.e.,
they are related to each other in unequal degrees, since they form
small groups arranged around certain species. In regard to the
varieties also the case is similar. In such genera the formation of
species is in active progress; but each species formation presup-
poses a high degree of variability.

 
HISTORY OF ZOOLOGY. 31

Phylogeny.—It is now clear that what has here been worked
out in the case of the species must also apply to the other cate-
gories of the system. Just as by divergent development varieties
become species, so must species by continued divergence become
so far removed from one another that we distinguish them as
genera. It will be only a question of time when these differences
will become still greater, and cause the establishment of orders,
classes, and branches, just as the tender shoots of the young
plantlet become in the strong tree the chief branches from which
spring lateral branches and twigs. If we pursue this train of
thought to its ultimate consequences, we reach the conception that
all the animals and plants living at present have arisen by means
of variation from a few primitive organisms. Inasmuch as at least
many thousands of years are required for the formation of several
new species through the variability of one, there must then have
been necessary for this historical development of the animal and
vegetable kingdoms a space of time greater than our mental
capacity can grasp. Since for the idea of the individual develop-
ment (embryology) of an animal the term Ontogeny has been
chosen, it has also proved convenient to apply to the historical
development of animals—though this has not been observed, but
only inferred—the term History of the Race or Phylogeny.

Spontaneous Generation.—If we attempt to derive all living
animals from a common primitive form, we are compelled to
assume that this was of extremely simple organization, that it was
unicellular; for the simpler, the less specialized, the organization,
so much the greater is its capacity for variation. Only from
simple organisms can the lower unicellular organisms, the
Protozoa, be derived. Finally, for the simple organisms alone
can we conceive a natural origin. Since there was undoubtedly a
time upon our earth when temperatures prevailed which made life
impossible, life must at some time have arisen, either through an
act of creation or in a natural way through spontaneous generation.
If, in agreement with the spirit of natural science, we invoke for
the explanation of natural facts only the forces of nature, we are
driven to the hypothesis of spontaneous generation, namely, that
by a peculiar combination of materials without life, the compli-
cated mechanism which we call ‘life’ has arisen. This hypothesis
also supposes that the first organisms possessed the simplest con-
ceivable structure.

Variability not proven to be a Universal Principle.—Starting
from a basis of facts, by generalization we reach a simple concep-

 

 
32 GHNHRAL PRINCIPLES OF ZOOLOGY.

tion of the origin of the animal kingdom, but we have in equal
measure departed from the results of direct observation. Observa-
tions only show us that species are capable of changes and can
from themselves produce new species. That this capacity for
variation is a universal principle, a principle which explains to us
the origin of the animal world, needs further demonstration.

Proofs of Phylogeny.—The rise of the existing animal world
is a process which has taken place in the thousands of years long
past, but is no longer accessible for direct observation, and there-
fore it can never be proved in the sense that we explain the indi-
vidual development of an organism. In regard to the conception
of the simple evolution of animals we can merely prove the
probability; yet it is shown that all our observations of accessible
facts not only agree with this conception, but find in it their only
simple explanation. Such facts are furnished to us by the classi-
fication of animals, paleontology, geographical distribution, com-
parative anatomy, and comparative embryology.

(1) Proofs from Classification.—For a long time it has been
recognized, and in recent times finds ever-increasing confirmation,
that if we wish to express graphically the relationships of animals,
theiz classes, orders, genera, and species, simple co-ordination and
subordination are not sufficient, but one must select a treelike
arrangement, in which the principal divisions, more closely or dis-
tantly related to one another,—the branches, phyla, or types,—
represent the main limbs, while the smaller branches and twigs
correspond to the several classes, orders, etc. This is, in fact,
the arrangement to which the theory of evolution, as seen above,
necessarily leads.

(2) Paleontological Demonstration approaches nearest to what
one might call direct proof; for paleontology gives us the only
traces of existence which the predecessors of the present animal
world have left. Even here a hypothetical element has crept into
the demonstration. We can only observe that various grades of
forms of an animal group are found in successive strata: if we
unite these into a developmental series, and regard the younger
as derived from the older by variation, we depart, strictly speak-
ing, from the basis of fact. But the value of paleontological
evidence is weakened much more by its extreme incompleteness.
In fossils only the hard parts are generally preserved; the soft
parts, on the other hand, which alone are present, or at least make
up the most important part of many animals, are almost always
lost. Only rarely are the soft parts (muscle of fishes, ink-bag of

 
HISTORY OF ZOOLOGY. 33

cephalopods, outlines of meduse) preserved in the rocks. Even
the hard parts remain connected only under exceptionally favor-
able conditions. If further we take into consideration the fact
that these treasures are buried in the bosom of the earth, and are
usually obtained only by accident, in quarrying and road-build-
ing, and besides only extremely seldom excavated with scientific
care, it becomes sufficiently clear how little is to be expected from
the past and indeed future material of paleontology.

Examples of Paleontological Proof.—Yet paleontology has
already furnished many important proofs of the theory of descent.

 

Fria. 2.—Archcopteryx lithographica. (After Zittel.) cl, clavicle; co, coracoid; h,
humerus; r, radius; u, ulna; c, carpus; I-IV, digits; sc, scapula.

It has shown that the lower forms appeared first, and the more

highly organized later, Among animals in general the latest to

appear were the vertebrates, and of these the mammals; among

the mammals man. For smaller groups genealogical material has
3 GENERAL PRINCIPLES OF ZOOLOGY.

fortunately been found. Transitional forms connect the single-
toed horse of the present with the four-toed Mohippos of the
eocene; for all the hoofed animals a common starting-point or
ancestral form has been found in the Condylarthra. Transitional
forms have also been found between the greater divisions, as, e.g.,
between reptiles and birds, the remarkable toothed birds, und the
Archeopteryx (fig. 2), a bird with a long, feathered, lizard-like
tail.

(3) Morphological Proofs. —When we employ comparative
anatomy and embryology in support of evolution, we find that the
two studies have so many points in common that they can best be
treated together.

Cuvier and von Baer taught that the separate types of the
animal kingdom are units, each with a special structure and plan
of development peculiar to it; farther, that there are no similari-
ties in structure and in the development forming a bridge from
type to type. The first of these two propositions is still regarded
as correct, but the second, which alone is important for the theory
of evolution, has become quite untenable. All animals have a
common organic basis in the cell and are thereby brought close
to one another; all multicellular animals agree in the principal
points during the first stages of their development, during the
fertilization, cleavage of the egg, and the formation of the first
two germ-layers, and vary from one another only in such differ-
ences as may occur within one and the same type. Also the
peculharities which distinguish each type in structure and in the
mode of development are not without intermediate phases.
Especially from the branch of the worms there lead cff transitional
forms to the other branches: Balanoglossus to both echinoderms
and chordates, the annelids and Peripatus to the arthropods, the
tunicates and Amphiowus to the vertebrates. In some representa-
tives of each type the structure and the mode of development are
simpler, thereby approaching to the conditions which obtain in
the other types. The existence of such transitional forms is one
of the most important proofs in favor of the theory of evolution,
and speaks against the assumption of a rigid unvarying type in
Cuvier’s sense.

Fundamental Law of Biogenesis.—A fact that weighs heavily
in the balance in favor of the theory of evolution is the fact that
the structure and mode of development of animals is ruled by a
law which at present can only be explained by the assumption of
iw common ancestry. Hach animal during its development passes
30

HISTORY OF ZOOLOGY.

through esseutially the stages which remain permanent in the case
of lower or at least more primitive animals of the same branch, as
£3 3° 4

 

Fig. 3.—Human Embryo, about third or fourth week. 1-4, visceral arches with gill-
slits between them: 1, mandibular arch ; 2, hyoid arch; 3 and 4, first and second
gill-arches. «a, eye; 1, nasal pit: h, cardiac region; ¢ Jande IJ, fore and hind
extremities; m, mesodermal somites.

 

Fic. 4.—Tadpoles of Rana temporaria. m, mouth; g, upper jaw; z, lower jaw; s,
sucking-disc ; kb, external gills; ik, region of the internal gills; m, nose; a, eye;
0, auditory vesicle ; h, cardiac region ; d, operculum.

the three following examples will show: (1) In the early stages of
development the human embryo (fig. 3) possesses remarkable
36 GENERAL PRINCIPLES OF ZOOLOGY.

resemblances to the lowest vertebrates, the fishes. Like these it
hag gill-shts, the same arrangement of the heart and of the arterial
vessels, certain fundamental features in the development of the
skeleton, etc. (2) Frogs in their tadpole stage have an organiza-
tion similar to that which remains permanent in the case of
certain Amphibia, the Perennibranchiata (fig. 5), which stand

 

Fra. 5.—Siredon pisciformis (larva of Amblystoma tigrinum). (After Duméril and
Bibron.)

lower in the system; they have a swimming tail and tuft-like gills,

which are lacking in the adult frog. (3) There are certain para-

sitic crustacea, which live upon the gills of fishes, and seem not

 

Fra. 6.—Achtheres percarwum. a, nauplius-, b, cyclops-stage ; ¢, adult female.
(After Claus.)

at all like their relatives. They are shapeless masses which were
formerly regarded as parasitic worms. Their systematic position
as only determined by their embryology (fig. 6). Tlere it is
HISTORY OF ZOOLOGY. BT

eo

shown that they pass through a nauplius-stage (fig. 6a), charac-
teristic of most crustacea, and that they then assume the shape of
small crustacea (fig. 6, ), like Cyclops (fig. 7, A), so widely dis-

 

Fia. 7.—Cyclops coronatus (A) and also the nauplius in lateral (B) and in ventral view
(C). T, head; TJ-V, the five thoracic, and behind these the five abdominal seg-
ments; F’, furca; 1, the first, 2, the second, antenne; 3, mandibles; 4, maxille;
5, maxillipeds ; (-9, the first four pairs of biramous feet, while the rudimentary
fifth pair are hidden; au, eye; 0, upper lip; e, egg-sacs; d, gut; m, muscle.

tributed in fresh waters. Very often the males inake a halt in the
cyclops-stage while the female develops farther and assumes a
shapeless form, so that there arises a very remarkable sexual
dimorphism (fig. 8). All these examples, which can be multiplied
by hundreds, can be explained in the same way. The higher forms
35 GENERAL PRINCIPLES OF ZOOLOGY.

pass through the stage of organization of the lower, because

they spring from ancestors which were
Ss" 4. more or less similar to the latter. Man
) in his embryological development passes
through the fish stage, the frog the per-
» ennibranchiate stage, the parasitic crus-
7 \ tacean first the nauplius- and then the
cyclops-stage, because their ancestors
were once fish-like, perennibranchiate-
like, nauplius- and cyclops-like. Here
is expressed a general phenomenon
which Haeckel has stated in a general
proposition under the name of ‘the
Fundamental) Law of Biogenesis.’ ‘* The

   

> development history (ontogeny) of an
A \ individual animal briefly recapitulates

 

the history of the race (phylogeny);
FIG. 8.— Philicthys siphie,, % aio i.e., the most important stages of organi-

iien Peresoel, ae zation which its ancestors have passed
through appear again, even if somewhat modified, in the develop-
ment of individual animals.”

Examples of the Application of this Law.—TZhe Nervous
System.—This law applies as well to single organs as to entire
animals. The central nervous system of the lower animals
(echinoderms, celenterates, many worms) forms part of the skin;
in its first appearance it belongs to the surface of the body, because
it has to mediate the relations with the external world. In the
case of higher animals, e.g., the vertebrates, the brain and spinal
cord lie deeply embedded in the interior of the body; but in the
embryo it is laid down likewise as a part of the skin (medullary
plate) and which gradually through infolding and cutting off from
this comes to lie internally. One can demonstrate this change of
position by cross-sections through the dorsal region of embryos of
different ages of any vertebrate (fig. ‘)).

The Skeletal System.—The skeleton of vertebrates is a further
example. In the lowest chordates, amphiorus and the cyclostomes,
the vertebre are lacking, and in their place we find a cylindrical
cord of tissue, the chorda dorsalis (notochord). In the fishes and
Amphibia the notochord usually persists; but it is partially
reduced and constricted by the vertebre, which in the lower forms
consist of cartilage, and in the higher of bone or a combination of
bone and cartilage. Mature birds and mammals finally have a
HISTORY OF ZOOLOGY. 39

 

FIG. 9.—Cross-sections through the dorsal region of Triton embryos at different Aer
(from O. Hertwig). In [the medullary plate (anlage of spinal cord) De eels
off from the skin (epidermis, ep) by the medullary folds (mf). In IT t he 1 eI
lary plate by inrolling of the medullary folds is converted into a groove. ee
the groove has closed into a tube (7), the spinal cord, which has Separates Tr de
the rest of the ectoderm (epidermis). C, body cavity (c@lom);_ ch, Bee .
cp, cavity of primitive somite (myotome); dz, yolk-cells; ik, entoderm; Ig, lume
of gut; mk}, mk*, somatic and splanchnic layers of mesoderm.
40 GENERAL PRINCIPLES OF ZOOLOGY.

completely ossified vertebral column; their embryos, on the other
hand, have in the early stages only the notochord (amphioxus
stage); later this notochord becomes constricted by the vertebra
(fish-ainphibian stage) and finally entirely replaced; the vertebral
column is in the beginning cartilaginous, only later becoming ossi-
fied. Comparative anatomy and embryology thus give the same
developmental stage of the axial skeleton: (1) notochord, (2)
notochord and vertebral column, the latter at first formed of
cartilage, then of bone.

We have here spoken of a parallelism between the facts of
comparative anatomy and those of embryology. But in reality we
should expect a threefold parallelism: for according to the theory
of evolution the systematic arrangement of animals is conditioned
by a third factor—the historical development of the animal world,
or phylogeny. The mile-stones of phylogenesis, the fossils, should
give the same progressive series in the successive geological strata
as the stages of forms found by comparative anatomy and embry-
ology. We actually know instances of such threefold parallelisms.
Comparative anatomy teaches that the lowest developed form of a
fish’s tail is the diphycercal (fig. 10, A); that from this the
heterocercal (/4), and from the heterocercal the homocercal form
of tail-fin (C, D) can be derived. Embryologically the most highly
developed fishes are first diphycercal, later heterocercal, and
finally become homocercal. Last of all, paleontologically the
oldest fishes are diphycercal or heterocercal, and only later do
homocercal forms appear.

What has here been referred to is only a small fraction of the
weighty proofs which morphology offers in favor of evolution; it
can only serve to show how morphological observations can be
employed. For the reflecting naturalist the facts of morphology
are a single great inductive proof in favor of the theory of evolu-
tion.

Distribution of Animals.—From Animal Geography we learn
that the present distribution of animals is the product of past
hundreds and thousands of years. It will therefore be possible
from this to figure out many of the earlier conditions of things,
hy proceeding with the utmost caution and overcoming extreme
difficulties.

If we assume that from the beginning all animal species were
constituted as they now are, they would then have been placed by
the purposeful Creator in the regions best suited to their organiza-
tion; their distribution would therefore have been determined by

 
HISTORY OF ZOOLOGY. 41

favorable or unfavorable conditions of life prevailing in the various
regions, as the climate, food-supply, ete. If, on the other hand,
we assume that the animal species have arisen from one another
through variation, then there must have been, as an influence
determining the manner of distribution, besides the conditions
of existence, still a second factor, which we will call the geo-

 

 

Fia. 10.—Tail-fins of various fishes. (From Zittel.) A, Diphycercal fin of Polypterus
bichir. (Vertebral column and notochord divide the tail into symmetrical dorsal
and ventral portions.) B, Heterocercal tail of the sturgeon. (Asa result of an
upward bending of the notochord and vertebral column _ the fin has become
asymmetrical, the ventral portion much larger than the dorsal.) C, D, Homo-
cercal fins, C, of Amia calva; D, of Truttu salar. (By a still greater upward bend-
ing of the notochord and vertebral column the dorsal portion has almost en-
tirely disappeared and the ventral portion almost alone forms the fin, externally
apparently symmetrical, but in its internal structure very asymmetrical.) ch,
chorda; a, b, c, cover-plates.

logical. We know that the configuration of the earth’s surface
has changed in many respects in the course of the enormous space
of time of the geological periods; that land areas, which earlier
were united, have become separated by the encroachments of the
sea; that by the upheaval of mountains important barriers to the
distribution of animals were also formed. From the fact that
42 GENERAL PRINCIPLES OF ZOOLOGY.

these two changes—the changes in the earth’s surface and in the
animal world established upon it—have gone on hand in hand
there follows necessarily the consequence that greater differences
in the faunal character of two lands must result the longer they
have developed independently of one another, without interchange
of their animal populations, and the Jonger the inhabitants have
been separated by impassable barriers. For the various groups
the character of the barriers is different; terrestrial animals, which
cannot fly, are hindered in their distribution by arms of the sea;
marine forms by land barriers; for terrestrial molluscs high moun-
tain ranges, which are dry and barren, or constantly snow-capped,
are effectual.

Instances of Proofs.—Since attention has been called to these
conditions, many geographical facts favorable to the theory of
evolution have been ascertained: (1) Of the various continents
Australia has faunally an independent character; when discovered
it contained almost none of the higher (placental) mammals,
except such as can fly (Chiroptera), or marine forms (Cetacea), or
such as are easily transported by floating wood (small rodents), or
such as could be introduced by man (dingo, the Australian dog);
instead, it had remarkable birdlike animals (with beak and cloaca),
and, the marsupials, which have become extinct in the Old World
and the opossums excepted, in America as well. The phenomenon
is explained by the geological fact that in the earth’s history
Australia, with its surrounding islands, was certainly the earliest
to lose its connexion with the other continents. While’ in the
other four parts of the earth the higher vertebrates, which were
developed from the marsupials and their lower contemporaries,
came, by way of the lands connecting the various continents, to
have a wide or even a cosmopolitan distribution, in isolated
Australia this process of evolution did not go on, and its ancient
faunal character was preserved. (2) As Wallace has shown, the
Malay Archipelago is divided faunally into an eastern and a western
half; within each group there are islands which, in spite of a
different climate, have a very similar fauna. On the other hand,
the faunal boundary (‘ Wallace’s line’) passes between the two
islands Bali and Lombok, which have the same climate and
geographically are very close together. But the depth of the strait
in this region shows that here runs a boundary of extraordinarily
long geological duration, and that in the earth’s history Bali has
developed in connexion with the western, Lombok with the eastern
chain of islands. More recent studies make it probable that there

 
HISTORY OF ZOOLOGY. 45

is an island zone between the two in which a mixture of faunas
occurs. Celebes especially belongs here. (3) A long time before
Darwin, the renowned geologist Leopold von Buch, from the dis-
tribution of plants on the Canary Islands, had come to the conclu-
sion of a change of species into new species; viz., on islands
peculiar species develop in secluded valleys, because high mountain-
chains isolate plants more effectually than do wide areas of water.
M. Wagner has collected many instances which prove that locali-
ties inhabited by certain species of beetles and snails have been
sharply divided by wide rivers or by mountain-chains, while in
neighboring regions related so-called ‘ vicarious species’ are found.
Causal Foundation of the Theory of Evolution.—The Dar-
winian theory, so far as the above exposition shows, is fundamen-
tally like the theories of descent advocated at the beginning of this
century by Lamarck and other zoologists; it is distinguished from
these only by its much more extensive foundation of facts, and
further in that it abandoned the successional arrangement oyer-
thrown by the type theory, and replaced it by the branched, tree-
like mode of arrangement,—the genealogical tree. But still more
important are those advances of Darwinism which relate to the
causal foundation of the descent theory. The doctrine of causes
which has brought about the change of species forms the nucleus
of the Darwinian theory, by which it is especially distinguished from
Lamarckism. In order to substantiate causally the change of
species, Darwin proposed his highly. important principle of
‘Natural Selection by means of the Struggle for Existence.’
Artificial Selection.—In the development of this principle
Darwin started from the limited and hence easily comprehended
subject of Domestication, the artificial breeding of our races of
domesticated animals. Many of these undoubtedly sprang from a
single wild living species; others arose from several species, but
now have the appearance of a single species. ILlow have arisen
such extraordinarily different races of pigeons—the fantail, the
pouter, long- and short-billed pigeons, etc., the long- and short-
horned cattle, the heavy, slow Percherons and the slenderly-built,
fleet-footed Arabian horses? Undoubtedly through that same
more or less conscious influence of man, which is still employed
by the skilful animal-breeder. If he wish to obtain a particular
form, he chooses from his stock suitable animals, which he pairs
together if they in ever so slight a manner approach nearer than
the others to the desired ideal. By repetition of this selection
according to plan, the breeder attains a slow but sure approx-
44 GENERAL PRINCIPLES OF ZOOLOGY.

imation to the goal, since he uses for breeding only the suitable
individuals from each new generation. If he wish, for example,
to breed fantail pigeons, he selects from his stock animals with
the most numerous and strongest tail-feathers. In the course of
generations, then, characteristics cumulate; the number of pigeons
having an increased number of tail-feathers becomes greater, and
thus material is obtained which is adapted to a further increase in
the number of feathers.

Factors of Evolution in Breeding.—The remarkable results of
breeding which are well known to every observer of our domesti-
cated animals depend mainly upon three factors: (1) Vartadbility;
the descendants of one pair of parents have the capability of
developing new characteristics, thereby differimg in appearance
from their parents. (2) Hereditability of newly-acquired charac-
ters. This consists in the tendency of the daughter-generation to
transmit the newly-developed characteristic to the succeeding
generation. (3) Artificial selection; man selects for breeding pur-
poses suitable individuals, and prevents a new character which has
arisen through variation from disappearing through crossing again
with animals of the opposite variational tendencies.

Factors of Evolution in Nature.—If we compare with the facts
of domestication the conditions of animals living in the state of
nature, we find again variability and heredity, as efficient forces,
inherent in all organisms, though the former is not everywhere of
the same intensity. There are many species which vary only
slightly or not at all, and therefore have remained unchanged for
thousands of years. But contrasted with these conservative species
are in every group progressive species, active species, which are
in the process of rapid change, and these alone are of importance
in causing the appearance of new species. Since heredity is
present in all organisms, there is only lacking a factor correspond-
ing to artificial selection, and this Darwin discovered in the
so-called ‘natural selection.’

Natural Selection: Struggle for Existence.— Natural selection
finds its basis in the enormous number of descendants which every
animal produces, There are animals (e.g., most fishes) which
produce many thousands of young in the course of their lives: not
to mention parasites, whose eges are numbered by millions. For
the development of this animal throng there is ‘ho room on the
earth; for even if we compute upon the basis of a slowly-multiply-
ing animal, like the elephant, and assume that all the progeny live
and reproduce normally, it would only be a few centuries before

 

 
HISTORY OF ZOOLOGY. 45

the entire earth would be occupied by herds of elephants. In
order to preserve the equilibrium in nature great numbers of
unfertilized and fertilized eggs, as well as young animals and many
that are mature but have not yet attained their physiological
destiny, must perish. Many individuals will undoubtedly be
blotted out by purely accidental causes; yet on the whole those
individuals which are best protected will best withstand adverse
conditions. Slight superiority in structure will be of importance
in this struggle for existence, and the possessors of this will gain
an advantage over their companions of the same species, just as in
domestication each character which is or is fancied to be useful to
man insures advantage to the possessor. Among the numerous
varieties that appear the fittest will survive, and in the course of
many generations the fortunate variations will increase by sum-
mation, while destruction overtakes the unsuitable varieties. Thus
will arise new forms, which owe their existence to ‘natural selec-
tion in the struggle for existence.’

The ‘Struggle for Existence.’—The expression ‘struggle for
existence’ is figurative, for only in rare cases does an active con-
scious struggle decide the question of an animal’s existence; for
example, in the case of the beasts of prey, that one which by
means of his bodily strength is best able to struggle with his com-
petitors for his prey is best provided in times of limited food-
supply. Much more common is the unconscious struggle: each
man who attains a more favorable position by special intelligence
and energy, limits to an equal degree the conditions of life for
many of his fellow men, however much he may interest himself in
humanity. The prey which by special craft or swiftness escapes
the pursuer turns the enemy upon, the less favored of its com-
panions. It is noticeable that in severe epidemics certain men do
not fall victims to the disease, because their organization better
withstands infection. Here the term ‘survival of the fittest,’
which Spencer has adopted in preference to ‘struggle for exist-
ence,’ is better.

Instances of the Struggle for Existence.—Although the fore-
going general considerations suffice to show that the struggle for
existence plays a very prominent réle in the organic world, yet on
account of the importance of this feature it will be illustrated by
a few concrete examples. The migratory rat (Mus decumanis),
which swarmed out from Asia at the beginning of the eighteenth
century, has since then almost completely exterminated the house-
rat (Mus rattus) in Europe, and has made existence impossible for
46 GHNERAL PRINCIPLES OF ZOOLOGY.

it in other parts of the world. Several European species of thistle
have increased so enormously in the La Plata states that they have
in places completely crowded out the native plants. Another
European plant (Hypocheris radicata) has become a weed, over-
running everything in New Zealand. Certain races of men, like
the Dravidian and Indian, die off to the same degree that other
races of men, like the Caucasian, Mongolian, and Negro, spread.
The more one attempts to explain that endlessly complicated web
of the relations of animals to one another, the relations of animals
to plants and to climatic conditions, as Darwin has done, so much
the more does he learn to appreciate the methods and results of
the struggle for existence. He will become conversant with many
interesting phenomena, formerly unintelligible, which immediately
find an explanation through this doctrine. Islands lying in the
midst of the ocean have a disproportionately large number of
species of wingless insects, because the flying forms are easily
carried out to sea. For example, on the Kerguelen Islands,
remarkably exposed to storms, the insects are wingless; among
them one species of butterfly, several flies, and numerous beetles.

Sympathetic Coloration.—Very often, in regions which have a
permanent or prevailing uniform color, the coat of the animals is
distinguished by the same or at least by a similar hue; this
phenomenon is called sympathetic coloration. Inhabitants of
regions of snow are white, desert animals have the pale yellow
color of the desert, animals which live at the surface of the sea are
transparent; representatives of the most diverse animal branches
show the same phenomenon. The advantages connected therewith
scarcely need an explanation. Every animal may have occasion
to conceal himself from his pursuers; or it may be his lot to
approach his prey by stealth: he is much better adapted for this
the closer he resembles his surroundings. Natural selection fixes
every advantage in either of these directions, and in the course of
many generations these advantages increase.

Mimicry is referable to the same principle, except that the
imitation is not here limited to the color, but also influences form
and marking. Frequently parts of plants are imitated, sometimes
leaves, sometimes stems. Certain butterflies with the upper sur-
faces of the wings beautifully colored escape their pursuers by the
rapidity of their flight; if they alight to rest, they are protected by
their great similarity to the leaves of the plants around which
they chiefly fly. When the wings are folded over the back, the
dark coloring of the under sides comes into sight and the color on

 
HISTORY OF ZOOLOGY. 47

the upper side is concealed. The parts are so arranged that the
whole takes on a leaf-like form, and certain markings heighten
the imitation of the neuration of the leat (fig. 11). Among the
numerous species of leaf-butterflies there are different grades of
completeness of mimicry; in many even the depredations of insects

 

Fia. age butterflies. .4, Kallima paralecta, flying; a, at.rest. (After Wallace.)
B, Siderone strigosus, flying; b, at rest. (After C. Sterne.)

is imitated; in others the form and marking are still incompletely
leaf-like, the marking being the first to come into existence.
Among the grasshoppers also there are imitations of leaves, like
the ‘walking-leaf,’ Phylliwm siccifolium, P. scythe, while other
nearly related forms more or less completely approach the appear-
ance of dried, sometimes of thorny twigs (fig. 12, a and 3).
Examples of Mimicry.—Very often insects are copied by other
animals. Certain butterflies, Heliconia, fly in large swarms,
48 GENERAL PRINCIPLES OF ZOOLOGY.

 

Fig. 18.—Methona psidii, a bad-tasting Heliconiid, copied by the Pierid, Leptatis orise.
(After Wallace.)
HISTORY OF ZOOLOGY. 49

clumsy and yet unmolested by birds, because they contain bad-
tasting fat bodies. Another species of butterfly accompanies them
(Pieridze), which does not taste bad, and yet are not eaten, because
in flight, in cut, and marking of the wings they imitate the
Heliconie so closely that even a systematist might easily be
confused (fig. 15). In a similar way bees and wasps, feared on
account of their sting, are imitated by other insects. In Borneo
there is a large black wasp, whose wings have a broad white spot

 

Fia. 14.--a, Mygnimia aviculus, a wasp imitated by a beetle; b, Coloborhombus fascia-
tipennis. (After Wallace.) } nat. size.

near the tip (Mygnimia aviculus). Its imitator is a heteromerous
beetle (Coloborhombus fasciatipennis), which, contrary to the habit
of beetles, keeps its hinder wings extended, showing the white spot
at their tips, while the wing-covers have become small oval scales
(fig. 14).

Sexual Selection is a special phase of natural selection, chiefly
observed in birds and hoofed animals. For the fulfilment of his
sexual instincts the male seeks to drive his competitors from the
field, either in battle or by impressing the female by his special
50 GENERAL PRINCIPLES OF ZOOLOGY.

excellences. With strong wings and with spurs the cock main-
tains possession of his flock, the stag by means of his antlers, the
bull with his horns. The birds of paradise by means of beautiful
coloring win the favor of the females, most singing-birds by means
of song; many species of the fowl by peculiar love-dances. Since

 

Fig. 15a.—Puaradisea apoda, male. (After Levaillant.)

all these characters belong chiefly to the male, and since it is
only exceptionally that they are inherited by the female (and even
then are less pronounced), it is almost certain that in a great
measure they have been acquired by the males through the struggle
for the female. In the case of birds a second factor has uwn-
doubtedly co-operated to impress distinctly the often enormous
difference between the feathers of the male and of the female—as
HISTORY OF ZOOLOGY. 51

is shown, for example, in the case of the birds of paradise
(fig. 15); for the nesting female inconspicuous colors and a close-
lying coat of feathers are necessary in order that, undisturbed by
enemies, she may devote herself to incubation.

On the Efficiency of Natural Selection.—In the course of the
last decade there has been much controversy as to how far natural
selection alone is a species-forming factor. A number of objectors
dispute the possibility of fortuitous variations being utilized in the
struggle for existence. It is not easy to see how many characters,

=

 

Fia. 158.—Paradisea apoda, female. (After Levaillant.)

especially such as are used in classification, can be of use to their
owners. It can only be said that they have developed in correla-
tion, that is in necessary organic connexion, with other important
characters. But useful characters must be considerable in order
to be seized upon by natural selection. Fortuitous variations with
which Darwinism deals are too inconsiderable to be utilized by the
organism and so to be of value in the struggle for existence. In
most cases, too, alteration in one organ alone is not enough to be
of value; usually a whole series of accessory structures must be
modified. In short, there must exist a harmonious co-operation
of parts, which presupposes a progressive and well-regulated
development extending through a long space of time during which
the struggle for existence could have exerted no directing influ-
ence. Thus, for example, the wing of a bird in order to be used
for flight must have already reached a considerable size; the
muscles for moving it, the supporting skeletal parts, the nerves
running to it must have a definite formation and arrangement.
Then there are difficulties in that most animals are bilaterally or
52 GENERAL PRINCIPLES OF ZOOLOGY.

radially symmetrical, many in addition segmented. In all these
cases the same organ is repeated two or more times. Organs which
are repeated symmetrically and usually those which are segmental
agree in general in structure. One must therefore admit that the
alterations of chance must have occurred at at least two points
simultaneously and in exactly the same way.

A further objection is that the action of natural selection would
under ordinary conditions be negatived by unhindered crossing of
the varying forms. If, for example, we do not isolate fantails from
other pigeons, they will cross with these, and their descendants
will soon resume the character of common pigeons. Finally, it
has been claimed that for the formation of new species a simple
variation of forms is not sufficient; it must reach still farther: (1)
a variation in different directions, a divergent development of the
individual members of a species; (2) the disappearance of the
transitional forms which unite the divergent forms.

The objection that the struggle for existence cannot bring
about the divergent development of individuals necessary for
improvement is of least importance. It need only be added that
of the many variations appearing at the same time in a species two
or more may be equally useful; that then one set of individuals
will seize upon one, another set upon the other advantage, and that
in consequence of this both sets will develop in different directions.
Consequently the intermediate forms which are not pronounced in
the one or the other direction will be in an unfavorable position,
and must carry on the struggle for existence with both groups of
partially differentiated companions of their species, and, being less
completely adapted, must fall.

More important are the first two objections; they have led to
theories which originally seemed destined to complete the Dar-
winian theory, but in the course of discussion they have more and
more raised the claim of entirely supplanting it. In the following
paragraphs will be found an outline of these theories, but it is to
be taken into consideration that, at the present time, we are still
in the midst of the reform movement, and it cannot yet be said
whether they will be able to stand beside the theory of the struggle
for existence or will supplant it.

Migration Theory.—T'o explain how characters newly formed
by variation become fixed, and do not disappear again through
crossing with differently modified individuals, M. Wagner has pro-
posed the Theory of Geographical Isolation, or the Migration
Theory. New species may arise if a part of the individuals of one
HISTORY OF ZOOLOGY. 53

species should take to wandering, or should be transplanted, and
thus come to a new place, in which crossing with the companions
of their species who were left behind is not possible. The same
might occur, if the region inhabited by a species should by
geological changes be divided into two parts, between which inter-
change of forms would be no longer possible. The animals
remaining under the old conditions would retain the original
characteristics; the wanderers, on the other hand, would change
into a new species. Direct observations support this theory. A
litter of rabbits placed at the beginning of the fifteenth century
on the island of Porto Santo has in the mean time increased
enormously and the descendants have taken on the characteristics
of a new species. The animals have become smaller and fiercer,
have acquired a uniformly reddish color, and no longer pair with
the European rabbit. A further proof in favor of the theory of
geographical isolation is the peculiar faunal character of regions
separated from adjacent lands by impassable barriers, broad rivers
or straits, or high mountains (comp. p. 42); especially instructive
in this regard is the peculiar faunal character of almost every
island. The fauna of an island resembles in general the fauna of
the mainland from which the island has become separated by
geological changes; it usually has not only these but also so-called
‘vicarious species,’ i.e., species which in certain characteristics
closely resemble the species of the mainland. Such vicarious
species have plainly arisen from the fact that isolated groups of
individuals, scattered over the islands, have taken on a develop-
ment divergent from the form from which they started. With all
due recognition of the migration theory, it will never be possible
by it alone to explain the multiformity of the organic world. In
addition, it must be assumed that formerly the earth’s surface
possessed an enormous capacity for change; but the more recent
investigations make it probable that the distribution of land and
water has not varied to the degree that was formerly believed.
The experience of botanists, too, teaches that several varieties can
arise in the same locality and become constant.
Lamarckism.—While the migration theory agrees with Dar-
winism in this, that the new characters appearing through varia-
tion are to be regarded as the products of chance, yet it is just
this part of the theory which has been subjected to searching
criticism. Many zoologists have again adopted the causal founda-
tion of the descent theory proposed by Lamarck and believe that
the cause of species formation is to be found in part in the

 
5+ GHNERAL PRINCIPLES OF ZOOLOGY.

immediate influence of ‘changing environment, in part in the
varying use and disuse of organs, brought about by alterations in
the conditions of life. Both principles, they say, are sufficient,
even without the help of the struggle for existence, to explain the
phylogenesis of organisms.

Influence of Environment.—T'o what extent can the environ-
ment bring about a permanent change in the structure of plants
and animals? To decide this is no simple problem, on account of
the complexity of the factors entering into the question.

In cases where the food-supply is altered, organisms change in
a very remarkable manner and within a short time; but these
changes (Niigeli’s ‘ Modifications through Nutrition’) seem to have
no permanence. Plants which, found in nature in poor soil, are
transplanted into rich soil, or vice versa, soon acquire quite a
different appearance, and preserve this through the following
generations, so long as they remain in the rich soil; but the plant
quickly returns to its former appearance when replaced in its
previous surroundings.

In general, a change seems to be the more permanent the more
slowly it has developed. In researches upon the influence of
environment, we can, therefore, rely soonest upon results if we
experiment with slowly-working factors, such as light and heat,
dry or moist air, different intensities of gravitation, of stimuli,
etc., which can be excluded from the environment of the organism.

Use and Disuse.—Regarding the efficiency of use and disuse,
there is no doubt that the shape of an animal is influenced to a
great extent by the manner in which the organs are used. The
organs which are much used will become especially strong and vice
versa those which are not used will become weak. The only ques-
tion is whether these, in the strict sense of the word, newly-acquired
characteristics are transmitted to the offspring, or whether the
descendants, in order to attain to the same stage, must not repeat
in the same way use and disuse. In the latter case the cumulation
of characteristics, and with it the possibility that these may
become permanent, is excluded. It is to be regretted that accurate
results are still lacking on a point so well adapted for experimental
treatment. At this time rudimentary organs strongly favor the
Lamarckian principle; for we see that cave animals, which for
many generations have lived in darkness, are blind, either having
no eyes, or only vestiges of them, incapable of function. This
seems to justify the view that this condition is attributable to lack
of use, since it has brought about a functional and anatomical

 

 
HISTORY OF ZOOLOGY. 5d

incapacity, which has increased from generation to generation.
Now we must believe that what is true for disuse must express
itself in the reverse sense in the case of use.

Nageli’s Principle of Progression.—In conclusion, there is still
to be considered the change of species from internal causes, to
which von Baer gave the poorly adapted because easily misleading
term ‘‘ Zielstrebigkeit ” (the striving toward an ideal), and which
Niigeli has termed the ‘ perfecting principle,’ or the ‘ principle of
progression.’ It cannot, indeed, be denied that each species is
compelled, by some peculiar internal cause, to develop into new
forms, independently of the environment, and up to a certain
degree, independently of the struggle for existence. In all animal
branches we see the progress from lower to higher going on, very
often in a quite similar way, in spite of the fact that the animals
live under very different conditions of development. We see how
the nervous system lying near the surface in the lower animals
becomes in the higher animals concealed in the depths of the body;
how the eye, at first a simple pigment-spot, becomes in worms,
arthropods, molluses, and vertebrates, provided with accessory
apparatus, as lens, vitreous body, iris, choroid, etc. Here we see
an energy for perfection which, since it occurs everywhere, must
be independent of the individual conditions of life, and must have
its special explanation in the character of the living substance.

It is by no means justifiable to call an assumption, as here
expressed, teleological, and to reject it as unscientific; rather the
organism seems to be just as mechanically conditioned as a billiard-
ball, whose course is determined not only by contact with the
cushions of the billiard-table, but also in a large measure by its
indwelling force, imparted to it by the stroke of the eve. An
organism, too, is a store of energy which must necessarily from
itself develop more, but it is of more extraordinary complexity,
and to an equal degree also is independent of the external world.
A complete independence is naturally never present, and Nigeli
has not so maintained. Along with it rather goes always an
‘action’ of the external world, a modifying influence which is
carried on by the external conditions of existence, either directly
or by the mediation of use and disuse.

This exposition of evolution has been given in a rather detailed
way, because in the history of zoology it is undoubtedly the most
important feature. No other theory in the course of the develop-
ment of zoological investigation has gained such a hold, none has
propounded so many new problems and opened so many new fields
56 GENERAL PRINCIPLES OF ZOOLOGY.

for research. There is no other zoological theory which compares
with it in value as a working hypothesis. To the many objections
which have been made that the theory is insufficiently grounded,
it can only be replied that in the present state of our knowledge it
is the only theory which agrees with our experiences and explains
these in a simple way and on a scientific basis. In this sentence
is given the merit of the theory, but at the same time also a limita-
tion of its applicability. For on the one side the statement attrib-
utes the merit in the applicability of the system to the necessity
of the human mind for simple explanations of the facts of natural
science, and on the other hand it makes the degree of correctness
dependent upon the state, whatever it may be, of our knowledge.
On both sides no constant quantities are involved. Many investi-
gators see no necessity of reconciling paleontology and our knowl-
edge of plants and animals. To such, therefore, the Darwinian
theory proves just as little as any opposing theory. Meanwhile
thoughtful naturalists will keep in mind that our knowledge of
nature is making considerable advances, and is visibly becoming
wider and deeper. It is possible, even probable, that these
advances will lead to many modifications of the theory. For
instance, the theory of the causes which condition the formation
of new species will undergo numerous changes. On the other
hand, we can affirm with great certainty that the principle of
descent, which first obtained credence through Darwinism, will be
a permanent landmark of zoological investigation.
GENERAL MORPHOLOGY AND PHYSIOLOGY

 

General Zoology: Animal Morphology.—In the vital phenom-
ena of animals a certain degree of similarity can be followed
through the entire animal kingdom; the way in which animals are
nourished and reproduce their kind, how they move, and how they
gain experience, is essentially the same in great groups, and even
widely separated forms show many agreements. Corresponding
to this, the apparatus which is concerned with the above-men-
tioned functions, the organs of nutrition and reproduction, of
motion and sensation in their grosser and finer structure, and in
their ontogeny, must be similar to one another and show evidence
of some fundamental characters which always or frequently recur.
All this needs a general explanation before we can go into a
description of the separate branches of animals. This explanation
is the subject of general zoology, specially of general anatomy and
embryology, or animal morphology.

cology or Biology.—If by means of anatomy and embryology
we have learned the general character of the animal organism, we
must yet farther study its relations to the environment. For this
study of the conditions of animal life, ecology or biology, we have
to consider the geographical range of animals, their distribution
over the surface of the earth and in the different depths of the
sea; further, the reciprocal relations of animals and plants, and
of beast to beast, as these find special expression in colony-build-
ing, symbiosis, parasitism, etc.

General Anatomy.—In the case of General Anatomy, with
which we shall begin, the fundamental proposition will be, //ow is
an organism formed from its constituent parts ? We shall thus in
spirit follow the opposite course from that which anatomy actually
takes, for this resolves the animal body into its elementary parts,
its organs, tissues, and cells. Instead of analytical we will pursue
synthetic anatomy.

The synthesis of an organism, of which by general anatomy we
can only gain an idea, actually takes place in nature during the

ny
oF

 
58 GENERAL PRINCIPLES OF ZOOLOGY.

development of every animal. Embryologically every organism
is at some time a simple element, a cell; this divides and gives rise
to tissues; from the tissues are formed organs, and from the
organs the regularly membered whole of the animal body is com-
bined. If the general ontogeny proceeds synthetically, it then
agrees in its manifestations with the processes which go on in
nature and which are accessible to direct observation.

GENERAL ANATOMY.

The Morphological Units.—The expression ‘constituent parts
of the animal body’ can be used in a double sense. We can speak
of the chemical units, the chemical combinations, which form the
tissues; these are the subject of animal chemistry, and may there-
fore be passed over here. But we may also speak of the constituent
units (morphological units) of the animal body; these are the cells.
These and their transformation into tissues, organs, and entire
animals are for us of vastly greater importance.

 

I. THe MorpnonocicaL UNITS oF THE ANIMAL Bopy.

The Cell.— The study of the morphological units of the organic
body first found a firm foundation in the cell theory. Every
scientific study of the anatomy of plants and animals must there-
fore take the cell as its starting-point.

 

History of the Cell Theory.—The conception of the cell of animals and
plants has in the course of time undergone many changes, which must
be known to some extent in order to understand completely the name and
the conception. When, in the seventeenth century, Hooker, Marcello
Malpighi, and Nehemia Grew introduced the term into vegetable anatomy,
they meant small chambers surrounded by firm walls and filled with air
or fluid contents. When, also, early in the nineteenth century, it was cor-
rectly recognized that the cell is the anatomical and physiological vegetable
unit from which all the other parts of the plant are formed, and when the
English botanist Brown discovered in the interior of the cell that small
body previously overlooked, the kernel or mzclews, the old conception
remained, and as such was accepted by Schleiden in his cell theory.
Schleiden added as new a completely erroneous view of the origin of cells:
that in a sort of matrix (the ‘cytoblast’) first a granule, the nuclear
body, was formed, then around this granule a membrane, the nuclear mem-
brane, arose by precipitation, and around the thus completed nucleus a
larger membrane (the cell membrane) was deposited. Hence for the
formation of the cell the nucleus would be of most importance,

The Schleiden-Schwann Cell Theory.—Since it is the nuclei which are
most easily seen in the animal body, and even now are particularly useful
GENERAL ANATOMY. 59

in deciding questions concerning the presence of cells, it is readily under-
stood how Schleiden’s theory, which placed the nucleus so much in the
foreground, should have led Schwann to apply the cell theory to the
animal kingdom, and thus raise it toa principle of general application.
We usually, therefore, speak of the Schleiden-Schwann cell theory.

As a result of this theory the walls, the cell membrane, were regarded
as most important for the function of the cell; through the cell mem-
brane diffusion-currents must pass between the surrounding medium
and the contents of the cell; the character of the membrane and of the
cell-sap must determine the condition of the diffusion-currents, and
hence the functional character of the cell; the different appearance of
tissues depends chiefly upon the fact that the cells, spherical in the
beginning, change their form; in the case of fibrillar connective tis-
sue, for example, they increase enormously in length and become fine
fibrillee. Since the life of an organism is nothing else than the co-operative
work of all its cells, they flattered themselves that through the cell theory
and the discovery, brought about by it, of the physical unity of the animal
and vegetable body they had made an important advance in the great
problem of the physical explanation of the phenomena of life. Cell gene-
sis also seemed, according to the theory, to be just as satisfactorily
explained on a mechanical basis as the formation of a crystal. In the
‘eytoblast’ the nuclear bodies, nuclear membrane, and cell membrane
must be formed by deposition just as in the process of crystallization.

Reform Movements,—Since that time our conception of the nature of
cells has completely changed. The cell does not, after the manner of a
crystal, arise as a new formation in a matrix, but it presupposes the
existence of a living mother-cell, from which it arises by division or bud-
ding. Just so also the cell is not a physical unit, but is itself an organism
which shows to us all the enigmas of life, the physical basis of which our
investigations must ever keep in view as a goal, though it be still indiscern-
ibly distant. The membrane and cell-sap are of quite subordinate impor-
tance for the existence of the cell; rather the most important thing in it is
the previously disregarded substance, for which von Mohl introduced the
name protoplasm. According to the newer conception the cell is practically
asmall mass of protoplasm, usually, probably always, provided with one
or more nuclei. This newer conception of the cell has developed so gradu-
ally, and has so slowly supplanted the Schleiden-Schwann view, that the
old name has been retained, although it no longer at all fits the new con-
ception. We have indeed become so thoroughly accustomed to the name
that we no longer notice the contradiction of terms when we call a solid
lump without a membrane a ‘ cell.’

Discovery of Protoplasm.—The reformation of the cell theory was begun
by discoveries which were made in very different regions and only lately
have been brought to a focus.

1. At about the beginning of the nineteenth century, Bonaventura
Corti and Treviranus had seen that the chlorophyl bodies, which cause the
green color of plants, in many species stream around in a lively manner in
the interior of the cell, but Moll was the first to find out that this motion
60 GENERAL PRINCIPLES OF ZOOLOGY.

was not active, but rather that they are moved by a homogeneous sub-
stance in which they are embedded. This substance, which Mohl, in
order to bring it into prominence, named protoplasma, became by other
studies still more important. In the reproduction of the simplest alge, it
was found that the protoplasm, together with the chlorophyl bodies, col-
lected itself into an oval mass, and that this body left the cell membrane
aud swam freely in the water. Since the cell-wall no longer showed signs
of life, while on the other hand the protoplasmic body came to rest and
formed a new plant, it was shown beyond doubt that this was the most
important constituent part of the cell (comp. fig. 115).

2. In the study of animal tissues the importance of the peculiar cell-
substance, the protoplasm, was still more plainly brought out. Here, in
spite of the long-prevailing preconceived idea, unbiassed observation led to
the discovery that most animal cells had no cell-membrane.

3. Very important, finally, was the study of the lowest organisms, the
Protozoa. Dujardin sought by extremely careful observations to prove
that these animals had no organs, but consisted of a uniform granular sub-
stance, the sarcode. The sarcode alone could produce all the vital phe-
nomena, such as movement, sensation, assimilation, previously ascribed to
many organs. Dujardin’s theory was stoutly contested by Ehrenberg and
his school, but finally attained general acceptance through the epoch-
making work of Max Schultze and Haeckel.

Schultze’s Protoplasm Theory.—On the basis of these three series of
observations, Max Schultze finally established the reformation of the cell
theory briefly sketched above, when by accurate study of the appearance
and the vital phenomena, and by means of numerous experiments, he
proved that the cell-substance of animals, the sarcode of Protozoa, and
the protoplasm of plants are identical, and that to this substance, for
which he retained the name protoplasm, all the vital phenomena of animals
and plants are referable in the ultimate analysis. The second important
modification concerns the changes of cells into tissues. These follow not
so much through changes of form and modification of the cells into the
tissue elements, as Schwann thought, but rather by means of chemical
changes. By means of its formative potentiality the protoplasm gives rise
to non-protoplasmic structural parts, as, for example, connective-tissue
fibrils, muscle fibrils, nerve fibres, etc. These give the various tissues their
specific character and perform their functions. The tissues also retain as
the source of life and formation the unemployed remnants of cells, the
connective-tissue corpuscles, muscle corpuscles, ete. We will now trace
out farther these two fundamental ideas in Max Schultze’s ‘protoplasm
theory,’ and thereby briefly sketch the elements of the modern theory of
tissues.

Nature of the Cell.—The size of the animal cell varies to a
considerable degree; the smallest elements are the male sexual
cells, the spermatozoa, whose bodies, particularly in case of the
mammals, are even less than 0.003 mm.; the largest, on the other
hand, with the exception of the giant plasmodia of some

 
GENERAL ANATOMY. 61

Mycetozoa, are the egg cells. The yolk of the bird’s egg, which
alone forms the egg in the narrower sense, apart from its coverings,
has for a time the morphological value of a cell, and in the case of
the ostrich egg may reach a diameter of several inches. The form
of the cell is hkewise variable. Free cells, whose form is not
determined by the environment, are usually spherical or oval in
the resting condition, as the egg cell shows; united into tissues,
the cells, on the contrary, may be pressed together into polygonal
or prismatic bodies, or may send out spindle- or star-shaped
branching processes.

Protoplasm.—So there is left to characterize the cell only the
constitution of its substance: the cell is a mass of protoplasm with
one or more nuclei. It is not known whether protoplasm is a
definite chemical body, which from its constitution is capable of
infinite variation, or whether it is a varying mixture of different
chemical substances. So, also, we are by no means certain whether
or not these substances (as one is inclined to believe) belong to
those other enigmatical substances, the proteids. We can only
say that the constitution of protoplasm must, with a certain degree
of homogeneity, have a very extraordinary diversity. For if we
see that from the egg of a dog there comes always and only a dog,
and indeed an animal with all his individual peculiarities, that a
sea-urchin’s egg, placed under the most diverse conditions, pro-
duces always a sea-urchin, that a species of amceba always performs
only the movements characteristic of that species, we must assume
that the functioning constituent part of this cell, the protoplasm,
has in each case its peculiarities. We are driven to the assumption
of an almost unlimited diversity of protoplasm, even if we concede
an impertant share in the prominent differences to the nucleus, of
which we shall speak later.

General Properties of Protoplasm.—The similarity of proto-
plasm, still recognizable through all its variations, expresses itself
in its appearance and in its vital phenomena. Under slight mag-
nification, protoplasm appears as a faintly-gray substance, some-
times colored yellowish, reddish, etc., by pigments taken up by
imbibition, in which numerous strongly-refracting granules are
embedded. The vital characteristics of this substance are move-
ment, irritability, power of assimilation and of reproduction.

By using higher powers a finer structure can be seen in the
protoplasmic substance, the ‘homogeneous protoplasm ’ of earher
writers. The nature of this is as yet in question: a fine-meshed
framework (filar substance, spongioplasm, cell reticulum) the
62 GENERAL PRINCIPLES OF ZOOLOGY.

interstices of which are filled with other material (interfilar sub-
stance, enchylema, ground substance). The dispute les especially
around the question whether this framework is formed of threads
and trabecule or whether the appearance is not formed by small
chambers, bounded by fine partition-walls (foam structure of
protoplasm).

Movement of Protoplasm.—Movement expresses itself first in
changes of form of the whole body—ameboid movement—and
secondly in the change of position
of the small granules in the interior
of the protoplasm—streaming of
granules. Examples of ameeboid
movement (fig. 16) are chiefly the
movements of many Protozoa, and
of the colorless blood-cells (leuco-
cytes) of multicellular animals;
here the protoplasmic body sends
out coarser and finer processes,
which may be again withdrawn,
serving for locomotion and hence
called pseudopodia or false feet.
The streaming of granules can be
observed in the interior of the cell-
body, as well as in the pseudopodia
Fic. 16. — Ameba proteus. (After extending from this. The pseudo-

Bees cis er eee podia may even be so fine as to be

Dns neles at the limits of visibility with our
strongest magnifications (fig. 17), yet in them it can still be
observed that the granules wander hither and thither like people
on a promenade, simultaneously centripetally and centrifugally,
some with greater, others with Jess speed. And yet the granules
are only passively moved by the protoplasm, for if we feed the
creature with some pigment granules, like finely-pulverized car-
mine, these granules show the same remarkable streaming. Indeed
nothing better illustrates the great complexity in the structure
of protoplasm than these extremely complicated phenomena of
motion in such narrow limits as pseudopodia in general.

Irritability of Protoplasm.—That ammboid movements and
streaming of granules can be induced, brought to a standstill, and
modified by mechanical, chemical, and thermal stimuli, is a sure
proof of the irritability of protoplasm. Most important are the
thermal stimuli; if the surrounding medium rise above the

 

 
GENERAL ANATOMY.

Fia. 17.—Gromia oviformis.

 

(From Lang, after M. Schultze.)
64 GENERAL PRINCIPLES OF ZOOLOGY.

ordinary temperature, the movements at first become more rapid
up to a maximum: from that point begins a slowing, finally coming
to a standstill.—eat-rigor. If the high temperature continue
much longer, or if it rise still higher, death results. The fatal
temperature is found for most animals between 40° and 50° C,
(104°-122° F.); its influence explains a part of the injurious
effects which high-fever temperatures have upon the human
organism. Like the heat-rigor, there is also a cold-rigor, induced
by a sharp sinking of the temperature below the normal. This
is accompanied by a gradual diminution of mobility; it results
in death by freezing, which is, however, not so easily produced as
death by heat. It is a remarkable fact that many animals, conse-
quently their cells, may be frozen; and in this condition can
endure still severer cold without dying. (For example: goldfish,
a temperature of — 8° to —15° C.; frogs, to — 28°; newts, to
= 26°),

Nutrition and Reproduction.—Irritability and power of motion
are the prerequisites of assimilation, the change of food-substance
into protoplasm. Most animal cells, for example almost all tissues
cells, are not suitable for studying assimilation, because they live
upon liquid nourishment. But certain cells of higher animals,
the colorless blood-cells, and most unicellular animals can be fed
also with solid substances; they take the food-particles into the
midst of the protoplasm by flowing around them with the pseudo-
podia. They extract all the assimilable and reject the indigestible
portions (fig. 16).

In the case of assimilation it is to be noted not only that the
cells use the food which they have taken for their own growth and
for replacing worn-out parts, but also that most of them have the
power of producing substances other than protoplasm; for
example, many Protozoa form organic shells or skeletons which
are hardened with silica or lime. This formative power, the
building of ‘plasmic products,’ 1s, as we shall shortly see, the
starting-point for tissue-formation.

Cell Nucleus.—The reproduction of protoplasmic bodies is
synonymous with the division of the cell: but to understand this
we must first consider the second important constituent, the
nucleus. This is a body enclosed in the protoplasm, whose form,
though definite for each kind of cell, shows in general wide varia-
tions. Usually it is a spherical or oval vesicle; but it may be
elongated or club-shaped, bent into a horseshoe, with constrictions
like a rosary, or even be branched, treelike (fig. 18); in many cells
GENERAL ANATOMY. 65

it is disproportionally large, so that the protoplasm surrounds it
only with a thin layer, in others again it is so small that it can
scarcely be found in the protoplasm among the other substances.
Formerly, on this account, it was in very many cases overlooked,
and even now it can often be demonstrated only by great care,

 

Fig. 18.—Various forms of nuclei. a, horseshoe-shaped nucleus of an Acinete; I»
branching nucleus from the Malpighian vessel of a Sphinyid larva; c, rosary-
shaped nucleus of Stentor cwruleus.

and by employment of a special technique based upon the micro-
chemical reaction of the nuclear substance.

The Nuclear Substance,—The nuclear substance is distin-
guished from protoplasm, among other ways, by its greater
coagulability in certain acids, e.g., acetic and chromic, which
therefore are often used for demonstrating the nucleus. If ina
living cell the nucleus be invisible on account of the similarity of
its refraction to that of the protoplasm, the addition of 2% acetic
acid will often bring it into sharp contour.

Structure of the Nucleus.—In its minute structure the nucleus
affords a wonderful variety of pictures varying according to the
objects chosen, but which are not sufficiently understood to permit
of a single description accepted by all. According to their reac-
tions to stains two substances in particular are distinguished:
chromatin or nuclein (fig. 19, ch), which is easily stained by certain
staining-fluids (carmine, hematoxylon, saffranin), and the achroma-
tin or linin, which stains not at all or only under special conditions.

The achromatin forms a network or reticulum (according to
another view a honeycomb structure) filled with a nuclear fluid,
66 GENERAL PRINCIPLES OF ZOOLOGY.

bounded externally by a nuclear membrane, easily isolated in large
nuclei. If little nuclear fluid be present, and the reticulum con-
sequently be coarse-meshed, the nucleus seems compact. If the
fluid be abundant, the nucleus appears vesicular. This is especially

ab

  
  
 

chp

6
4

Fic. 19.—Vesicular nuclei with achromatic reticulum and different arrangements of
the chromatin and nucleolar substance. p, plastin (nucleolar substance); ch,
chromatin; chp, chromatin plus plastin. land 2, nuclei of Actinospharium; 3,
of Ceratium hirundella (after Lauterborn); 4, germinal vesicle of Unio (after
Flemming); 5, nucleus with many chromatin nucleoli.

the case when the lines of the framework are separated by con-
siderable amounts of nuclear fluid (fig. 19, 4).

The chromatin enters into close relations with a less stainable
substance, the plastin or paranuclein (also sharply distinct from
achromatin). In the nuclei of Protozoa plastin and chromatin
are usually intimately united, the first forming a substratum in
which the latter is embedded (chp). The united substances
are most frequently closely and regularly distributed as fine gran-
wes on the reticulum, so that the entire nucleus appears uni-
formly chromatic (fig. 18). More rarely the mixture collects into
one or more special bodies, the chromatic nucleoli (7, 2). The
nucleolus is ordinarily a rounded body, more rarely branched
(fig. 19, 7).

In the nuclei of the Metazoa there may occur the same intimate
mixture of plastin and chromatin (6). As a rule, however, the
plastin (apparently not the whole, but a surplus) is separate from
the chromatin. Thus there occur in the nuclei of many eggs
GENERAL ANATOMY. 67

nuclei which contain, the one chromatin, the other exclusively
plastin (4). In tissue cells only the plastin has the form of
nucieoli (true or chromatin-free nucleoli, 5), while the chromatin
is distributed on the nuclear reticulum (chromatin reticulum).
Somewhat the same may occur in the Protozoa (fig. 19, 2).

Significance of the Cell Nucleus.—For a long time the func-
tional significance of the nucleus in the cell was shrouded in
complete darkness, so that it began to be regarded, in comparison
with the protoplasm, as a thing of little importance. The evidence
that the nucleus plays the most prominent role in fertilization has
altered this conception. Then arose the view that the nucleus
determines the character of the cell; that the potentiality of the
protoplasm is influenced by the nucleus. If from the egg a definite
kind of animal develop, if a cell in the animal’s body assume a
definite histological character, we are, at the present time, inclined
to ascribe this to the nucleus. From this, then, it follows farther
that the nucleus ts also the bearer of heredity; for the transmission
of the parental characteristics to the children (a fact shown to us
by our daily experience) can only be accomplished through the
sexual cells of the parents, the egg and sperm cells. Again, since
the character of the sexual cells is determined by the nucleus, the
transmission in its ultimate analysis is carried on by the nucleus.
This idea has a further support in experiments on Protozoa. If
one of these unicellular animals be cut into nucleate and anucleate
halves, the latter sooner or later degenerates, the former persists
and regenerates the lost parts. Within the nucleus it is probably
the chromatin which controls the functions of the protoplasm and
is accordingly (as observations on fertilization also seem to show)
the bearer of heredity, while the achromatin is the seat of contrac-
tility, and as such plays a part in cell multiplication.

The Centrosome.—Besides the nucleus there frequently occurs
a special body in the protoplasm, the centrosome, which on
account of its small size and a behavior similar to achromatin
with reference to staining-fluids was long overlooked, and even
now its demonstration is difficult. It is apparently well distributed
among the Metazoa, but is absent from most Protozoa. In many
it appears only at certain times and then disappears. What 1s
known of it makes it probable that it is a derivative of the nucleus,
a part of the achromatin which has left the nucleus; in other cases
possibly a second nucleus which by degeneration has lost the
chromatin and retained only the active nuclear substance, the
achromatin. In its function the centrosome is a specific organ of

 
68 GENERAL PRINCIPLES OF ZOOLOGY.

cell division which controls both the division of the nucleus and
that of the cell itself.

Multiplication of Cells.—Increase in cells occurs exclusively
by division or by budding (gemmaticn). Most common is binary
division in which a circular furrow appears on the surface of the
cell, deepens and cuts the cell into two equal parts. Multiple
division is more rare and can only occur in multinucleate cells,
Here the cell divides simultaneously into as many (sometimes

hundreds) daughter-cells as

there were nuclei present. In
all forms of division the simi-
larity of the products is char-
acteristic, while in budding the
resulting parts are unequal.

In budding one or more

smaller daughter-cells, the
buds, are constructed from a
a large mother-cell (fig. 20).

i Direct Cell Division. —
Every cell division is accom-
panied by nuclear division or
2 ee en ee Oe

Se pone UD. melons. ivision has previously oc-
curred. Direct and indirect
division are recognized. Direct division is most common in
Protozoa, and especially in nuclei with abundant chromatin (fig.
20, 145). The nucleus is elongated and is divided by constriction,
in the same way that the cell itself constricts. Since the proto-
plasm has no special arrangement with regard to the dividing
nucleus (the latter besides protected by its membrane), we must
conclude that the nucleus divides itself and is not passively
divided. The dividing force resides in the achromatic framework,
which correspondingly often exhibits a certain arrangement, a
fibrous structure in the direction of the elongating nucleus.

Indirect Cell Division, Karyokinesis.—Indirect cell division,
karyokinesis or mitosis, is most beautifully shown in cells, poor in
chromatin, which possess a centrosome. The process is introduced
by a division of the centrosome (fig. 21). The daughter centro-
somes migrate to two opposite poles of the nucleus, which now
loses its membrane and becomes the nuclear spindle. The
characteristics of the spindle are that it is drawn out into points
at two poles which are indicated by the position of the centro-

 

 
GENERAL ANATOMY. 69

somes, while from these poles fine threads, the spindle-fibres,
run to the centre or equator of the
nucleus. These fibres are in many
cases certainly derived from the
achromatic nuclear reticulum, while
in others a greater or less part in
their formation is taken by the
protoplasm. <A debated point is the
relations of the fibres in the equa-
torial plane of the spindle. Do all
the fibres extend from pole to pole ?
Do all of them end in the equatorial Bie. elle tear a
plane, so that the spindle consists of a Macainole, | (Cee Sennen dG
two cones of fibres separated at the
equator? Or, lastly, are fibres of both kinds present in the same
spindle? It would appear that differences exist in these respects
in different objects.

All of the chromatin of the nucleus lies in the equator, united
in the ‘equatorial plate,’ but by this must not be understood a
connected mass but a layer of separate bodies, the chromosomes,
for the chromatin of the nucleus divides early into particles which
are rarely spherical or rodlike, but usually have the shape of
U-shaped loops. These chromosomes are of equal size in the same

 

 

Fic. 22.—Cell division in the skin of Salamandra maculosa. (After Rabl.)

cell, and, what is of greater theoretical significance, their number
is identical in all the cells of all the tissues of one and the same
species.
70 GENERAL PRINCIPLES OF ZOOLOGY.

The first step in the karyokinetic formation of the daughter
nuclei is the division of the chromosomes, which is usually com-
pleted in the equatorial plate (division of the equatorial plate), but
occasionally may be completed at an earlier stage. The division
is an accurate halving (fig. 22, 6). The two halves of a mother-
chromosome, the daughter chromosomes, now travel, under the
influence of the spindle-fibres, towards the opposite poles of the
spindle. In this way, by a splitting of the equatorial plate, the
lateral plates arise, the elements of each uniting and producing
the daughter nuclei. The centrosomes remain separate as division
organs for the next nuclear division (fig. 22, ¢, d, ¢).

What further distinguishes the indirect from the direct cell
division is the active participation of the protoplasm. The
centrosome is the centre of a marked radiation of the protoplasmic
reticulum (fig. 27). When the centrosome divides a double radia-
tion (amphiaster) appears. Not only the spindle-fibres but the
protoplasmic rays extend from the daughter chromosomes. Since
the arrangement and degree of development of the protoplasmic
radiations stand in certain relation to the phases of cell division
we must recognize in them the expression of the effective forces
(apparently contractile) in the protoplasm which cause cell
division.

Between these two extreme cases of direct and indirect division are
all possible transitions which show how the mechanism of nuclear divi-
sion has been completed step by step, first, by the fibrous arrangement of
the nuclear reticulum (spindle structure); second, through the develop-
ment of the centrosome by which the division obtains an influence on the
protoplasm ; and third, by the development of the chromosomes. In
reference to the latter the irregular division of the chromatin mass in
direct division is relatively crude in comparison with the complicated
processes involved in the formation and division of the chromosomes.
These become intelligible if we regard the chromatin as the controller of
the cellular processes and the bearer of heredity (cf. fertilization, infra).
The more highly organized the animal, the more its cells have to inherit
and the more important it is that the physical basis of heredity should be
accurately divided in amount and in quality between the daughter cells.
This is accomplished by mitosis.

Nuclear Fragmentation is to be distinguished from direct division ; by it
the nucleus becomes broken up into numerous parts which alone cannot
live and as a rule degenerate. A typical example is afforded by the
breaking up of the macronucleus during conjugation in the Infusoria
(fig. 146),

Multinuclearity, Multicellularity.—Nuclear division and cell
division commonly constitute a well-arranged mechanical process,
GENERAL HISTOLOGY. ea

the separate phases of which follow one another according to

a definite law. The plane of division is per-
pendicular to the long axis uniting the two
poles of the spindle. But the interrelation of
cytoplasm and nucleus is by no means an unchange-
able and ijndissoluble one, for very often nuclear
division takes place without participation of the
cytoplasm. If this process be repeated several
times, there results a mass of protoplasm with many
nuclei (fig. 23), which now on its part may become
many cells, if subsequently the protoplasm divides

according to the number of nuclei. Hence multi- Pi, 3. Gant
nucleated protoplasmic masses are transitional stages uClei.
between the simple mononucleated cell and a collection of several
mononucleated cells, and in consequence of this are sometimes
regarded as the equivalent of one cell, sometimes as equivalent to
many cells, and are called sometimes multinucleated giant-cells,
sometimes cell-complexes or syncytia. In the following pages a
multinucleated mass of protoplasm will be considered as a single
cell, because the essential feature of the cell is that it constitutes a
vital unit, it has a physiological individuality, and in this respect
a multinucleated mass of protoplasm behaves like a mononucleated ;
as the tissue cells and the Protozoa show, the plane of organization
is not raised in the least by the multinuclearity. A change only
begins at the moment when many particles of protoplasm are
separated from one another, and many vital units are formed, Le.,
when in place of multinuclearity a true multicellularity appears.

 

II. Tue Tissues of THE ANIMAL Bopy.

Definition of Tissue.—In the formation of tissues two processes
are operative: (1) the multiplication of cells by means of division
into cell-complexes, and (2) the histological differentiation of cells.
A tissue, therefore, can be defined as a complex of differentiated
cells histologically similar.

Nature of Histological Differentiation.—The histological differ-
entiation consists chiefly in that the cells have definite form and
definite position relative to the neighboring cells; in addition,
there almost always occurs, as a second and more important
feature, the histological modification of the cell. The fact has
already been mentioned that the cell uses its food-material, not
only for its own growth, for increase of its protoplasm, but also,
in another manner, for forming substances, protoplasmic products,

 
72 GENERAL PRINCIPLES OF ZOOLOGY.

either in its interior (internal plasmic products), or more often on
its surface (external plasmic products). The histological change is
the formation of specifically functioning plasmic products. If we
take as an example the manner in which a cell
becomes a muscle fibre (fig. 24), we see that it
continually secretes upon its surface new fibrille
of specific muscle substance (in the case of the
vertebrates, new cross-striated muscle fibrillee),
until finally the remnant of the formative cell,
the muscle corpuscle, is contained in a mantle of
muscle fibrille. In an analogous way, each
tissue, upon histological examination, is seen to
be composed of cells and plasmic products. The
former control the formation, the renewal, and
the sustenance of the tissue; the latter are the
agents of its physiological function. The advan-
anal meade tages of tissue formation are far-reaching, since
oh ae ae ia in general they are connected with division of
gram.) a, forma- dabor (frequently referred to later). So long as
tive cell; h, forma- ‘ f . 3 5
tive cell with two the cell unites in itself all the vital functions,
ied cicele Abeer these are incomplete because they mutually
Se bee hinder each other in their free development; the
muscle fibrils. ylasmie product, on the other hand, has only the
single function peculiar to it and can therefore discharge its duties
with greater completeness. The muscle fibrille, the characteristic
elements formed by the muscle cells, have preserved of the various
properties of protoplasm only the capability of contraction; but
this power of contraction is much more energetic and stronger than
the mere movement of protoplasm. The nerve fibrille serve only
for the transmission of stimuli, but in an extraordinarily more
rapid and orderly manner than does simple protoplasm.
Classification of Tissues.—Since in every tissue its function
interests us most, it would be natural to base the classification of
tissues upon the function and the intimate structure connected
therewith. For a long time the tissues have been arranged in four
groups: 1. Epithelial tissue; 2. Supporting tissue: 3. Muscular
tissue; 4. Nervous tissue. Within these, however, certain con-
stituent parts of the animal body, to which indeed the term
‘tissue’ is scarcely applicable, find no shelter: these are the sexual
cells, the blood, and the lymph. The former may be spoken of
in connexion with the epithelium, the latter in connexion with
the supporting substances.

 

 
GENERAL HISTOLOGY. 73

1. Epithelial Tissues.

Morphology of Epithelial Tissues.—On several grounds the
epithelia must be considered first. They are the oldest tissues;
they are the first to appear in the animal kingdom, there being
animals which consist only of epithelia. Further, each separate
organism during the first stages of embryonic life consists only of
epithelial layers, the germ-layers. With this is also connected the
fact that in epithelial tissues the cells have undergone the least
degree of histological change, and that the formation of plasmic
products is subordinated.

Function of Epithelium.—The most important purpose of the
epithelium is to form a protecting and excluding covering over
surfaces, equally valuable whether the surfaces are external (surface
of the body) or caused by cavities in the interior of the body (the
body cavity, lumen of the gut and blood-vessels). The importance
of the epithelia in this respect is shown by the fact that if the
protecting layer be removed, inflammation arises and continues
until the epithelium is regenerated. Only exceptionally do areas
occur which are free from epithelium; the teeth of vertebrates, the
antlers of stags, are parts of the body which, on account of their
hardness, can exist, at least for a more or less considerable time,
without epithelial covering.

Glandular and Sensory Epithelia.—By their superficial posi-
tion epithelia are suited for presiding over two other functions: all
substances which ought to be removed from the body—some
because they have become useless, and consequently injurious
(excreta), and others, as, for example, the digestive fluids, because
they have to perform important functions (secreta)—must pass the
surface, and are therefore separated by the epithelia; these are the
glandular epithelia. Further, all influences of the external world
chiefly impress the surface of the body, causing sensations; hence
also certain epithelia are of the greatest importance for the recep-
tion of sensory stimuli, and serve for hearing, seeing, smelling,
tasting, and touching. Such areas of epithelium are called sensory
epithelia.

Covering Epithelium.—The covering epithelium consists of
cells which, in order to serve the function of the tissue, are united
by a small quantity of cementing substance. We speak of simple
or of stratified epithelia, according as we find in sections running
perpendicularly to the surface one or several superimposed layers
(figs. 25, 26, 27).

 

 

 
T4 GENERAL PRINCIPLES OF ZOOLOGY.

 

Simple Epithelium. — Exclusively one-layered epithelia are
found in all invertebrated animals and in Amphioxus; in the
vertebrates, on the other hand, they are limited to the internal
cavities of the body, and even here are occasionally, as always in
the skin, replaced by a many-layered epithelium. According to

 

Fig. 25 —Various forms of epithelia. a, flattened epithelium of Sycandra raphanus,
a’ in cross-section, a/’ in surface view; b and c, cuboidal and columnar epithe-
lium of a mollusc (Haliotis tuberculata); d, flagellated epithelium of an actinian
(Calliactis parasitica); e, ciliated epithelium from the intestine of the fresh-water
mussel; f, epithelium (e¢) with cuticle (c) of Cimbex coronatus (a wasp).

the shape of the cells we distinguish cuboidal or pavement, flat,

and columnar epithelium. In the case of pavement epithelium

(fig. 25, b) the cells are all developed about equally in all direc-

tions of space, and because they have become compressed by

lateral pressure have the appearance of cubical blocks or paving-
stones. In columnar epithelium the long axis, the distance from
the deeper to the peripheral end of the cell, is especially great

(fig. 25, ¢); finally, in flat or squamous epithelium this is greatly
GENERAL HISTOLOG Y. 75

shortened (fig. 25, a) and the separate cells have become changed
into thin plates.

Flagellated and Ciliated Epithelia.—Further differences which
obtain in the three kinds of epithelium mentioned above are
caused by the presence or absence of processes (cilia, or flagella)
on the peripheral end of the cells. Both are fine threads which
arise from the body of the cell, extend above the surface and here
maintain an extremely lively motion. In case of flagellated
epithelium (fig. 25, «/) each cell has only one vibratile projection,
but this is strongly developed; in the case of ciliated epithelium
(fig. 24, e), on the other hand, the surface of the cell is covered
with a thick forest of minute threads moving in unison.

Cuticle.—The majority of the one-layered epithelia are covered
by a cuticle, a membrane which is secreted by the epithelial cells
in general, and hence very frequently shows the impression of the
celis as polygonal markings. In many cases thin and inconspic-
uous, it may in other instances become thickened into a very con-
siderable layer, much thicker than the matrix layer of epithelium
which secretes this cuticle. The cuticle is plainly composed of
layers parallel with the surface, and forms a more effective protec-
tion for the surface of the body than does the epithelium; it
becomes a protective armor, as shown, among other examples,
by the calcareous shells of molluscs and the chitinous integument
of insects (fig. 25, f).

Stratified Epithelia.—The protection furnished by the cuticle
in the case of simple epithelium, may in the stratified be obtained
immediately through a chemical change of a part of the cells
themselves. In the stratified epithelia the cells of the various
layers always can be distinguished by their form. The deepest
layer consists of cylindrical cells; the superficial, on the other
hand, of more or less flattened elements; between lie several layers
of transitional forms, so that starting from the cylindrical cells we
gradually pass through the cubical cells to the flat cells of the sur-
face. As this arrangement shows, there exists a genetic con-
nexion between the cell-layers: the lower cylindrical cells are in a
state of active multiplication; their descendants, with gradual
changes of form, become the superficial layers, here to replace an
equal quantity of worn-out cells (fig. 26).

In the course of this change of position, the protoplasmic
bodies may undergo an alteration; in the reptiles, birds, and
mammals (fig. 27) they became cornified, first the margins, then
the inner part of the cell, changing into horn. Of the lving cell

 
76 GENERAL PRINCIPLES OF ZOOLOGY.

the nucleus remains for some time, until at length this vanishes,
and then the cell becomes completely changed into a dead, horny
scale. In the skin of the higher vertebrates the zones of the living
protoplasmic, and the cornified cells no longer capable of life, are
sharply marked off from one another. In cross-section they are
readily distinguished as the stratum corneum (sc) and the stratum

PB F os

  

 

Fig. 27.— Stratified epithe-
lium of man. sM, stratum
Malpighi; sc, stratum “cor-
neum.

 

  

FI@. 26.—Section through the skin of Petromyzon Fic. 28.—Single-layered
planeri. Ep, the many-layered epithelium of the epithelium of a snail. e¢,
epidermis, including B, goblet cells; A6, granu- cuticle; d, goblet cells.

lar cells; Ko, Co, dermis (with blood-vessels

G), consisting of bundles of fibrils running hori-

zontally (W) and vertically (S). (From Wieders-

heim.)
Malpighi (sJ/) of the skin (fig. 27). In the many-layered epithelia
the cuticle has lost its importance, and it is either an inconspicu-
ous boundary line or is entirely absent.

Glandular Epithelium.—Glandular epithelium is distinguished
physiologically from ordinary pavement epithelium by the fact
that it also produces secretions or excretions: anatomically it is
recognizable by the presence of « gland cells,’ cells which carry on
the secretion and, to a greater or less extent, reveal their character
by their structure. Characteristic glandular cells are, for example,

 
GENERAL HISTOLOGY. Tere

the goblet cells; here the secretion, usually mucus, is collected as
a clear mass in the interior of the cell, the cytoplasm being com-
pressed into a thin external wall, reminding one of a goblet con-
taining the nucleus at its base (fig. 26, 28, d). Other gland cells
are the granular cells, swollen bodies completely filled with secre-
tory granules (fig. 26, AG). Naturally all grades of transition
between pavement and glandular epithelium occur. Commonly
the latter name is only employed when the gland cells are especially
numerous, thereby giving to the epithelial area a pre-eminently
secretory character. This is especially the case with the structures
which have the name of glands, among which we distinguish
unicellular and multicellular glands.

Unicellular Glands.—Unicellular and multicellular glands
increase the secretory surface by invagination. Invagination of a
single cell produces the unicellular glands which are chiefly found
among the invetebrate animals (fig.
29); a gland cell here becomes so
enormous that there is no room for it
in the epithelium, but it is pushed
into the deeper, the subepithelial
layers, the nucleated cell body, dis-
tended by secretion, sending up a
slender process, the duct, to the
epithelial surface.

Multicellular Glands.—In the for-
mation of multicellular glands a con-
siderable area of glandular epithelium
grows as a cylindrical cord or tube
from the surface down into the deeper
tissues 5 this cord of cells seldom eee eof ine wom eeu oe
remains simple; it usually branches (ere a ee ara tae cele
and forms the compound glands,
which may consist of hundreds or thousands of glandular sacs, al]
emptying into a common duct. Among the multicellular glands
are to be distinguished tubular and acinous (racemose) forms. In
tubular glands (fig. 30) the simple or branched glandular pouches
preserve the same tubular diameter from beginning to end; in the
acinous glands (fig. 31), on the contrary, the blind end of the
glandular pouch widens into a sac (acinus), largely composed of
secretory cells, and related to the outer part of the glandular
pouch, the duct, as grapes are to their stem. To the tubular
glands belong the liver, kidney and sweat glands of man; to the

 
-
4

; GENERAL PRINCIPLES OF ZOOLOGY.

GG

acinous belong the salivary glands, not only of the vertebrates, but
also of the arthropods and molluscs.

eT

SG

"a,

 

Fia. 30.—Tubular glands. (After Toldt.) 4, glands of Lieberkiihn from the human
intestine; A’, of the conjunctiva of the eye; B, of the cat’s stomach; C, from the
ey pyramids of the dog’s kidney; D, fromthe cortex of the rabbit's

idney.

Sexual Epithelium.—The sexual cells may be considered in
connexion with glandular epithelium. As the secretion of some

 

 

Fra. 31.—Acinous salivary gland of the aphid Orthezia cataphracta, (After List.) In
some acini the nuclei and boundaries of the cells are shown.

glands must be expelled from the body, so the sexual cells are

elements which differ from the rest of the organism, and must

reach the exterior in order to perform their function. Just as the
GENERAL HISTOLOGY. rey)

gland-cells are usually scattered among ordinary epithelial cells,
so the sexual cells, almost without exception, lie embedded in
epithelium ; it may be in the epithelium of the skin (fig. 32), of the

 

Fia. 32.—Germinal epithelium of a medusa. ek, ectoderm; en, entoderm; 0, eg
e, epithelium.

oe
Sy

gut, of the body cavity, or of parts cut off from this (fig. 33).
This connexion of the sexual cells with the epithelium has a
deeper meaning in the fact that many organisms, and particularly
organisms of low structure, consist exclusively of epithelia and

 

FG. 33.—Section through the ovary of a new-born child. (After Waldeyer.) (qe,
germinal epithelium; pe, primitive egg in the germinal epithelium; p, egg-pouch;
y. egg-nest constricted off from the pouchlike growth (p); f, single egg with fol-
licle; v, blood-vessel.
therefore must necessarily develop their sexual products in
epithelium. In other words, sexual and epithelial cells are the
oldest elements of the animal body, and hence very early came
into relation with one another.
Sexual epithelium (or, as it is often called, germinal epithe-
lium) like glandular epithelium has a tendency to grow into the
subepithelial tissues in the form of isolated or branching tubes
SO GHNERAL PRINCIPLES OF ZCOLOGY.,

(figs. 33, p, 34), and thus in many groups of animals the sexuai
* organs bear the character of branched
penta: for this reason one speaks as often
of sexual glands as of sexual organs (fig. 34).
The male and female cells, the specific ele-
ments of the germinal epithelia and of the
sexual glands, differ in the fact that the eggs
are generally the largest, the spermatczoa
the smallest, cells of the animal body.
The egg-cell (fig. 35) as it is
formed in the ovary varies in size according
to the animal group: in case of the micro-
scopic Gastrotricha it is less than 0.04 mm.,
in man about 0.2 mm., in the frog se fecal
millimetres, and in the large birds often
several inches; however, only the yolk of the
bird’s egg is the egg-cell, the white of the
egg and the shell are structures which are
formed outside of the ovary in the oviduct.
These remarkable differences in size are
caused less by the quantity of the peculiar
cell-substance, protoplasm (formative or
primary yolk), than by the accumulation of
deutoplasm (food or accessory yolk, also

        

  

Fria. 35.

Fia. 84.—Ovarian tube of an insect, Vanessa urticer. a, formative cell;_b, follicular
epithelium; c, nutritive cells; d, egg-cells; f, fibrous covering extending out into
the terminal fibres (g). (After Waldeyer.)

Fia. 35.—Egg-cell of Strongylocentrotus lividus.

briefly called yolk). The function of the deutoplasm is to nourish
the embryo during development, and hence consists of substances
rich in fat and proteid, arranged in spherical oil-drops, or in fine
GENERAL HISTOLOGY, Sl

granules or polygonal bodies, the yolk-granules. Its quantity,
and therefore the size of the egg, isin part proportional to the
length of time which the egg is cut off from any other supply of
nourishment. In general we find the largest eggs in the case of
the highly organized oviparous animals, where a long-continued
course of development is necessary to lay the foundation of the
manifold organs. Besides the protoplasm and deutoplasm, a cell
nucleus or germinal vesicle (sometimes yisible to the naked eye)
surrounded by a membrane always occurs in the egg. Its contents
are mainly the nuclear fluid, through which is distributed an
achromatic network, and in addition the nucleolus, called also
the germinal spot. Often there are multinucleolated germinal
vesicles, especially in eggs which contain very much yolk.

The Spermatozoa, the morphological elements of the male
reproductive product, are so small that their finer structure can be
studied only with the strongest powers of the microscope (fig. 36,
aand f). Easiest to recognize in them is the head, which from.

C 2p

\,

 

Fig. 36 —Various spermatozoa. a, of the night-hawk; 8, of the green frog; y, of the
crayfish ; 6, of a crab; ¢, of the round worm (Ascaris) 2, nucleus; m, middle
piece; s, flagellum; ik, homogeneous body.

its variety of form—spherical, oval, sickle-shaped, ete.—often

renders possible the specifie determination of the spermatozoa,

The head is the closely compacted chromatic part of the nucleus,

and hence colors very deeply in staining fluids. Next comes an
82 GENERAL PRINCIPLES OF ZCOLOGY.

unstaining second part, the middle piece, and then the tail, a long
flagellum, which causes the active motility of the ripe sperma-
tozoon. Cytoplasm is usually present only in an extremely thin
layer surrounding the nucleus.

The spermatozoa of nearly all animals, except the nematodes
and crustaceans, are constructed according to this type. In these
two groups it is worthy of notice that the spermatozoa are
remarkably large and incapable of motion, and that they enclose a
homogeneous strongly refractive body (fig. 36, 4), previously not
found, the significance of which is not clear. The spermatozoon
of Ascaris (fig. 36, ¢) has the form of a sugar-loaf with a broad
rounded end, containing the nucleus; the spermatozoon of the
crayfish (fig. 36, y), on the other hand, has the shape of a cake-
pan, from whose periphery springs a circle of fine, stiff, and
pointed fibres.

The two kinds of spermatozoa found in a few animals are. problem-
atical. In the testis of one and the same individual of Paludina vivipara
occur together hair-like spermatozoa with corkscrew heads and vermiform
spermatozoa with a bunch of cilia on the hinder end. The first accomplish
fertilization ; the physiological significance of the second is unknown.

The last modification of epithelium of which we have to speak
is sensory epithelium, characterized by the connexion of certain
of its cells, the sensory cells, with
the finest twigs of branching nerves
which arise in the central nervous
system. This connexion may be of
two kinds. In the first the cell
(primary sense cell) is slender and
filiform, the position of the nucleus
being indicated bya swelling. The
peripheral end is concerned with
the reception of sensatory stimuli,
while the deeper end is continued
directly into the nerve ends and
correspondingly is branched into two
or more extremely fine processes
Fig. 87.—Sensory epithelium. a,ofan Which take on the character of

thelium of many a. supporting sche; Herve fibrille (fig. 8%). In the
si peupany eels: second type the sensory nerve ends
ina ganglion cell beneath the epithelium, which sends processes
into the latter, the ends of these being applied to the sensory cell
(secondary sense cell), the connexion being one of contact, not of

 
GENERAL HiSTOLOGY. 83

continuity. In both the peripheral end of the cell bears appen-
dages for sense perception; auditory and tactile hairs, stronger
processes in the case of olfactory and taste cells, conspicuous rods
in visual cells. Almost without exception the sensory cells are
part of the skin (ectoderm), or at least arise from it in develop-
ment. This is true for sense organs like the eye and ear of verte-
brates, which are separated from the skin by thick intermediate
tissue, for in these the sensory epithelium (retina, crista acustica)
is derived from the ectoderm.

Supporting Cells.—In the region of the sensory epithelium
and between the sensory cells are found still other epithelial cells,
which are not connected with nerves, but have accessory functions:
they serve as supports for the sensory cells; in the eyes they con-
tain pigment; in the auditory organs they often bear the otoliths,
etc. They have the general name of supporting or sustentative
cells.

2. Connective Tissues.

Contrast of Epithelium with Connective Tissue.—From a his-
tological point of view there can be found no greater difference
than exists between epithelium and connective tissue; the former
belongs to the surface, the latter to the interior of the body; in
the former the cells play the chief réle, in the latter, on the con-
trary, their importance is subordinate to the plasmic products, the
‘intercellular substances’ which chiefly determine the character
of the various kinds of connective tissue.

In spite of this contrast the connective tissues are genetically
connected with epithelium. Jn embryos which at first consist
only of epithelia the connexion can be directly seen. The
epithelia secrete a gelatinous substance from their deeper surfaces
into which separate cells migrate. Thus arises the embryonic
connective tissue, the mesenchyme (fig. 107).

Function of Connective Tissue.—The primary function of con-
nective tissue is to fill the spaces between the various organs in
the interior of the body, thus connecting not only the single parts
of the organs, but also the various organs themselves. In conse-
quence of this the connective tissues contribute to the firmness of
body, and are frequently employed in building up a skeleton. To
accomplish this, substances which are usually firmer than proto-
plasm are formed on the surface of the cells, and, since they lie
between the cells, these are called intercellular substances. In
+ GENERAL PRINCIPLES OF ZOOLOGY.

CO

proportion as the intercellular substance increases in volume the
cells themselves diminish and become inconspicuous corpuscles,
the connective-tissue corpuscles, or, as seldom happens, entirely
disappear. Since, in the connective tissues, the intercellular sub-
stances are most important, it is readily understood that the dis-
tinctions between the various kinds of connective tissue rest chiefly
upon the differences of this intercellular substance. The following
forms are to he distinguished: (1) cellular connective tissue; (2)
homogeneous connective tissue; (3) fibrous connective tissue; (4)
cartilage; (5) bone.

Cellular Connective Tissue shows the characteristics of the
group least distinctly. It owes its name to the fact that the cells
make up the chief mass, while the cell-products are inconsiderable.
The cells are large and vesicular bodies which, like plant cells, are
closely pressed together and are consequently polygonal (fig. 38).
They have secreted between them a firm but thin layer of inter-
cellular substance.

 

Fig. 38.—Cellular connective substance. Fic. 39.—Homogeneous connective sub-
Cross-section through the notochord Stance of Sycandra raphanus. (After
of a newly hatched Trout. F. E. Schulze.)

Homogeneous Connective Tissue.—In the case of homogeneous
connective substance the intercellular substance (or matrix) is
usually present in considerable quantity as a transparent mass,
nearly invisible under the microscope, sometimes soft like jelly,
often firmer (fig. 39). The gelatinous cells lying in it are either
spherical or send branching processes into the matrix. Such
processes may unite to form meshes which, like a pseudopodial
network, unite cell to cell. Frequently the matrix contains, in
addition, isolated firm fibres or threads, which, on account of
GENERAL HISTOLOGY. 85

their physical characteristics, are called elastic fibres, and consist
of a substance (elastin) exceedingly resistant to all reagents.
Finally, in the matrix there may develop the finer connective-
tissue fibrils, the characteristic element of the next group; they
may become so prominent by increase in number as to determine
the character of the tissue.

Fibrous Connective Tissue is characterized by the rich supply
of connective-tissue fibrille; these are fibres of extraordinary fine-
ness, lying in a homogeneous basal substance, which is the less
evident the richer it is in fibres. The fibres may be either con-
fusedly arranged, crossing in all directions, or may run essentially
parallel and in a definite direction. Between them are found the
rounded, spindle-shaped or branched connective-tissue corpuscles
(fig. 40). It is characteristic of vertebrates that the fibres are
grouped into bundles. Each bundle is generally surrounded by
connective-tissue corpuscles, metamorphosed into flat cells. The

it
We

 

i ai 5c ective tissue of 2 Fig. 41.— Areolar fibrous. connective
ee ee ea tissue. (After Geyenbaur.)
bundles, loosely interwoven, run in all direction (areolar connec-
tive tissue, ‘cellular tissue’ of the earlier authors) (fig. 41), or
they may be almost parallel, forming a compact mass of fibres
(tendinous tissue) (fig. 42). Since the fibrils of the fibrous con-
nective tissue of the vertebrates have another peculiarity not met
with elsewhere, in that they are composed of glutin, and upon
boiling become gelatine or glue, 1t 1s well to reserve for these
forms of tissue the special name connective tissue.

Elastic Tissue.—In all fibrous connective tissue there may
appear, as a further constituent, elastic fibres; they may indeed
86 GENERAL PRINCIPLES OF ZOOLOGY.

supplant the ordinary connective-tissue fibrils and become the
predominant element of the connective tissue, which is then
spoken of as elastic tissue.

 

  

Fic. 42.

Fic. 42.—Tendinous tissue. (After Gegenbaur.)

Fic. 43.—Cartilage. (After Gegenbaur.) c, perichondrium; b, transition into typical
cartilage (a).

Cartilage.—Cartilage and bone are likewise tissues which find
their characteristic development only in the vertebrates. In its
appearance cartilage is similar to the homogeneous connective
substance of many invertebrated animals; the matrix is homo-
geneous and, at first glance, appears quite structureless (fig. 43),
but, under the action of certain reagents, assumes a fibrous condi-
tion. This conduct, as well as the fact that the cartilage grows
through changes of the perichondrium,—a thin, fibrillar skin
covering its surface,—makes it more certainly evident that it is
homogeneously fibrillar; and it is thereby distinguished from
homogeneous connective substance since it is not, like the latter,
a lower but a higher stage cf tissue formation. It is worthy of
note that the matrix of cartilage (chondrin) by cooking produces
a kind of glue which differs from true or glutin glue in that it is
precipitated by acetic acid. In the matrix the cartilage cells lie
united in groups and nests, a mode of grouping pointing to their
origin, since each group of cells has arisen from a single mother-
cell by successive divisions. In cartilage also, elastic fibres are
found; if present in great number, these change the bluish shiny,
hyaline cartilage into the yellow-colored elasiie cartilage.

Bone is the most complicated structure in the series of connec-
tive tissues. It consists of a matrix (ossein), closely allied to
GENERAL HISTOLOGY. 87

glutin, so intimately combined with inorganic constituents that it
appears under the microscope as
a homogeneous mass. The propor-
tion of organic and inorganic sub-
stances varies according to the age
and species of animal: in man, for
example, there is 65% inorganic to
35% organic substance; in the
turtle, 63% to 37%. Of the in-
organic constituents, the most im-
portant is calcic phosphate, 842;
in smaller quantities, combinations
of fluoric, chloric, carbonic acids
and magnesia. Morphologically
the matrix is composed of the bone
lamelle (fig. 44), whose arrange-
ment is determined by the surfaces
present in and upon the bone. In
a hollow bone (like that of the
upper arm or of the hand) there is
an outer surface to which a fibrous
skin, the bone-skin or periosteum,
is closely apphed; the presence of
the marrow-cavity necessitates a
second surface. Finally, the solid
mass of the bone is permeated by
the Haversian canals, which run
chiefly in a longitudinal direction,

 

through the

 

ction

united into a network by cross or
oblique canals, and serve for the
passage of blood-vessels. Since the

 
  

human metacarpus.
surface of the periosteum;
of the marrow-cavity; s
tions of the Haversian canals anil
their system of lamelle; d, funda-
mental lamelke; ¢, bone corpuscles.

  

bone lamelle arrange themselves
parallel to the surfaces mentioned, two systems may be distin-
guished in cross-section, the fundamental lamelle and the
Haversian lamelle. The former are arranged paraliel to the sur-
face of the periosteum and of the marrow-cavity and form a

mantle of concentric layers around the marrow-cavity. Into this
groundwork the UHaversian canals with their lamelle enter,

destroying and superseding the fundamental lamellae coming in
their way. The Haversian lamelle are concentrically arranged
around the lumen of the Haversian canals just as the fundamental
lamelle are around the marrow-cavity.
88 GENERAL PRINCIPLES OF ZOOLOGY.

Formation of Bone.—The stratification of bone is caused by its
mode of origin. Where the bone borders upon the Taversian
canals, the marrow-cavity, and the periosteum, there is transiently
or permanently an epithelial-like layer of cells, osteoblasts, which
secrete the bone-substance on their surface. Certain cells in the
matrix participate in this secretion, and here give rise to the
bone-corpuscles, which are distinguished from the cartilage-cells
by their numerous processes ramifying through the matrix. The
processes of a bone-corpuscle branch, and unite with the neighbor-
ing cells through fusion of the processes, an arrangement most
beautifully seen in dried bone, because here the cavities and the
canals of the matrix are filled with air. Special modification of
bony tissue, the substance of fish-scales and of the teeth, called
also ivory or dentine, should be mentioned.

Blood and Lymph, here treated in connexion with the connec-
tive substances, are in reality not tissues at all, but nutritive
fluids. Two kinds of nutritive fluids occur in the vertebrates, red
blood and the colorless, weakly opalescent, or cloudy white lymph.
The blood of man and other vertebrates, consists of a fluid and
the organized constituents. The fluid or blood-plasma is, apart
from inorganic constituents, specially rich in proteids; after the
removal of the blood from the blood-vessels a part of these separate
by coagulation and form the blood-clot, made up of fibrin, leaving
a fluid poor in proteids, the blood-serum. The organized con-
stituents, the dlood-cells, are distin-
guished as red and white blood-cor-
puscles. The latter, the leucocytes,
are present in smaller numbers and
have great similarity to the ameeb
found in water: they are particles
of protoplasm, contain a nucleus,

     

Oe BONS.

SSS Oe
PO L,0§
eso ORS

devour foreign bodies (for example,
carmine granules injected into the
blood), and move in the ‘ameeboid’
manner by putting out pseudopodia
(fig. 45).

Red Blood-corpuscles.—In the
Fia_ 45 — White hlood corpuscles. a, mature condition the red blood-

of man; b, of the crab (n, the nu-

cleus) corpuscles of vertebrates (fig. 46)
are cireular or oval dises, which by external influences (e.g., by
pressure) may temporarily be bent, incised, or otherwise modified

in form, but cannot actively change their shape, because they no
GENERAL HISTOLOGY. 89

longer consist of protoplasm. Embryologically they arise from
true, nucleated, protoplasmic cells; whether these cells are iden-

   

Fic. 46.—Red blood-corpuscles. a,of man; }, of the camel; c,of the adder; d’, of
Proteus (seen from the edge): «’’, surface view; e, of a ray; f, of Petromyzon; 1,
nucleus (all the blood-corpuscles are magnified 700 times, except d, which is mag-
nified 350 times).

tical with the leucocytes or are special ‘erythroblasts’ is still
undertermined; but gradually the protoplasmic cell-body changes
completely into a plasmic product, the stroma of the blood-
corpuscle. If the nucleus be retained in this metamorphosis, there
is a slight swelling in the centre of the disc; if, however, the
nucleus degenerate, the bilateral convexity is replaced by a shallow
concavity. In the latter case, one has, in reality, no right longer
to speak of blood-cells, since all the characteristic constituents of
the cell—nucleus and protoplasm—have disappeared. Systemati-
cally the red blood-corpuscles are of interest, since non-nucleate
forms are found only in the mammals (fig. 46, a, 4), nucleated
ones in all the other vertebrates (c, d). The mammals also have
circular, the other vertebrates oval, discs. To this, however,
exceptions occur, since among the mammals the Typloda (camel,
llama) have oval, the Cyclostomes have circular, blood-corpuscles.

Hemoglobin.—The red blood-corpuscles are the cause of the
color of the blood, as well as the agents of one of its most impor-
tant functions, the interchange of gases; both are connected with
the fact that the stroma contains the coloring matter of the blood
or hemoglobin. Wemoglobin belongs to the few crystallizable
proteids and is remarkable for the presence of a small, though
extremely important, quantity of iron, and also for its affinity for
oxygen. Hemoglobin containing oxygen, oxy-hemoglobin, causes
the carmine-like color of the so-called arterial blood; oxygen-free,
‘reduced’ hemoglobin causes the dark red, faintly bluish color of
vevous blood.
90 GHNERAL PRINCIPLES OF ZOOLOGY.

Lymph is distinguished from blood by the entire lack of red
blood-corpuscles and the slight coagulability of its plasma.
Lymph is accordingly a proteid-containing fluid with leucocytes,
which are here called lymph-corpuscles.

In the majority of invertebrated animals there is present only
one kind of nutritive fluid, and not even this in every class; the
fluid is called blood, although it is usually colorless. | Where
color is present, it is generally, if not always, a yellowish red or
an intense red; this may, even as in the vertebrates, be caused by
hemoglobin (among the molluscs in Planorbis, Arca tetragona,
A. now, Solen legumen, Tellina planata, Pectunculus glycimerts,
and others; among the annelids in the Capitellide, Glycera,
Polycirrus, Leprea, leeches, and earthworms; among insects in
Chironomus). Often other coloring matter occurs instead of
hemoglobin: in the cuttlefish, many snails, and in the lobster and
Limulus, the oxygen is taken up by the bluish hemocyanin, which
contains a trace of copper; in the Sipunculids by hemoerythrin,
etc. The blood-plasma, as a rule, is the seat of the color (Chiro-
nomus, Hirudinea, earthworms, and most other annelids); only
exceptionally do colored blood-corpuscles occur, as in the case of
Arca, Solen, and the other mussels mentioned above, and also in
the genus Phoronis. Colored elements containing hemoglobin,
identical with blood-corpuscles, are found besides in the coelomic
fluid of many annelids (Capitellide, Glycera, Leprea, Polycirrus),
and in the ambulacral vessels of echinoderms (Ophiactis virens,
some Holothurians). Most widely distributed in the iny¥ertebrate
animals are the leucocytes, which are distinguished by their active
amcboid movements; still even these may be absent, and then the
blood is a fluid without any organized corpuscles.

3. Muscular Tissue,

Characteristics of Muscular Tissue.—Most sharply character-
ized functionally is the muscle-tissue, inasmuch as it is the agent
of active movements in the animal body. Since active mobility
occurs in protoplasm, it is important to notice the differences
between the two kinds of movement. The distinctions lie in
the direction and in the intensity of the movement. A mass of
protoplasm has the capacity to move hither and thither in all
directions, because in it there is a high degree of mobility be-
tween the smallest particles. Muscles and hence their separate
GENERAL HISTOLOGY. 91

elements, the muscle-fibres and muscle-fibrils, on the contrary,
can shorten only by correspondingly increasing in
diameter (fig. 47); they can therefore accomplish
motion only in a definite direction, that of the axis
of the muscle. The muscle-substance consequently
is more limited in its movement than is protoplasm,
but on the other hand it has the advantages of
greater energy and greater rapidity. An observer
conversant with the different kinds of motion is
able to decide with considerable accuracy, from the fe, 47—Trans-
intensity and rapidity, whether in a given case a Versely stri

ated muscle-

7 qq ‘ 7 . fibrils. (After
movement has been brought about by the agency of  fprils, (Alter

protoplasm or by the contractile substance in the fhe resting, },
narrower sense (muscle-substance). tracted state.

Formation of Muscle-substance.—These physiological con-
siderations show that protoplasm and the contractile substance are
morphologically different, and that therefore one must distinguish
sharply between formative cells, or muscle-corpuscles, and the
product of these cells, the contractile substance, just as in the
case of connective tissue, between the connective-tissue corpuscles
and the connective-tissue fibrils. This distinction actually occurs,
but optically it is not equally demonstrable, for the reason that it
is not prominent histologically. In animal histology there are
recognized two kinds, it might even be said two stages, in the
formation of muscle-substance, the homogeneous, or smooth, and
the cross-striated. Since the former looks very similar to non-
granular protoplasm, the boundary-line between it and the
muscle-corpuscle is more difficult to recognize than in the case of
the cross-striated muscle-substance, which in its minute structure
is quite different in appearance from protoplasm. In cross-striated
muscles the contractile portion consists of two substances regularly
alternating with one another in the direction of the contraction of
the muscle, of which the one is doubly, the other singly, refractive
(figs. 24, 47, 50),

Smooth and Cross-striated Muscle-fibres.—The smooth muscle-
substance represents a lower stage of development than the cross-
striated, since it chiefly occurs in the less highly organized and
more inactive animals. Interesting in this respect is the fact that
in the two stages of development of one and the same animal the
simple and inert polyp has smooth muscles, while the more highly
organized and actively motile medusa has cross-striated muscles
(fig. 48). The difference in their action has led in the vertebrates

 

 
92 GENERAL PRINCIPLES OF ZOOLOGY,

to a peculiar distribution of the muscle-substance, the smooth
musculature being chiefly distributed to the internal organs,
which are not under control of the will (involuntary muscles),
while the musculature of the body, subject to the will and hence
demanding more rapid action, is cross-striated (voluntary muscles).
We must not conclude that the difference between smooth and
cross-striated musculature coincides with the distinction between
visceral and body musculature; it should be noticed that the body
musculature of all molluscs is smooth, the visceral as well as the

 

Fia. 48.—Epithelial muscle-cells. a, of a medusa; b, of an actinian.

body muscles of many insects and crustacea, and the muscles of
the heart of vertebrates are cross-striated.

It was pointed out above, in connexion with epithelia and
connective tissue, that these tissues differed fundamentally. This
contrast has its bearing in dealing with the muscles, for both
epithelial and mesenchymatous cells may form contractile sub-
stances and therefore there are two genetically different kinds of
muscles, the epithelial and the mesenchymatous (contractile fibre-
cell). Both kinds of muscle-cells can a@ prior? form smooth as
well as cross-striated muscle-substance; but the collection of con-
nective (mesenchymatous) tissue around inner organs has caused —
most contractile fibre-cells to be smooth, while most of the
epithelial muscle-cells are cross-striated.

Epithelial muscle-cells are cells of which one end extends to the
surface of the body or the surface of an internal cavity (body
vavity, lumen of the gut, etc.), and may here have a cuticle, cilia,
or flagella, while at the opposite end it has secreted contractile
substance in the form of muscle-fibrils (fig. 48). They combine
the double function of epithelial and muscle cells.

Contractile fibre-cells, on the other hand, are connective-tissue
cells, which usuaily have surrounded themselves with a layer of
contractile substance; corresponding to their origin, they have the
form of connective-tissue cells, and are spindle-formed or
branched; the branches arising from the ends of the cells (fig. 49).
The similarity of form renders the distinction between ordinary
comnective-tissue cells and fibre-cells difficult; if the contractile
GENERAL HISTOLOGY. 93

layer on the surface be slightly developed, the distinction is im-
possible. To recognize the character of the elements, therefore,
we must choose well-defined examples, in which the uninucleated
or the multinucleated mass, the ‘axial substance,’ is sharply
marked off from the muscle-mass, the ‘cortical layer’ (fig. 49,
c, d, é). -

 

 

 

 

 

 

Bia. 49. Fig. dv.

Fia. 49.—Contractile fibre-cells. a,of man; b-e, of Beroé (a Ctenophore); b, young
fibres; c, branched ends; d@, middle portion of a fibre; ¢, cross-section.
Fria. 50.—Cross-striated primary bundle. (After Gegenbaur.) mn, nuclei; s, a point
where the sarcolemma is plainly shown on account of the tearing of the fibres.
In vertebrates and arthropods the contractile fibre-cells occur
in the vegetative organs as elements of the ‘ organic musculature’;
on the other hand we find here the epithelial musculature in the
cross-striated primary bundles, separated from the epithelium,
and only developmentally referable to the epithelium of the body
cavity (fig. 50). A primary bundle is a cylindrical mass, bounded
externally by a structureless envelope, the sarcolemma. Its con-
tents consist of fine fibrils, which, closely parallel to one another
and pressed closely together, run from one end of the mass to the
94 GENERAL PRINCIPLES OF ZOOLOGY.

other. Each fibril is formed of singly and doubly refractive
parts, which alternate with one another in more or less compli-
cated arrangement. Since now the doubly refracting parts of the
fibrils within a bundle lie at about the same level, there is caused
a cross-striation extending through the whole bundle. Finally,
scattered here and there between the muscle-fibrils are the muscle-
corpuscles, spindle-shaped protoplasmic bodies with a nucleus, the
remnants of the cells which have formed the musculature.

4. Nervous Tissue.

Function of Nervous Tissue.—As the muscular tissue brings
about motion, so the nervous tissue serves for the transmission of
stimuli. It communicates the stimulations of the sense-organs at
the periphery to the central nervous system, the seat of conscious-
ness, and here brings about perception (centripetal nerve tracts) ;
further, it transmits the voluntary impulses to the periphery, par-
ticularly to the musculature (centrifugal nerve tracts). By the
nervous system, finally, the stimuli arising in various places are
co-ordinated, thus furnishing the elements for that which we call
independent psychic activity.

Elements of Nervous Tissue.—The agent of the transmission
of stimuli is undoubtedly a specific nerve-substance different from
protoplasm. Hence we speak of nerve fibrille as of muscle fibrille,
the product of the special nerve-cells, but the relations involved
are not sufficiently understood.

The elements of the nervous system are divided into ganglion
cells and nerve-fibres, but it must be remembered that these are
not independent of each other, but that the fibres are enormously
elongated processes of the ganglion cells. In the vertebrates the
ganglion cells vary greatly in size; besides small elements there
are large cells, only exceeded by the eggs in size which correspond-
ingly have large nuclei recalling the germinal vesicles. Unipolar,
bipolar, and multipolar ganglion cells are recognized, the differ-
ences depending upon the number of processes (nerve-fibres) which
arise. In multipolar cells the number is very large (fig. 51) and
are of two kinds, dendrites and axons or neurites. Dendrites are
so called because they branch again and again, not far from their
origin from the cell. The axons (of which there is usually but
one to a ganghon cell) can be followed to a long distance with-
out giving off branches, except here and there lateral side twigs
(collaterals) which arise at right angles to the main fibre; they

 
GENERAL HISTOLOGY. 95

often pass over into peripheral nerves.

so the morphological distinc-
tion from dendrites lies in the
greater distance of the region
of branching from the body
of the ganglion cell. In bi-
polar ganglion cells both pro-
cesses are neurites, the cell
itself thus being an element
intercalated in the course of
a nerve-fibre, as also is a uni-
polar ganglion cell. The single
process of this divides near
the cell in a T-shaped man-
ner, so that the unipolar cell
is to be regarded as a bipolar
ganglion cell in which the two
neurites are united for a short
distance.

This conception is intel-
ligible in the light of recent
researches on the structure of
the ganglion cell and its pro-
cesses (fig. 52). Both consist

They branch at their tips,

 

i

Fig. 51.—Multipolar ganglion cell of man.

(After Gegenbaur.) a, axon.

 

Fiq. 52.—Motor ganglion cell from the thoracic region of the spinal cord of a dog.

(After Bethe.)

n, nucleus.

of extremely fine fibrille, and inter- and perifibrillar substances

cementing them together,

Each process brings a bundle of
96 GENERAL PRINCIPLES OF ZOOLOGY.

fibrille to the ganglion cell, in which they spread out and pass
over into other processes. The branching of neurites and
dendrites ig a separation of the contained fibrille; the ganglion
cell, the place of exchange of fibrille between the various processes.
Hence the ganglion cell is not a simple cell, but a cell plus plasma
products.

The similar fibrillar structure of nerve-fibres has long been
known. In the central nervous system of vertebrates the most
minute elements are the nerve fibrille, distinguished from muscle
fibrille by the absence of cross-
striation; from connective - tissue
fibrillee by the ease with which they
are injured; in preserved material
they frequently swell and show vari-
cosities (fig. 53). Many fibrille
united in a bundle form a nerve-
fibre (fig. 54, A) which is called a
gray nerve-fibre in distinction from
the white or medullated fibres. In
the latter the fibre or azis-cylinder
is surrounded by a medullary sheath
(fig. 54, B) composed of myelin, a fat-
like substance, blackened by osmic
acid and separated into variously
shaped ‘myelin drops.” The medul-
lary sheath appears to act as an
insulator.

 

 

 

 

 

 

 

valk
\ B A
bia. 53. Fia. 54, Fic. 55.
Fig. 53.—Nerve fibrille with varicosi | Both medullated and non-med-
Ret ee | nerve-ibres, Wated fibres can be enclosed in a
Hig to. edalies ith sheath of Sheath of Sebwann.? ‘This is a
eet iat RE RORT Maver) feature of the fibres composing the
peripheral nervous system and is lacking in brain and spinal cord.
Tt is a delicate envelope with nuclei here and there (fig. 55). At
times it forms constructions which cut through the medullary
sheath to the axis-cylinder (nodes of Ranvier).

Multipolar and bipolar ganglion cells also occur in the inverte-
brates, most commonly in the calenterates (fig. 56), more rarely in
worms (e.g., Lambricus), arthropods, and molluses, and then
chiefly in the peripheral nervous system. In the ganglia (the
nervous centres of the last three groups) the ganglion cell usually
gives rise to a single strong process, which, however, is richly pro-
vided with lateral branches or dendrites (fig. 74). The medullary

  
GENERAL HISTOLOGY. 97

sheath and sheath of Schwann are usually absent in invertebrates
even in the peripheral nerves. A thin myelin layer has been rarely
observed in arthropods and annelids. On the other hand the true
conducting elements, the nerve fibrille, have been seen in inverte-

 

Fic. 56.—Ganglion cells of an actinian.

brate nerve-fibres, and these have been followed into the ganglion
cell in which the afferent and efferent fibrille are united in a
lattice-like manner.

SUMMARY OF HISTOLOGICAL FACTS.

Cells.—1. The most important morphological element of all
tissues is the cell.

2. The cell is a mass of protoplasm which contains one or
several nuclei (uninucleated, multinucleated cells).

3. The nucleus probably determines the specific character of
the cell, since it influences its functions; accordingly it is also the
bearer of heredity.

4, Cells and nuclei increase exclusively by division or budding.

Tissues.—5}. Tissues are complexes of numerous similar his-
tologically differentiated cells.

6. Histological differentiation rests in part upon the fact that
the cells take on a definite form and arrangement, in part upon the
formation of plasmic products, which determine the character of
the tissue (muscle-fibres, connective-tissue fibrils).
98 GHNERAL PRINCIPLES OF ZOOLOGY.

Classification of Tissues.—’7. According to function and struc-
ture (1) epithelia, (2) connective tissue, (3) muscular tissue, (4)
nervous tissue are distinguished.

8. The physiological character of epithelia is determined by the
fact that they cover the surfaces of the body, their morphological
character in that they consist of closely compressed cells united
only by a cementing substance.

9. According to their further functional character epithelia
are divided into glandular epithelia (unicellular and multicellular
glands), sensory, germinal, and protective epithelia.

10. According to the structure are distinguished simple (cubi-
cal, cylindrical, pavement epithelia) and stratified epithelia,
ciliated and flagellated epithelia, epithelia with or without cuticle.

11. The physiological characteristic of the connective tissues is
that they fill up spaces between other tissues in the interior of the
body.

12. The morphological distinction depends upon the presence
of the intercellular substance.

13. According to the quantity and the structure of the inter-
cellular substance the connective substances are divided into (1)
cellular (scanty intercellular substance); (2) homogeneous; (3)
fibrous connective tissue; (4) cartilage; (5) bone.

14. The physiological character of muscular tisswe is its
increased capacity for contraction.

15. The morphological characteristic is the fact that the cells
have secreted muscle-substance.

16. According to the nature of the muscle-substance are dis-
tinguished smooth and cross-striated muscle-fibres.

17. According to the character and origin of the cells (muscle-
corpuscles) the muscles are divided into epithelial (epithelial
muscle-cells, primary bundles) and connective-tisue muscle-cells
(contractile fibre-cells).

18. The physiological distinction of nervous tissue rests upon
the transmission of sensory stimuli and voluntary impulses, and
upon the co-ordination of these into unified psychic activity.

19. The conduction takes place by means of nerve-fibres (non-
medullated and medullated fibrils and bundles of fibrils); the
co-ordination of stimuli by means of ganglion-cells (bipolar,
multipolar ganglion-cells).

20. Blood and lymph are proteid-containing fluids; rarely
without cells, they may contain only colorless ameboid cells (white
GENERAL ORGANOLOGY. 99

blood-corpuscles, leucocytes), or in addition to these also red
blood-corpuscles.

21. Red blood-corpuscles occur, in the main, only in verte-
brates and cause the redness of the blood; they are absent in most
invertebrate animals.

22. When invertebrate animals have colored blood (red,
yellow), this is usually due to the color of the blood-plasma.

23. The red blood-corpuseles are nonnucleated in mammals,
nucleated in all the other vertebrates.

Ill. THe ComBinatTion or TISSUES INTO ORGANS.

An Organ Defined.—Organs are formed from the tissues. An
organ isa tissue complex, marked off from the other tissues, which
has tuken on a definite form for carrying on a special function.
Thus a single muscle is an organ which consists of a certain
amount of muscular tissue; with scalpel and scissors it can be
removed from its environment as a connected whole and can still
accomplish a definite movement.

Principal and Accessory Tissues.—In each organ there is a
tissue which determines the function of the organ, and therefore
its physiological character. This may be called the principal
tissue, for there may be other accessory tissues present, which
merely support or render possible the function of the principal
tissue. In the muscle of the vertebrates we find, besides the
muscle-fibres, connective tissue which, like a kind of cement,
unites the bundles of muscle; blood-vessels which provide nourish-
ment; finally, nerves by which the muscles are aroused to action.
In the human liver also, besides the functionally most important
part, the liver-cells, blood-vessels, nervous and connective tissues
are present. These accessory tissues are usually found only in the
highly developed organs; in the case of the lower animals they
may be absent; thus the digestive tract of ccelenterates has only
an epithelial lining; their nervous system consists merely of a
cord of nerve-fibres and ganglion-cells.

Effect of Use and Disuse.—It is of the greatest importance for
the permanency of an organ that it be constantly in function.
Living substance is distinguished from the non-living by the fact
that, if it be destroyed by use, it is immediately replaced, often by
more than sufficient to make good the loss. Functioning tissues
and organs under favorable conditions increase in volume; on the

 
100 GENERAL PRINCIPLES OF ZOOLOGY.

other hand, functionless parts undergo a gradual decrease, which
finally leads to their disappearance.

Change of Function of Organs.—The two factors mentioned,
that the permanence of the tissues depends upon continued use,
and that usually several tissues enter into the structure of an
organ, are important for the understanding of the principle of
change of function which plays a prominent role in the meta-
morphosis of animal form. It may happen that an organ is
brought under changed conditions and no longer has an oppor-
tunity to function as before. In that case the functioning tissue,
from lack of use, gradually degenerates, but the organ may persist
by means of its accessory tissues if the new conditions make it
possible for one of them to attain to functional activity, and to
give the organ a new physiological character.

Examples of Change of Function.—A muscle, for example, may
become functionless from many causes. Should the muscle-tissue
disappear there are still left the accessory tissues, particularly
connective tissue permeated by blood-vessels; this may remain in-
tact and form a protecting band, a tendon, or fascia. We have
then, morphologically, the same organ, changed in its physio-
logical character; the muscle has undergone a change of function,
and has become a ligamentous band. The visceral arches of fishes
afford another example; these primarily are supports for the gills;
if now by the acquirement of terrestrial habits the gills be lost,
the visceral arches become functionless and correspondingly under-
goa partial degeneration; but a part persists by assuming a new
function, and forms the jaws, the hyoid bone, and the small bones
of the ear, which, in spite of their quite different functions, are
morphologically the same structures as the gill-arches.

Homology and Analogy.—In the History of Zoology (page 13)
it was shown that comparative anatomy has caused a discrimina-
tion between homology or morphological equivalence, and analogy
or physiological equivalence, i.e., between organs which appear in
the same relative positions and relations, and organs which have
the same function. What we have here learned of the structure
of organs makes it evident that morphological and physiological
characters do not necessarily coincide, that morphologically similar
organs may have different functions, morphologically different
organs the same functions.

Systems of Organs.—Organs wholly identical, or, at least,
functioning in an equivalent manner, may occur in considerable
numbers in the same body. A man has many muscles, and many

 

 
GENERAL ORGANOLOGY. ‘ 101

organs which carry on digestion. Hence we may group all organs
which in the body have equivalent or similar functions, and speak
of systems of organs. In all we recognize nine such systems: (1)
skeletal, (2) digestive, (3) respiratory, (4) circulatory, (5) excre-
tory, (6) genital, (7) muscular, (8) nervous, and (9) sensory
systems. Not all are necessarily present; a skeleton, for instance,
is frequently lacking. The most different functions which in man
are divided among different complicated and specialized systems
may be performed in a lower animal by one and the same
apparatus. Yet according to the fundamental functions the fol-
lowing groups of organs may be recognized: I. Organs of assimila-
tion (2-5); II. Organs of reproduction (6); III. Organs of motion
(7); IV. Organs of perception (8 and 9).

Vegetative and Animal Organs.—The organs of assimilation and of repro-
duction (I and II) are grouped together as vegetative, the others (III and
TV) as animal organs. The older zoologists used to say that assimilation
and reproduction are functions which are common to animals and plants ;
that, on the contrary, sensation and motion are lacking in plants, and are
exclusively characteristic of animals. The atom of truth in the funda-
mental idea needs reconsideration in the light of our present knowledge.
We have seen that the protoplasm of plants and animals has not only the
power of assimilation and reproduction, but also power of motion and of
irritability. The latter characteristics consequently cannot be entirely
lacking in all the plants, for they are found in the most important. In
fact many plants, as the mimosas, the compass-plants, insectivorous plants
show great irritability ; many low plants, the reproductive states of alge,
move quite as actively as, or even more actively than, many of the lower
animals. On the other hand, there are many animals which in the mature
condition are fixed in position like plants. Many Protozoa and worms,
most of the zoophytes, some echinoderms like the Crinoids, even many
Crustacea, the cirripedes (barnacles), can change their location only during
the earlier stages of development, in later life being limited to movements
of single parts of the body, the arms, tentacles, etc. In the sponges
motions are so insignificant that they cannot be seen at all by the naked
eye, and scarcely even with the aid of the microscope.

Nevertheless the two terms, animal and vegetative, must be retained.
For although motion and sensation occur in the vegetable kingdom, still
they reach no high development ; indeed we may say they become more
and more inconspicuous the higher the plants; in the animal kingdom, on
the contrary, they are unfolded in extraordinary perfection and lie at the
basis of its most characteristic features.
102 GENERAL PRINCIPLES OF ZOOLOGY.

Vegetative Organs.
A. Organs of Assimilation.

Assimilation Defined.—If the term assimilation be used in its
widest sense, one must speak in this connection of all the con-
trivances in the animal body which render growth possible during
the period of progressive development, and, during mature life,
compensate for the loss of energy connected with each period of
functional activity, in order to preserve to the body its functional
powers. In each period of functional activity organic compounds
are oxidized. Compounds which are especially rich in carbon and
hydrogen (as well as some nitrogen and sulphur) and are poor in
oxygen are changed by oxidation into carbon dioxide, water, and
various nitrogenous products, like urea, uric acid, etc. A com-
pensation takes place, for not only is the useless substance
removed, but also compounds of oxygen and materials rich in
carbon are furnished to the tissues to replace the material oxidized.

Assimilation in Animals.—In lowly organized animals all the
processes connected with compensative assimilative changes take
place through the agency of one and the same organ, the digestive
tract; but in the higher animals, through specialization, normal
assimilation is a definite series of separate phenomena. Between
the lower and the higher animals there are evidently intermediate
conditions where specialization has halted at an earlier or a later
stage.

Different Organs of Assimilation.— Assimilation begins with
the presence of suitable food; the solid and quid constituent
parts of the body must digest and incorporate this, ie., it must
be altered so that it can be absorbed and distributed to the tissues.
All this takes place through the agency of the digestive tract,
which is provided with accessory organs, the digestive glands; the
digestive tract likewise removes all matter remaining undigested
(the faeces). The necessary oxygen, gaseous food, so to speak, is
usually taken, however, by particular parts of the body, the
respiratory organs, the gills or lungs. The oxygen and the
digested (consequently liquefied) organic and inorganic compounds
must further be distributed in the body to the organs and tissues
according to their needs. Therefore there are usually blood-
vessels or etreulatory organs, which permeate the body in all
directions. But the tissues need not only a means of obtaining
but also of getting rid of certain compounds. The accumulation

 
GENERAL ORGANOLOGY, 108
of the oxidation products arising from functional activity is
injurious, to some extent even poisonous, to the organism; conse-
quently they must be removed, and in a dissolved state they are
taken up by the blood-vascular apparatus, and are brought to
definite places for expulsion or excretion. Fluid wastes are
expelled by the kidneys of vertebrates, the Malpighian vessels of
insects, the water-vascular system of worms; these, together with
their accessory apparatus, are embraced under the name ¢ excretory
organs.’ Eucreta are to be distinguished from feces; excreta are
substances which have been a part of the tissues of the body itself,
and, through oxidation, have become useless; while those sub-
stances which constitute the feces were useless from the beginning,
and have never belonged to the body, but have remained separated
from the tissues by the boundary of the epithelium of the digestive
tract. The gaseous oxidation product of the animal body, carbon
dioxide, is removed by the blood-vascular apparatus through the
agency of the respiratory organs. Since in the respiratory organs
there takes place an exchange of the useless carbon dioxide for
the oxygen necessary to life, these organs have a double function,

being, at the same time, excretory organs and organs for taking
up food.

After this general survey, we must enter somewhat more
minutely into a discussion of the various systems of organs.

I. The Digestive Tract.

Archenteron or Primitive Digestive Tract.—Since the taking
in of food and its assimilation are functions most important for
the well-being of the animal, it is to be expected that of all the
organs in the animal series the digestive tract should be formed
first, and also in almost every case should be earhest established in
the embryo. The fact that many worms (cestodes) and crustacea
(Rhizocephala) have no digestive tract does not alter this state-
ment; for it can be definitely affirmed that, in adaptation to
special conditions of life, particularly parasitism, the digestive
tract has degenerated. The simplest multicellular, free-living
animals are merely simple or branched digestive pouches which
have only a single opening, functioning both as mouth and anus
(fig. 57). Such an animal has necessarily two epithelial layers,
one of which lines the digestive tract, the other covers the surface
of the body. These two fundamental cell-layers are called ento-
derm and ectoderm. In many cw@lenterates they are the only
104 GENERAL PRINCIPLES OF ZOOLOGY.

layers of the body. In most animals they are separated by inter-
mediate tissues, called collectively mesoderm. The higher the
animal, the more differentiated is the mesodermal layer. The
primitive digestive cavity lined by entoderm is called the archen-

      

ee

callin

 

Fia. 57. Fia. 58,

Fic. 57.—Longitudinal section through the nutritive poly
Haeckle.)

Fia. 58.—Stenostoma leucops, in division.
for the hinder animal; m, the blindly ending entodermal mid-gut; e, ectodermal

ciliated epithelium; g, ganglion with ciliated pit; w, water-vascular canal; g’,
ganglion of the hinder animal.

of a siphonophore. (After
0, mouth-opening; en, entoderm ; ek, ectoderm.

a, ectodermal fore-gut, at a’ forming anew

teron. Inthe case of meduse and polyps it forms the entire diges-
tive tract, but in most animals this is not sufficient for the needs
of digestion and the alimentary tract is increased by invaginations
of parts of the surface of the body.

Stomodeum and Proctodeum.—Even in many celenterates
and lower worms an invagination arises at the anterior end of the
digestive tract, forming the ectodermal fore-gut or stomodeum
(fig. 58). From the higher worms onwards, it is accompanied by
a second invagination at the hinder end, the ectodermal hind-gut,
or proctodeum (fig. 56); embryologically, this is formed as a blind
GENERAL ORGANOLOGY. 105

sac whose closed end unites with the likewise closed posterior part
of the archenteron (now called also mesenteron or mid-gut) until
the separating wall disappears, whereupon mid- and end-gut com-
municate with each other, and the digestive tract becomes a canal
extending through the entire body.

Divisions and Appendages of the Digestive Tract.—The part
which the archenteron takes in comparison with the ectodermal

 

 

Fia. 59. Fia. 60.

Fra. 59.—Bee-larva just after hatching: seen from the ventral surface. The diges-
tive tract consists of three portions; a, fore-gut; m, mid-gut; e, hind-gut (not yet
connected with the mid-gut); sy, limits of segments; st, stigma; ¢t, trachea; 1,
ventral nerve-cord. (After Biitschli.)

Fra. 60.—Digestive tract of the domestic fowl. a, esophagus; h, crop; ¢, glandular
stomach: d, gizzard; e, liver; f, gall-bladder; y, pancreas; h, i, small intestine;
k, ceca; 1, large intestine ; m, ureters; n, oviduct; 0, cloaca.

proctodzeum and stomodeum in making up the completed diges-

tive tract is very different in the various groups. On one side the
y g

crustacea, on the other side the vertebrates, offer the strongest
106 GHNERAL PRINCIPLES OF ZOOLOGY.

contrast; the crustacea have a very short mid-gut and consequently
a long extent of fore- and hind-gut formed from the ectoderm; in
vertebrates, on the contrary, the ectodermal portions are extremely
short.

The width of the lumen varies in the course of the alimentary
canal and renders possible the distinction of different divisions,
which, so far as possible, have been provided with uniform names.
Fig. 57, drawn from a domestic fowl, illustrates the usual terms.
The mouth-opening leads into a wider cavity, which is usually
divided into an anterior division, the duccal cavity, and a posterior
one, the pharynz. The narrow tube leading from this is the
esophagus (a); here and there it may widen, or bear a pouchlike
evagination, the crop or ingluvies (b), for the temporary reception
of food. From the wsophagus the food passes into a considerable
enlargement, the stomach. Birds, ike many other animals, have
a double stomach, a thin-walled portion rich in glands, and a
second part, the walls of which are remarkable for the thick
masses of muscle; the former is the glandular stomach (c), the
latter is the grinding stomach or gizzard (d), serving for comminu-
tion of the food. Behind the stomach the digestive tube narrows
into the small intestine (h), the hinder widened part of which is
the large intestine (1), terminating in the anus. The limit of the
small and large intestine is usually marked by blind pouches, the
ceca (k). Connected with the anal gut also are the outlets of the
kidneys (m) and of the sexual apparatus (7); hence the terminal
portion, serving as the outlet for the urine and fesces, and also for
the sexual products, is called the cloaca (0).

In animals which require abundant food the area of the
alimentary tract is not sufficient to furnish the digestive fluids, so
that evaginations of the wall (glands) serve to increase this. Into
the mouth empty the salivary glands; into the first part of the
small intestine, close behind the stomach, the Jiver (e) and the
pancreas (g) (or a single glandular apparatus, whose secretion
combines the characters of gall and of pancreatic juice, the hepato-
pancreas). Finally, in the hind-gut there sometimes oceur glands
which form a fetid secretion—the anal glands. The length of the
digestive tract is chiefly influenced by the kind of food. In many
groups of animals there is found a difference between herbivores
and carnivores, the former having a very long and consequently
convoluted digestive tract. That of a carnivore is about four or
five times the length of the body, while in an herbivorous ungulate,
on the other hand, it is twenty to twenty-eight times. Similar,
GHNERAL ORGANOLOGY. 107

though not so great, are the differences between carnivorous and
plant-eating beetles.

II. Respiratory Organs.

 

Sources of the Oxygen used in Breathing.—The oxygen which
each animal must obtain in exchange for the carbon dioxide formed
in the tissues is derived either from the air or from the water,
according as the animal is terrestrial or aquatic. Less frequently
it is the case that water-dwellers breathe air, and hence are com-
pelled, from time to time, to rise to the surface of the water for a

 

Fic. 61.—Left second foot of a crayfish with attached gill (br). (After Huxley.) czp,
coxopodite; hp, basipodite; ip, ischiopodite; mp, meropodite; cp, carpopodite; pp,
propodite; dp, dactylopodite; cs, bristles of the coxopodite; ¢, lamina of the vill.

supply of air; this is true for the great marine mammals, and for
many insects, spiders, and snails which are found in fresh water.
Air- and water-breathing takes place exclusively through the skin,
so long as this is delicate and readily permeable, and so long as
no higher development of organization necessitates a more active
interchange of material. If, on the other hand, the demand for
oxveen be greater, other more special breathing-organs are found
ede = fi se ze E ovens

—gills for water-breathing, lungs and trachee for air-breathing, in
108 GENERAL PRINCIPLES OF ZOOLOGY.

addition to which the skin functions as an accessory organ of more
or less importance.

Gills.—The gills are usually thin-walled areas of the skin
which are abundantly supplied with blood-vessels, and where richly
branched tuftlike projections or broad leaves have grown out, thus
furnishing the largest possible surface for the interchange of gases;
these occur in such a position as to be most exposed to fresh water;
in the crayfish, for example, they are on the legs, where the motion
drives fresh water constantly through them (fig. 61); in the
swimming worms, on the back; in the tube-dwelling worms, at
the anterior end, projecting out of the tube (fig. 62); in most

 

 

Fia. 62.—Anterior end of Terebella nebulosa. (After Milne Edwards.) ph, pharynx;
vd, dorsal, vv, ventral, blood-vessel; br, gills; t, tentacles. 7

amphibians (fig. 4), on each side of the neck. More rarely the
digestive tract functions for water-breathing; in the fishes,
Enteropneusta, and tunicates gills have been formed in connection
with the pharynx, its lateral walls being pierced by the gill-slits,
which open to the exterior on the surface of the body. The water
containing oxygen in solution passes out through the gill-slits, and
bathes the gill-filaments, which are richly provided with blood-
vessels. The hind-gut also in many fishes, insects, and worms
GENERAL ORGANOLOGY. 109

may become an accessory respiratory organ, being filled from time
to time with fresh water.

Aerial Respiration.—In the air-breathing animals the respira-
tory apparatus is derived either from the digestive canal or from
the skin. With the vertebrates the former is the case, since the
lungs, either directly or by the mediation of the trachea and bronchi,
are in connexion with the lumen of the digestive tract. On the
contrary, in the case of invertebrate animals (snails and spiders)
when the term ‘lung’ is used, it refers always to an invagination
or sac of the skin; of such a nature are the trachex of insects,
tubes containing air, beginning at the surface of the body with a
hole or stigma, and branching internally (fig. 59, sf).

Distinctions between the Respiratory Systems of Chordates
and Invertebrates.—In general, then, a distinction can be drawn
between the respiratory systems of vertebrate and invertebrate
animals: in the former, the digestive tract, or derivatives from it,
are respiratory; in the latter, on the contrary, it is the skin. On
the side of the vertebrates the only exceptions are most amphibians
and a few fishes (Profopterus), in which the gills are tuftlike pro-
jections of the skin (figs. 4 and 5); while among the invertebrates
some aquatic insects respire by the hinder end of the digestive
tract.

III. Circulatory Apparatus.

In order that the oxygen, taken up by the respiratory organs,
and the constituents of the food digested in the alimentary canal
may reach the tissues, there is need of no special organs, so long
as the body consists of only two thin epithelial layers, the ectoderm
and entoderm. When, however, a third, a mesodermal, layer is
interpolated between these, and the body consequently becomes
more bulky, there is usually some apparatus for distributing the
food. The simplest is when the digestive tract departs from the
character of a straight tube and branches, and by means of these
branches extends into the various parts of the body. We speak
then of a gastro-vascular system, because the alimentary canal
itself takes on the function and the branching arrangement
generally characteristic of the vessels or ‘ vascula’ (fig. 63).

Celom.—The celom or enterocwle is apparently derived from
a pair of gastric diverticula which have become completely cut off
from the archenteron (compare development of mesoderm, tnfrv).
It is a cavity pushed in between the intestinal tract and the body-
wall, is lined by a special epithelium, the peritonewm, and encloses
110 GENERAL PRINCIPLES OF ZOOLOGY.

most of the vegetative organs. If the two halves of the celom
approach, without uniting, dorsal and ventral to the gut, the
result is dorsal and ventral membranes, the mesenteries, which
support the alimentary canal. Of these the ventral is most fre-
quently, the dorsal least often, degenerate. In many invertebrates
the celom plays an important réle in nutrition since it contains a
lymphoid fluid, rich in proteids and containing cellular corpuscles.
It loses this significance the more the blood system is developed,

 

lia. 63. Fia. 64.

Fia. 63.—Leptoplana tremellaris. a, mouth; hb, buccal cavity; ¢, opening of the head
of the pharynx into the buccal cavity; d, central stomach; ¢, branched ento-
dermal gut; f, ganglion; g, testicle; h, seminal vesicle; k, uterus: 1, receptaculum
seminis ; m, female sexual opening.

Fig. 64,—Schema of circulation of the blood. a, arteries; c, capillaries; h, auricle;
k, ventricle ; kl, valves; p, pericardium ; v, veins.

and in the vertebrates, so far as nutrition is concerned, it is a

rudimentary organ.

A sharp distinction should be drawn between the celom and other
cavities in the body. Not every ‘ body cavity’ is a ealom, but frequently
there occur large spaces which are entirely different. in origin and in
relations, Frequently, as in arthropods, these ‘body cavities’ contain
blood and are in reality but expansions of the vascular system. To such
cavities the term hamocarle has been given.

Heart, Arteries, Veins, Capillaries.—The most complete
method of food distribution is accomplished by the dlood-vessel

 

Ss
GENERAL ORGANOLOGY. Ta.

which, therefore, belong generally to the higher animals, and
function whether a body cavity is present or not. Blood-vessels
are tubes with fluid contents, the blood, which transports the
oxygen received through the respiratory organs, as well as the food
absorbed from the digestive tract, and later gives these up to the
tissues. Since such an interchange of substances presupposes that
the blood circulates in the vessels, definite parts in the course of
the blood-vessels are contractile; they are covered by muscles which
by their contraction narrow the tube and push the fluid forwards.
In the lower forms wide areas in the course of the blood-vessels are
contractile; in higher animals a greater regularity of circulation
is reached; a definite specialized muscular part of the course, the
heart, alone propels the blood.

The Higher Development of the Heart.—<A free motion of the
heart is only possible when it is separated from the contiguous
tissues and enclosed in a special cavity (fig. 64). Hence we see
that the heart always les either free in the body cavity or enclosed
in a special pouch (p), the pericardium (in all cases a portion of
the general body cavity, but not always of the caelom, which has
become independent). The division of the heart into a part which
receives the blood, the atrium or auricle (h), and a part which
drives the blood onward, the ventricle (k:), is of less functional
importance; hence this division is not carried out in all cases.
There are also special mechanisms within the heart, the valves
(47), which, by closing, prevent the blood from flowing back when
the walls relax at the end of the contraction.

Blood-vessels.—In order that the blood system may properly
perform its function, in addition to circulation, it is necessary that
the nutritive substances be readily taken up and given out again
to the tissues. The part of the course of circulation concerned in
this must have easily permeable walls, must be widely distributed
in the body, and have a large superficial area. These demands are
met by the capillaries (c), extremely fine and thin-walled tubes,
which surround and permeate all organs. Through their walls,
usually formed of a thin epithelial layer alone, the proteid substances
for nourishing the tissues can pass, and the oxygen can be
exchanged for carbon dioxide. Between the heart and the capil-
laries there exists, corresponding to their different functions, great
differences in structure; they must therefore be united by special
transitional vessels—vessels which begin large and thick-walled at
the heart, and by branching, and thinning of their walls, pass
gradually into the capillaries; of such vessels there are two kinds,

 

 
112 GENERAL PRINCIPLES OF ZOOLOGY.

the firmer arteries (a) leading to the capillary region, and the
thinner-walled veins (v) leading back to the heart.

Correlation of Respiratory Organs and Blood System.—It is a
law that in all animals the blood-vascular system has been influ-
enced in its arrangement and structure more by respiration than

 

 

 

Fia. 65.—Scheme of circulation in a fish. a’, ascending (ventral) aorta; a2, descend-
ing (dorsal) aorta; c, carotid , da, intestinal arteries; dc, intestinal capillaries;
dv, intestinal veins; h, auricle; k, ventricle; ka, afferent gill-arteries; kv, efferent
gill-arteries; lc, liver-capillaries; sc, body-capillaries; ve, cardinal veins; wh,
hepatic vein ; vj, jugular vein.

by nutrition in the narrower sense; there exists a correlation
between the organs of respiration and of circulation. A double
capillary region must be distinguished; besides the body capillary
system already mentioned there is the respiratory capillary region,
whose exclusive office is to remove the carbon dioxide from the
GENERAL ORGANOLOGY. 113

blood and to furnish oxygen to it (gill and lung capillaries). A
twofold capillary region makes necessary also a twofold system of
arteries and veins (systemic arteries and systemic veins, respira-
tory arteries and respiratory veins). The accompanying diagram
(fig. 65) of the blood circulation of fishes illustrates this. Veins
lead from the capillary region of the tissues of the body to the
auricle of the heart; from the auricle the blood flows into the
ventricle, and through the afferent gill-arteries into the gill-capil-
laries. Thence it is conducted through the <gill-veins’ (efferent
arteries), which unite into a single large trunk; this again gives
off lateral branches passing into the capillary region of the body.
Since the branches of the main trunk formed by the < gill-veins’
lead again into a capillary region, they must, like the main stem,
be called arteries.

Arterial and Venous Blood.—During its course through the
body the blood twice changes its chemical character and corre-
spondingly its color. The blood which flows from the body
capillary region has given up its oxygen to the tissnes, receiving
in exchange carbon dioxide, and has become dark red. This
character is maintained until, in the gill-capillaries, it again
becomes oxygenated, giving up the carbou dioxide and becoming
bright red. The different character of the blood can be recognized
in the arteries and veins of the systemic circulatory system; the
dark blood containing carbon dioxide is called venous, and the
bright red, containing oxygen, arferial blood, since the former
flows in the veins, the latter in the arteries. These terms are
entirely unsuitable, as can readily be seen from the above diagram
(fig. 65), because they easily lead to the false assumption that veins
must always conduct blood containing carbon dioxide, and arteries
always oxygenated blood. In opposition to this, the diagram
shows that, in the respiratory circulation (the shorter course), the
conditions must be the reverse of those in the systemic circula-
tion, since here the arteries contain ‘venous,’
contain ‘arterial,’ blood.

Closed and Lacunar Blood-vascular Systems.—Such a blood-
vascular system as has here been described is called a closed one,
because the blood always flows in special tubes provided with their
own walls. Opposed to the closed stands the lacunar blood-vascular
system; here the blood-vessels lose, after a time, the character of
tubes and become wide cavities, or sinuses, which, without special
walls, are enclosed between the intestines and other organs
(hemoceele, supra).

while the veins
114 GENERAL PRINCIPLES OF ZOOLOGY.

Example of Lacunar Blood-vascular System.—The best exam-
ple of a lacunar blood-vascular system is furnished by the insects
and myriapods, which have only the heart and
short arterial trunks; from the ends of the
arteries the blood enters the hemocele, and
from this through lateral slits (ostia) again
enters the heart (fig. 66). In the groups of
arthropods and molluscs are found all transi-
tions between so extreme a case of a lacunar
blood-vascular system and the almost com-
pletely closed one. Tlere appears again a
close correlation of the circulatory and respir-
atory organs, the latter determining the
development of the former. If the respira-
tion be diffusely distributed over or through
the body, and the distribution of the oxygen
goes on without special vessels, the circula-
tory apparatus is very simple; on the other
hand, if the respiration be connected with

definitely restricted areas, and a regular dis-
Fig, 66.—Anterior end of : ete : *
the heart of Scolopen- tribution of oxygen be necessary, the appara-
dra. (After Newport.) . : 3 : i :
ac. cephalic artery; ab, tusis differentiated into heart, arteries, veins,

arterial arch; al, later- _ : : td : i o_ mr
aT lnterys fm, alary and capillaries. Details may be found in the

muscles (ale cord): sections on crustaceans, spiders, and insects,
o, ostia. infra.

Lymph-vessels.—A special part of the vascular system is the
lymph system, which is known only in vertebrates. In the capil-
lary region of the body, it is true, proteids may pass over into the
tissues, but it is evident that a possible overflow cannot re-enter
the blood-vessels in the same way, on account of the higher pres-
sure prevailing in the capillaries. This overflow is conducted back
to the veins through the lymph-vessels. The lymph-vessels begin
with lacune in the tissues, and gradually pass into vessels with
definite walls. The lymph-vessels of the digestive tract are par-
ticularly important since, during digestion, they become filled with
the proteid and fatty constituents of the digested food; they are
called the chyle-vessels, because they contain the chyle, distin-
guished from ordinary lymph by its milky color.

Cold- and Warm-blooded Animals.—In connexion with the
blood-vaseular system, two expressions are much used but not
generally correctly understood by the general public, viz., cold-
blooded and) warm-blooded—or, more correctly, animals with

 
GENERAL ORGANOLOGY. 115

variable and animals with definite temperatures. Under the head
of animals with varying temperature (poikilothermous) or cold
blood are placed forms whose temperature is largely dependent
upon the temperature of the environment, rising and falling with
it, but usually a few degrees above it. In our climate, where the
atmospheric temperature is considerably lower than the tempera-
ture of the human body, such animals, for example the frog,
would feel cold to our touch, since they, particularly in the cool
season, have a much lower temperature than we.

Such creatures as, living under any thermal condition, maintain
about the same temperature, are termed warm-blooded or definite-
temperatured (idiothermous, homoiothermous) animals. Man in
summer and winter, under the equator and at the north pole, has
approximately a temperature of 36° C. (982° F.), showing higher
temperatures only in fever. In order to maintain a constant tem-
perature during the varying external conditions, the animal must
have a heat-regulator; it must have the power to regulate the
warmth of its body, on the one hand by hmiting the production of
heat, on the other by controlling its loss. If the environment be
warmer than is suitable for the body temperature, then the pro-
duction of heat must be limited to the smallest quantity com-
patible with the vital processes; but, if this does not suffice, the
loss of heat must be increased by evaporation from the surface,
usually accomplished by active perspiration. If, on the contrary,
the environment be cold, then, conversely, every unnecessary Joss
of heat must be avoided, while the production of heat must be
increased. It is clear that idiothermy, since it requires compl-
rated apparatus, can occur ouly in the highly organized animals.

IV. Excretory Organs.

Nature of the Organs of Excretion.—The excretory organs are
tubes or glandular canals which open upon the surface of the body,
either directly or by way of an end-gut (cloaca), and conduct sub-
stances which have become useless to the body to the exterior.

The presence of a blood-vascular system or a caelom or both
together exercises an important influence on their structure.
When neither are developed the excretory tubules in order to
remove the excreta from the tissues must branch and penetrate
the body in all directions like a drainage system, being frequently
connected in a network recalling the blood-capillaries (proto-
nephridia or water-vascular system of parenchymatous worms,
116 GENERAL PRINCIPLES OF ZOOLOGY.

fig. 67). The canals begin with closed tubes, which are provided
internally at the end with a bundle of
actively vibrating cilia, the ‘flame’ (fig.
68). One or more main trunks lead from
the canal system to the exterior. A little
before the external opening (excretory
pore) there is frequently a contractile
enlargement, the urinary bladder.

With the appearance of a ccelom there
is a central place for the collection of
excreta. The nephridia or segmental
organs — usually simple tubes (rarely
branched) open at both ends—lead from
this to the exterior. One opening is ex-
ternal (fig. 69), the other communicates
with the celom by means of a ciliated

iL

  

Fie, 67. Fig, 68.

Fig. 67.—Distomum hepaticum with water-vascular system. (From Hatschek.)
p, porus excretorius; 0, mouth.

Fic. 68.—Blind end of one of the finest water-vascular canals (k) of a Turbellarian.
(From Lang.) 7, nucleus; f, processes of the terminal cell; wf, ‘flame’ of the
terminal cell; v, vacuole. ;

funnel, the nephrostome, a wide mouth with active cilia which

connects with the canal of the tube. Through this the ex-

cretion (in annelids peritoneal cells laden with guanin—the dis-
integrated ‘chloragogue’ cells) is carried to the outside.

The excretory organs (kidneys) of vertebrates are derived from
such nephridia. The fact that in the embryos (and frequently in
the adults) these open into the celom by nephrostomes mukes it
probable that also in the vertebrates the ewlom was once important
in excretion (fig. 70). The increasing importance of the blood-
vessels which envelop the nephridial canals and bring to them
the waste matter taken from the tissues is probably the cause of
the loss of connexion of the kidneys with the coelom by degenera-
GENERAL ORGANOLOGY. 117

tion of the nephrostomata. The relation of the blood-vessels to
the nephridial tubes becomes specially close by the development
of the glomeruli (Malpighian corpuscles); bundles of capillaries

Ns MW

   

 

 

 

 

 

   

Fia 70.

Fic. 69.—Segmental organ of an Oligochete. (From Lang.) tz, ciliated funnel; dis,
septum ; ng!, non-glandular, ng?, glandular, part of the canal; eh, terminal ves-
icle; ln, body-wall.

Fic. 70.—Diagram of the primitive kidney of a vertebrate. (From Hatschek.) Dotted
lines mark the limits of the segments. 4, anal opening; P, mouth of the duct of
the primitive kidney (W); Ns, nephrostome; M, Malpighian bodies of the seg-
mental tubules (S),

carrying the walls of the canal before them and so projecting into
the lumen of the tube.

B. Sexual Organs.

 

Sexual Glands and Ducts.—In the sexual apparatus of animals
are distinguished the areas where the germinal cells are produced,
the sexual glands or gonads, and the ducts for these. The former
are present, temporarily or permanently, in all multicellular
animals; the latter, on the contrary, may be completely absent.
If the sexual products arise in the skin or in the walls of the
digestive tract, as is usually the case in the celenterates, then
118 GENERAL PRINCIPLES OF ZOOLOGY.

special outlets are superfluous, since the ripe elements can reach
the exterior directly by rupture of their covering or by means of
the digestive tract.

Germinal Epithelium and Germinal Glands.—Male and female
sexual cells, as we have seen, originate from an undifferentiated
incipient organ, or anlage, which is called the germinal epithelium.
Usually it forms a part of the epithelial lining of the body cavity,
in many animals permanently, in others only temporarily; in the
latter case it separates, usually by constriction, and forms gland-
like bodies, the gonads or sexual glands.

Gonochorism and Hermaphroditism.—In most animals the
germinal epithelium produces either only female or only male
sexual cells; such animals are called separate-sexed, diecious or

 

 

 

 

 

at ow’ bm dt,to at

Fia. 71.—Sexual organs of Lignbricus agricola. (From Lang, after Vogt and Yung.)
The seminal vesicles of the right side are removed. Dm, ventral nerve-cord: bv
and Dl, ventral and lateral rows of setae; st}, sf?, receptacula seminis; sb}, ab?, sb8,
the three seminal vesicles of the left side, which are connected with a median
unpaired seminal capsule (shu). Enclosed in the latter are the anterior and pos-
terior testes (M1, 2), and the anterior and posterior seminal funnels (f}, (2), which
lead into the vas deferens (td). 0, ovaries: to, ciliated funnels lenaing into the
oviducts (ov); di, dissepiments; VIII-XV 5 eighth to fifteenth segments.

   

   

gonochoristic, in opposition to the hermaphroditic forms, in which
both kinds of sexual glands are contained in one and the same
individual. Different degrees of hermaphroditism can be distin-
guished; commonly testes and ovary are contained in the same
GENERAL ORGANOLOGY, 19

animal, some distance apart, as in the earthworm, in which two
segments are male, while a third segment is female (fig. 71).
More rarely there is a union of testes and ovary into a single
glandular body or hermaphroditie gland; our land-snails have an
hermaphroditic gland, which produces spermatozoa and eggs in
the same follicle.

Occurrence of Hermaphroditism. —ermaphroditism is, in
general, of more frequent occurrence in the lower than in the
higher animals. Insects and vertebrates are, almost without
exception, dicecious; only two cases of normal hermaphroditism
are known among them, a sea-perch, Serranus scriba, a bony fish,
and Myxine gluitinosa, the hagtish. More commonly hermaph-
roditism occurs as an abnormalty; a striking form is lateral
hermaphroditism, in which one half of the animal has only male,
the other half only female, gonads. If the males and females of
a species be distinguishable by their appearance, then lateral
hermaphroditism is expressed in their external form, since oue half

 

Fia. 72.—Lateral hermaphroditism of a gipsy moth (Ocneria dispar). Left female,
right male. (After Taschenberg.)

of the animal has the characteristic marks of the male, the other
half those of the female. Hermaphroditic lepidoptera and bees
are known in which the male half bears the special form of the
male antenne, eyes, and wings, and thus is essentially different from
the female half (fig. 72). Still it must be noted here that, in
many instances where the external appearance pointed towards
hermaphroditism, anatomical investigation has disclosed either
only male or only female sexual glands in a rudimentary condition
(gynandromorphism). True hermaphroditism (the presence of
both kinds of sexual glands in the same animal) is extremely rare
"in mammals and in man. What is described as hermaphroditism
does not in the majority of cases deserve the name.
120 GENERAL PRINCIPLES OF ZOOLOGY.

 

Genital Ducts.—Very frequently in the animal kingdom the
excretory apparatus furnishes outlets for the sexual products. In
the annelids and in the vertebrates portions
of the nephridial system, either exclusively
or in addition to their excretory function,
become accessory sexual organs. Hence we
speak of a wrogenttal system. This remark-
able connexion of genital and excretory
organs has a double cause, a physiological and
an anatomical. Physiologically important is
the fact that eggs and spermatozoa behave
like excreta; being substances which are no
longer destined for the use of the individual,
but must reach the exterior in order to
become efficient. The morphological cause
is the relation to the celom. A urogenital
system is formed only in animals in which the
germinal epithelium arises from the epithe-

lium of the eewlom, and in which the kidneys
eee ee ee apa, or their rudiments stand permanently in
(iran Gegen panna’ connexion with the body cavity and thus
v4, vasa deterentia: ® form the natural outlet for its products.
ee ee Whether the accessory sexual parts are por-
ee one tions of the excretory organs or are inde-
glands. pendent structures, they have in the animal
series a definite arrangement adapted to their function (fig. 73).
Canals lead from the sexual glands to the exterior, the ovtducts in
the female, the vasa deferentia in the male (and the herma-
phroditic duct from the hermaphroditic gland),

Accessory Sexual Apparatus.—The terminal portion of the vas
deferens is often very muscular and is called the ductus ejacula-
torius; it may be evaginated as a penis or cirrus, or project
permanently beyond the surface of the body. The terminal
portion of the oviduct is often widened so that two portions may
be distinguished, the wferws, which harbors the eggs during their
development, and the vagina, which serves for copulation. In
addition there may occur in both sexes other accessory glands of
the most diverse character. Oviduct and vas deferens may be
provided with sac-like evaginations which serve for the reception
of the sperm. In the female these are called receptacula seminis,
in the male vesicule seminales; the former give lodgment to sperm
which enters the female sexual passages during coition, the latter

o

 

 
GENERAL ORGANOLOGY. 121

to sperm which has been formed in the testes of the same indi-
vidual.

Animal Organs.

I. Organs of Locomotion.

Voluntary Locomotion.—The power to change their location
voluntarily is a peculiarity so prominent in animals that to the
general public it is sufficient for deciding whether an organism
belongs to the vegetable or to the animal kingdom. On this
account it is necessary to call attention to the fact that numerous
animals lose the power of locomotion, becoming fixed to the
ground, to plants, or to other animals. All sponges and corals,
most hydroid polyps, and the crinoids among the echinoderms,
have actively swimming larve, but become fixed in the adult and
thus obtain such a marked similarity to plants that, although true
animals, they were long regarded as plants. Further, many mol-
luses and worms are firmly fixed by their shells; indeed, many
crustacean forms, the cirripeds, have completely lost their free
motility. Buta more careful investigation in all these cases will
show that the power of moving the separate parts exists, for the
corals can retract their tentacles, the cirripeds their featherlike
feet, and the clam can close its shell.

Locomotion among Lower Animals.—The lowest forms, the
Protozoa, progress almost exclusively by processes of the cell:
pseudopodia, cilia, or flagella. In the metazoa this is rarely the
case. Ameboid movements of the epithelial cells, indeed, occur
in the celenterates and also in many worms, but do not suffice
for change of position. More effective is the ciliated or flagellated
epithelium, by which ctenophores, turbellarians, and rotifers
swim; this occurs, besides, in many larve of animals which, in
the mature state, are unable to change their location or do so only
by the aid of muscles. Nearly all ccelenterates, echinoderms,
molluses, and the majority of the worms leave the egg-membranes
in the form of the planula, i.e., as a larva swimming by means of

 

cilia.

Locomotion among Higher Animals.—The musculature is alone
adapted for energetic motions. The arrangement of this varies
with and depends upon the constitution of the skeleton. Forms
without a skeleton have commonly the ‘dermo-muscular tunic,’ a
sac of circular and longitudinal muscle fibres which is firmly united
122 GENERAL PRINCIPLES OF ZOOLOGY.

with the skin. If a skeleton be formed by the skin, as in the
arthropods, then the sac breaks up into groups of muscles, which
find points of attachment upon the dermal skeleton; if, on the
other hand, as in the vertebrates, an axial skeleton be formed, a
fixed point is furnished for muscular action, so that the muscula-
ture obtains a quite new character, in particular a deeper position.
A locomotor apparatus quite unique is the ambulacral system of
the echinoderms, a system of delicate little tubes with protrusible
portions which function as feet, described in connexion with that
group.

II. Nervous System.

Scarcely a system of organs in the animal series shows such a
recular development as the nervous system. The different stages
which can be grouped may be termed the diffuse, the linear, the
ganglionic, and the tubular types.

Diffuse Nervous System.—The diffuse type is certainly the
most ancestral; it shows the two elements, nerve fibres and
ganglion cells, regularly distributed through the whole body, or,
at least, through certain layers of the body. The skin of the
body, the ectoderm, is to be looked upon as one of the fundamental
elements in the nervous system, since it is related to the external
world, and hence receives the sensory impressions, so important
for the development of nervous tissue. The corals and hydroid
polyps are examples, since in them the ectoderm is permeated in
all directions by a delicate, subepithelial spider-weblike network
of nerve fibres and ganglion cells, which encroach even upon the
entoderm.

Linear Nervous System.—From the diffuse type the other
chief types can be derived through concentration, which is chiefly
conditioned by the fact that there are a few points which are most
advantageously located for the reception of sensory stimuli, and
hence for the development of nervous elements. In the meduse
such a place is the rim of the bell; consequently a stronger nerve-
cord remarkably rich in ganglion cells is found here. This, as
well as the nerve-ring and the five ambulacral nerves of echino-
derms, may be called a central system, thereby distinguishing the
rest of the nervous network as the peripheral nervous system.

Ganglionic Central Nervous System.—Numerous transitional
forms lead to the ganglionic central nervous system of the worms,
molluses, and arthropods (fig. 74). The central nervous system
here consists of two or more ganglia; each ganglion being a

 

 
GENERAL ORGANOLOGY. 123

rounded bunch of regularly ar ranged nerve-fibres and ganglion-

vey 1 n . ~ ay at} ms 7

cells. The former constitute the centre of the mass, and, since

they cross in all directions, give the appearance of fine granula-

tions; this fact has led to the unsuitable, because misleading, name
‘ Tove Joe ate ? 9 ] 5

of « Leydig’s dotted substance.’ The ganglion-cells, on the other

 
 

  
    

DoE.

    
  

a

  

Fic. 74.—Third abdominal ganglion of a crayfish. (After Retzius.) ©, connective or
longitudinal commissure; y, ganglion cell layer; y’ ganglion cell whose neurites
enter the connective; yg’, ganglion cell whose neurites enter the peripheral nerve;
L, Leydig’s dotted substance; N, peripheral nerve.

hand, collect in a thick layer around the dotted substance. The

peripheral nerves, and also the commissures, the cords connecting

similar ganglionic masses, extend outwards from the ganglia.
Supraesophageal (or Cerebral) Ganglia.—Since most animals
are symmetrical, the ganglia occur in pairs; left and right ganglia
correspond to one another and are connected simply by a cord of
nerve-fibres, the transverse commissure. Of most constant occur-
rence are two ganglia, which lie dorsally above the pharynx, and
hence are called the suprawsophageal or cerebral ganglia. Tf other
ganglia occur, they lie ventrally and below the digestive tract

 

(ventral nerve-cord).
124 GENERAL PRINCIPLES OF ZOOLOGY.

Ladder Nervous System.—A widely recurring arrangement is
that termed the ladder nervous system (of annelids and arthropods)

(fig. 75). Numerous pairs of ganglia (in
OD. the example before us, nine) lie in serial
1 SS ‘ order on the ventral side of the animal, and
jare connected by longitudinal commissures
(connectives), and also by transverse com-
missures connecting the left and right
ganglia. The first pair of the series is
formed by the infra-cesophageal ganglion,
which sends out commissures right and left,
surrounding the pharynx, to the supra-
cesophageal ganglion. The supra- and infra-
esophageal ganglion together with the
esophageal commmissures form the wsopha-
geal ring, &% nerve-ring surrounding the
esophagus.

Tubular System.—The tubular type of
nervous system is found only in the chordates
(fig. 76). The vertebrate brain and spinal
cord may be regarded as parts of a tube with
greatly thickened walls, developed in differ-
ent ways. In the centre lies the extremely
narrow central canal, which widens anteriorly
into the several ventricles of the brain. In

a transverse section the nervous elements
i ivstom of Duvatlinwiber are seen grouped around the central canal in
(gowbug). (After Leva'é) a manner almost the reverse of that of the
connected with the brain 4, anglionic type. On the periphery lies a

by the cesophageal com- ga

Se tee ate ei layer of nerve-fibres (the ‘white matter’ of

shicus. human anatomy); next is a central portion
formed of ganglion-cells and nerve-fibres (the ‘gray matter’),
which is marked off from the central canal by a special epithelium
(ependyma).

Relations between the Nervous System and the Skin.—For
almost all animals it has been ascertained that the nervous system
arises from the ectoderm. Therefore, in many animals, the nerve-
cords and the ganglionic masses lie pemanently in the skin: in
others only during the development, later becoming separated by
splitting off.or by infolding, and thus coming to lie in the deeper

layers of the body (fig. 9).

  
  

 

 

 
Or

GENERAL ORGANOLOG ¥. 12

III. Sensory Organs.

Sensations of the Lower Animals.—What we know of the
character of the external world is founded upon experiences gained
through our sensory organs. We thus know the external world
only in so far ag it is accessible to the senses, controlled by the
judgment. If things exist outside of ourselves which have no
influence upon our senses, we can form no conception of them.
It follows from this proposition that we can gain knowledge of the

 

 

ae /
Wp
Fie, 16.—Cross-section of the human spinal cord. (From Wiedersheim.) Black repre-
sents the gray, white the white substance of the cord; (Cc, central canal, sur-
rounded by the anterior and posterior commissures (© and (’); Sa, Sp, anterior
and posterior fissures; VV, HW, anterior and_posterior nerve-roots : VH, HH.
anterior and posterior horns of gray matter; V,S, H, anterior, lateral, and pos-
terior columns of white matter.

iH

natural capacity of the sensory organs of animals only by analogy
with our own experiences. Hence the distinction of five senses,
touch, taste, smell, hearing, and sight, based upon human
physiology has been extended to the whole animal kingdom. A
priori, however, it cannot be denied that sensations may occur in
animals which we do not experience; following out this course of
thought has led to the idea of a ‘sixth sense,’ which, however,
must remain to us a meaningless abstraction, since it is impossible
for us to conceive of the character of a sense which we lack.

Anatomy gives Insufficient Knowledge of Sensory Organs.—
A further, and still more important reason for our very fragmen-
tary knowledge of animal sensations is the fact that, in regard to
the physiological meaning of the sensory apparatus, it is seldom
that we can depend upon experiments, and consequently we must
base our conclusions upon structure. But the anatomy of many
sensory organs, like those of smell and taste, is hy no means so
characteristic that it alone is sufficient to determine the physio-
logical significance.

Tactile Organs.—The skin of animals functions as a tactile
organ, usually over the whole area, although not everywhere with

 
°

126 GENERAL PRINCIPLES OF ZOOLOGY.

equal intensity. Prominent parts, like the tentacles of polyps
and of many worms, the antenne of arthropods and the snails,
need only mention. Special epithelial cells with stiff hairs pro-
jecting above the surface, the tactile bristles or tactile hairs, are

  

Fia. 78.

Fic. 77.—Skin of an insect with an ordinary hair (h) and_a tactile hair (t); n, nerve;

s, Sensory cell; e, epithelium; ¢, cuticle. (After vom Rath.)

Fig. 78.—Vater-Pacinian corpuscle of the mesentery of a cat. a, axis cylinder: f, fat;

y, blood-vessel; i, inner bulb; k, capsule with nuclei; 1, medullated nerve-fibre.
tactile (fig. 77). Only in the vertebrates do the nerves of touch
terminate in specially modified end organs (Vater-Pacinian cor-
puscles, corpuscles of Meissner, etc., fig. 78); these usually lie
under the epithelium.

Organs of Smell and of Taste are accurately known only in
vertebrates. The olfactory organ of fishes consists of two small
pits in the skin, above or in front of the mouth.

In the air-breathing vertebrates this pair of pits which here
also arise from the skin are taken into the dorsal wall of the two
respiratory canals leading from the outside to the pharynx. Now
since the olfactory cells distributed in these pits (fig. 37, 4) are
frequently characterized by bundles of olfactory hairs, while the
surrounding epithelium is often ciliated, one is inclined to regard
as organs of smell sensory organs of invertebrates (e.g., meduse,
cephalopods), which have the form of ciliated pits and lie near the
respiratory apparatus (e.g., the osphradium of molluscs). Yet
there are exceptions. Experiments seem to show that in. the
arthropods the antennew probably serve for smelling. Tere the
sensory perception can be connected only with certain modified
GENERAL ORGANOLOG ¥. 127

hairs, the olfactory tubules of the Crustacea and the olfactory
cones of insects. In a similar way certain nerve end organs in
the region of the mouth are considered as organs of taste, since the
taste organs of vertebrates, the so-called taste buds, are abundant
in the mouth cavity, especially on the tongue.

Organs of Hearing and of Sight are called the higher sense-
organs, because they are of much greater importance for the
totality of our perceptions than the other organs, since they fur-
nish sensations which are quantitatively and qualitatively much
more definite. Ears and eyes have therefore a complicated and
characteristic structure, which renders them easily recognizable
by the almost invariable presence of certain structures accessory to
their functions.

History of the Auditory Organs.—The auditory organs of
vertebrates and of most of the other animal groups can be traced
back to a simple fundamental form, the auditory vesicle (fig. 79).

 

 

Fic. 79.—Auditory vesicle of a molluse (Pterotrachea). N, auditory nerve : /1z, audi-
tory cells with the central cell; Cz, ciliated cells; Wz, ciliated cells; Ot, otolith.
(After Claus.)

This has an epithelial wall, a fluid contents, the endolymph, and
an auditory ossicle or ofolith, formed from a single or from several
fused auditory concretions. In some instances the otoliths, to the
number of thousands, may remain separate. In a definite region
of the epithelial wall the cells are developed into the crista
acustica, the auditory ridge; they are in connexion with the
auditory nerve and bear the auditory hairs projecting into the
endolymph. The otoliths themselves are concretions of carbonate
or of phosphate of lime (exceptionally in Jfysts of fluoride of
calcium). They usually float free in the centre of the vesicle, and
are often held in place by bundles of cilia which project from the
non-sensitive epithelial cells.
128 GENERAL PRINCIPLES OF ZOOLOGY.

 

Auditory Pit.—Every auditory vesicle develops from a pitlike
invagination of the skin, and consequently is for a time an auditory
pit. Therefore it is not surprising that in many animals the organ
has stopped at the lower stage of development; for example, the
erayfish has an open auditory pit. On the other hand, the
auditory vesicle may develop a complicated system of cavities as
in mammals (fig. 80), where it is divided by a constriction into

 

Fic. 80.—Diagram of the human labyrinth. U, utriculus with the semicircular
canals; N, sacculus connected with the cochlea (C) by the canalis reuniens; R,
recessus labyrinthi; V, blind sac of the cochlea; K, apex of the cochlea.

the sacculus and the utriculus. The sacculus is provided with a
spirally-wound blind sac, the cochlea, the utriculus with the three
semicircular canals. In addition there is formed in the mammals,
as also in most vertebrates, a sound-conducting apparatus, so that
the auditory organ acquires an extremely complicated structure.

Other Forms of Auditory Organs.—Since there are animals
without auditory vesicles which hear well, like the spiders and
insects, we must assume that there are auditory organs which are
formed after another type. Still we have no certain knowledge of
these except in the case of the tympanal auditory organs of the
grasshoppers (which compare).

Function of the Semicircular Canals. — Experiments upon
representatives of the most diverse classes of vertebrates have led
to the conclusion that the three semicircular canals, standing at
right angles to each other, are organs of equilibrium, for, after
these canals are destroyed, the animals begin to stagger and lose
their balance. It is probable that in fishes this is the sole function
of the labyrinth; for it bas not been determined that fishes hear.
Starting from this assumption, recent investigators have attempted
to prove that the auditory vesicles of invertebrated animals are
exclusively, or at least largely, organs of equilibration. This would
explain the otoliths, for these bodies, of relatively large specific
gravity, would affect the crista in different ways according to the
position of equilibrium of the body. Statoliths would thus be a
better name.

 

 
GENERAL ORGANOLOGY. 129

The Eye is in all animals recognized by the character of the
sensory epithelium, the retina. This always has a large amount
of pigment which lies either in the sensory cells or in special cells
arranged between or behind them. The simplest-formed eye,
therefore, appears as a sharply circumscribed pigment-spot in the
epithelium of the skin, provided with nerves, commonly also with
a lens (fig. 81).

Rods and Cones.—The sensory cell itself bears usually at its
peripheral end a process, the rhabdom. This is a cuticular struc-
ture, probably serving to collect the
rays of light and thus to stimulate the
cell, and has, particularly in the verte-
brates, a complicated structure, each
rhabdom consisting of an inner and an
outer portion. Here can be frequently
distinguished two kinds of rhabdoms,
rods and cones (fig. 82).

The Optic Ganglion.—Before the
optic nerve divides into the separate
visual cells it forms a swelling, the

 
 
 
 
 
 
  
 
 
 
 
     

 

 

 

iG, 51.

Fia. 81.—Ocellus (0c) of a medusa (Lizzia koellikeri) with lens (D.

Fia. 82.—Human. retina. (After Gegenbaur.) P, pigment-layer; E, layer of sensory
cells: G, optic ganglion; 1, limitans interna; 2, nerve-tibre layers: 3, ganglion-
cells; 4, inner reticular layer; 5, inner granular layer; 6, outer reticular layer; 7,

outer granular layer; 8, imitans externa; 9, rods and cones; 10. tapetum nigrum;

M, Miiller’s fibres. ey i

 

  

optic ganglion, which either lies as a detached body outside of the
eye, or is united with the retina into a connected whole. The
130 GENERAL PRINCIPLES OF ZOOLOGY.

considerable thickness of the vertebrate retina is due to the fact
that it includes the optic ganglion. The parts (fig. 82) called
reticular layers, inner granular layer, ganglion cells, and nerve-
fibre layer, constitute the optic ganglion; the layer of visual cells
consists only of the outer granular layer and the connected rods
and cones. The outer granules are the nuclei of the visual cells
to which rods and cones belong.

Accessory Structures.—The eye may be further complicated
by special refractive bodies (cornea, lens, vitreous body) which

 

L

 

 

 

 

C NO VO

Fia. 83.—Horizontal section through the human eye. (After Arlt, from Hatschek.)
#K, epithelium of the cornea (conjunctiva); C, cornea; v4, anterior chamber of
the eye; I, iris; NA, posterior chamber of the eye; Z, zonula Zinnii; Os, ora ser-
rata ; Sc, sclerotic coat ; Ch, choroidea; R, retina; p, papilla of optic nerve; m,
macula lutea, area of most distinct vision; VO, sheath of the optic nerve; VO,
optic nerve; C, arteria centralis; Ce, corpus ciliare; L, lens; Cv, vitreous body.

concentrate the light in order to cast an image upon the retina;

and an iris to regulate the amount of light. Then, too, means for
nutrition (the choroid coat) and for protection (sclerotic coat)
must be provided. Jf all these parts be present, a structure results

such as is found in the squid and in the vertebrates (fig. 83).
GENERAL ORGANOLOGY. 181

The Eye of the Vertebrates.—The eye of the vertebrates
usually is an approximately spherical body whose surface is formed
by a firm membrane. Over the greater part of the circumference
this is an oqapue, fibrous or cartilaginous covering, called the
sclera, or sclerotica; it is transparent only in the most anterior
part, and here it forms by its greater convexity a projecting por-
tion like a watch-glass, the cornea. Internally to the sclera lies
the choroidea, an envelope of connective tissue, rich in pigment
and blood-vessels, which, at the junction of sclera and cornea, is
changed into the iris. The iris, the seat of the color of the eye,
is pierced in the centre by the pupil, an opening the varying size
of which regulates the amount of light. Next internal to the
choroid follows a layer of black cells, the tapetwmn nigrum (pig-
mented epithelium), and finally the retina itself, the expansion of
the optic nerve which enters the eye at the hinder part. The
tapetum nigrum and the retina arise together, and hence both end
at the edge of the pupil, although the retina loses its nervous
character at the ora serrata, some distance from the outer edge of
the iris.

The cavity of the eye is completely filled by the vitreous body,
aqueous humor, and the lens. For vision the lens is the most
important, since, next to the cornea, it influences most the course
of the rays of light. It lies behind the iris, fixed to the anterior
wall of the choroidea, which here is changed into the ciliary
process. In front of it is a serous fluid, the aqueous humor, partly
in the so-called posterior chamber of the eye, between the lens
and iris, partly in the anterior chamber, between the iris and
cornea. The single, larger cavity behind the lens is filled up by
a jelly-like mass of tissue, the vitreous body. The image formed
on the retina is inverted.

The Various Types of Eyes.—Between the simple pigment-
spot and the highly organized vertebrate eye are many transitional
stages: pigment-spots with lens and vitreous body, with enveloping
and nourishing coverings, etc. The faceted eye of insects and
Crustacea shows a special type of development, described later
under the Arthropoda.

SUMMARY OF THE MOST IMPORTANT POINTS OF ORGANOLOGY.

1. Organs are composed of tissues, and by their environment
are led to the formation of a body of definite shape and to the
performance of a single function; consequently every organ is
182 GENERAL PRINCIPLES OF ZOOLOGY.

characterized morphologically (according to its structure and its
relations) and physiologically (according to its function}.

2. Organs of different animals may be physiologically equiva-
lent, analogous organs (i.e., with similar functions).

3. Organs of different animals may be morphologically equiva-
lent, homologous (developing in similar relations).

4. In the comparison of the organs of two animals three
possibilities become evident.

a. They may be at the same time homologous and analogous.

i. They may be homologous, but not analogous (swim-bladder
of fishes, lungs of mammals).

c. They may be analogous, but not homologous (gills of fishes,
lungs of mammals).

5. Organs are divided into animal and vegetative.

6. Animal functions are those which are not completely foreign
to plants, but are only slightly developed in them; in the animal
kingdom, on the contrary, they undergo an increase and become
characteristic.

V. Vegetative functions are developed with equal completeness,
though in a different manner, in plants and animals.

8. To the animal organs belong the organs of motion and
sensation, such as the muscles, the sense-organs, the nervous
system.

9, To the vegetative organs belong the organs of nutrition and
reproduction.

10. Under natrition, in the widest sense, are included not only
the taking in and digestion of food and drink, but also the taking
in of oxygen (respiration), the distribution of food to the parts of
the body, and the removal of matter which has become useless.

11. With nutrition, therefore, are concerned not only the
digestive tract and its accessory glands, but also the organs of
respiration, the blood-vascular system, and the excretory organs
(kidneys). /

12. The male and female sexual organs serve for reproduction.

13, The male and female organs may oceur in different indi-
viduals (diwcious), or both may be found in one and the same
animal (hermaphrodditic).

14. The highest degree of hermaphroditism is attained when
one and the same gland (the hermaphroditic gland) gives rise to
both eggs and spermatozoa. :

15. Very often the sexual organs and the duets from the
kidneys are closely united; we then speak of a urogenital system.
PROMORPHOLOG Y. 1383

IV. PRoMORPHOLOGY, OR STUDY OF THE FUNDAMENTAL FORMS.

Organic and Inorganic Bodies.—The structure of the individual
animal rests upon the regular combination of differently-function-
ing organs. The organs thus assume a relation to one another
which is definite for each animal group, or varies only in subordi-
nate ways. If the various groups be compared with reference to
the principle of the arrangement of parts, there appear a few
fundamental forms which play a réle in morphology similar to that
of the fundamental forms of crystals in mineralogy. But we must
not carry this comparison too far, and attempt to compare the
study of the fundamental forms, the promorphology, of animals

 

Fic. $4.—Spongilla fluviatilis, fresh-water sponge. (After Huxley.) a, superficial layer
with dermal pores; be, region of the ampulle; d, osculum.
with crystallography as of equal value. A crystal is a mass made
up of similar parts; its form is the necessary and immediate result
of the chemico-physical constitution of its molecules. A direct
connection of this kind between molecular structure and funda-
mental form does not, and cannot, exist in the organism, since
each organ is composed of many chemical combinations. Conse-
quently there is lacking also the mathematical regularity which
occurs in crystals. Even in the case of animals which have the
greatest regularity in the arrangement of their parts there is not
an entire conformity to the demands of the fundamental form, so
that we are compelled to ignore certain greater or less variations.
If, for example, we call man bilaterally symmetrical, we overlook
not only the shght asymmetry of a nose awry, etc., but also what
is more important—that the liver has been pushed to the right,
134 GENERAL PRINCIPLES OF ZOOLOGY.

the heart to the left; and that the digestive tract throughout its
entire course runs asymmetrically.

 

 

Fie. 85.—Haliomma erinaceus, a radiolarian. a, external, i, internal, latticed spheri-
cal skeleton; ck, central capsule; wk, extra capsular soft parts; m, nucleus.

 

Fia. 86.—Nausithoé, an acraspedote medusa (after Lang), seen from the end of the
greatly shortened main axis. pr, perradii; ir, interradii; ar, adradii (perradii
and interradii mark the four planes of symmetry of the animal); sr, subradii: 7,
mantle-lobes; t, tentacles; sk, sensory organs: g, sexual organs; yf, gastric fila-
ments; m, subumbrellar circular muscle; in the centre the cross-shaped mouth-
opening.

 
 

Symmetry.—Now, according to the three dimensions of space,
we can pass three axes, perpendicular to each other, through the

 
PROMORPHOLOGY. 135

body of an animal, and up to a certain degree may characterize it
according to the nature of these axes; further, we may characterize
it according to the planes by which it can be symmetrically halved,
the planes of symmetry. Thus we find the following fundamental
forms:

1. Anaxial, asymmetrical, irregular, or amorphous funda-
mental form (fig. 84).

2. Homaxial, symmetrical in all directions, spherical funda-
mental form (fig. 85).

3. Monaxial, radially symmetrical (fig. 86).

4. Simple heteraxial, biradially symmetrical (figs. 87, 88).

5. Double heteraxial, bilaterally symmetrical (fig. 89).

1. Anavial or asymmetrical animals, so called, are those in which the
arrangement of parts is not regularly de-
fined in any direction or space, and they
therefore may grow irregularly in any direc-
tion. There is no fixed central point, and
there is no possibility of running definite
axes through the body or of dividing it into
symmetrical parts. (Many sponges and
many Protozoa.)

2. Homaxial or spherical animals have
the sphere as their fundamental form; the
parts of the body are arranged concentri-
cally around a fixed central point, so that
any number of axes and planes of symmetry
can be passed through it; that is to say,
all lines and planes which run through the :
central point of the sphere. (A few spheri- Fig. 87.— Diagram of anactinian
eal Protozoa, chiefly radiolariaus.) Ce eae eae oe

3. Monaxial or radial symmetry is ™Much-lengthened main axis.
brought about, if growth go on in a definite direction, and correspond-
ingly also if the formation of organs take place in directions other than
perpendicular to this. The line which marks this direction of growth is
the main axis, in distinction from the accessory axes or radii, which are
all similar to each other. The main axis can be determined, because it is
longer or shorter than the accessory axes; but it may also be of the same
length and still be entirely distinct, since it contains certain organs (e.g.,
the mouth-opening) which are lacking in the other planes. In radially
symmetrical animals the same organs are always present in greater num-
ber and are distributed regularly around the main axis in the direction of
the radii. Through such an animal a great number of sections can be
made, which pass through the long axis and halve the body symmetri-
cally. If we cut the animal in the direction of all the possible planes of
symmetry, we obtain pieces which, in essential points, are similarly con-

 
186 GENERAL PRINCIPLES OF ZOOLOGY.

structed. Great groups of animals, as most echinoderms and ccelenter-
ates, are more or less completely radially symmetrical.

4 and 5. The next two fundamental forms have in common the fact
that three unequal axes perpendicular to each other are distinguishable ;
these may be designated as the main axis, the transverse axis, and the
sagittal axis: this is the case if, leaving the main axis out of considera-
tion, an arrangement of organs occur different in the sagittal direction
from that in the transverse direction—if organs lie in the former which

 

Fic. 88.—Cross-section of an actinian (Adamsia diaphana). AB, directive septa,
which are at the same time ends of the sagittal axis, which marks one plane of
symmetry of the body, while the second stands perpendicular to it; I-IV, cir-
cles of paired septa of first to fourth order.

are lacking in the latter or the reverse. There are then, if we take into
consideration the dissimilarity of the axes, two possible planes of sym-
metry: the animal can be symmetrically divided, (1) if the division passes
through the main and transverse axes, (2) if it passes through the main
and the sagittal axes. Such béradially symmetrical animals are the
ctenophores, actinians (figs. 87, 88), and corals,

Bilateral Symmetry.—If now we further suppose that the ends of the
sagittal axes become unlike, that at one end are organs quite different
from those of the other, we then reach the most usual form, bilateral
symmetry. The dissimilar ends of the sagittal axes are called ‘dorsal’
and ‘ventral,’ and further the terms ‘right’ and ‘left? are given to the
ends of the transverse axis; a bilaterally symmetrical animal can be
divided symmetrically into aright and a left half by one plane, the median,
passing in the direction of the longitudinal sagittal axis; a frontal see-
tion (a section through the longitudinal and transverse axes) always gives
dissimilar parts, dorsal and ventral sides.
PROMORPHOLOGY. 137

 

 

 

Fia. 89.—Cross-section of a fish passing through the fore limbs. DY, sagittal axis;
RL, transverse axis; a, dorsal aorta; c, body cavity; d, gut; ch, notochord; g,
shoulder-girdle; h, heart; m, muscles; n, anterior end of the kidneys; p, peri-
cardium ; of, neural arch; wh, hemal arch; 7, spinal cord.

 

Antimeres and Metameres.—The symmetrical parts of an
animal are called wntimeres; each antimere has organs which occur
likewise in its adjacent antimere. The right arm of man corre-
sponds to the left, the right eye to the left, etc; the same organs
are repeated in the direction of the transverse axis. Frequently,
however, the repetition of organs occurs not only in the direction
of the transverse axis, but also im the direction of the long axis.
Thus the body is made up not only of symmetrical parts, the
antimeres, but also of similar parts placed one behind the other,
the metameres.

Internal and External Metamerism.—Metamerism or seymen-
tation ig spoken of when the body of an animal consists of
numerous segments or metameres (consult fig. 59). Very often
it is recognizable externally—when, for instance, the limits of the
segments are marked on the surface by constrictions (arthropods
and annelids). But this exlernal metamerism may be entirely
lacking, and the metamerism find expression only tnfernailly in
the serial succession of organs, in metameric or segmental arrange-
ment. Man, for example, is segmented only internally; in his
skeleton there are numerous similar parts, the vertebre, which

 
138 GENERAL PRINCIPLES OF ZOOLOGY.

follow one another in the long axis. In fishes the musculature also
is made up of numerous muscle segments, as any one can readily
see by examining a cooked fish. In the case of the externally
segmented earthworm also, the ganglia of the nervous system, the
vascular arches, the nephridia or segmental organs, the sete, and
the septa of the body cavity are repeated metamerically.

Homonomous and Heteronomous Metamerism.—The examples
mentioned are well adapted for illustrating the different forms, the
homonomous and the heteronomous, of metamerism. The earth-
worm is homonomously metameric, because the single segments are
much alike in structure, and only slight differences exist between
the anterior, the posterior, and the genital segments. Man and
all vertebrates, on the contrary, are heteronomously metameric,
because the successive segments, in spite of many points of agree-
ment with one another, have become very unlike. The segments
of the head have an importance, for the organism as a whole, quite
different from those of the neck, the thorax, or the tail. A
division of labor has taken place among the segments of an
heteronomous animal.

Heteronomy and Homonomy.—The distinction between heter-
onomy and homonomy is of great physiological interest. The
more different the segments of an animal become the more
dependent they are upon one another in order to be able to func-
tion normally; so much has the whole become unified that the
single parts can live only while the continuity is maintained. On
the contrary, if the connexion between the parts be less intimate,
they are more similar, and the more able to exist after separation
from one another. This is most beautifully shown in instances of
mutilation. It hag been observed that when many species of
Lumbricide are cut in two each part not only lives on by itself, but
it even regenerates the part which is lacking; if, on the other
hand, the same thing is done te a heteronomously segmented
animal, either death immediately ensues, as in the case of the
higher vertebrates, or the parts live for a short time a hopeless
existence, as can be seen in the case of frogs, snakes, insects, ete.
In metamerism a phenomenon is repeated which obtains widely in
the animal kingdom, and contributes towards its higher develop-
ment; first there is a reduplication of parts, then a division of
labor, so that the final result isa whole composed of many parts,
yet uniformly organized.

 
GENERAL EMBRYOLOGY. 139

II. GENERAL EMBRYOLOGY.

Origin of Organisms.—Since the development of every indi-
vidual begins with an act of generation, the ways by which new
organisms may arise should be mentioned first in this chapter.
If we wish to limit ourselves to that which has been actually
observed, we must still cling to the old expression of the renowned
Harvey, ‘“‘Omne vivum ex ovo,” and modifying it somewhat say,
Omne vivum e vivo: that every living organism is derived from
another living organism. We must limit ourselves to the mode of
origin which has been termed tocogony, or generation by parents.
The great importance which the question of generation without
parents, or spontaneous generation, has obtained through the
evolution theory renders a consideration of this question necessary
at this point.

I. GENERATIO SPONTANEA, ARCHEGONY.

Theory of Spontaneous Generation.—The old zoologists, even Aristotle
himself, believed that many animals, including even highly organized
forms, like frogs and most insects, arose by spontaneous generation from
the mud. Not until the seventeenth and eighteenth centuries did this
doctrine find energetic opponents, in Spallanzani, Francesco Redi, Risel
von, Rosenhof, Swammerdam, and others, who endeavored to prove
experimentally that all animals lay eggs which must be fertilized by the
spermatozoon in order to develop further. By their convincing investi-
gations the doctrine of spontaneous generation was driven into the realm
of the lower animals. Here it found a new foundation in the occurrence
of parasites inside of animals which, at the beginning of their life, without
doubt must have been free from these internal inhabitants, Parasitolo-
gists maintained that the parasites arose quite anew from the superfluous
plastic material of their host. At last, by a series of epoch-making
researches, the way was discovered by which the young of the parasite,
developing from eggs, find their way into the body of their host. It was
until recently considered a proof of the doctrine of spontaneous generation
that, after a time, animal and plant life (unicellular organisms, infusorian
animalcules, etc.) appears in water supposed to contain no living thing
whatever ; further, that organic fluids became foul by the development of
the lowest of the plants, the bacteria. At present we know that in all
these cases germs of organisms, carried about by the air, are the cause of
the new development of life. If the germs be killed by heating the
glass and boiling the fluid, and if by suitable means the entrance of new
germs be prevented, then such a ‘sterilized fluid’ remains permanently
unchanged. It has been found, indeed, that spores, particularly of bac-
teria, have an extreme power of resistance, and in many cases must be
140 GENERAL PRINCIPLES OF ZOOLOGY.

boiled more than ten minutes before they are destroyed. As the final
result of all the recent experiments and observations it can only be said
that the present existence of spontaneous generation is not proved. Now
the question is, With what right can one conclude that spontaneous gene-
ration neither occurs nor has ever occurred ?

First Origin of Life.—Whoever, in agreement with the teachings of
astronomy, adopts the view that our earth was at one time in a molten
condition and has gradually cooled, must assume that life on the earth
has not existed from eternity, but at some time has had its beginning.
If he wish to base his explanation, not upon a supernatural act of crea-
tion, nor upon hypotheses, like that of the transference of living germs
from other worlds through the agency of meteors, there is left only the
hypothesis that, according to the generally prevailing and still to be
observed laws of chemical affinity, compounds of carbon, oxygen, hydro-
gen, nitrogen, and sulphur have been brought together to produce living
substance. This process is called spontaneous generation. If the carbon,
oxygen, nitrogen, etc., which are now combined in a stable manner in
organisms were formerly unstable, the conditions for the origin of organic
compounds, through whose wider combination life would be possible, may
have been more favorable. Thus the hypothesis of the first origin of life
through spontaneous generation ts carried to a logical postulate.

Bat the postulate cannot be extended to affirm that spontaneous gen-
eration must even now exist. Since there are neither observations nor
convincing theoretical considerations for such a view, there is no necessity
to discuss the objections here.

IJ. GENERATION BY PARENTS, OR TocoGony.

As mentioned above, we shall deal here only with those methods
of reproduction which have actually been observed, i.e., generation
by parents. These methods fall mainly into two great groups,
asexual and sexual generation, monogony and amphigony, to which
may be added a third group, a combination of these two methods
of reproduction.

a. Asexual Reproduction.  Monogony.

 

Monogony Defined.—The chief characteristic of asexual repro-
duction is the fact that for it only a single organism is necessary.
But since, in certain modes of sexual reproduction (herma-
phroditism, parthenogenesis), this also holds true, further explana-
tion is necessary. Asexual reproduction must be a result of the
growth of the organism. This growth may be general and result
in an equal growth of all parts; or it may be local and consequently
lead to the formation of an outgrowth in the region of greatest
increase. In the first case division takes place, in the latter
budding.
GENERAL EMBRYOLOGY. 141

Division.—In the case of division (cf. figs. 119, 122, 145) an
animal separates into two or more equivalent parts, so that it is
not possible to distinguish the mother and the daughter animal;
for the original animal has completely disappeared in the young
generation. The division is commonly a transverse one, in which
the plane of division stands perpendicular to the long axis of the
animal; less common is longitudinal division, rarest is oblique
division (the planes of division passing in the direction of the long
axis, or forming an acute angle with it).

Budding.—In the case of budding, the products are unequal.
One animal maintains the identity of the mother animal; on the
contrary, the bud, the outgrowth caused by local increase, appears
as a new formation, as the daughter individual. Yet the differ-
ence between division and budding is bridged by intermediate
conditions; for, if we start with binary division, this will approach
budding in the same degree as the division products become
unlike, so that the one takes on more and more the character of a

 

Fie. .—A, Hydra grisea in optical section with a bud ; also B, first stage of a bud.
en, entoderm; ek, ectoderm; s, supporting lamella; t’, tentacle of the mother ani-
mal; t’’, tentacle of the bud; m, stomach; 0, mouth.

bud, the other retaining the character of the mother organism.
Such transitions are chiefly possible in the case of terminal
budding, where the buds appear at one end of the main axis of the
maternal organism. The character of budding is, on the contrary,
unmistakable if the buds arise as lateral outgrowths of the mother
142 GENERAL PRINCIPLES OF ZOOLOGY.

(fig. 90), or if from the same mother numerous buds are simul-
taneously cut off (lateral and multiple budding) (compare fig. 20).

b. Sexual Reproduction : Amphigony.

Amphigony Defined.—For sexual reproduction two animals are
commonly necessary, a female and a male; the reproductive cells
—the eggs—of one must be fertilized by the reproductive cells—
the spermatozoa—of the other, and thus acquire the capacity of
giving rise to a new organism. Now, since there are hermaph-
roditic animals which produce simultaneously eggs and sperma-
tozoa, and since with many of them at least the possibility of
self-fertilization has been demonstrated, it becomes clear that the
emphasis in the definition of sexual reproduction must be laid, not
upon the individual, but upon the sexual products. Consequently
the essential point of sexual reproduction is to be sought in the
union of male and female sexual cells.

Parthenogenesis and Pedogenesis.—This explanation is appli-
cable to by far the greater majority of cases, namely, to all cases
where the term sexual reproduction can be applied. Still, in the
course of the last thirty years it has been demonstrated in many
instances that two modes of reproduction formerly considered as
monogony, parthenogenesis and pedogenesis, must be regarded as
special modifications of sexual reproduction, although the above-
mentioned conditions are not strictly satisfied. In both cases the
eggs develop on account of some peculiar internal stimulus,
without the occurrence of fertilization by spermatozoa. In ease of
pedogenesis there is the additional circumstance that reproduction
is accomplished by animals which have not completed their normal
development; for example, the larvee of certain flies reproduce
before they have passed through the pupal stage and become flies.
Pedogenesis consequently is parthenogenesis in an immature
organism.

Parthenogenesis and Typical Amphigony.—Some have at-
tempted to exclude parthenogenesis from sexual reproduction by
claiming that those eggs which develop parthenogenetically are
pseudova, structures which are not actual eggs. This view is
absolutely untenable in view of the proof that the ‘pseudova’
arise just like ordinary eggs and develop like them, since they
cleave and form germ-layers. The equivalence of parthenogenetic
eggs to those which are fertilized is best shown in the case of the
bee, where similar cells give rise to a female or a male insect

 

 
GENERAL EMBRYOLOGY. 143

according as they are or are not furnished by the queen during
oviposition with a spermatozoon. Parthenogenesis is, therefore,
not an asexual reproduction which was antecedent to sexual repro-
duction, but rather a reproduction which must have been derived
from the sexual; it is @ sexual reproduction in which a degeneration
of fertilization has taken place. Such facts show that, for the
essential point of sexual reproduction, fertilization (the entrance
of the spermatozoon) forms indeed an extremely important, but a
by no means indispensable, characteristic. To all cases comprised
under amphigony this definition alone applies: sexual reproduction
is a reproduction by means of sexual cells.

Sexual and Somatic Cells.—The distinction of sexual cells from the
asexual reproductive bodies, the parts arising by division and budding, is
shown by their relations to the vital processes of animals, The cells of a
bud have had a share in the vital processes of the animal before the begin-
ning of reproduction; they were functional or ‘somatic’ cells. In the
fresh-water polyp (fig. 90), when a bud arises, the cellular material em-
ployed is that which was previously related to the mother animal in
exactly the same manner as the other parts of the body wall. The sexual
cells of an animal, on the contrary, are permanently, or at least for a long
time, excluded from the vital processes, remaining in a resting condition,
and conserving their vital energies, Therefore there are also lacking in
sexual reproduction the relations to growth which are so remarkable in
asexual reproduction. For, although very often sexual reproduction does
not begin until the bodily growth is completed, yet it is found repeatedly
that animals, as for example all fishes, continue to grow after the begin-
ning of sexual maturity, until they are double or many times their size at
that time. Sexual reproduction is not even a special form of growth, but
a complete renewal of the organism, a rejuvenescence of it. This explains
the important fact that asexual reproduction is most common in the lower
animals (celenterates, worms), but is lacking from vertebrates, molluscs,
and arthropods. The higher the organization of the animal the more
the vital energies of its cells must be employed to meet the increasing
demands upon their functional capacity, and so the more necessary is
sexual reproduction.

c. Combined Modes of Reproduction.

Occurrence in the Same Species.—Very often two modes of
reproduction occur in one and the same species of animal side by
side. Many corals and worms have the power of multiplying by
division or budding, and also of forming eggs and spermatozoa;
again, others have no asexual reproduction, but their eggs develop
according to circumstances, either parthenogenetically or after
fertilization. The appearance of two kinds of reproduction is very
often governed by the fact that individuals with different modes of

 
144 GHNERAL PRINCIPLES OF ZOOLOGY.

reproduction alternate in a quite definite rhythm with each other.
Such a development is called alternation of generations in the wider
sense, and of this two special forms are distinguished: meta-
genesis, or alternation of generations in the narrower sense (pro-
gressive alternation of generations), and heterogony (regressive
alternation of generations).

Progressive Alternation of Generations. Metagenesis.— Alter-
nation of generations in the narrower sense, or melagenesis, is the
alternation of at least two generations, of which one reproduces
only asexually, by division or budding, the other either exclusively,
or at least to a great extent sexually. The first generation is called
the nurse, the second the sexual animal. The reproduction of
hydromeduse furnishes the best example (fig. 91). The nurses

 

 

 

Fie. 91.—Bougainvilled ramosa. (From Lang.) hy lydranths (nurse) which have given
rise to medusa-buds (it); mm, Separated medusa, Margelis ramosa (sexual animal).

here are the polyps, which, wnited with one another usually in
numbers into a colony, never produce sexual organs, but bud sexual
animals, the meduse. The meduse are altogether unlike the
polyps, being much more highly organized, and freely motile; only
very rarely have they preserved the asexual mode of reproduction ;
on the other hand, they develop eggs and spermatozoa, from which
the non-motile nurses, the polyps, develop. This example shows
GENERAL EMBRYOLOGY. 145

that, in alternation of generations, there is not only a difference in
the mode of reproduction, but usually, in addition, a difference in
form and organization. Between polyp and medusa the difference
is so great that for a long time these two, though representatives
of the same species, were referred to quite different classes of the
animal kingdom. In many cases the alternation of generations
may be still further complicated by two asexual generations fol-
lowing each other, before the return to the sexual generation takes
place; one speaks ‘then of erand-nurse, nurse, and sexual animal.

Heterogony is distin died from inetagenesis by the fact that
the asexual generation is replaced by parthenoge nesis. Couse-
quently there alternate animals of sometimes quite different struc-
ture, of which the one arises from fertilized, the other from
unfertilized, eggs. Certain Crustacea, the Daphnide, show heter-
ogony ina typical manner. During a large part of the year only
females are found; these increase parthenogenetically by ‘summer
eggs’; then males appear for a short time; they fertilize the
‘winter eggs,’ which now are formed, from which again partheno-
genetic generations arise. Very often heterogony has been insuffi-
ciently distinguished from metagenesis, chiefly for the reason that
parthenogenetic reproduction was regarded as an asexual mode, as
vas the case in the trematodes. The sexually ripe Distomum
produces very peculiar sporocysts; these again give rise partheno-
genetically to the larvae of Distomum, the cercarie, For a long
time the erroneous view was held that the cells from which the
cerearie arose were not eggs, but ‘internal buds,’ ‘germinal
granules.’ On the other hand there have been included under
heterogony modes of reproduction in which no parthenogenesis
whatever occurs. Cases have been called heterogony when two
generations which have only different forms and organization
alternate. clscaris nigrovenosa, an hermaphroditic worm, lives in
the frog’s lungs; it produces the separate-sexed Lhabdonema
nigrovenosum living in mud, from whose eggs the ascarid of the
frog is again produced.

GENERAL PHENOMENA OF SEXUAL REPRODUCTION.

In sexual reproduction a series of developmental processes is
observed which is repeated in an essentially similar manner in all
multicellular animals, and hence these should be spoken of here
together. They are: (1) the maturation of the egg; (2) the
process of fertilization; (3) the process of cleavage; (4) the forma-
tion of the three germ-layers.
146 GENERAL PRINCIPLES OF ZOOLOGY.

1. Maturation.

The egg with the large vesicular nucleus (germinal vesicle)
cannot yet be fertilized; to render it capable of fertilization it
must undergo a series of changes—the process of maturation,
which consists in the replacement of the germinal vesicle by a
much smaller egg-nucleus, and simultaneously the formation at
one pole of the egg of the ‘directive corpuscles’ or ‘ polar bodies.’

Formation of the Polar Bodies. —The germinal vesicle initiates
the changes, its walls disappearing, its contents in part mingling
with the cytoplasm of the egg, in part being employed for the
formation of a nuclear spindle (directive spindle). The latter
places itself with its axis in a radius of the egg so that one pole
is turned towards the centre, the other being in the superficial
layer of the egg (fig. 92, a). Now begins a regular cell-division,

 

Fig. 92.—Successive stages in the formation of the polar bodies of Asterias glacialis.
sp, directive spindle; rk’, first polar body; rk?, second polar body; ck,egg-nucleus
in process of formation.

 

 

but the products of the division are of very unequal size; the larger
part is the egg, the smaller quite insignificant part is the polar
body (fig. 92, 4, ¢). The latter projects above the surface carrying
with it one half of the spindle, and when the globule is cut off half
of the spindle is included in it.

The Second Polar Body.—The part of the directive spindle
remaining in the egg immediately forms a new spindle; the cell-
budding is repeated and leads to the formation of the second polar
body. Asa result two small cells (fig. 92, d, ¢, 7) lie at one pole
of the egg, in many cases even three, since during the formation
of the second polar body the first may have again divided. The
part of the directive spindle still remaining after the second divi-
sion becomes a vesicular resting nucleus, the egg-nucleus, the
GENERAL EMBRYOLOGY. 147

characteristic feature of the ripe egg capable of fertilization. In
other words, by a double division there have been formed from the
immature egg four (sometimes three) cells, of which one has
received by far the greatest part of the original mass of the cell
and constitutes the ripe egg, while the others are small bodies like
rudimentary eggs. The name directive corpuscles was given to
them because in very many cases their position renders possible a
definite orientation of the egg; i.e., a diameter, the long axis, can
be passed through the egg, one end of which is marked by the
directive corpuscles. With reference to later processes of develop-
ment this end is called the animal pole of the egg, the opposite
end the vegetative pole.

Relation between Maturation and Fertilization.—In many cases the
maturation takes place before the entrance of the sperm, either in the
ovary or at the beginning of the oviduct ; in many animals, on the con-
trary, there ensues a pause after the first polar body has been formed ; the
egg then requires the penetration of a spermatozoon in order to complete
the further changes, i.e., the formation of the second polar body and
reconstruction of the egg-nucleus. This dependence of the last phenomena
of maturation upon the beginning of fertilization led for a long time to
the error that the formation of the polar bodies was a part of the fertiliza-
tion process itself,

2. Fertilization.

Copulation and Artificial Fecundation.—The term ‘ fertiliza-
tion’ in the scientific sense refers to the internal processes which,
after the meeting of the egg and spermatozoon, go on in the
interior of the former and end with a complete fusion of the two
sexual cells; on the other hand, special expressions are necessary
for those preparatory processes whose purpose is to render fertiliza-
tion possible. Very often, but not in all cases, there is necessary
un active transfer of the sperm from the male to the female, a
copulation. Tn case of many marine animals, particularly most
fishes, echinoderms, ccelenterates, the eggs and the spermatozoa
are discharged into the water, and the union of these (impregna-
tion or fecundation) depends upon chance. One can bring about
then artificially what is accomplished by nature, by obtaining from
the sexual organs the ripe products and bringing them together.
For example, by suitable pressure upon the body of sexually ripe
fishes the eggs may be collected in one dish, the sperm in another,
and the contents of the latter poured cver the former, and thus
in many cases an entirely normal development may be obtained.
Such a proceeding is called artificial impregnation.

 
148 GHNERAL PRINCIPLES OF ZOOLOGY.

Fertilization.—T'he process of fertilization in the narrower
sense begins with the entrance of the spermatozoon into the egg.
Usually the egg is surrounded by a gelatinous envelope, the
chorion, to the surface of which the spermatozoa adhere, and
through which they bore until they reach the surface of the egg
(fig. 93). But since the chorion, particularly in eggs which are
laid in the air, may be hard and resisting, there exists in it very
often a special arrangement, the micropylar apparatus, rendering
possible the entrance of the spermatozoon; this may be a single
canal extending through the chorion, as in the eggs of fishes, or a
group of such canals, as in those of almost all insects.

Monospermy and Polyspermy.—Many spermatozoa may pass
through the gelatinous envelope, or through the micropyle canal,
but under normal conditions only one serves for fertilization.
The spermatozoon which is in the slightest degree ahead of the
others is met by a process of the protoplasm by means of which it
enters the egg. The egg is now impervious to all others. Only
in the case of pathological eggs can two or more spermatozoa
enter and then multiple impregnation (di- or polyspermy) occurs, a
pathological phenomenon. There are means of protection against

 

e Co ( Ox

  

Fic. 93.—Egg of Asterias glacialis during fecundation. (After Fol.) 4, entrance of the
spermatozoon; B, the s,ermatozoon has entered: the yolk-membrane has formed.
this abnormal fertilization. One, though by no means the only
one, is the formation of the yolk-membrane, an impermeable
envelope which is suddenly secreted from the surface of the egg,
as soon as the spermatozoon has accomplished the impregnation.
Within the yolk-membrane the body of the egg contracts into a
smaller volume by discharging some of the more fluid constituents,
so that between the yolk-membrane and the surface of the egg a
cavity is formed casily recognized in smaller fertilized eggs
(fig. 93, B).

In the large yolk-laden eggs of many insects and vertebrates several
spermatozoa may normally enter. But this does not alter the conception
GENERAL EMBRYOLOGY. 149

of fertilization, for even here but one spermatozoon fuses with the egg-
nucleus, the others degenerating sooner or later.

Essential Feature of Fertilization.—After the spermatozoon
has penetrated into the egg, the head and the middle piece which
contains the centrosome can still be recognized, as the chromatic
and achromatic parts of the spermatozoon or sperm-nucleus (male
pronucleus), while the tail and the slight amount of protoplasm
disappear in the yolk. In the cytoplasm of the egg the centrosome
of the sperm-nucleus gives rise to conspicuous rays, like those
observed during division. Preceded by these rays the sperm-
nucleus travels towards the egg-nucleus until it reaches (fig. 94);
and fuses with it to form a single cleavage nucleus. Now the
centrosome divides into two, which migrate to opposite poles of
the nucleus, while the cleavage nucleus changes to a cleavage
spindle, which divides and thus initiates the embryonic develop-
ment, the successive divisions being known as the cleavage or seg-
mentation of the egg. Since not until this point is fertilization
complete, we arrive at the fundamentally important proposition
that the essential feature of fertilization consisls in the union of
egg and sperm nuclet.

 

 

Tic. 94.—Stages in the fertilization of the egg of the sea-urchin. (After O. Hertwig.)
The sperm-nucleus (ss) with its rays is near the surface in one egg, in the other
near the egg-nucleus (ck).

 

Part Played by the Two Nuclei in Fertilization.—In many
cases an abbreviation of development may take place, the stage of
the cleavage nucleus being omitted, and the egg and sperm nuclei,
without previously uniting, pass directly into the cleavage spindle.
This fact in no wise alters the above-mentioned proposition, but
yet it is important, because it shows more plainly in what way the
two nuclei participate in the formation of the cleavage spindle.
It shows that of the chromosomes which form the equatorial plate
of the nucleus, exactly one half are furnished by the egg-nucleus,
150 GENERAL PRINCIPLES OF ZOOLOGY.

the other by the sperm-nucleus. For, even before the spindle has
been formed and the contour of the two nuclei has disappeared,
the chromosomes destined for the spindle are completely developed
in exactly the same number in each of these (fig. 95).

    

i

Fic. 95.—Fertilization of Ascaris megalocephala, (After Boveri.) A, the ends (centro-
somes) of the spindle formed; B, the spindle completed; sp, sperm-nucleus with
its chromosomes; e/, egg-nucleus; p, polar bodies.

Heredity.—Recent observations have furnished a certain basis
for the doctrine of heredity. By heredity we understand the
transmission of parental characteristics to the offspring. This
transmission, on the whole, takes place with equal energy from
the father’s and from the mother’s side; if we take the average of
numerous cases, the result is that the child’s pecularities hold the
mean between the peculiarities of father and mother; or, in other
words, male and female individuals in the average have an equal
power of transmitting characteristics.

The Physical Basis of Heredity.—Since in case of all animals
with external fertilization a material connexion between parents
and offspring can exist only through the sexual cells, these latter
must contain the substances which render heredity possible:
further, the two hereditary substances, in cases of equal capacity for
transmission, must be present in the egg and in the spermatozoon
in equal quantity. By this course of reasoning, the chromatic
nuclear substance which forms the chromosomes has come to be
regarded as the bearer of heredity; for we know that the egg con-
tains a great quantity of cytoplasm, but the spermatozoon only the
slightest trace of it; that, on the other hand, egg-nucleus and
sperm-nucleus furnish equivalent substances, and particularly the
same quantity of chromosomes, to the cleavage spindles: hence only
the chromatin can be regarded as the hereditary substance (idio-
plasm). This supports the view expressed before (p. 67) that the
nucleus is the bearer of hereditary qualities and determines the
character of the cell.

 

 
GENERAL EMBRYOLOGY. 152

3. Cleavage Process.

 

Arrangement of the Cleavage Planes.—The fertilized egg-cell
divides in rapid succession into 2, 4, 8, 16, ete., cells. which
become continually smaller, since the mass of the egg does not
increase. The ceils are called cleavage spheres, or blastomeres,
the whole process the cleavage process, or segmentation,
because, at each division, furrows arise on the surface which
continue to penetrate more deeply (fig. 93). As a rule each

I I

COO)

{

A
| OD) G ce
Z Roe. .

Fic. 96.—The equal cleavage of Amphiorus lanceolutus. (After Hatschek.) 1, division
into two (formation of the first meridional furrow); II, division into four (second
meridional furrow) forming four cleavage spheres (fourth is hidden); III, division
into eight (equatorial furrow; the seventh and eighth cleavage spheres’ hidden);

IV, blastula in optical section. A single layer of cells surrounds the cleavage
cavity. In IJ, II, III, a polar body is shown.

 

new plane of cleavage is as nearly as possible perpendicular to the
preceding. Hence the first three cleavage planes, which cause the
division into 2, 4, and 8 parts, are similarly arranged in almost all
animals. Using the globe asa basis for comparison, one speaks
of a first and a second meridional furrow (I, II), and calls the third
the equatorial furrow (III). The intersections of the two meridi-
onal furrows form the poles of the egg, the animal and the
vegetative, so called because the material of the one is used chiefly
for animal organs (nervous system), the material of the other for
vegetative organs (digestive tract).

Influence of the Yolk upon Segmentation.—Different kinds of
cleavage processes are distinguished, the peculiarities of which
depend upon two factors: (1) upon the quantity of material, food-
yolk, serving for nourishment of the egg; (2) upon the arrange-
ment of this. The food-yolk hinders the division, since it is a
material which is incapable of active movement, and is only
passively divided through the activity of the protoplasm in the
cleavage cells. The more the mass of this increases in proportion
to the protoplasm, the more slowly does the cleavage process pro-
ceed. Finally there comes a point where the resistance of the
yolk becomes so great that the protoplasm is no longer able to
carry out the work completely; then only the protoplasmic part

 
152 GENERAL PRINCIPLES OF ZOOLOG ¥.

Or

of the ege is divided, that which is rich in yolk remaining an
undivided mass. In this case one speaks of a partial cleavage in
comparison with the ordinary and more primitive mode, the total
cleavage; further, the eggs which show a partial cleavage are called
meroblastic, because only the segmented part of the egg is directly
employed in the formation of the embryo or bud (fAacros), while
the undivided main mass serves merely as food-material in the
course of growth. Eggs with total cleavage, on the contrary, are
called holoblastic.

Distribution of the Yolk.—The arrangement of the yolk is
connected with the position of the nucleus; either the egg-nucleus
maintains a central position and collects the yolk concentrically
around itself (rentrolecithal eggs) (fig. 97), or it is pushed, together

 

 

Fie. 97. Fig. Ys.

Fia. 97.—Centrolecithal egg. (After O. Hertwig.) n, nucleus; p, portion of the egg
rich in protoplasm; d, portion rich in yolk.
Fie. 98.—Telolecithal egg.‘ (After O. Hertwig.) Letters as in fig. 97.

with the greater part of the protoplasm, to one pole of the egg,
while at the other pole the yolk predominates ((elolecithal eggs).
Since the nuclear pole, in the course of development, always
becomes the animal pole, there can be distinguished in the egg an
animal part rich in protoplasm and a vegetative part rich in yolk
(fig. 98). In many telolecithal eggs the two regions pass gradually
into one another, but in others a distinct boundary separates an
almost purely protoplasmic animal portion from a yolk-containing
vegetative portion. This condition is well shown in the bird’s egg
(fig. 99). Here only the yolk is to be regarded as an egg in the
embryological sense, while the white, the egg-membrane, and the
calcareous shell are only later depositions upon the surface of the
egg. The chief mass of the yolk is deutoplasm, upon which rests
a thin layer of protoplasm, the germinal disc, always wppermost
GENERAL EMBRYOLOGY, 153

whatever the position of the egg. The protoplasmic layer contains
the egg-nucleus, and, after fertilization, by progressive develop-

 

Fie. 99.—Diagrammatic longitudinal section through a bird’s egg. (After Balfour.)
(1) The egg: IL, blastoderm; w.y., white yolk: y y., yellow yolk. (2) Coverings of
the egg: v.t., yolk membrane (vitelline membrane); ©. and w., inner and outer
layers of white; ch.l., chalazee; ison. and s.m., inner and outer shell-membrane;
between them at the right end is the air-chamber («.c.h.); s, shell.

ment continually separates itself (blastoderm) more and more

sharply from the underlying yolk.

Various Types of Cleavage.—After the foregoing remarks a
brief explanation will suffice to render intelligible the following
figures of the various modes of cleavage.

a, Holoblastic Eggs with Total Cleavage.

 

1. Lqual Cleavage.—The yolk, present only in small quantity,
is distributed equally through the egg; upon cleaving, the egg
divides into parts of approximately the same size and equally rich
in yolk (alecithal eggs, fig. 96).

2. Unequal Cleavage.—The yolk is abundant, but not in such
a quantity as to prevent complete cleavage; it lies especially at the
vegetative pole of the egg, causing the cleavage in this region to
progress more slowly; here larger cleavage spheres are formed,
because richer in yolk; hence the embryo, at the very first, is
found to be composed of smaller animal cells poor in yolk, and
larger vegetative cells rich in yolk (telolecithal, holoblastic eggs,
figs. 100 and 101). In some instances of unequal cleavage the
154 GENERAL PRINCIPLES OF ZOOLOGY.

A

 

Fie. 100.—Unequal cleavage of the egg of Petromyzon. (After Shipley, from Hat-
schek.) A, stage of eight cleavage spheres; B, blastula in meridional section.
The dissimilarity of the cleavage spheres begins with the equatorial furrow.

 

Fig. 101.—Unequal cleavage of a snail’s egg, Nassa mutabilis. (After Bobretzky.) I,
the first meridional furrow has divided the egg into unequal parts; II, the second
meridional furrow has formed three smaller and one larger cleavage sphere
(seen from the side); III, the equatorial furrow has formed four smaller animal
and four larger but unequal vegetative cells (seen from the animal pole).

cells are in straight lines (fig.
100, 4), but in others the cells
alternate (fig. 102); this is called
spiral cleavage and is common in
several groups.
6. Meroblastic Eggs with Partial
Cleavage.

3. Discotdal Cleavage.-—The
yolk is so collected in the vegeta-
tive portion of the egg that it
prevents cleavage; cleavage, there-
Fra. 102.—Spiral cleavage in Crepidula. fore, is limited to the region

ee around the animal pole and here
forms a disc of small cells, the anlage of the embryo, or dlasto-
derm (telolecithal, meroblastie eggs) (figs. 99, 103).

4. Superficial Cleavage.—The yolk is collected in the centre of
the egg and prevents cleavage; in consequence of this only the
outer layer of the egg divides into cells, which, in the form of a
continuous superficial layer, enclose the unsegmented central mass
(centrolecithal eggs) (fig. 104).

 
GHNERAL HMBRYOLOGY. 155

Distribution of the Types of Cleavage.—Of the four types of
cleavage mentioned the superficial one has an interest from the
point of view of the systematist, since it occurs exclusively in the
arthropods. The other modes of cleavage are distributed as fol-
lows: the discoidal has been observed in the majority of the
vertebrates and in the most highly organized molluscs, the

 

koi ee
Vic. 103.— Discoidal cleavage of the egg of a cephalopod (Loligo pealii).
(After Watase.)

&
we
Las
e
Se

  

Fic. 104.—Superficial cleavage of an insect egg (Pieris crategi). (After Bobretzky.)
A, division of the cleavage nucleus; B, movement of the nuclei to the periphery
to form the blastoderm; C, formation of the blastoderm.

cuttlefishes, while the equal and the unequal cleavage can be found
in all the groups of the Metazoa.

Blastula.—Sometimes during the first stages of segmentation,
sometimes later, there is usually formed a cavity, the cleavage or
segmentation cavity, between the cells in the interior of the ege;
with the progress of development this cavity becomes continually
larger (fig. 100, B). Around it the cells lie in the form of a one-
layered or of a many-layered epithelium and form the blastoderm;
hence the name for this stage, blastodermic vesicle, or, briefly,
blastula. The more yolk there is present, the smaller is the
cleavage cavity; in centrolecithal eggs with superficial cleavage it
is entirely absent.

 
GENERAL PRINCIPLES OF ZOOLOGY.

4. Formation of the Germ-layers.

Gastrula.—Besides the blastula there is still a second stage of
development, the gastrula or the two-layered embryo, which is
common to all the Metazoa. This stage is
understood easiest in the case of eggs which
have an equal cleavage (fig. 105, B); here it
has the form of a double-walled cup with a
wider or narrower mouth. The cavity of the
cup (the primitive digestive tract or archen-
teron) is the beginning of the most important
part of the digestive system; the opening is
the primitive mouth or dlastopore ( prostoma).
Of the two layers of cells forming the wall of
the cap and uniting at the blastopore, the
external is the ectoderm or outer germ-layer,
the internal the entoderm or inner germ-layer.
In the gastrula we meet for the first time

 

Fra. 105. — Gastrulation

of Amphiovus. (After

Hatschek.)Theanimal
pole here is above, and
the vegetative pole
below, in comparison
with fig. 93. In fig.
A the cells of the
vegetative pole are be-
ginning to sink in; B,
the invagination com-
pleted, the cleavage
cavity reduced to a
slit between _the ento-
derm (en) and the ecto-
derm (ck); 0, blasto-
pore.

the formation of germ-layers, 1.e., the forma-
tion of definite embryonic layers marked off
from each other, the cells not yet differen-
tiated, from which organs arise through
organological and histological differentiation.

Invagination.—The gastrula is formed
from the blastula by tvagination (fig. 105, A).
The result is the same as when by pressure of
the finger upon a hollow india-rubber ball

one side is pressed in against the other; the layer of vegetative
cells gradually sinks in and becomes surrounded by the cells of the
animal pole (fig. 105, B). Thus there arises in the egg, in addi-
tion to the cleavage cavity, a new cavity, the anlage of the lumen
of the digestive tract; this increases and finally obliterates the
cleavage cavity, so that the invaginated part of the blastoderm,
the entoderm, becomes pressed against the part which remains
external, the ectoderm.

Modified Modes of Gastrulation.—In the case of eggs with mueb food-
yolk the relation of the structure and of the mode of formation of the
gastrula is more difficult to understand. Here, however, it is sufficient to
mention the fact that the gastrula stage has fortunately been discovered
for almost all eggs with a great quantity of food-yolk, and that-the yolk-
material finds lodgment principally in the entodermal cells.

Epiblast and Hypoblast.—For outer and inner germ-layer the terms
epiblast and hypoblast, upper and lower germ-layer, have been much
GENERAL EMBRYOLOGY, Poe

used ; these names are strictly applicable only to those eggs with discoidal
cleavage. In the bird’s egg, for example, the two germ-layers form over
the unsegmented yolk, from which they become separated by the gas-
trular cavity ; thus, then, the external germ-layer actually lies above, the
internal below. Other terms for the two germ-layers are entoblast and
ectoblast.

Delamination.—In regard to the mode of development of the gastrula
many controversies have arisen which are not yet finally settled ; in addi-
tion to invagination there may exist a second, but very much less frequently

  

    

' x :

Fiac. 106.—Delamination of the egg of a Geryonid. (After Fol, from Korschelt-
Heider.) h, cleavage cavity ; y, jelly.

 

Srey

occurring mode of development, delamination. In delamination the
blastula may become two-layered by tangential division of its cells (fig,
106) ; each single biastoderm-cell, or, at least, the majority of these cells,
by this division falls into a peripheral ectodermic and a central ento-
dermic cell. In case of delamination the cleavage cavity becomes directly
the cavity of the digestive track, a fact which renders it difficult to
regard delamination and invagination as modifications of one and the
same process,

Formation of the Mesoderm. The Mesenchyme.—Many lower
animals, e.g. most ccelenterates, have in general only two germ-
layers. When these are laid down there begins immediately the
differentiation of muscle and nerve fibres and the other processes
of histological changes of the cells, as well as a series of changes
of form, by which the gastrula becomes the adult animal. In
higher organisms, on the other hand, before further differentiation
begins, there arises still a third germ-layer, which, owing to its
position between the first two, is called the mesoderm, mesoblast,
or middle germ-layer; this naturally can come only from the cell
material of the existing germ-layers, and indeed only the entoderm
seems to participate in it. Two methods can be distinguished in
its formation. In one the space between ectoderm and entoderm
becomes widened by the secretion of gelatinous substance, and
from the entoderm isolated cells push into this jelly; thus there
arises a middle layer, the mesenchyme (fig. 107), somewhat similar
158 GENERAL PRINCIPLES OF ZOOLOGY.

to gelatinous connective tissue, from which certain organs either
wholly or in part take their origin.

Mesothelium.—In the second case the mesoderm may preserve
the epithelial character of the two primary germ-layers, so that it

 

Fig. 107.—Formation of the mesenchyme and beginning of gastrulation in Holothuria
tubulosa, (After Selenka, from Balfour.) cv, cleavage cavity; ep, ectoderm; hy,
entoderm; ms, mesenchyme cells; ae, archenteron.

is called mesothelium. The mesothelium is cut off from the

entoderm, the mode of development being shown in the embryology

of the worm Sagitla (fig. 108).

 

Fic. 108.—Formation of the mesothelium and ccelom of Sagitta. 4. From the bottom
of the gastrula arise two folds, which divide the archenteron into the permanent
digestive tract and the ccelomie diverticula. B. The separation is almost com-
pleted by the pushing up of the folds. ak, outer, mk, middle, ik, inner germ-
layer; nie!, somatic layer; mie?, splanchnic layer; Ih, body-cavity.

 

Celomic Pouches.—When the gastrula of Sagiffa has been
formed two folds arise from the archenteric walls opposite the
Dlastopore (4), thus partially separating a pair of lateral chambers
from the rest. The process continues; the blastopore closes, while
GENERAL EMBRYOLOGY. 159

the entodermal folds extend to the opposite side, where they fuse
with the walls (2). In this way a pair of celomic pouches are
cut off from the rest of the archenteron which forms the lumen of
the digestive tract and its derivatives, while the walls of the
pouches form the mesothelium, that of the digestive region the
secondary entoderm. In each ceelomic pouch two walls are recog-
nizable, an inner or splanchnic layer which unites with the
entoderm to form the wall of the digestive tract, the splanchno-
pleure, while the somatic layer unites similarly with ectoderm to
form an outer body wall, the somatopleure. From the foregoing it
is evident that the mesothelium is strictly not a single layer, but
consists of two layers which pass into each other, and that its
origin is closely connected with the formation of the body cavity.

Occurrence of Mesenchyme and Mesothelium.—There are three
possible methods for the distribution of mesenchyme and meso-
thelium, and these actually occur. There are purely mesenchy-
matous animals, like the flat-worms, and purely mesothelial, like
Sagitta, many annelids, and Amphioxus; but there are also
animals in which the mesoderm consists of mesenchyme and
mesothelium: either the mesenchyme arises first and later the
mesothelium, as in the echinoderms, or the reverse order is fol-
lowed, as in most vertebrates.

Histological and Organological Differentiation.—A]1 the organs
of an animal arise from the three germ-layers in this way: first,
embryonic cell material is marked off into separate complexes,
usually by infolding (organological differentiation), and then later
these become changed into tissues (histological differentiation).
The details differ in the various animal groups; the following is
the most general: from the ectoderm arise the skin with its glands
and appendages, the nervous system, and the sensory epithelium;
the entoderm gives rise to the most important part of the digestive
tract with its glands; while muscles, blood, supporting and con-
nective substances, excretory organs, in whole or in part, arise in
the mesoderm; the sexual organs are also usually mesodermal.

Relations of the Germ-layers in Budding.—Of late the question has often
been raised as to how far the germ-layer theory is applicable to the occur-
rences in asexual reproduction. At first one would expect in budding,
and still more in the case of division, that each organ of the daughter
animal would arise from the corresponding organ of the maternal animal,
or, if that be impossible by conditions of space, from a mass of tissue
belonging to one of the same germ-layers. In many instances this is cer-
tainly the case, as, for example, in the budding of hydroids the entoderm
and ectoderm of the bud arise from the corresponding layers of the maternal
160 GENERAL PRINCIPLES OF ZOOLOGY.

body (fig. 90). But through recent investigations exceptions to this rule
have become known. In polyzoans and tunicates there are undifferen-
tiated cells which are employed in cases of budding; these are elements
without the characteristics of a definite body layer which, independently
of the position they assume in the maternal animal, can be employed,
according to need, in the building up of organs. In the regeneration of
lost parts investigations show that it is not necessary that the missing
structure, in worms and even in vertebrates, should be re-formed by the
same layer from which it originally arose. The lens of Triton arises
ontogenetically from the epithelium of the skin. If extirpated, it is regen-
erated from the pigmented epithelium of the iris.

5. The Different Forms of Secual Development.

Embryonic and Postembryonic Development. — While the
occurrences described (fertilization and cleavage of the egg, forma-
tion of the germ-layers) are going on the young animals are
usually enclosed within a firm protective covering, or even in the
maternal sexual apparatus (uterus), and are hence called embryos.
Later stages, even the formation of the most important organs,
may occur during embryonic life, as we see in case of the
mammals, birds, reptiles, many fishes, worms, and crabs, which,
at the end of their embryonic existence, are complete in all their
parts, and need only the maturity of the sexual organs, and growth
of the body as a whole, in order to reach the climax of their
development. On the other hand, there are animals, chiefly
aquatic, which, after leaving the egg, undergo important changes,
like the cceelenterates, echinoderms, insects, amphibians, etc. The
ccelenterates, echinoderms, and many worms usually escape from
the egg even before the formation of the germ-layers, and, as free-
swimming ciliated ‘planule,’ form the germ-layers and organs.
Since there is here a more or less extensive post-embryonic develop-
ment, it is a misnomer to apply the term ‘embryology’ to both
stages; it is necessary, rather, to limit the name to the develop-
mental processes inside the egg, and, on the other hand, to speak
generally of the history of the development of the individual, or
ontogeny. As the undeveloped animal within its membrane is
called an embryo, so the name larva is applicable to the free-living
but not completely matured animal.

Direct and Indirect Development—Metamorphosis.—Larval
development may be either direct or indirect. Tn direct develop-
ment, as the term implies, the larva pursues the direct way towards
the sexually mature animal, the lacking organs being outlined one
after another; hence it is continually becoming more like the

 
GENERAL EMBRYOLOGY. 161

sexually mature animal. In indirect development, on the con-
trary, organs belonging only to the larval life, and hence called
larval organs, are formed and later are destroyed. Therefore in
the definition of indirect development, or as it is commonly called
metamorphosis, special emphasis is laid upon the presence of larval
organs. Thus the caterpillars of butterflies are distinguished not
only by the absence of compound eyes and wings, but also by the
presence of anal feet and spinning-glands, which are absent in the
butterfly, and further by the different shape of the jaws, antenne,
and legs, the different arrangement of the trache» and nervous
system, ete. Tadpoles are distinguished from frogs not only by
the absence of lungs and extremities, but also by the presence of
gills and tail. The more numerous the larval organs, the more
pronounced, therefore, will be the metamorphosis.

Oviparous and Viviparous Animals.—The time at which the
egg leaves the mother’s body is independent of that at which the
embryo escapes from the egg membranes. Two extremes are
known, the oviparous or egg-laying animals, and the viviparous or
those which give birth to living young. Only those forms can be
considered as strictly ov/parous in which the egg at the time of
laying is a single cell, in which case it is either not fertilized until
after extrusion, as in the case of most fishes, sea-urchins, etc., or
during extrusion, as in batrachians and insects. In viviparous
animals, on the contrary, birth and the rupture of the egg mem-
branes occur quite, or almost, at the same time, and from the
mother there emerges an animal which has completed its develop-
ment or, at least, has progressed so far that it is able to hve with-
out protective coverings.

Ovo-viviparous Animals.—Varying degrees of ovo-viriparous develop-
ment connect these two extremes. What here appears at birth at first
impresses us, On account of its covering, as being an egg; but the first
stages of development have already passed, so that, by artificial rupture
of the egg membranes, an embryo more or less developed, but usually not
yet capable of independent life, is exposed. Birds really belong in the
category of ovo-viviparous animals, for their eggs are fertilized some time
before they are laid, and have already completed the formation of the
blastoderm, In the case of many worms the egg-shell may contain, even
at the time of laying, an animal all ready for hatching.

No Sharp Line between Oviparous and Viviparous.—Transitional forms
of this kind show that no sharp line can be drawn between ‘ egg-laying’
and ‘ bearing living young’ and one must guard against attributing too
much importance to the apparent distinctions. Linnzus, following the
example of Aristotle, was in error in regarding the time of birth as of
162 GENERAL PRINCIPLES OF ZOOLOGY.

systematic importance. In many divisions of animals oviparous as well
as viviparous forms are found. The majority of sharks are viviparous,
but a few species lay eggs ; on the contrary, for bony fishes the rule holds
that the eggs are laid before fertilization. Exceptions are the viviparous
surf perches, Embiotocide, of the Pacific coast and many Cyprinodonts of
fresh water. Most of the Amphibia, reptiles, and insects are egg-layers,
but not a few forms are viviparous. Even among the mammals, for
which for a long time the ‘bearing young alive’ was regarded as diag-
nostic, it has been discovered lately that the Hchidna and Ornitho-
rhynchus lay eggs. Finally, exceptions to the rule occur in one and the
same species. Adders commonly lay eggs, but under unfavorable condi-
tions they retain them inside their body until ready to hatch,

SUMMARY OF THE FACTS OF ONTOGENY.

1. The development of an animal begins with an act of genera-
tion; spontaneous generation and generation by parents are to be
distinguished.

2. Spontaneous generation (generatio equivoca, or spontanea;
abiogenesis) is the origin of living beings from lifeless matter
(without pre-existing organisms).

3. The present existence of spontaneous generation is neither
shown by observation, nor is it, on the whole, probable; yet spon-
taneous generation is a logical postulate, in order to explain the
first origin of life on our globe.

4. (reneration by parents (Tocogony), derivation of an animal
from an animal of the same or similar structure, can take place
either by the sexual or the asexual mode.

5. Asexual generation may be either by division or by budding.

6. In case of division an organism grows regularly in all its
parts, and by constriction falls into two or more equivalent new
pieces.

7. According to the direction of the plane of division in refer-
ence to the long axis of the animal we speak of longitudinal,
transverse, and oblique division.

8. In case of budding a local growth occurs; the local out-
growth, the bud, separates from the mother as a smaller, usually
incompletely formed, animal.

9. According to the position and number of the buds we dis-
tinguish lateral, terminal, and multiple budding.

10. Seawal reproduction is reproduction by means of special
sexual cells, which do not take part in the ordinary functions of
the body.

11. In sexual reproduction two kinds of cells unite, the female
egg and the male spermatozoon (fertilization).
GHNERAL EMBRYOLOGY. 163

12. In rare cases the egg develops without fertilization:
parthenogenesis; this is a sexual reproduction with degencrated
fertilization.

13. Peedogenesis is parthenogenetic reproduction by a young
(i.e., incompletely developed) animal.

14. Different modes of reproduction (asexual, sexual, partheno-
genesis, pedogenesis) may occur in the same species; then these
often occur in a regular order, and in such a way that individuals
with different modes of reproduction alternate with one another:
alternation of generations in the wider sense.

15. Alternation of generations in the strict sense (progressive
generation, metagenesis) is the alternation of two generations, of
which one reproduces by division or budding, the other sexually.
The former is called the nurse, the latter the sexual animal.

16. The alternation of parthenogenesis or pzdogenesis with
pronounced sexual reproduction is called regressive alternation of
generations, or heterogony.

17. Development which is inaugurated by sexual reproduction
shows in nearly all multicellular animals a general agreement in
the incipient stages: fertilization, cleavage, formation of germ-
layers.

18. The essential point of fertilization lies in the complete
fusion of egg and spermatozoon, particularly in the fusion of the
nuclei, egg and sperm nuclei, to form the cleavage nucleus.

19. The cleavage of the egg is a cell division, a division of the
fertilized egg into the cleavage spheres (blastomeres). The cleav-
age may be total (holoblastic egg) or partial (meroblastic egg);
total cleavage is either equal or unequal, the partial either discoidal
or superfictal.

20. By progressive division of the cleavage spheres, and by the
formation of a cleavage cavity, there arises a one-layered embryo,
the dlastula (vesicula blastodermica).

21. By the invagination of the blastula the gastrula or two-
layered embryo arises.

22. The gastrula contains a cavity, the primitive digestive
tract or archenteron, opening to the exterior through the blasto-
pore; it consists of two epithelial layers, the entoderm (hypoblast)
or the inner germ-layer, lining the archenteron, and the ectoderm
(epiblast) or outer germ-layer.

23. Between the inner and the outer germ-layer still a third,
the middle germ-layer, mesoderm, may be formed.

24. The middle germ-layer arises either by an infolding or
164 CENERAL PRINCIPLES OF ZOOLOGY.

a cutting off of a part of the entodermal epithelium, epithelial
mesoderm, mesothelium; or by the migration of separate cells to
form a gelatinous tissue (mesenchyme).

25. Many animals deposit their eggs before or shortly after
fertilization (oviparous); others lay eggs which have already been
fertilized in the maternal body, and at the time of laying have
passed through some of the stages of development (ovo-viviporous).
A third series of animals give birth to living young (viviparous).

26. The development of an animal is either direct or indirect
(metamorphosis).

27. Indirect development or metamorphosis is where the young
animal, as it comes from the egg, differs from the sexually mature
animal in two points:

(a) by the lack of certain organs which occur in the
sexually mature animals;

(2) by the appearance of organs, larval organs, which are
lacking in the sexually mature animals.

Ill. RELATION OF ANIMALS TO ONE ANOTHER.

General Relations.—Just as between the organs of one and the
same animal there exists a regular relation which is termed corre-
lation of parts, so also the different individuals of the animal
population stand in manifold and intimate reciprocal relations to
one another. Darwin has shown, in a great number of instances,
how the conditions of existence of many animal species are altered,
if other forms appear or disappear, or undergo an extraordinary
reduction or increase in number of individuals. Such reciprocal
effects are usually of a more special nature and can be understood
only by individual study; a few conditions are of wide occurrence
and are hence suitable for a general consideration; to such belong
colony and society formation, parasitism, and symbiosis.

J. RELATIONS BETWEEN INDIVIDUALS OF THE SAME SPECIES.

Colony Formation.—Colony and society formation are condi-
tions which exist between individuals of the same species. An
animal colony is a union of numerous individual animals by an
organic bodily connexion; the latter may arise in two ways: first,
by animals, originally separate, approaching one another and
partially fusing together; secondly, by individuals, formed by
division and budding, remaining united with one another instead
of separating. The first is extremely rare, and in the animal
kingdom plays no réle whatever.

 

 
GCOLOGY. 165

Colony Formation by Fusion.—Many Protozoa fuse with one
another and form larger bodies in which the individual animals
can still be recognized. Among the multicellular animals, that of
Diplozoon paradoxum (fig. 109) is the only case known where two
animals (Diporpa), sprung from different eggs, normally unite
into a double animal, which recalls certain double monsters, as, for
example, the Siamese twins.

 

Fic. 109.—Development of Diplozoon paradorum. (From Boas.) (1) Larva, from
which comes (2) ‘Diporpa.’ (3) Two Diporpe uniting. (4) The Diporpe have
united into Diplozoon. m, mouth; d, digestive tract; h, posterior adhering appa-
ratus; b, ventral sucking-disc, which serves for attachment to the dorsal cone, r.

 

Colony Formation by Incomplete Division and Budding.—In
general it can be said that the important instances of colony
formation rest upon incomplete asexual reproduction. An animal
forms new individuals by division or by budding, but the process
is not completed since the new generation does not separate from
the parent. There remain connexions of tissue uniting the buds
with the mother or the sisters with each other. The colonies of
marine hydroids and corals (figs. 91, 207) may consist of thousands
of individuals which, by repeated incomplete budding or division,
have sprung from a single sexually produced mother animal.

Community of Functions.—In the majority of cases the con-
nexion of the tissues results in a considerable degree of community
of functions. Stimuli which affect one individual are transmitted
by common nerves to the others of the colony; thus movements in
common are rendered possible. In a similar way the food captured
and digested by one animal serves for all. On account of the
community of its functions, a colony appears like a unified whole,
like an individual of a higher order; the same process which led to
the formation of multicellular organisms is repeated. Just as
there the elementary organisms, the cells, are united into a single
animal, so here the single animals are united into a colony.

Polymorphism.—When a whole is made up of numerous
equivalent parts, the conditions for division of labor are present.
Instead of the functions of the entire organism being distributed

 

 
166 GENERAL PRINCIPLES OF ZOOLOGY.

equally to the individual parts, many of the latter become employed
solely for this, others again solely for that function, and acquire a
corresponding structure. In case of such animal colonies one
speaks then of multiformity or polymorphism. Polymorphism
appears oftenest in connexion with the vegetative functions, lead-
ing to a distinction between sexual animals and nutritive animals,
as in the case of most Hydrozoa, where often nutrition is accom-
plished by animals without sexual organs, and reproduction is
carried on by animals without a mouth. But other functions,
movement, sensation, offence and defence, may also become
specialized. Siphonophores are the classical examples of poly-
morphism (fig. 110). Here united into a single body are locomotor

B

  

A

|\

Fia. 110.—Praya diphyes. (After Gegenbaur.) 4, the entire animal; B, a single
group of individuals greatly magnified (Hudoria). 1, covering scale; 2, nutritive
polyp; 3, nettle-threads; 4, sexual bell. .,

animals, the swimming-bells, serving only for locomotion; cover-

ing scales, which serve only to protect the others; nutritive polyps,
which alone take in and digest food; sexual animals and tactile
polyps, which are concerned only in sexual reproduction and with
sensation. In regard to the other functions each animal is related
GCOLOGY. 167

to its brothers and sisters; its very existence therefore has become
dependent upon these; the single individual can live only while a
part of a whole. Thus also division of labor leads to greater
centralization; the more polymorphic an animal colony ‘becomes,
the more unified it is, the more it gives the impression of being a
single animal instead of an aggregation of single animals.

In Social Animals the reciprocal dependence of the individuals
is much less, since here there exists no organic connexion, only a
voluntary communal life. As asexual reproduction is of impor-
tance in the case of colonies, so here the sexual plays a prominent
role. Under the influence of the sexual impulse, many animals,
even some of the lowest organisms, flock together, either per-
muanently or periodically; sea-urchins, sea-cucumbers, many fishes,
collect near the coast at the time of egg-laying. The sexual im-
pulse draws together herds of deer, elephants, etc. The care of the
young offspring further leads to a closer organization, to a society.
All insect societies are built up on this basis. Consequently,
since the sexual life is the starting-point of social life, it is further
comprehensible that, in the different groups of individuals forming
the community, the sexual organs may be influenced in their
development. Besides males and females (kings and queens)
there are still other animals with degenerated sexual organs
incapable of function, the workers; the latter are either only
females (bees and ants) or females and males (termites). While
the kings and queens give rise to the next generation, the workers
care for the young, look after the hive, provide food and protec-
tion, and also serve for defence, if the latter is not delegated to a
special class, the soldiers (termites).

II. RELATIONS BETWEEN INDIVIDUALS OF DIFFERENT SPECIES.

Causes of Close Relation.—Where individuals of different
species stand in close reciprocal relations to each other the cause
is to be found in the advantages which the one species derives from
the other, or which these both furnish reciprocally; the former
condition is called parasitism, the latter symbiosis.

Parasitism.—Parasites are animals which find their dwelling-
place upon or in another animal, the host, and obtain nourishment
from it. They have consequently come into a dependent condi-
tion and have undergone a more or less extensive change in their
organization.

True Parasitism.—The fact that an animal has settled down upon
another is not sufficient to characterize it as a parasite. There are many
163 GENERAL PRINCIPLES OF ZOOLOGY.

‘sedentary animals which, when opportunity offers, attach themselves toa
stone, a plant, or another animal; in such cases the term parasitism is a
misnomer, because it cannot be called a dependent condition. If a
hydroid fasten itself upon the back of a crab instead of on a stone, it is the
result of chance, in which the nature of the hydroid is in no way con-
cerned. The case would be different if the polyp were able to live only
upon the crab, and perished if in any other place. Such a dependent
condition usualiy occurs only when the mode of nutrition is also depend-
ent upon the place of abode; when the host not only serves for a dwell-
ing-place, but also furnishes the dweller with food ; when, consequently,
the dweller lives at the expense of the host.

Degeneration Caused by Parasitism.—The degree to which a
parasite has become dependent upon its host varies in the different
species; it is determined by the extent to which the parasite has
adapted itself to the organization of its host. Therefore it is
necessary in speaking of parasitism to consider the changes of form
which the parasitic mode of life has caused in the structure of
animals. These concern most immediately the organs of locomo-
tion and nutrition. Since a parasite needs to fix itself as firmly as
possible to the host, the locomotor apparatus more or less com-
pletely disappears and an apparatus for fixation to the host
becomes necessary; parasites of different groups are provided with
hooks, claspers, sucking-dises, etc. The blood, tissue-fluids, or
liquid food of the host furnishes nourishment to the parasite:
these are substances in solution which scarcely need digestion.
Usually, therefore, the digestive canal is simplified or quite dis-
uppears; among the parasites there are gutless worms as well as
gutless Crustacea. The mode of life of a parasite is also simpli-
fied, since it is no longer compelled to seek its food; in all parasites
the nervous system and sense-organs undergo a high degree of
degeneration; the former becomes limited usually to the most
indispensable portion; the latter, except those of touch, may
entirely disappear.

Modification of the Sexual Apparatus by Parasitism.—The
sexual apparatus, on the contrary, undergoes a strong develop-
ment. While it becomes easier for the parasite to maintain itself,
the existence of the species is more precarious. If a man die,
then most of his parasites die with him, especially those which
exist in the interior of his body. In order that a parasitic species
may not become extinct in a short time, it is necessary that the
eges be introduced into anew host. Since this transmission is

 

attended with difficulties, the parasites must produce an enormous
number of eggs. The eggs, too, are distinguished by great resist-
GCOLOG Y. 169

ing power and well-developed protective organs, such as strong
shells, etc. ; thus it is known, for example, that the eggs of Ascarids
continue to develop for some time in alcohol, being protected by
their impermeable shell.

Ectoparasites and Entoparasites.—All the above-mentioned
phenomena are more conspicuous in the case of parasites which
live inside of other animals, enfoparu-
sites, than in the case of the dwellers
upon the skin or other superficial
organs, the ectoparasites. In case of
entoparasites the transforming influ-
ence of parasitism is so considerable
that representatives of the most diverse
animal groups take on a remarkable
similarity of appearance and structure.
Pentastomum tenioides (fig. 112), for
example, belongs in the same class with
the spiders, the Arachnida, but in
external appearance it is entirely unlike
them, resembling the tape-worms (fig.
111). Hence for a long time all ento-
parasites, on account of their simi-
larity, were united into a_ single
systematic group under the name of
‘ Helminthes,’ comprising members of
the crustaceans, worms, and spiders, | F!¢- 11.
as well as animals of entirely different ee HENS ee nee
groups of the animal kingdom. Only Fic. 112.—Pentastomum tenioides

female. (After Leuckart.) h,
by embryology was the unnaturalness 20°Ks right and left of mouth ;
of this grouping recognized. Ento- into two oviducts, which unite

 

 

Fig. 112.

ov, unpaired ovary, branching
into the unpaired vagina (va):
parasitism therefore is one of the best tee pe
examples for illustrating convergent eno ee digestive
development, i.e., animals of different
systematic position acquiring, under similar conditions of life, a
great similarity of structure and appearance.

Symbiosis.—Less frequent than parasitism is symbiosis, or the
association of animals for reciprocal advantages. Social animals
frequently not only hold certain animals in bondage, but even seek
to protect and serve them; as, for example, in the company of
ants are found certain blind beetles, like Claviger (Myrmecophily),
or some species of plant-lice, or even ants of other species and
genera. But such cases of association correspond in part to the

 
170 GENERAL PRINCIPLES OF ZOOLOGY.

domestication of animals, or to slavery, as carried on by man.
The ants keep the plant-lice in order to lick the sweet juice which
is secreted in their honey-tubes; they steal the pup of other ants
and rear them, to use them later as slaves. This state of things
rests, consequently, not upon equal rights, since the one animal,
in the present example the ant, brings about the association, while
the other animal is passively led into it.

An instance of most complete equal rights and true symbiosis is
furnished us, however, by a hermit-crab and an actinian (fig. 113), Hupa-
gurus pubiscens and Hpizoanthus ameri-
canus. Like every species of hermit-crab
this also inhabits a snail-shell from the
opening of which only his legs and pin-
cers are protruded. Upon this shell an
Epizoanthus becomes attached and by
budding soon covers it with a colony of
Hird: 11— A ealony of Muiesantiine polyps. ‘‘ After thus covering the shell

americanus on the shell occu- it is not only capable of extending the

pied by a hermit-crab. (From os ‘ .

Verrill.) aperture by its own growth, but bas the

power of entirely dissolving and absorb-
ing the substance of the shell so that no trace of it can be found, though
the form is perfectly preserved by the somewhat rigid membrane of the
polyp.” The advantage which the actinian derives from this symbiosis is
clear: it gains a share of the food which the crab obtains. It is less
clear what the crab gains by the association ; however, the polyp is perhaps
a protection to him, by means of its batteries of nettle cells, while by
growth it increases the size of the ‘house’ occupied by the hermit and
thus saves him periodic changes of abode.

Occurrence of Symbiosis.—That animals in general rarely live sym-
biotically with one another rests mainly upon the fact that the conditions
of life of all animals to a certain point are similar or identical. They all
take in compounds rich in carbon and nitrogen, decompose them, and, in
the presence of oxygen, separate them into carbon dioxide, water, and
oxidation products containing nitrogen. All animals consequently are
competitors in the struggle for food. For the same reason, conversely,
symbiosis between plants and animals is not at all uncommon. In
particular there are certain lower algi, the Zooxanthellie, which often
live in animals. The radiolarians contain with such constancy in their
soft bodies green- or yellow-colored cells that for a long time these were
regarded as constituent parts of the animal. Quite similar yellow and
green cells inhabit the stomach epithelium of many actinians, corals, and
even of many worms. The Zooxanthell~ are nourished by the carbon
dioxide which is formed by the animal tissues, and breathe out oxygen,
which in turn serves as food for the animal; further, they form starch
and other carbohydrates, and there is nothing to prevent any surplus thus

eis

   
GCOLOGY. 171

formed from becoming food material for the animal. Thus there is on a
small scale that cycle of matter which exists on a grand scale in Nature
between the animal and vegetable kingdoms. By aid of chlorophyl and
of the chemical influence of sunlight the plants decompose water and
carbon dioxide and form from them oxygen, which they breathe out, and
compounds rich in carbon, which they store up in their tissues: they
are reducing organisms. On the contrary, animals give off carbon
dioxide and water, but take their oxygen from the air, and carbon com-
pounds in their food; they use oxygen to break down the chemical
combinations, to oxidize: they are oxidizing organisms. This explains
why the favorable influence of plants upon animals ceases immediately
when they change the character of their metabolism. With the disap-
pearance of their chlorophyl moulds and bacteria lose the power of reduc-
ing carbon dioxide; they derive their food from other organisms and
decompose this into carbon dioxide, water, etc. ; like animals, they are
oxidizing organisms, and consequently dangerous competitors. When
they establish themselves upon the animal body, they almost always work
injury to it ; hence in animals they are the cause of many extremely dan-
gerous ailments,

IV. ANIMAL AND PLANT.

 

Distinction between Animal and Plant.—The consideration of
symbiosis has led us up to the fact that a distinction exists between
plants and animals in the mode of metabolism, which may be
expressed thus: plants usually take in carbon dioxide and give off
oxygen, while animals breathe in oxygen and give out carbon
dioxide. ence it might be concluded that it is easy to discover
differences which generally obtain between plants and animals,
for, as a matter of fact, the laity are never in doubt in deciding to
which realm of nature the more highly organized animals and
plants, which are the only ones known to them, belong.

Doubtful Cases.— But the more one studies this question, the
more difficult becomes its solution. The old zoologists indeed
formed the conception that there are organisms which stand on the
limits between the animal kingdom and the vegetable, and Wotton
named these directly zoophytes or plant-animals. Now we know
that Wotton’s plant-animals are true animals with but a superficial
similarity to plants; but, by means of the microscope, we have
become acquainted with numerous lower organisms, and it is still
doubtful in which of the two realms of nature these belong. As
such may be mentioned the Myxomycetes and many Flagellata.

 
172 GENERAL PRINCIPLES OF ZOOLOGY.

 

Physiological Distinctions.—I{ one wish to discover sharp
distinctions between animals and plants, he may take into con-
sideration on the one side physiological, on
the other morphological, characters. Start-
ing from the physiological point of view,
Linnezus ascribed to plants only the capacity
of reproduction and nutrition, but to animals
the power of motion and sensation in addi-
tion. However, we know that vegetable,
like animal, protoplasm is irritable and is
capable of movement, as is shown by the
active movements of the lower Algz, the
great sensitiveness of the Mimosa, and other
Fic. ld.— Lepas anati- plants; but further, we know that even many

fera. (After Schmar- a ‘ ‘

da.) c, carina; t, ter- of the more highly organized animals, e.g.,

no ee Crustacea (fig. 114), lose the power of loco-
motion and become fixed, and many fixed forms, e.g., the sponges
(fig. 84), even under the closest examination appear immovable
and unaffected by stimulation; thus we are led to abandon the
idea that the so-called animal functions are to be regarded as
accurate distinctions.

Metabolism not a Safe Criterion.—Even the difference in met-
abolism is by no means sufficient. Every plant has a double
exchange of material. In its movements and other vital functions
the vegetable protoplasm produces carbon dioxide and consumes
oxygen; at the same time there goes on, under the influence of
sunlight and of chlorophyl, the reduction of carbon dioxide and
the giving off of oxygen. In chlorophyl-containing plants the
reducing process preponderates so considerably during the day
that there is evident, as the final result, the giving off of a greater
quantity of oxygen, and only at night, when the reducing process
becomes interrupted on account of the lack of sunlight, does the
production of carbonic-acid compounds become perceptible. But
the reducing processes become immediately preponderant if the
chlorophyl be absent; chlorophylless moulds and bacteria have,
therefore, the same metabolism, so far as carbon dioxide is con-
cerned, as animals.

Cellulose not a Sure Test.—So also it is incorrect to say that
only plants have the power to make cellulose, for cellulose is found
in many lower animals, the rhizopods, and in the highly organized
group of tunicates; according to recent investigations it appears
to be found even among the arthropods.

 

 
2

GCOLOGY. Li:

G

 

Morphological Distinctions.—Turning to the morphological
characteristics, multicellular animals and multicellular plants are
readily distinguished, since the former in the germ-layers have a
principle of cell arrangement peculiar to them. With the appear-
ance of the gastrula each organism is undoubtedly an animal.
But in unicellular animals the arrangement of the cells is lacking,
and only the constitution of the single cell guides us. Now are
there unmistakable morphological differences between the animal
and the vegetable cell ?

Plant-cells have a Cellulose Membrane.—In the structure of
plant and animal cells an important distinction is found in the
fact that the former has a cellulose membrane, but the latter is
usually membraneless. To this distinction must be referred in the
last analysis the widely different appearance of the two realms.
Since the plant-cell is early surrounded with a firm coat, it loses a
large part of its power of further changing its form; hence vege-
table tissues and organs are uniform in comparison with the incon-
ceivable multiformity which animal histology and organology
disclose. The numerous higher stages of organization which the
animal kingdom reaches, even in its lower classes, is in great part,
indeed, the result of the fact that the cells of animals do not
become encapsuled, but have preserved the capacity for more
varied and higher development.

Transitions.— But even here transitions are found between the
lower plants and animals. In the lower
Alge the cells have power to emerge from
their cellulose membrane, and to swim
about freely (fig. 115), before they encapsule
themselves anew. On the other hand, most
unicellular animals encyst; they pause in
their ordinary functions of hfe, become
spherical, and surround themselves with a
firm membrane, in some cases even of cel-
lulose. Since in both cases an alternation
between the encapsuled and the free-living
condition occurs, only the longer duration
of the one or of the other can lead to a pre. y5—Cedogoniun in
distinction. But here occurs the possibility  Pore;formation. “After

Sachs.) .A, a piece of

: a ¢ J; a : _ the filament of the alga
that undifferentiated intermediate forms ex- tn escaping cell-con.

ist; their actual existence prevents, even yet, tents: B. Zoospore

har istincti animal anc tents; C, zoospore fixed
a sharp distinction between the animal and — tent ne
vegetable kingdoms.

 

 

 
174 GENERAL PRINCIPLES OF ZOOLOGY.

V. GEOGRAPHICAL DISTRIBUTION OF ANIMALS.

The Different Faunal Regions.—Even a superficial knowledge
of the mode of distribution of animals shows that the animal fauna
in different regions of the earth has an essentially different char-
acter. In part this difference of fauna is the immediate result of
climatic differences. The polar bear, arctic fox, eider-ducks, and
many aquatic birds are restricted to the polar zones, because they
cannot endure more than a certain degree of warmth; on the other
hand, the larger species of cats, the apes, the hamming-birds, etc.,
occur only in tropical or sub-tropical regions, because they are not
sufficiently protected against cooler weather.

Climate not the Only Factor.—If climate were the sole factor
determining distribution, the faunal character of two lands which
have similar climatic conditions would be essentially the same;
conversely, the separate regions within a continuous territory
extending through several climatic zones must have quite different
faunas, according as they are nearer the equator or the poles. But
such is not the fact; two tropical countries may differ more widely
in the characteristics of their fauna than the hot and cold regions
of one and the same country.

Factors in Distribution.— Modern zoology endeavors to explain
these peculfar conditions by regarding the present distribution of
animals as the product of two factors: the gradual changes of the
animal world, and further the gradual changes of the earth’s sur-
face on which the animals are distributed. The history of the
earth as disclosed by geology shows two facts: (1) that the con-
nexions between parts of the earth have varied greatly; that, for
example, at a time when the Mediterranean had not yet reached
its present extent, Morocco, Algiers, Tunis, and Egypt were more
closely united with the European coast of the Mediterranean than
with the southern part of the African continent separated from
them by the Sahara; (2) that considerable variations of climate
have taken place: there prevailed in Europe in the tertiary period
a subtropical climate which rendered possible the existence of
animals which now occur in Algeria (lions). But later a glacial
period began, which introduced over a wide area of the European
continent the conditions of arctic life, and consequently a fauna
of northern animals (reindeer). Hand in hand with the geological
changes went changes in the animal world, the then existing
species dying out under the change of conditions, or forming new

 

 
DISTRIBUTION. 175

species through gradual variations. Thus the distribution of
animals constitutes an extremely complicated problem, the solu-
tion of which necessitates comprehensive preliminary work. It
must be known with certainty how the connexions between the
continents and the climates have changed, particularly in the
later geological periods; further, we must study, not only how
animals are distributed over the earth’s surface at the present time,
but also how they were distributed in earlier times. Finally, by
means of comparative anatomy and embryology we must have clear
and detailed ideas of the relationships and interrelationships of
animals.

It will be an extremely long task to solve all the problems of
the subject here sketched in outline. What has been investigated
thus far can only be regarded as a preliminary proof that zoology
with its prevailing views of the changes of animals and of the earth
is on the right track. It would be a test of the correctness of this
view if it were proved that the faunal resemblances of two countries
depends, in the first place, upon how long they have been in close
connexion with each other, consequently allowing an interchange
of the animals inhabiting them. Two regions, separated early in
the earth’s history and never again connected, must have greater
differences in faunal characters than two lands still connected or
only recently separated. It is instructive when we travel in the
northern hemisphere and find in widely separated regions strik-
ingly similar faune, while under the equator or in the southern
hemisphere under the same conditions striking differences are seen.
This is explained on the hypothesis that in all past periods as now
the land masses of the northern hemisphere have been closely con-
nected, while the parts of the continents extending to the south—
aside from hypothetical temporary connexions between South
America, Africa, and Australia—have been separated through
most of the earth’s history.

In carrying out more closely the points of view mentioned,
students of distribution have attempted to mark off the great
faunal areas of the earth, the faunal provinces or regions, and
within these again less important divisions, subregions. These
provinces have been based chiefly upon the distribution of
mammals, less upon that of birds and other animals; for the dis-
tribution of mammals is chiefly determined by those changes of
the earth’s surface which are best known geologically and possess
most interest. Elevation or depression of the earth’s surface often
opposes impassable barriers to most mammals: rising, if it lead to

 
176 GENERAL PRINCIPLES OF ZOOLOGY.

the formation of glaciered mountain-chains; sinking, when arms
of the sea are formed, which, even if only narrow, interpose
between two hitherto connected land areas straits which are
impassable for most mammals. Birds and insects which fly well
are less affected by all such changes of the earth’s surface; the
majority of them can fly over arms of the sea and mountain-chains,
for there are birds which can even cross the Atlantic Ocean.

The Six Primary Regions.—Of the systems of animal geography
proposed up to the present time, the divisions advocated by
Sclater and Wallace finds most favor. These English scholars
distinguish the six following primary regions: (1) the palwarctic,
comprising all Europe, northern Africa as far as the Sahara, and
northern Asia as far as the Himalayas; (2) the Ethiopian, all of
Africa south of the Sahara; (3) the orvental, including upper and
farther India, southern China, and the western Malay Islands;
(4) and (5) the nearctic and the neotropical regions, which make
up the American continent and are divided by a line drawn at
about the northern border of Mexico; (6) the Australian, in
which, besides Australia itself, are included the larger and smaller
islands of the Pacific Ocean and the eastern Malay Islands, east of
Celebes and Lombok.

(1) The Australian region is most sharply distinguished from
all the others and by many is set apart as a distinct division called
‘Notogea.’ Its isolated geographical position together with the
fact that it has long been separated from other countries
(apparently since the beginning of the tertiary) explains the fact
that only the oldest mammals, the monotremes and marsupials,
have entered the region, while the placental mammals have not
been able to follow. While the marsupials, which in the secondary
period also inhabited the northern hemisphere, were replaced there
in tertiary times by the placentals, they were able to develop
farther in the Australian region. Australia and the adjacent
islands are thus the land of marsupials, which have persisted else-
where only in South America (Cenolestes, Didelphide), the
opossum ranging north into the United States. On the other
hand, at the time of discovery Australia lacked all placental
mammals except those (whales, dugong, seals, bats) which were
not restricted by water and the Muride, easily transported on
floating wood. Two larger mammals, the wild dog or dingo
(Canis dingo) and the pig of New Guinea (Sus papuan ws), may
have accompanied man, this being the most probable for the dingo
in spite of the fact that his remains oceur in the pleistocene along

 
. DISTRIBUTION. 177

with those of the giant marsupials. Further peculiarities of the
Australian region are the birds-of-paradise in New Guinea, the
egg-laying mammals Ornithorhynchus, Echidna, and Proechidna,
and the cassowaries and the Australian ostrich (Dromeus nove-
hollandie).

It is easily understood that the isolated island groups of the
South Sea (Polynesia) have developed many faunistic peculiarities,
as well as that an exchange of forms may have taken place between
the islands of the oriental province and the islands faunally related
to Australia, and that * Wallace’s Line’ is not so sharp a boundary
as it was once thought to be (extension of marsupials into Celebes,
of placentals into the Moluccas). On the other hand the distinct-
ness of New Zealand needs mention. It is distinguished from
Australia by a large number of peculiar birds (Apteryx and the
extinct Dinornithide), reptiles (the ancient Sphenodon), and
molluses. If the bats and mice—unimportant in matters of dis-
tribution—hbe excepted, New Zealand lacks all native mammals,
even marsupials.

(2) The neotropical province (South and Central America) is,
next to Australia, the most sharply characterized, and, like that
region, has been set aside as a special division + Neogea,’ especially
when considered with reference to its geological history, which
shows that during the cretaceous and early tertiary time it was
separated from North America by the sea and had developed a
peculiar fauna (e.g., gigantic edentates, no carnivores). These
peculiarities disappeared towards the end of the tertiary by the
entrance of carnivores and ungulates from the north and an
extension of the edentates to the northern hemisphere. To the
Neogea belong the platyrhine apes, the catarrhine to the Old
World. Characteristic edentates are the armadillos, sloths, and
ant-eaters; the marsupials are represented by the opossums and
Cenolestes; among birds the hamming-birds, toucans, the peculiar
Cotingide, Tanagride, Tinamous, Palamedide, Rhea, etc. The
almost entire absence of insectivores and the considerable develop-
ment of rodents (cavies, agoutis, chinchillas) are noteworthy.

The four remaining provinces are still closely connected
geographically and form a third great division, * Arctogma,’ charac-
terized by the entire absence of platyrhine apes, monotremes, and,
except the North American opossum, of marsupials. In the
secondary and tertiary times the northern parts of these lands were
connected and an interchange of faunas occurred, this being the
easier on account of the extension of the warm climate to the far
178 GENERAL PRINCIPLES OF ZOOLOGY.

north. Hence many unite the palearctic and nearctic provinces
into a ‘holarctic’ province.

(3) The nearctic region has peculiar to it three mammalian
families, the prong-horned antelope, the opossums, and the
Haplodonte; of the group of Amphibia, the Sirenid# and Amphi-
umide. The Nearctic is to be distinguished from the nearest
related palearctic region through the crowding in of neotropical
forms like the raccoon, opossum, humming-birds, etc.

(4) The palearctic region covers the greatest area and conse-
quently abuts upon many other provinces. Hence there exist on
the one side important differences between the various local
faunas, which are conditioned by climate and great distances, but
on the other it explains the fact that the palearctic region has no
peculiar families. The families which here have reached a great
development are the deer, cattle, sheep, and camels; especially
conspicuous genera are the chamois, squirrel, badger, and marmot.

(5) The Ethiopian region has many animals found only there;
among these the hippopotamus and giraffe, the aardvark, and, if
we include Madagascar, the lemurs are most characteristic. To
these are added a rich development of antelopes and zebras and
the gorilla and chimpanzee. Equally noteworthy is the entire
absence of striking families and genera, such as the bears, moles,
deer, goats, tapirs, sheep, the true cattle and swine, provided they
have not been domesticated and introduced.

Within the region the island of Madagascar occupies a remark-
able position. This island is the land of lemurs and Insectivora;
no land is so rich in lemurs, as the majority of the genera live
exclusively in Madagascar. On the other hand the large beasts
of prey. the cats, hyenas, dogs, and the bears (which, however, do
occur in Africa), all the true apes, antelopes, elephants, and the
various species of rhinoceros are absent. Consequently, since
Madagascar is distinguished quite conspicuously from Africa, many
zoologists separate the island from the Ethiopian region; many
even give it the rank of an independent province.

(6) The oriental region contains, next to Madagascar, the most
Jemurs; among which the Tarsid and Galeopithecide (the latter
often considered an insectivore) are exclusively oriental. Remark-
able inhabitants of the province are the gibbons and orang-utans,
the musk-deer, numerous families and genera of birds.

Arctic and Antarctic Provinces.—Of late the view has gained
ground that, besides these six, two other, cireumpolar, provinces
must. be distinguished, the arctic and the antarctic. Both have a
DISTRIBUTION. 179

fauna consisting of few species but numerous individuals, of which
the auks, polar bear, reindeer, and arctic foxes are characteristic of
the northern or arctic region, the penguins and the entire absence
of land mammals of the antarctic.

The Distribution of Aquatic Animals.—Since most seas are
connected, the faunal regions cannot be distinguished so sharply
as in the case of the land faunas; conspicuous differences are
present only when two oceans are separated by continents extend-
ing far to the north and south; such, for example, exist between
the Red Sea and the geographically neighboring Mediterranean,
between the east and west coasts of North America, even where
they are separated only by the narrow isthmus of Panama. Then,
too, considerable differences may exist where currents of greatly
different temperatures meet.

Changes in the Fauna Conditioned by Depth.— Much more
remarkable in the marine fauna are certain differences brought
about by the changes of the conditions of life in the different
depths of the sea. <A deep-sea fauna, a coast fauna, and a pelagic
fauna can be distinguished. The coast fauna embraces the
animals, some free, some fixed, which inhabit the plant-covered
rocky or sandy shore to a depth of a few hundred feet. The
deep-sea fauna swims, creeps, or is fixed at the bottom of the
ocean at depths of 1000 to almost 9000 meters; it is distinguished
from the coast fauna in part by its archaic character, for here very
often genera and entire groups of animals exist, like the Hexac-
tinellide, crinoids, certain starfishes and sea-urchins, etc., which
for a long time were chiefly known through fossils from earlier
geological ages.

The Plankton.—The pelagic animal world comprises all forms
which swim freely in the water, the ‘plankton’; here belong many
celenterates, medusx, and ctenophores, entire groups of Protozoa,
like the radiolarians, many Crustacea and crustacean larvae; of the
molluses the heteropods and, pteropods. These animals live either
at the surface of the sea itself or floating at greater or lesser
depths, to 8000 meters or even more. Usually they are gelatinous
and of glasslike transparency; this must be regarded as sympa-
thetic coloring and adaptation to the transparency of the water.

Distribution of Fresh-water Animals.—In fresh water two
groups of animals must be distinguished, of which the one com-
prises mainly the more highly organized forms, the molluscs,
fishes, and Crustacea, the other the ower animal world. The
distribution of the former is mainly determined by the same factors

 

 
180 GENERAL PRINCIPLES OF ZOOLOGY.

which influence terrestrial forms; the distribution of the latter,
however, is cosmopolitan. The same infusorians and rhizopods,
copepods, fresh-water sponges and polyps which occur in America
seem to be distributed over the entire earth. This is connected
with the fact that all these animals have resting stages in which
they endure desiccation. The resting stage, be it as a hard-shelled
egg or as an encysted animal, may be borne about by the wind, or
may be carried with the mud by aquatic birds, and upon again
reaching the water resume its active state.

VI. DISTRIBUTION OF ANIMALS IN TIME.

It is the province of a special science, paleontology or paleo-
zoology, to treat of the character and distribution of animals in
the earlier periods of the earth’s history, but since it is necessary
to draw upon paleontological facts to understand the living forms
an outline of the geological periods with the characteristic animals
may be given here.

I. Azorc ok ARCHEAN ERA.

No organisms are certainly known from this age. The animal
nature of Hozoon canadense of the Laurentian beds, once referred
to the Foraminifera, is more than doubtful.

II. Patmozorc Era.

1. Cambrian. 4. Carboniferous.
2. Silurian. 5. Permian.
3. Devonian.

The oldest paleozoic period, the Cambrian, contains only
invertebrate fossils: trilobites, gigantostraca, cystoids, nautiloids,
gasteropods, and a few lamellibranchs. — Trilobites, cystoids,
gigantostraca, and the blastoids and tetracoralla, which appear in
the Silurian, reach their culmination and become extinct in the
paleozoic. Fishes appear in the Silurian, and acquire a great
development in the Devonian. The earliest Amphibia come from
the carboniferous, the reptiles appear in the Permian.

III. Mersozotc Era.
1. Triassic. 2. Jurassic. 3. Cretaceous.

The mesozoic era was the age of reptiles, which were repre-
sented by numerous forms, some of gigantic size; most of them
DISTRIBUTION. 181

becoming extinct in the cretaceous. The first mammals appear in
the triassic, the birds in the Jurassic. Among the invertebrates
the ammonites, which appeared in the Devonian, reached their
greatest development and became extinct in this era.

IV. Crenozoic Era.
(a) Tertiary.

1. Eocene. 3. Miocene.

2. Oligocene. 4, Pliocene.
(6) Quaternary.

5. Pleistocene. 6. Recent.

In the tertiary all of the now living orders of mammals and
birds uppeared, among them man, whose remains have been traced
with certainty to the pleistocene.
SPECIAL ZOOLOGY.

SINCE comparative anatomy and the theory of evolution have
made their impression upon systematic zoology one recognizes in
classification not only a means of arranging the species, but also
the possibility of expressing the relations which the larger and
smaller groups bear to each other. The solution of these prob-
lems demands an accurate knowledge of comparative anatomy and
embryology and a complete knowledge of animal forms based upon
them. We are yet far from such a knowledge, farther with regard
to some groups than others, and as a consequence systematic prob-
lems are not all equally advanced towards solution.

In general it may be said that certain natural groups are
recognized: (1) Chordata; (2) Mollusca (after the elimination
of the Brachiopoda); (3) Arthropoda; (4) Echinoderma; (5)
Celenterata (after the separation of sponges); (6) Protozoa. On the
other hand, it is yet uncertain exactly how to regard the worms,
brachiopods, polyzoa, and a few other forms. The general ten-
dency is to distribute the worms into at least three branches (flat
worms, round worms, and annelids) and to unite the Polyzoa and
Brachiopoda in a branch of Molluscoida. In this way groups poor
in species and of little importance in a general account of the ani-
mal kingdom are placed on the same basis as the large and exceed-
ingly important groups of vertebrates, arthropods, and molluscs,
and thus obtain, especially in the eyes of the beginner, an impor-
tance which does not belong tothem. It therefore seems better in
an elementary work to pursue a more conservative course.

1

D

9
PROTOZOA. 183

PHYLUM I—PROTOZOA.

All of the Protozoa are small; some may be seen by a sharp eye
as minute points, but the majority are so minute that they are
invisible except with a microscope. On the other hand, there are
afew which have a diameter to be measured by millimeters, this
being especially the case where hundreds of individuals are united
in colonies.

This small size is a necessary result of the fact that the Protozoa
are single-celled animals. Like all cells they consist of that pe-
culiar substance, protoplasm, and they have the further cell attri-
bute, the possession of one or more nuclei. Being unicellular, it
follows that they lack true tissues and true organs. They lack
alimentary canal, nervous system, sexual organs, etc. The funda-
mental functions of nourishment, sensation, movement, and repro-
duction are performed more or less directly by the protoplasm.

In nutrition, in so far as it is not produced by substances in
solution, foreign particles pass into the protoplasm and are digested
by it. They usually lie during digestion in special collections of
fluid, the food vacuoles (figs. 120, 144, etc, na), more rarely in the
protoplasm itself. All indigestible portions are cast out after a
time. This taking in and casting out of foreign matter can take
place in the lower Protozoa at any point of the surface, while in
the more highly organized species there are definite openings which
according to analogy with many-celled animals are spoken of as
mouth and anus, or more precisely, cyfostome and cytopyge. The
mouth may connect with a tube, the esophagus or cylopharynz,
which ends free in the protoplasm.

Structures may occur within the protozoan cell which recall
the organs of higher animals, and which are called cell organs.
While motion is usually produced by the protoplasm and its pro-
cesses—pseudopodia, flagella, and cilia—there are Protozoa, like
Stentor and the Vorticellide, which have true muscular fibrille.
The sensitiveness to light is often increased by the formation of an
eye spot, a small pigment body in which even a lens may occur.
More “constant of cell organs are the contractile vacuoles (fig. 116,
etc., cv), structures rarely absent from fresh-water species, but
commonly lacking from marine forms. These are distinguished
from the food vacuoles by three characters: they have a definite
184 PROTOZOA.

place in the cell; their number is approximately constant in most
species; they exhibit extremely constant phenomena. The walls
contract and empty the fluid contents to the exterior, often
through a special duct. When one empties it completely disap-
pears and is formed again anew in a short time, and is filled with
fluid from the surrounding protoplasm. It thus resembles the
contractile vacuoles in the water vascular system (excretory organs)
of the worms to be described later. Apparently the contractile
vacuoles are for the elimination of injurious substances in solution
produced by the vital processes, among them possibly carbon
dioxide, like a respiratory organ.

Apparently all the vital functions are under the control of the
nucleus. Experiments show that Protozoa, artificially deprived of
their nuclei, perform their functions incompletely and soon perish,
while fragments containing a nucleus remain alive. Young Pro-
tozoa usually have a single nucleus, and many have but one
throughout life; but others early become multinucleate, Such
multinucleate forms are frequently regarded as cell complexes or
syncitia, but unnecessarily, for aside from the fact that in animal
and plant histology polynucleate masses of protoplasm are regarded
as cells, this term makes a distinction between the uni- and the
multi-nucleate forms, which does not correspond to the actual
relations, since the phenomena of both are completely alike.

Reproduction is accomplished exclusively by fission or budding,
and under suitable conditions, such as abundance of nourishment,
occurs so rapidly that many Protozoa inside a few weeks can
number their descendants by millions. Many divide in the free
state while they are creeping or swimming about; others become
encysted before division. They become spherical and secrete a
protecting membrane around themselves (figs. 121, 122). En-
cysted individuals usually divide into more than two pieces, in
four, eight, or even many hundreds of reproductive bodies. It fre-
quently happens that multinucleate species divide into as many
parts as there are nuclei.

In the Protozoa may occur a fusion of individuals—conjuga-
tion—which in many respects has much similarity to the process
of fertilization in Metazoa and in plants. In some (conjugation
of many Rhizopods) this does not correspond to trne fertilization,
since only the protoplasm unites (plastogamy), while the fusion of
nuclei (caryogaimy) necessary to fertilization does not occur. In
others a fusion of nuclei takes place. In the eases which have
been accurately studied there has been seen, before the fusion of
PROTOZOA. 185

the nuclei, a process comparable to the formation of the polar glob-
ules in the egg, to this extent, that in each of the conjugating
individuals the nucleus divides twice and of the products of divi-
sion only one, the nucleus intended for caryogamy, persists while
the others (polar globules) degenerate.

These cases of true fertilization permit of great diversity. The
conjugating individuals can be equal in size (most Infusoria, many
Rhizopoda), or there is a disparity in size (sexual dimorphism), in
which smaller and consequently more mobile ‘males’ (microga-
metes, zoospores) fertilize the larger fixed or slowly moving
‘females’ (macrogametes, oospores) as in Volvow globator, Vorti-
cellide, and many Sporozoa. In conjugation of individuals of
equal size there is frequently a mutual fertilization—A fertilizes
B, and is in turn fertilized by B—after which the animals sepa-
rate (most Infusoria, Gregarines, Noctiluca).

 

Twenty years ago it could be laid down as a universal fact that the
Protozoa in contrast to the Metazoa lacked sexuality. In the mean time
observations on Protozoa belonging to different classes, even the Rhizop-
oda, have so increased that the conclusion is that fertilization occurs in
all Protozoa, although the rarity of the process in many species renders
the complete demonstration difficult. Still there remain certain interest-
ing differences from the Metazoa. The Protozoa lack special sexual cells
—eggs and spermatozoa. On the contrary, the whole body functions as a
sexual cell. Further, the relations of fertilization to reproduction are not
the same as in the Metazoa. It does indeed occur (swarm-spore formation
in Noctiluca, formation of pseudonavicelle in gregarines) that fertilization
precedes rapid division, but much more commonly fertilization is the
result of rapid division and a cause of slower reproduction (Jnfusoria) or
even of complete rest (Actinospherium, Actinophrys, Volvow). One can
therefore only speak of fertilization, not of sexual reproduction, in the
Protozoa. These facts are of great importance in the consideration of the
vature of impregnation, for they show that it has not only the purpose of
stimulating the developmental processes, but that it accomplishes other
functions, and that these functions, obscure as they at present are, are
the more important since they are the more primitive and the more widely
distributed.

With Noctiluca, many Sporozoa, and perbaps in Rhizopods a period
follows impregnation in which the division (‘sexual reproduction’) has a
special character (swarm-spore formation in Voetiluca, formation of sporo-
blasts and sporozoites in the Sporozoa) and differs from the customary
‘vegetative’ reproduction. This alternation of methods of reproduction
recalls the alternation of generations of the Metazoa and is called by the
same name.

The Protozoa with thin small and soft protoplasmic bodies are
but little if at all protected, against drying up, and therefore they
186 PROTOZOA.

are aquatic. A few forms, like Ameba terricola, are terrestrial, and
these only occur in moist places. Salt and fresh water, of the latter
stagnant pools rich in vegetation, are the favorite places for Pro-
tozoa. The fresh-water forms are cosmopolitan, so that the forms
in the most diverse lands are very similar. This depends upon cer-
tain peculiarities. The fresh-water Protozoa can become encysted
independent of reproduction, and in the encysted stage can endure
times of unfavorable conditions such as lack of food, freezing, or
complete evaporation of the water. When thus protected they
may be blown about by the wind or carried far on the feet of
birds. Hence it is that one group bears the name Infusoria, for if
dry earth or dry plants (¢.g., hay) be soaked in water and this
infusion allowed to stand for some time, a more or less rich
Protozoan fauna will develop in it. The encysted animals in the
earth or on the plants are awakened by the moisture to new life
and leave the cyst. Spontaneous germination, as was once believed,
does not occur here, for if one sterilize the materials and prevent
the entrance of germs the water will remain uninhabited.

Historical.—On account of their practical invisibility the Protozoa were
unknown until 1675; they were discovered in infusions by the Dutch
Leeuwenhoek, the discoveror of the microscope. Wrisberg in the eight-
eenth century called them Animalcula infusoria—infusion animals, and
Siebold in the century just closed gave them the name Protozoa. The
proposition of Haeckel to place a portion of the Protozoa in a kingdom
Protista between animals and plants has found but little acceptance. In
the accounts of the structure the views of Dujardin and Ehrenberg were
long at variance. Ehrenberg maintained with all confidence that the
Protozoa like all animals possessed the most important organs, alimen-
tary canal, nervous system, muscles, excretory and sexual organs. Du-
jardin denied all this and ascribed to the Protozoa only a single homo-
geneous substance, ‘sarcode’ (p. 60) which was sufficient to produce all
vital phenomena. Dujardin’s view later found important support in
Siebold’s discovery that the Protozoa were unicellular. Still for a long
time Ehrenberg’s ideas persisted in various modified forms and were not
totally overthrown until after the middle of the nineteenth century. The
fact that there are unicellular animals without organs and yet capable of
existence was an extremely valuable addition to knowledge, for it not only
broadens our conception of animal life, but it furnishes for the theory of
evolution from simple organisms the strongest link in the chain, the first
of the series.

The different appearances of Protozoa depend upon the grade of
organological and histological differentiation, Since these are most promi-
nent in the nourishing and locomotor structures, these become important
in subdividing the group. The organs for these purposes—pseudopodia,
I. RHIZOPODA. 187

flagella, cilia—furnish the basis for the differentation of these classes, to
which are added forms—the class of Sporozoa—modified by parasitism.

Class I. Rhizopoda.

In the lowest position in the Protozoa must be placed those
organisms which lack permanent structures for locomotion and
nourishment, but in which the protoplasm of the body performs
these functions. The term Rhizopoda refers to the fact that the
protoplasm sends out root-like processes—false feet or pseudopodia
—for locomotion and for taking nourishment. These differ from
true appendages in that they are not constant cell organs, but are
formed according to demand and again disappear. A pseudopo-
dium arises when the protoplasm
streams to one point of the body and
extends as a process beyond the sur-
face. Since the process becomes
attached and draws the body after
it, or since the protoplasm of the
body may flow into it, a slow change
of place occurs. Thus the process *
disappears and is absorbed in the
organism, and new pseudopodia are
formed at other places which after a
time are retracted in turn. This
type of locomotion is called amceboid
after the Ameba, in which it was
first accurately studied. When the
Rhizopoda in their wanderings meet
particles of nourishment, they en-
close them with their protoplasm He bs = Aree 2 (After

i : eidy.) cv, contractile vacuole; en,
and take them into the interior of GHUDRATO; Git, GELORAEDS Th nucleus;
the body (fig. 116, WV). ; ;

The form of the pseudopodia is approximately constant for
each species, but as a whole very variable, so that it may be used
not only for separating species but families and larger groups. On
the one hand, there are finger-like pseudopodia (fig. 116), on the
other, those of such delicacy that even under strong magnification
they appear like fine threads (fig. 117). Between these two
extremes are many intermediate forms. Thread-like pseudopodia
usually branch, and when the branches meet they may fuse and
form anastomoses, from which it follows that it is not true, as was
once supposed, that the pseudopodia are covered by a membrane.

  
188 PROTOZOA.

The fine granules of the protoplasm can enter the pseudopodia
and produce here, as they move back and forth, the phenomenon
of ‘streaming.’ Since foreign particles, like grains of carmine
taken up by the protoplasm, can participate in this streaming, it

  

Fria. 117.—Rotatia freyeri. (From Lang, after M. Schultze.)
follows that the movements depend not upon the granules but on
the protoplasm itself. We have already used the fact (p. 62)
that granules in the finest thread can move in opposite directions
at the same time, to demonstrate the extraordinary complexity of
protoplasmic structure.

When Rhizopoda, in the free or en-
cysted condition, increase by division, the
division products frequently exchange the
amceboid motion for that of the Flagellata,
and become flagellate spores or zoospores.
The body becomes oval and develops, on the
anterior end which contains the nucleus,
one or more flagella, which move more ener-
getically than pseudopodia, and are perma-
nent as long as the zoospore stage persists
(fig. 121). Since many Protozoa possess
flagella along with pseudopodia, the
Wigs Tid Mestlianaba: eke boundary between Rhizopods and Flagel-
vera. (Atter Py. Schulze) lates is not distinet: (fig. 118).

The Rhizopoda form an ascending series in which the systematic char-
acters become more and more pronounced; such are the assumption of a

 
I. RHIZOPODA: MONERA, LOBOSA., 189

definite form, as in the Radiolaria and Heliozoa, the formation of a skele-
ton of regular character, as in the Thalamophora, or the development of
a peculiar reproduction, as in the Mycetozoa. At the bottom stand the
Monera and the Lobosa whose characters are mostly negative, for neither
form, skeleton, nor reproduction affords systematic distinctions.

Order I. Monera.

The most important character of the Monera is the lack of a nucleus.
As with other negative characters this is somewhat uncertain. In many
cases, especially when the protoplasm is filled with chromatin granules,
the nucleus is recognized with difficulty, and hence animals have been
described as anucleate in which the nucleus was overlooked. The num-
ber of ‘Monera’ was formerly very great, but has diminished with the
development of microscopic technique. So it is possible, even probable,
that, in the few forms now remaining in the group, the nucleus has merely
escaped observation. On the other hand there are several theoretical
reasons which support the idea of anucleate organisms. It is easier to
suppose that with the appearance of life there were organisms consisting
of but a single substance than that these organisms had nucleus and
protoplasm already differentiated. Several species of Protameba are
placed in the Monera.

Order II. Lobosa (Amebina).

Lobosa are primitive Rhizopoda with
one or several nuclei. The species of
Ameba, forms which owe their name to
their constant change of shape, are typical
(figs. 116, 119). This change of form is
due to the formation and disappearance
of a few finger-like (lobose) pseudopodia.
Body and pseudopodia consist of two
layers, a soft granular inner entosarc
(en) and a firmer, clear, outer ectosarc
(ek). In the entosarc is usually a single
(sometimes several) nucleus (7), which is
vesicular, and contains either one large
or several smaller nucleoli. A contractile
vacuole is usually present. Reproduction
occurs by division (fig. 119), and in some
instances encystment has been observed, Ft4- H9.— Amaba. polypodia

 

in cna (After F. E.

s es eee ae int ‘ Be Schulze.) ev, contractile

the protoplasm dividing into many hun- Schulte) st tosate een
dred small amcebee. entosarc; n, nucleus.

Most Lobosa occur in fresh water; the larger forms, like Pelomyxa
palustris (2 mm. in diameter) live in the ooze of pools, the smaller, like
Ameba proteus and A. princeps, on plants or free in the water. The very
small A. terricola lives in moistearth. There are also parasites among the
Amebe, like A. coli (0.02 to 0.035 mm. large), rare in colder climates, fre-
190 PROTOZOA.

quently observed in the tropics in liver abscesses and in ulcers of the
colon of men ill with dysentery, and perhaps the cause of the disease.
Some of the Monothalamia (p. 198) are sometimes referred to the Lobosa.

Order III. Heliozoa, Sun Animalcules.

The Heliozoa owe their name to the shape of the body, with
the pseudopodia arranged like rays. In the pseudopodia are a
firm axial thread forming a skeleton and a thin coating of
protoplasm. Branching and anastomoses of the pseudopodia
are rare, and usually occur only when the radial arrangement is
modified by pressure. The axial threads frequently converge at
the centre of the body. Here lies a granule, the centrosome
separated from the nucleus, which plays an important part in
division. The body consists of cortical and medullary portions

 

cv Na

His. 120 cortical substanes with contractile vaskolen cae, Pot ee Ds
(fig. 120), distinguished by differences in the protoplasm, but
not separated by membrane. In the cortex are the contractile
vacuoles (cv); the medulla contains the usually single nucleus.
Among the polynucleate forms is the largest and one of the most
heautiful of fresh-water species, Actinospherium eichhorni. Many
I. RHIZOPODA: HELIOZOA. 191

Heliozoa possess a silicious skeleton, which may be a lattice-work
sphere (fig. 121), needles radially arranged or placed tangen-

 

Fia. 121.—Clathrulina elegans. A, with extended pseudopodia; B, divided into two
cysts; C, zoospore; n, nucleus; cv, contractile vacuole.

tially. The forms without skeletons are few, but these have the
power (fig. 122) of forming silicious envelopes during encystment.

Reproduction takes place by divi- a
sion, and one or both moieties may
become swarm spores, 7.e., assume an
oval form bearing at one pole one or two
flagella (fig. 121, @). These swarm /
spores become widely distributed by |
the flagella before they resume the|/
spherical shape, lose their flagella and |
form pseudopodia. It frequently occurs \ ‘|
that several Heliozoa of the same
species become connected by proto-
plasmic bridges, and so form unions of
from two to ten individuals. True fer-
tilization preceded by a kind of polar- pre. 122.— Cyst with germina.
globule formation has only been ob- jpoperes OE eee
served in Actinophrys sol and Actinospherium eichhorini.

 
192 PROTOZOA.

Forms with skeleton and those without are distinguished. To the first
belong Clathrulina elegans, with aspherical lattice-work skeleton supported
ona stalk (fig. 121). Acanthocystis turfacea, skeleton of radial, branching
needles. To the forms without skeleton belong first Actinospheriwm
eichhorni, as large as a pin-head, milk-white, protoplasm foamy from the
numerous fluid vacuoles, the different sizes of which markedly distinguish
cortical from medullary proportions. The contractile vacuoles are in the
cortex, the nuclei in the medulla. In encystment the foamy appearance
and most of the nuclei are lost, and a cyst is formed with numerous uni-
nucleate daughter cysts. Each daughter cyst divides into secondary cysts
which, after the formation of polar globules, fuse (fertilization) and
produce germ spheres. From these then escape, after a long rest, the
young Actinospheria. The reproduction of Actinophrys sol, a smaller
form, is essentially similar.

Order IV. Radiolaria.
The Radiolaria, the most beautiful and most highly organ-

ized of the Rhizopoda, strongly recall, in their appearance, the

 

Fie, 123.—Thalassicolla pelagica. In the centre the nucleus with coiled nucleolus,
around it the central capsule with oil globules; still outside the extracapsular
aot Feae with vacuoles (extracapsular alveoli), yellow cells (black) and pseu-
dopodia.

Heliozoa. They are spherical, only rarely by flattening converted
into disks, or by unequal growth into conical or lobular shapes.
I. RHIZOPODA: RADIOLARIA 193

A second resemblance les in the delicate pseudopodia, often
with an axial filament. The distinguishing characteristic is the
central capsule. This is the central portion of the body surrouded
by a membrane, outside of which is the extracapsulum. The
central capsule is the most important part of the animal. If it be
dissected out from the extracapsulum it not only lives but regen-
erates the lost parts, while the extracapsular portions die. Since
the protoplasm of both parts is identical, the difference in regen-
erative powers can only depend on the nuclei, which are confined
to the central capsule.

The central capsule may be uni- or polynucleate. In the first case the
nucleus (fig. 123), a vesicle of appreciable size, lies in the centre of the
capsule, in the others the capsule is thronged by hundreds of small homo-
geneous nuclei. All Radiolaria are uninucleate in the young stage, and only
at the time of swarm-spore formation polynucleate. The fact that certain
species have almost always one nucleus, while others usually have many, is
explained in the first case by the long continuance of the uninucleate con-
dition, only giving place to the polynucleate condition just before the
formation of swarm-spores, while in the second the polynucleate condition
is reached early. In the central capsule are also included various
deposits which serve as food during reproduction, such as concretions, oil
globules, etc.

The membrane surrounding the central capsule is either per-
forated on all sides by numerous pore-
canals or by small openings in certain
places. Through these pores and open-
ings the intracapsular protoplasm passes
out and spreads itself in the extracap-
sulum. | This consists of a gelatinous
mantle through which the protoplasm
extends as a fine network before it forms
pseudopodia on the surface. In the
larger Radiolaria it contains vacuoles
(extracapsular alveoli) developed in the
protoplasmic net (fig. 123).

With few exceptions the Radiolaria
possess skeletons of wonderful beauty;
latticed spheres, single or one within
another, and bound together with radial
rods (fig. 85), frequently ornamented on #19, 124 —Eugyrtiqium, crani-
the outer surface with spines, or latticed
discs, helmet-like or cage-like structures (fig. 124) or spongy
structures. In other cases occur rings, tubes, spines, which meet

 
194 PROTOZOA.

 

Fia. 125.—Acanthometra elastica. ck, central capsule; n, nucleus; p, pseudopodia;
St, spines; Wk, extracapsulum.

 

Fra. 126.—Collozowm inerme. a, jelly: b, oil globules in the central capsule; cd, yellow
I
cells; e, vacuoles.
I. RHIZOPODA: RADIOLARIA. 195

in the central capsule (fig. 125), etc. In rare cases the skeleton is
formed solely of organic substance (acanthin); usually it is sili-
cious and of much firmness. Hence skeletons of Radiolaria occur
in rocks of various ages, as in Caltanisetta, Sicily, the Nicobars
(both tertiary), and the Barbadoes.

In reproduction first comes fission, which begins a division of
the central capsule (in uni-nucleate forms with a division of the
nucleus) and usually extends through the extracapsulum. If this
latter does not divide a colony results, in which a gradually
increasing jelly contains numerous central capsules, bound together
by protoplasmic threads, which form the pseudopodia on the sur-
face (fig. 126). A second type is reproduction
by swarm spores, which begins when the a 2
nucleus has divided into hundreds or thousands oY
of daughter nuclei. The central capsule then @ y)
divides into as many portions as there are L
nuclei, which become oval and develop two
flagella (fig. 127), which soon begin to vibrate
so that the central capsule is filled with a
tumultuous crowd. With the breaking of the yye. 127 goospores of
capsular membrane these swarm spores escape, Sie eae sete
and at this point our knowledge of this type of ane ae
development ceases. Since in many species
there are large and small spores—macrospores and microspores—
it is probable that for the further development a copulation of
different swarm spores is necessary.

 

Common, if not constant, in the bodies of the Radiolaria are the yellow
cells which were formerly regarded as a part of the animal; they are uni-
cellular algve (Zooranthelle), which are also present in other animals.
(Thalamophora, actinians, sponges, etc.) They afford an instance of sym-
biosis, or the living together of different organisms for mutual good. This
new view rests upon the facts that the Zoovanthelle have a membrane,
secrete starch-like substances, divide independently of the radiolarian and
continue to live after its death,

The Radiolaria are exclusively marine. In fair weather they float at
the surface, but sink in times of storm. Certain species and even large
groups like the Pheeodaria occur only at great depths (1500-4000 fathoms)
where the temperature is about 0° C.

Sub Order I. PERIPYLEA or SPUMELLARIA. The capsule mem-
brane everywhere perforated by pore canals; skeleton absent or formed of
loose needles, of silicious latticed spheres, often reduced to a spongy net-
work or flattened to discs. The latticed spheres can be provided with
spines and connecting rods. SPHAROZOIDA, colonial (fig. 126); THALAssI-
196 PROTOZOA.

COLID# (fig, 123); HALIOMMIDZ with latticed spheres (fig. 85}: DisciD®, dise
like.

Sub Order II. ACANTHARIA. Capsular membrane perforated every-
where by pore canals; twenty spines of acanthin which radiate from the
centre in an extremely regular manner, form the skeleton, as in Acantho-
metra (fig. 125), or the spines are bound together by a latticed sphere
formed of twenty separate plates, as in Acanthophracta,.

Sub Order III. MONOPYLEA or NASSELLARIA. The pores of the
central capsule occupy a pore field at one end. Best known are the Cyrtip£
(Hucyrtidium, fig. 124) with graceful helmet or cage-like skeletons.

Sub Order IV. PHAHODARIA. The central capsule has a principal
opening, often drawn out into a tube and surrounded by dark pigment
(pheeodium) around which may be smaller openings. The skeleton is sili-
cious and formed of hollow pieces. Mostly from the deep seas. Auzla-
cantha, Aulosphera, Celodendron, measuring from 0.5 to 1.0 mm., are
pelagic.

Order V. Foraminifera (Thalamophora, Reticularia).

The Foraminifera, though not equalling the Radiolaria in
beauty and variety of forms, excel them in numbers of indi-
viduals, and hence have a greater importance in the history of the
earth. No group of animals at present or in the past have had
so great a part in the formation of beds of rock.

The most prominent characteristic of the order is afforded by
the shell. This is an envelope, closed
at one pole, and usually opening to the
exterior at the other, the pseudopodia
passing through the aperture (fig. 128).
Accordingly as the axis connecting
these poles is shortened or lengthened,
the shell becomes dise-like, spherical,
flask-formed or even coiled in a spiral.
An additionalfeature, the division of the
interior of the shell by transverse par-
titions into numerous chambers, is com-
mon. Such many-chambered shells
(Polythalamia) are at first small, and
consist of one or few chambers, but
ee SEG cer Ui as the animal grows new chambers are

ees u, nucleus; continually added at the mouth of the

shell. Openings in the walls of the

shell (foramina) connect the adjacent chambers. The spiral

shells with many chambers have a striking resemblance to the

much larger shells of the Nautilus (fig. 386), which led to the
view once held that the Foraminifera were small cephalopods.

 

 
I. RHIZOPODA: FORAMINIFERA. 197

In the fresh-water forms the shell is built of a chitinuous
organic substance which may be strengthened by silica or the
incorporation of foreign particles. The more typical members,
exclusively marine, have almost invariably calcareous shells which,
when dissolved by acid, leave but the slightest trace of organic
matter. The presence of minute pores in the shell is of syste-
matic importance, the group of Perforata (fig. 117) being char-
acterized by them.

The animal portions form a more or less complete cast of the
inside of the shell (fig. 129), and in the polythalamate forms con-
sist of as many pieces as there are chambers in the shell, connected

 

Fic. 129.—Protoplasm of Globigerina Fia. 130.—Young Miliola with several
after solution of the shell. 7, nucleus. nuclei. (From Lang.)

together by plasma bridges passing through the foramina of the
partitions. In the protoplasm there is a large nucleus (figs. 128,
130, x), which in some cases is early replaced by a daughter gen-
eration of small nuclei. Contractile vacuoles usually occur only
in the fresh-water forms. The pseudopodia project through the
chief opening of the shell and in the Perforata probably through
the pores in the shell wall. They are rarely finger-like (fig. 128);
usually they are thread-like, branched, richly granular and
anastomosing, and hence favorable objects for the study of the
streaming of protoplasm.

Reproduction is generally accomplished by fission, but presents
many variations. Only rarely do both animal and shell divide;
frequently the protoplasm protrudes from the mouth of the shell,
a new shell is formed on the outgrowth and the protoplasm then
divides, one of the resulting individuals retaining the old shell. In
the marine Polythalamia the following process is general: The
198 PROTOZOA.

polynucleate protoplasm divides into numerous uninucleate por-
tions (‘embryos’) which frequently, while still within the
mother, develop a shell of one or several chambers. In several
species (Polystomella crispa, Hyalopus dujardinit, Miliolide, etc.)
there is apparently a dimorphic alternation of generations. <A
megalospheric generation (characterized by the long persistence of
a large ‘principal nucleus,’ and often of a large central chamber)
produces zoospores. These develop into the microspheric genera-
tion (early polynucleate, often with small central chamber), which
form ‘embryos’ (supra), which in turn become megalospheres.
Sub Order I. MONOTHALAMIA. Mostly fresh-water forms. These
species never have calcareous shells, but shells of chitin or silica, often
strengthened by foreign bodies. Contractile vacuole usually present.
Pseudopodia either lobose or filiform, branched or simple. A. Forms
with finger-form pseudopodia: Arcella, brown disc-like shell, two or
several nuclei; Quadriila, shell of many square plates (fig. 128); Diffiugia,
(fig. 1381 with shells of sand. These forms may be regarded as merely
shelled Amebe and are frequently referred to the
Lobosa. All other Foraminifera are characterized
by the filiform branching or anastomosing pseudo-
podia. B. Forms with branching and anastomosing

    
        

           

Me i
Ue aaah filiform pseudopodia. Huglypha, shell of oval plates;
\e TS Gromia (fig. 17), marine, shell a horny sac.

\ 4 Sub Order I]. POLYTHALAMIA. Exclusively

marine, living on aquatic plants, on the bottom or
pelagic. The shells, when not dissolved, fall to the
bottom in such numbers that a gram of the sand
Fig. 131.—Diglugia, MAY contain 50,000 of them. Thick beds of rock like
(Orig.) the chalk, the nummulitic hmestone, and the green-
sand are largely foraminiferal in origin. The living species have an average
diameter of about 1 mm. Rarely species have a diameter of several cen-
timeters (Psammonyx vulcanicus, 5-6em.). The fossil nummulites reach
a diameter of 6cm. A. Shell wall massive, the terminal pseudopodal open-
ing being the only communication with the exterior. Miliola (fig. 130).
B. PerroraTa. Shell perforated by many pores; the terminal opening may
be lacking. Polystomella, Rotalia (fig. 117), bottom dwelling; Globigerina
bulloides (fig. 129), pelagic. Among the fossils the Nummulites need men-
tion as well as the Hozoon canadense of the extremely old Laurentian beds
of Canada, the animal nature of which is denied by most students.

Order VI. Mycetozoa.

The Mycetozoa or slime animals are regarded by some as ani-
mals, by others as plants under the older name Myxomycetes
(shme moulds). The first position is supported by the structure
of the motile stage, the plasmodium, the second by the reproduc-
tion resembling that of many fungi. The plasmodia appear in
LL RHIZOPODA: MYCETOZOA. 199

dump weather as networks of bright-red, orange or yellow slime on
decaying wood. They are giant Amebe, several centimeters in
extent, of reticulate protoplasm containing many nuclei and much
foreign matter taken as food. They creep slowly by means of
pseudopdia (fig. 132). On drying the plasmodium encysts in a

 

a
Fig. 132. Fig. 133.

Fig. 182.—Chondrioderma difforme. (After Strasburger.) a, dry spore; b, swollen in
water; c, spore with escaping contents; d, zoospore; c, amoeboid modification of
zoospores which are uniting to form a plasmodium; f, part of a plasmodium;
in dand e, nuclei and contractile vacuoles.

Fic. 133.—Spore-sacs of Arcyria incarnata, (After de Bary.) At the left the sporan-
gium ruptured by the expanding capillitium, which has discharged the spores.

peculiar manner, and if at the proper stage of maturity, it forms the

reproductive bodies, the sporangia (fig. 135). These are firm-walled
vesicles, frequently stalked, the stalk sometimes extending into the
axis of the sporangium as a columella. The space between the
wall of the sporangium and the columella is filled with fine powdery
spores and an exploding apparatus, either a network of fine fila-
ments (capillitium) or many spirally coiled threads (elaters).

When wet, as by rain, the elaters or capillitium expand, rupture

the sporangium and scatter the spores. The spores germinate in

water or on moist surfaces, and from each comes out a small

amceba-like embryo, frequently furnished with a flagellum (fig. 132).

Several of these embryos fuse to form a plasmodium: Hthalium

septicum, flowers of tan, plasmodium yellow, on spent tanbark;

Comatricha, Arcyria (fig. 133).
200 PROTOZOA.

Class II. Flagellata (Mastigophora).

In many Rhizopoda, as described in the foregoing pages, the
pseudopodia disappear from time to time and are replaced by one
or two flagella; others have, besides pseudopodia, permanent
flagella for locomotion and taking of food. Such flagellate spores
and flagellate Rhizopods form the transition to the Mastigophora,
which are permanently flagellate, the flagella serving as organs of
locomotion and feeding. There are three orders which must be
described separately.

Order I. Autoflagellata.

All autoflagellates at first sight are closely similar, a usually
oval body with a vesicular nucleus at one end, a contractile vacuole
at the other. At the anterior end there is often added a small red
or brown pigment spot (fig. 134), apparently for the recognition
of light, and hence a primitive eye. At this same pole are also
one or two flagella; when a greater number occur they are scat-

 

Tia. 134. Tia. 135, Fig, 136,

Wie, 134.—Euglena viridis, (After Stein.) c, contractile vacuole; n, nucleus; 0, pig-
ment spot. :

Fic. 135,—Dinobryon sertularia, (After Stein.) aa parasitic flagellate often found in

y the lorica: }, contractile vacuole; 1, nucleus. :

Fig. 186.—Conocladitun wnbellatiom. (After Stein.)

tered over the body. The body surface is frequently naked, and
may be capable of am@boid motions; at other times it is covered
with a more or Jess evident cuticle. Very common are closed
envelopes and open goblet-shaped cases (loriew, fig. 135), and

a

also simple or branched stalks (fig. 136), on which the animals
Il, FLAGELLATA: AUIOFLAGELLATA. 201

sit in small groups. There are great differences in the feeding
and in the organs connected therewith. Many feed like animals,
being provided with pseudopodia like the Rhizopoda or with a
mouth like the Infusoria. In the Choanoflagellata there is an
interesting structure, the collar. This is a funnel-like process of
the body protoplasm on which foreign particles are thrown by
the flagellum in the centre (fig. 136) and thence are conveyed
to the interior. (According to recent researches the collar
consists of a plasma membrane rolled up spirally with two turns. )
Besides these animal forms are plant-like species which contain
chlorophy] (Volvocine, Euglenide) or brown chromatophores
(Chromomonadinex), aiding in assimilation and enabling the
organism to produce paramylum or even starch. It is note-
worthy that forms which are plant-like in this respect are
closely allied anatomically to forms which resemble the animals.
Indeed, there are species which possess a cytostome without taking
solid nourishment, assimilating by means of chlorophy! or living
on fluid food (fig. 137). All this renders more difficult the syste-

 

Fie. 137. Fig. 138.

Fia. 1387.—Chilimonas paramecium. (After Biitschli.) oe, cytostome; u, nucleus; v,
contractile vacuole.

Fic. 138.—Flagellata. 4, Mastigameba; B, Codosiga; C, Bicoseca; D, Hexamita; EF,
Noctiluca, side view.

matic valuation of the differences appearing in the food, and also
shows that the Flagellata have relations in different directions:
with the Rhizopoda, the Infusoria, and the lower plants.
202 PROTOZOA.

Reproduction is nearly always by fission. In many species
conjugation is known, best in those plant-like forms, the Volvo-
cina, where two individuals fuse completely to a resting spore. In
the colonial Volvocina the conjugating individuals are unequal in
size, some animals of the colony growing to large immobile
oospheres, while others by continued division form groups of
minute active zoospores or spermatozoids. | When fertilized by
the zoospore the oospheres fall to the ground, become encysted,
become brown in color, and enter a resting stage before they form
a new colony by division.

Sub Order I. PHYTOFLAGELLATA.  Plant-like chlorophyl-bearing
flagellates, mostly with eye-specks. Volvocina: Volvox globator,* green
sphere 0.2-0.7 mm. in diameter, consisting of thousands of individuals
which propel the colony by their flagella. Euglenide : Euglena viridis *
(fig. 184), solitary, coloring small pools bright green (a red variety colors
them purple) by their immense numbers. Chrysomonadina, plant-like in
nourishment but rarely taking solid food : Dinobryon * (fig. 136).

Sub Order TI. CHOANOFLAGELLATA. With collars; mostly small
colonial forms. Codosiga * (fig. 188, B); Conocladium, numerous indi-
viduals united on a stalk (fig. 136).

Sub Order II]. EUFLAGELLATA. Animal flagellates, taking solid
particles of food either by pseudopodia or by a more or less developed
cytostome. Monapina. Here belong, besides numerous free forms, several
parasites of man: Lamblia (Cercomonas) intestinalis, fig. 189 (Mega-

oy

F1a. 139. Fia. 140.

mm

 

Fig. 139.—Lamblia intestinalis. (After Grassi.) Front and side views. , nucleus
Fia. 140,—Trichomonas vaginalis. (After Blochmann.) n, nucleus (fourth flagellum
lacking in figure).

stoma entericum); also in rats and mice: Trichomonas hominis (T. intes-
tinalis), both in small intestine. 7. vaginalis (fig. 140).
Il, FLAGELLATA: DINOFLAGELLATA., 203

Order II. Dinoflagellata (Cilioflagellata).

These forms, occurring in both fresh water and the sea, are
recently placed near the plants because, OPO
with their brown chromatophores, their
food relations are like those of plants,
although the taking of solid food by a
mouth opening has been observed. The
armor formed of cellulose plates is also
plant-like. This armor is divided by a
transverse groove into two parts which
recall somewhat a box and its lid. There
is also a longitudinal furrow which crosses
the other. At the point of crossing
are two flagella, one of which lies in
the transverse groove and was for a long
time regarded as a circle of cilia, whence
the old name cilioflagellates given the TStleee Sicies api enierioe

(After Stein.) apo, anterior

aus ots horn with opening; aah,
order. Peridinium tabulatum and Cera- ysh. posterion and’ right
° . . horn; g, flagellum ; gs, flagel-
tium cornutum (fig. 141); Ceratium tri- jar groove: If, longigudinal
* : groove; r, rhomboidal plate;
pos,* marine. v, vacuole.

 

Order III. Cystoflagellata.

The cystoflagellates, characterized by a gelatinous body sur-
rounded by a membrane, include two very interesting species,
both marine, which differ markedly in external appearance.

Noctiluca miliaris* (figs. 142, 138, #), among all marine

 

Fie. 142.—Noctiluca miliaris (in part after Cienkowski). A, entire animal; f, flagel-
lum; n, nucleus; 0, cytostome, beside it the ‘tooth’ and ‘lip’; t, tentacle; B, C,
upper end with two stages in the formation of zoospores; D, zoospores.

animals, best shows the phenomenon of phosphorescence. These
spherical forms, about 1 mm. large, sometimes occur in such
204 PROTOZOA.

numbers at night as to make the whole surface light at the slight-
est agitation. The phosphorescence is apparently caused by oxi-
dation processes in the protroplasm, but it persists for some time
after deprivation of oxygen. The membrane covering the body is
interrupted by a pit at one point, the cytostome, near which is the
nucleus surrounded by an aggregation of protoplasm which sends
branching threads into the jelly of the body. At the entrance to
the cytostome is a flagellum, of no use in locomotion, and a band-
like tentacle consisting of an outgrowth of the body membrane
with a transversely banded muscular axis; it moves slowly with a
swinging motion.

Noctiluca reproduces by simple fission and by formation of
swarm spores (fig. 142, B, C, D). In the latter two individuals
lose tentacles, flagella, and cytostomes, and conjugate ; after
mutual nuclear fertilization the
animals separate, while the proto-
plasm in each collects in a disc
which, by successive divisions, is
converted into numerous uni-
nucleate oval germs. These at first
project from the sphere, but later
separate and form small flagellate
spores whose later history is not
certainly known.

Leptodiscus medusoides of Eu-
rope (fig. 145) has the appearance
of a medusa 1 to 1.5 mm. in
diameter. Its gelatinous dise is
covered by a membrane, and at the
Fie. 143.—Leptodiscus medusoides, sur. highest point of the concave surface

ee ini : Pao is a mass of protoplasm with a sin-

tract; p, protoplasmic band. gle nucleus. On one side of this a
band goes to the mouth, on the other a canal bearing a fine
flagellum at its end. The animals swim well, like meduse, by
closing the umbrella, the motions of which are caused by delicate
muscles on the concave side.

 

Class III. Ciliata.

The Ciliata rival the Rhizopoda in numbers and variety of
form. They are so complicated in structure that they were long
held as multicellular, a view which was entirely abandoned only
in the last quarter-century. All have a form definite for the
ITI. CILIATA. 205

species; this in the ‘ametabolous’ forms is unalterable, in the
‘metabola’ it can be pressed out of shape in passing through a
narrow space. This constancy of form is due to the development
of more or less cuticle on the outside of the body, which in the
‘ametabola’ acquires an armor-like firmness; in the others is more
flexible. The cuticle is covered with cilia—small vibrating pro-
cesses which move not singly but together in large numbers, and
serve not only as organs of locomotion, but by
creating vortices in the water bring food to the
organism. They furnish the most important
characteristic of the class (fig. 144).

The presence of a cuticle necessitates a
eytostome (except in the parasitic species),
since food particles cannot be taken in at any
point. At the cytostome the cuticle with its
cilia forms a funnel-like extension (cyto-
pharynx) into the protoplasm. At the bottom
the cuticle is interrupted so that water and pro-
toplasm are in contact. By the action of the
cilia food particles are taken into the cyto-
pharynx and pressed into the protoplasm,
forming asmall enlargement which finally sinks Ee eee
into the substance as a food vacuole (2),  grammatic). cv, con-

3 4 . 4 tractile vacuole in
which by the streaming of the protoplasm is systole, cv’, in dias-
carried about in the body. The digestible por- foo iatble moe in
tions are absorbed, and those not capable of pucionse.  *uricnes
digestion are cast out of the body at a fixed © at" protruded.
point (cytopyge) usually not recognizable at other times (fig. 151).

Contractile vacuoles (¢v) are lacking only in parasites and
marine species. They are constant in number and position, and
frequently have afferent ducts which empty into the vacuole, the
vacuole in turn forcing the fluid to the exterior. Trichocysts, nettle
bodies, and muscular fibrille occur in some species. Trichocysts
are minute rods placed vertically to the surface in the cortical
layer, which under the influence of reagents (chromic acid is best)
elongate into threads penetrating the cuticula. These have been
compared by some to the nettle cells of cwlenterates, and have
been ascribed defensive functions; others regard them as tactile
structures. They have no connexion with the cilia. Nettle
bodies are extremely rare. Muscle fibres are more common; they
lie between ectosarce and cuticle, and cause quick convulsive motions
of the animal.

 

 
206 PROTOZOA.

The nuclear relations are extremely interesting in that there are
two nuclei physiologically unlike. The larger of these (nucleus
of older writers, macronucleus) is a large oval, rod-like, or spiral
body, readily and deeply staining with microscopic stains, and sur-
rounded with a membrane. It appears to control all the common

vital functions of the animal (motion,
feeding, etc.). Beside it or in a depres-
sion in it is the much smaller micro-
nucleus (nucleolus or paranucleus of
older authors) which stains less deeply
and only plays a part in reproduction.
In all sexual processes it comes to the
front and can be called the sexual
nucleus.
Multiplication of Ciliata occurs by
binary fission (fig. 145); more rarely, and
0 then only in the encysted condition, by
Ee division into numerous (up to 64) parts.
Budding is known in the Peritricha and
ad Suctoria. First the micronucleus divides
ia: 1 Pareieriime anaes Oa, ond then the macronucleus
in division. k, macronucleus; separates by elongation and construction.
qh, micronuclei; 0, eytostome x ie P
gy Sonera ne sngividnels: ‘he old cytostome persists in the anterior
of cytostome. offspring, but often an outgrowth from
it (fig. 145, 2, 0’) passes into the posterior half and develops into
a new mouth.

The periods of fission are interrupted from time to time by the
sexual process of conjugation, which will be described as it occurs
in Paramecium (fig. 146). Two individuals touch at first in front,
and then by their whole ventral surfaces, so that their cytostomes
come together. In the neighborhood of the latter a plasma bridge
connects the two. Later the individuals separate. While these
easily observable external processes are occurring there is a com-
plete modification of the nuclear apparatus in the interior. The
macronucleus increases in size, and breaks into small portions
which disappear within the first week after copulation (probably
wbsorption), and give place to a new nucleus derived from the
micronucleus. At the beginning of copulation the micronucleus
becomes spindle-shaped, divides and repeats the process, the result
being the formation of four spindles, three of which break down,
thus recalling the polar globules in the maturation of the egg
(p. 146). The fourth or primary spindle places itself in the neigh-

   
III. CILIATA. 207

borhood of the cytostome at right angles to the surface and divides
into two nuclei, the superficial being called the wandering or

E T.

    

Fia. 146.—Conjugation in Paramecium. hk, macronucleus; nk, micronucleus; 0, cyto-
stomes.

I. Changes of micronucleus; left sickle stage, right spindle stage.

II. Second division of micronucleus into primary spindles (1, 5) and secondary
spindles (2, 3, 4; 6, 7, 8).

III. Degeneration of secondary spindles (2, 3, 4; 6, 7, 8); division of primary spindle
into male (1m, 5m) and female spindles (17, 5w).

IV. Exchange of male spindles nearly complete (fertilization), one end still in the
parent animal, the other united with the female spindle, lm with 5w and 5m with 1w;
macronucleus broken up.

V. The cleavage spindle t formed by male and female spindles dividing into the
secondary cleavage spindles t’, t”’. ‘ ae

VI, VIL. End of conjugation. The secondary cleavage spindle dividing into the
anlage of the new micronucleus (vk’), and that of the new macronucleus, pt
(placenta). The fragments of the old macronucleus begin to degenerate.

Since P. caudatum shows the earlier and P. aurelia the later stages better, these
forms have been used, P. caudatum for I-III, P. aurelia for the rest. The differences
consist in the existence of one micronucleus in P. caudatum, two in P. aurelia, and

that in the latter the nuclear degeneration begins in I.

male nucleus, the deeper, the stationary or female nucleus. The
male nuclei of the two copulating animals are exchanged, travers-

 
208 PROTOZOA.

ing the protoplasmic bridge in their course. Both male and
female nuclei become spindle-shaped, and the immigrant male
spindle fuses with the female spindle, forming a single spindle of
division. At last the division spindle produces (usually by indi-
rect means) two nuclei, one of which becomes the new macronu-
cleus, the other the new micronucleus.

In a comparison of the fertilization of the Metazoa, the female
nucleus corresponds to the egg nucleus, the male nucleus to that
of the spermatozoa. As the fusion of egg and sperm nuclei forms
a segmentation nucleus, so here the division nucleus is formed in
a similar manner. As the egg cell through fertilization acquires
the capacity not only to produce sex cells but somatic cells—cells
which carry on the common functions of the body—the fertilized
micronucleus forms not only the new micronucleus, but also the
macronucleus which controls the body processes, and hence is the
somatic nucleus. In other words, fertilization in the Ciliates
leads to a complete new formation of the nucleus and thus toa
new organization of the organism.

In most Ciliata the conjugating individuals are equivalent,
the fertilization is mutual, and the individuals separate later. In
the Peritricha (mostly sessile forms, fig. 147), on the contrary, the

 

Fia. 147.—Epistylis umbellaria, (After Greeff.) Part of a colony in ‘bud-like’ conju-
gation. r, microspores arising by division; k, microspore conjugating with a
macrospore. ;

resemblance to fertilization in the Metazoa is strengthened in that

there is a sexual differentiation and a permanent fusion of the
conjugating individuals. Some animals—the macrospores—retain
their size and sessile habits; others by rapid division produce
Ill, CILIATA,: HOLOTRICHA, HETEROTRICHA. 209

groups of markedly smaller microspores. The latter separate and
fuse completely with the macrospores, only a small cuticular sac
persisting to indicate the fusion. The nuclear phenomena are
much the same as with Paramecium, allowance being made for
the permanance of the fusion.

Order I. Holotricha.

The Holotricha are doubtless the most primitive Ciliates, since
the cilia on all parts of the body are similar; being at most slightly
stronger at one end of the body or on the inside of the cytostome.
Best known are the species of Paramuciwmn* (fig. 144) occurring
in stagnant water. Opalina ranarum * lives in the intestine of the
frog. It lacks mouth, has numerous similar nuclei, no micronu-
cleus and no conjugation, The small encysted Opaling pass out
with the faces, and are eaten by the tadpoles, which thus become
infected.

Order II. Heterotricha.

Like the Tolotricha the Heterotricha
are everywhere ciliated, but they have a
tract of stronger cilla, the adoral ciliated
spiral. This is a band of cila beginning
at some distance from the cytostome and
leading in a spiral course into the mouth.
It consists of rows of cilia united into
‘membranellie’ placed at right angles to
the course of the spiral. In the best-
known heterotrichans, the Slentors * (fig.
148), the peristomial area, surrounded by

    

i
Fie. 148. Fie. 149.
FIG. 148.—Stentor polymorphus. (After Stein.) a, peristomial area; hb, roof of hypo-
stome; gy, contractile vacuole; x, nucleus; 0, cytostome; r, adoral ciliated spiral;
t, hy postome (excavation for mouth).
Fig. 149.—Balantidium coli. (After Leuckart.)
210 PROTOZOA.

the spiral, forms the broader end of the body, which gradually
tapers toward the other end, by which the animal may attach
itself by small plasma threads. Muscle fibres which run length-
wise immediately under the cuticle produce energetic move-
meuts. Stentor polymorphus* when attached builds a gelatinous
case. JS. ceruleus.* Balantidiwm coli (fig. 149) appears in the
large intestine of men ill with diarrhwa; it also occurs in swine
without causing sickness. Other parasites of man are B. minu-
tum and Nyctotherus faba.

Order III. Peritricha.

In the Peritricha there is always a broad peristome area with
the cytostome; the opposite end has a corresponding pedal disc
or is narrowed like a goblet and ends in a stalk (fig. 150). Only

rs,

 

Fie. 150.—Carchesium polypinum. (After Biitschli.) Left, a single animal; right, three
stages of division. cv, contractile vacuole; 2, macronucleus: 1’, micronucleus;
Nv, food vacuoles; os, cytopharynx; per, peristome; vs, reservoir of contractile
vacuole; wn, undulating membrane ; vst, vestibule; wk, ring on which a posterior
circle of cilia may develop.

the adoral ciliated spiral is constant. It arises from the swollen

margin of the peristomial area, and continues on the ‘ operculum,’

a ciliated disc which projects free from the peristomial area, but

in contraction is drawn close against it, the peristome lips folding

over all, Besides, there may be a temporary or permanent circle
of cilia near the hinder end. The nucleus is usually sausage-
shaped, much bent, and with the small micronucleus in its hinder

angle (fig. 150, 2’).

The best known representatives are the VoRTICELLID# (figs. 147, 150),
attached by a long stalk which is usually hollow and contains a slightly
4

Il, CILIATA: HYPOTRICHA. 211

spiral muscle. This extends into the body and divides up into fine fibrille
which extend under the cuticle to the peristome. When the muscle in the
stalk contracts it becomes coiled into a corkscrew spiral, drawing back the
animal, and folding in the anterior end. Vorticella* is solitary; Carche-
stum * forms colonies with dichotomously branched stalks; Zoothaminon,*
colonies imbedded in a common jelly ; Epistylis * (fig. 147), branched col-
onies with rigid stalks, the muscle being confined to the base of the body.

Order IV. Hypotricha.

In this order the body is more or less flattened and a ventral
and a weakly arched dorsal surface are differentiated. The back
lacks cilia, but often bears spines and tactile bristles. On the ven-
tral side are several longitudinal rows of cilia, and besides straight

     

I

 

Fia. 151. Fia. 152.

Fic. 151.—Stylonychia mytilus. (After Stein.) a, anal hooks; 6, ventral hooks; ¢, con-
tractile vacuole; d, frontal ridge; g, canal leading to contractile vacuole; l, upper
lip; m, nucleus with micronucleus; p, adoral ciliated spiral; 7, marginal cilia; s,
caudal cilia; st, frontal spines; z, anus (cytopyge). ;

Fic. 152. Division of Stylonychia mytilus. (After Stein.) c, ¢’, contractile vacuoles of
the two individuals; n, 2’, nucleus and micronucleus; p, p’, adoral ciliated spiral;
7,7’, marginal cilia; w, w’, ciliated ridges.

 

 

spines and hooked cilia composed of united cilia. These latter ure
of use in creeping. The strongly developed adoral cilia are of use
in locomotion and in producing vortices which bring food. The
macronucleus is often divided into two oval bodies connected by ¢
thread; the micronuclei vary in number from » to 4 in the same
212 PROTOZOA.

species. These are the best forms for studying the micronuclei.
The species of Stylonychia* (figs. 151, 152) are best known.

Order V. Suctoria (Acinetaria).

The Suctoria differ from other Infusoria in the absence of cilia
from the adult and consequently have no means of locomotion.
They are fixed to some support either by the base or by a slender
stalk. The body is usually spherical and is covered with a cuticle,
which in the genus Acineta is produced into a cup-like lorica.
There is no mouth, but in its place tentacles or sucking feet, very
fine tubes with contractile walls which begin in the protoplasm and
protrude through the cuticle (fig. 153, #’). The Acinetaria kill
other animals, especially infusoria, with their tentacles, and then

 

Fia. 153.—Forms of Suctoria. (After various writers.) 4, Dendrosoma; B, Rhyncheta:
C, Ophryodendron; D, Tokophrya, E, ciliated young of Spherophrya; F, diagram of
structure showing capitate and styliform tentacles arising from the ectosarc
and corresponding canals in the entosarc.

suck the substance through these tubes. The contractile vacuole,
rarely lacking, lies near the compact macronucleus; micronuclei
are generally present.

In contrast to the immobile adults the young which are ciliated
(fig. 153, #) after the pattern of ciliates, are good swimmers.
They arise either as buds from the surface of the mother or as
‘embryos’ in her interior. This latter condition is only a modifi-
cation of the other, for parts of the outer surface become pushed
into the interior, and there form a brood cavity in which the
embryos arise. After swimming for a while the young come to
rest, lose the cilia, and develop the tentacles.

Some species of Podophrya are widely distributed in fresh water, also
Spherophrya, parasitic in Infusoria. The species of Acineta as well as
Podophrya gemmipara (fig. 20) are marine, living on hydroids and Poly-
zon.
IV. SPOROZOA: GREGARINA. 213

Class IV. Sporozoa.

Under the name Sporozoa are united several groups of Protozoa
which, while they differ much in structure, have much in common
in life and development. They are parasites in Metazoa, many of
them in the cells themselves, causing their degeneration (Cytospo-

eS S

 

Fic. 154.—Sporozoa. A, cyst of Clepsidrina with sporoducts: B, Clepsidrina, two indi-
viduals (after Schneider); C, Himeria falciformis, from mouse; PD, same, falciform
embryos; &£, Hoplorhynchus dujardinii, from Lithobius; F, Gregarina gigantea, from
lobster; G, Sarcocystis miescheri, from pig; H, Myxidium (after Théolan); J, Rhopa-
locephalus, alleged cause of cancer (after Korotneff).

ride). They take no solid food, but are nourished by fluid mate-
rial absorbed through the whole surface. In reproduction they
form a large number of ‘ sporoblasts,’? which when enveloped with
a membrane are called ‘spores,’ the contents of which usually
break up into several small bodies or ‘sporozoites.’ The sporozo-
ites for their development must leave the host. The resemblances
to the Rhizopods (Mycetozoa) are unmistakable, especially those
Sporozoa which have pseudopodia for much of their hfe.

Order I. Gregarina.

The typical and longest known sporozoa are the Gregarines,
parasites of oval or thread-like form (recalling round worms),
usually somewhat flattened, which so far have only been found in
invertebrates, where they live in the intestine or gonads, more
rarely in the body cavity. The protoplasm (fig. 155, /) is sepa-
rated more sharply than in other Protozoa into a clear ectosare
(ek) and a granular entosare (ev). The ectosare is covered by
a cuticle (not always easily seen, but frequently with a double con-
tour) (cw), which must be permeable by fluid food, for no cyto-
stome exists. In many (perhaps all) there is a double striping
of the body, a longitudinal recognizable by furrows on the outer
surface and hence cuticular, and a transverse marking in the
214 PROTOZOA.

ectosarc, produced by circular or spiral muscle fibrille. These
muscles explain the peristaltic motion and the occasional sharp
bending of the body, but not the peculiar gliding motion like that
of diatoms by which locomotion is usually effected. This is

 

Fic. 155.—Development of Gregarina blattarum. I, conjugation; II, A-C, a cyst in
transformation into pseudonavicelle; III,_A, a pseudonavicella greatly enlarged;
B, same with sickle-formed sporozoites; cu, cuticle; dm, deutomerite; ek, ecto-
sare; en, entosarc; 2, nucleus; pm, protomerite; pn, pseudonavicelle; rk, re-
sidual body; sk, sickle-form sporozoites.

explained by the view that the gregarines secrete stiff gelatinous
threads from the posterior end, and the elongation of these forces
the body forward.

In many gregarines (Polycystida) the body is divided by a cir-
cular incision into a smaller anterior part, the protomerite, and a
larger deutomerite. Internally this division is marked by a bridge
of ectosare across the entosare. The vesicular nucleus (there is
but one in any gregarine) lies in the deutomerite. An epimerite—
a structure connected with the peculiar type of parasitism—occurs
in many species. All gregarines are parasitic in youth inside of
cells. They later leave these, but many remain for a long time
with a process of the protomerite imbedded in the cells. This
process—the epimerite—is provided with threads or hooks for
IV. SPOROZOA: COCCIDLA. 215

anchorage, and is lost when the animal gives up its connexion
with the host cell. Among the intestinal gregarines frequently
occur ‘associations’ where two or more animals are fastened to-
gether head to tailin a row. Perhaps these associations are prep-
arations for conjugation which occurs in development.

Reproduction occurs exclusively in an encysted condition (fig.
155, I], A). Usually two animals (sometimes one, rarely more than
two) occur ina cyst. <A fusion of the two encysted animals does
not take place, but it is probable that a nuclear exchange (recalling
that of ciliates) takes place. After each individual has become
polynucleate by division of its nucleus, it divides at first super-
ficially, later internally into small particles, the sporoblasts (II, B),
which change into spores, here called pseudonavicelle. The
pseudonavicelle are mononucleate bodies with firm membrane and
usually spindle form in shape (III, 44). In these processes a
part of the gregarine takes no part. This residual body appears
under proper conditions to swell up and rupture the cyst, thus
freeing the pseudonavicelle. In many gregarines there are sporo-
ducts for the escape of the pseudonavicelle (fig. 154, A). The
contents of the pseudonavicelle divides into (usually eight) sporo-
zoites or falciform spores which must pass out from the spores and
into the cells of the host in order to form gregarines. This escape
of the sporozoites depends upon entrance into the proper host.
Often the transformation of the contents of the cysts into pseudo-
navicelle takes place when the cysts have left the original host.

Best known are the Ifonocystis tenax of the spermatheca of earthworms,
and Gregarina (Clepsidrina) blattarum of the cockroach. The American
species have scarcely been touched. One species is abundant in the intes-
tine of Geophilus.

Order II. Coccidie.

The gregarines of all Sporozoa are nearest the Coccidiw, which
are also cell parasites with a single nucleus, but without either cell
membrane or division into protomerite and deutomerite. In most
species, asin Coccidium cuniculi, there are two types of reproduc-
tion, an endogenous, leading to ‘ autoinfection,’ and an exogenous,
concerned in the transfer of the germs to other hosts. In the
first (lacking in many species) the Coceidiwm divides into many
falciform germs which separate from each other and, without
alternation of hosts, enter other cells. The second type is begun
by fertilization. Certain individuals, by rapid division form
microgametes, small bodies swimming with serpentine motions or
by one or two flagella. Other individuals do not divide, but form
216 PROTOZOA.

mucrogametes which are fertilized by the microgametes, and then
eneyst, pass to the outside, and serve for the infection of other
animals. The coutents of the cyst begin to divide, sooner or
later, into sporoblasts (in Coccidium, four) containing spores, the

a b c d €

 

Fig. 156.—Coccidium cuniculi, from the liver of the rabbit (from Wasielewski. a, b,
young Coccidia in the epithelial cells of bile duct, the nucleus of the cell in the
upper process; c, encysted; d,e, contraction of protoplasm; y, li, i, spore forma-
tion; kh, ripe spore with two germs and a residual body.

process being completed only after entrance into a new host.
Each spore forms one or more sporozoites, a portion of the sub-
stance being left behind.  Cocetdinm cunteuli (oviforme) in the
liver of mammals, especially rabbits (rare in man), producing
cheesy granules. C. perforans in the intestine of rabbits, rare in
man.

Order III. Haemosporida.

In structure and development these are much like the Coccidix;
they live in blood corpuscles. The forms occurring in man pro-
duce malaria. ILere, also, there are endogenous (autoinfecting)
and exogenous generations transferring the parasites to other
hosts. The parasites in the corpuscles (fig. 157, @ to @) grow and

 

Ta. 157.—Plasmodium laverni, var. quartuna (from Wasielewski, after Labbé), from
the blood of a malarial man. a, newly infected blood corpuscle; b, somewhat
larger germs; ¢, full-grown parasite with strong pigmentation; « Younded form
with large nucleus; ¢, beginning of germ formation; f, rosette of cserms around a
residual body; gy, germs set free by degeneration of corpuscle, se c

divide, producing ‘daisy-like forms’ characterized by little accu-
mulations of pigment derived from the hemoglobin of the blood.
IV. SPOROZOA: MYXOSPORIDA., 217
These division products are set free by a breaking down of the
corpuscle (period of chill) and infect other corpuscles. Thus
autoinfection can continue until at length sexual forms appear—
‘spheres’ or macrogametes, flagellate microgametes—incapable of
infecting the corpuscles. The conjugation of these seems to take
place when they are taken into the stomach of a blood-sucking
mosquito. After fertilization, the oosphere wanders into the
intestinal wall of the mosquito, grows larger, encysts, and produces
many sporoblasts, which in time form many sporozoites. These
migrate into the salivary glands of the mosquito and thence are
transferred to man with the sting of these insects. Since a tem-
perature above 20° C. (68° F.) is best for the development of the
stages in the mosquito, and since mosquitos need water for their
development, the prevalence of the disease in moist, warm regions
is easily understood. For the transfer of human malaria not all
mosquitos will serve, but apparently only those of the genus
Anopheles. The species of Culex convey bird malaria. The differ-
ent kinds of malaria seem to be produced by different parasites.

Order IV. Myxosporida.

The Myxosporida (fig. 158) are mostly large (sometimes visible
to the naked eye) and occur especially
in fish and arthropods. When they occur
in hollow organs they are naked and have
pseudopodia, but in parenchymatous or-
gans like the heart, liver, braim, kidney,
etc., they are usually enclosed in a mem-
brane, and here they produce the great-
est injury. At first binucleate, they soon
become polynucleate, and it would appear
that they can reproduce by fission. Even

 

before the growth is ended they begin
the process of sporulation. In the in-
terior single spherical protoplasmic bodies
separate, these having at first a single
nucleus, later more (as many as ten).
From each of these bodies arise from two

to many spores, the so-called psorosperms.

Fie. 158.—Myxosporid , 4% My.2-
obolus miller’, from gills of a
fish; 2,3, psorosperms of Myc-
idium lieberkuhne; k, degen-
erating nucleus; , vacuole
formerly regarded as w nu-
cleus; p, cnidocil-like pole
body, in ~” with exserted
threads.

These (fig. 158, 4) are

enclosed in a bivalve shell which ineludes, besides a binucleate
germ, one, two, or four polar capsules, these resembling somewhut
the nettle organs of the cwlenterates. They are oval and contain
threads which, under certain conditions, are protruded (fig. 158, ”)

5
218 PROTOZOA.

and serve to fix the capsule, while the ameboid germ creeps out
and penetrates the tissues of the host. Experiment shows that
fishes are infected through the alimentary canal.

The Myxosporida frequently cause serious epidemics in fish. This was
noticeably the case with the fish in the aquaria at the Chicago exposition.
Myxobolus, Myxidium. Invertebrates may also be infected, the celebrated
pébrine of the silkworm being caused by Nosema (Glugea) bombycis.

Order V. Sarcosporida.

The Sarcosporida (fig. 159)—also called
Rainey’s or Miescher’s corpuscles—occur in
the voluntary muscles of vertebrates, especially
mammals. They are oval cysts lying in sar-
colemma sacs between the fibrille. They have
a cyst, the wall of which is radially striped,
and inside this, in the ripe condition, are
spores, imbedded in a stroma, each spore con-
taining numerous reniform or falciform sporo-
zoites. Sarcocystis miescheriana in muscles of
pig; S. merits in the mouse; S. lindemannt
rare in human muscle.

Summary of Important Facts.

1. The Protozoa are unicellular organisms

Fra. 159.— Sarcocystis Without true organs or true tissues.

 

diaphragm of Ok 2. All vital processes are accomplished by
(After Biitschli. S, ee : hese : me Pe Sha
cyst: sp, spheres of the protoplasm (sarcode), digestion directly
spores. by its substance, locomotion and the taking

of food by means of protoplasmic processes (pseudopodia) or by
appendages (cilia and flagella).

3. Excretion takes place by special accumulations of fluid, the
contractile vacuoles.

4. Reproduction is by budding or by fission. Conjugation has
been witnessed in many, and possibly occurs in all. True con-
jugation is a process of fertilization (caryogamy), in contrast to
fusion of plasma (plasmogamy).

5. Protozoa are aquatic, a few living in moist earth; they can
only exist in dry air in the encysted condition, surrounded by a
capsule which prevents desiccation.

6. Since encysted Protozoa are easily carried by the wind, the
occurrence of these animals in water which originally contained
none is easily explained. ;

7. The mode of locomotion serves as the basis for division of
IV. SPOROZOA: SUMMARY OF IMPORTANT FACTS. 219

the Protozoa into the classes Rhizopoda, Flagellata, Ciliata, and
Sporozoa.

8. The Rutzopopa have changeable protoplasmic processes,
the pseudopodia.

9. The Rhizopoda are subdivided into Monera, Lobosa, Helio-
zoa, Radiolaria, Foraminifera, and Mycetozoa.

10. The Lolosa and Monera have no definite shape. The
Lobosa have a nucleus, the Monera are anucleate.

11. Heliozoa and Radiolaria are spherical and have fine radiat-
ing pseudopodia and frequently silicious skeletons. They are dis-
tinguished by the occurrence of a central capsule in the Radiolaria
which is lacking in the Heliozoa.

12. The Thalamophora (Foraminifera) have a shell, closed at
one end, at the other with opening for the extension of pseudopodia.
The shell is chitinous or calcareous, one or several chambered,
straight or spiral, sometimes with cloge walls, sometimes perforated
with pores; the pseudopodia are occasionally lobular, but usually
filiform, branching and anastomosing.

13. The Foraminifera are of great geological importance on
account of their numbers and their shells, which have built and
are still building extensive beds of rock (chalk, nummulitic lime-
stone). The silicious skeletons of the Radiolaria are less important.

14. Mycetozoa (Myxomycetes of botanists) are mostly enormous
Amcebee with branched reticulate protoplasm (plasmodium). They
form complex reproductive structures (sporangia, etc.), recalling
those of the fungi.

15. FLAGELLATA have one or a few long vibratile processes—
flagella—which serve for locomotion and for the taking of food.

16. The Awtoflagellata have only flagella; they feed like plants
(Volvocina) by means of chlorophyl, or have a mouth for the tak-
ing of food, or a collar (Choanoflagellata).

7. The Dinoflagellata have two kinds of flagella and usually
an armor of cellulose.

18. The Cystoflagellata have a gelatinous body enclosed in a
firm membrane (Noctiluca).

19. The Ciniara (INFUSORIA in the narrower sense) have
numerous slender vibrating processes, the cilia, a cuticle, and hence
fixed openings for the ingestion of food (cytostome) and for extru-
sion of indigestible matter (cytopyge).

21. Of great interest is the occurrence of two kinds of nuclei,
a functional nucleus (macronucleus) and a sexual nucleus (micro-
nucleus, paranucleus).
220 PROTOZOA,

22. In conjugation portions of the micronucleus are exchanged
and accomplish impregnation. The macronucleus degenerates and
is replaced by part of the fecundated micronucleus.

22. The classification of the Ciliata is based on the structure
and arrangement of the cilia.

23. The Holotricha have similar cilia over the whole body.
The Heterotricha have besides the total ciliation stronger cilia in
the neighborhood of the mouth (adoral ciliary spiral). The Pert-
tricha have only adoral ciliation. The “Mypotricha have, on the
ventral surface, the ciliary spiral and rows of cilia and coalesced
cilia. The Suctoria have cilia only in the young, later they
become attached and feed through suctorial tentacles.

24. Sporozoa are parasitic Protozoa, usually without organs
of locomotion or mouth. They take no solid food, but live by
osmosis on tissue fluids. In reproduction the encysted animals
produce spores (apparently always beginning with fecundation and
accompanied by a change of host). The spores divide again into
sporozoites. Besides, multiplication without change of host (auto-
infection) can occur.

25. The Gregarinida are temporary or permanent parasites in
cells. (Spores = pseudonavicelle, sporozoite = falciform embryo).
Coccidie, Hemosporida (cause of malaria, parasitic in blood
corpuscles).

26. The Sarcosporida (Rainey’s or Miescher’s corpuscles of
mammalian muscles) and Mycosporida (psorosperm capsules of
fishes, psorosperm = spore) live in tissues or hollow organs.

APPENDIX.

According to the evolution theory one should expect forms between the
Protozoa and Metazoa. The CaTaLLAcTa—spheres of ciliated cells which in
reproduction break il into single cells—have been described as such.

 

 

Fia. 160.—Section of half of Trichoplax adherens. (After Schulze.)
Peculiar many-celled animals whose position in the system is difficult to
decide are, further, Trichoplax adharens, Salinella salve, the ORTHO-
NecTIDA and the Dicyemipa. Z'richoplax (fig. 160) is discoid and consis‘s
of two epithelial-like cell layers separated by gelatinous tissue. The Ortho-
PORIFERA.

to

21

nectida and Dicyemida have a many-celled ectoderm, enclosing a solid mass
of cells in the Orthonectida, a single giant cell in the Dicyemida. Sadé-
nella consists of a single layer of cells enclosing a central digestive space.
Since the Dicyemida live as parasites in the nephridia of cephalopods, the
Orthonectida in worms and echinoderms, it is possible that their low or-
ganization is the result of degeneration.

METAZOA.

Excluding the Protozoa, all the branches of the animal kingdom
may be included under the head Metazoa, i.e. higher animals.
The point of union is that they consist of numerous distinct cells,
and that these cells are arranged in several layers. At least two
layers are present, a layer—the ectoderm—which bounds the body
externally, and a second, lining the digestive tract—the entoderm.
Between these two a third layer can oceur, which frequently is
separated by a body cavity into an outer or somatic layer forming
part of the body wall, and an inner or splanchnic layer forming
part of the intestinal wall. This middle layer is called mesoderm
no matter whether there be a body cavity or not.

The multicellular condition allows a higher development of the
organization, which appears in varying grades in the specialization
of tissues and organs. In no metazoan is there lacking a true sexual
reproduction, that is one by sexual cells, but the fact must not be
overlooked that many species reproduce (possibly exclusively) by
unfertilized eggs in a parthenogenetic manner. Besides the sexual
reproduction many species, especially the lower worms and colen-
terates, reproduce by budding and fission.

For all the Metazoa the segmentation of the egg is characteristic
to a high degree. The fecundated egg divides into numerous
cells which, as segmentation cells (blastomeres), remain united and
formthe germ. No Protozoan has a true segmentation. Division
there produces new individuals which either separate completely or
exceptionally remain in slight connexion as a colony.

PHYLUM II. PORIFERA (SPONGIDA).

The Porifera, or sponges, the most familiar representative of
which is the bath sponge (Luspongia officinalis), are, with few
exceptions, marine. In fresh water occur but a few species of
Spongilla (recently subdivided into several subgenera). The ani-
mals have no powers of locomotion, but are attached to stones or
plants, along the shores or at depths up to 6000 metres (4000
222 PORIFERA.

fathoms), They form spherical masses, thin crusts, small cylinders,
or upright branching forms. Frequently the shape varies so that
one cannot speak of a typical form. It was also difficult to decide
about the animal nature of the sponges. Striking movements of
the body are rare; only by aid of the microscope can one see
motion—the opening and closing of the pores and the streaming
of the gastrovascular system.

The simplest sponges, the Ascons (fig. 161), are thin-walled
sacs, fixed at one end, and with an opening, the osculum (func-
tional anus), at the other. The cavity of the sac, the ‘stomach,’ is
a wide digestive cavity into which water bearing food obtains
entrance through numerous small openings or pores in the body
wall. The basis of the body isa homogeneous or fibrous connective

    

Fie. 161. Fie. 162.

Fia. 161.—Olynthus. (After Haeckel.) e, spicules; i, eggs; 0, osculum; p, pores; u
stomach.’

Fig. 162.—Section of wall of Sycandra raphanus. (After Schulze.) e, ectodermal
epithelium; en, collared flagellate cells; m, mesoderm with connective-tissue
cells; 0, eggs; st, calcareous spicules.

tissue permeated with branching cells (fig. 162) covered externally

by a thin layer of pavement epithelium which is easily destroyed.

This epithelium (earlier called ectoderm) and the connective

tissue (mesoderm) are now regarded as a common layer, ‘meso-

ectoderm,’ since it has been shown that the pavement epithelium
is often genetically only connective-tissue cells which have spread
over the surface. On the other hand there is a distinctly differen-
tiated entoderm in the shape of a one-layered flagellate epithelium
lining the stomach, the cells of which (e) recall the Choanoflagellata
PORIFERA. O28

(p. 202), since they have collars surrounding the flagella. It has
therefore been attempted to regard each flagellate cell as an indi-
vidual, and the whole sponge as a colony of Flagellata, a view which
neglects the other tissues, not only the connective tissue and the
epithelium already mentioned, but sex cells, ameboid wandering
cells, and contractile fibre cells which close the pores. The taking
of food is accomplished by the collared cells.

Sponges of this simple ascon type are few. Asa rule the
sponges are more massive and have a more complicated canal
system (figs. 164-166). The first step towards complication is
seen in the Sycon sponges (fig. 163), in which the gastral cavity

 

Fic. 163.—Stereogram of Sycon sponge (orig.). a, ampulla with pores in their walls;
c, cloacal chamber, with the openings of excurrent canals; i, incurrent canals;
o; osculum,

Fic. 164.—Section of Leucortis pulvinar. (After Haeckel.) a, aboral pole; c, efferent
tee from the ampulle to the cloaca; e, ampulle; 7, mesoderm; 9, osculum,; v,
cloaca,

consists of numerous radial outpushings (the flagellate chambers

or ampulle) which alone contain the collared cells, while the cen-

tral cavity, now called cloaca, is here lined with pavement epi-
thelium. By increase of mesoderm and corresponding thickening
of the body wall the ampulle become separated from external and
cloacal surfaces by the ingrowth of tissue (Leucon type). The
ampullew nevertheless retain their connexion with both surfaces
by means of a system of canals. This canal system is double; one
part is incurrent and leads from the dermal pores to the ampulle ;
224 PORIFERA.

the other or excurrent from the ampulle to the cloaca, the two
being connected by the ampulle alone. Both may consist of lacunar
spaces (fig. 164), or have a more regular arrangement (fig. 165),

    
 

ae 2 \
t\ fee (eo 2
a Gas 4 [Ko ey -
Fig. 165.—Section of cortex of Chondrilla nucula, the skeleton omitted. (After Schulze.)
c}, afferent canals; c?, efferent canals; g, ampulla; m, cloaca; 0, osculum.

   

the canals from the pores uniting in trunks and these in turn
branching to go to the ampulla. The excurrent canals also show
a similar tree-like arrangement. Not infrequently extensive
subdermal or subcloacal spaces occur. The relations may be more
complicated by the development of several cloace, or these may be

  

Fic. 166.

Fia. 166.—Surface view of dermal pores of Aplysina aérophobia. (After Schulze.)
Fia. 167.—Ascyssa acufera. (After Haeckel.)

repressed; again by the branching of the sponge (fig. 167), while
still further the branches may anastomose (fig. 168), giving rise to
a network.

Sponges may reproduce asexually, small portions separating as
buds and producing new animals. Usually sexual reproduction
prevails. Hggs and spermatozoa arise from mesoderm cells (fig.
162), are fertilized and undergo segmentation at the point of origin,
and leave the parent as flagellate larva (fig. 169, 4). At fixation
CALCISPONGLE. 225

a kind of gastrulation takes place, the blastopore (fig. 169, 2)
closes, and the osculum, an entirely new formation, arises at the
opposite pole.

    

Fig, 168, Te. 169.

Fie. 168.—Leucetta sagittata. (After Haeckel.)
Fig. 169.—Development of Sycandravaphanus. (After Schulze.) 4, blastula; B, gas-

trula at the moment of fixation; ek, ectomesoderm; en, entoderm.

The sponges are frequently regarded as Ceelenterata, but seareely a
single homology can be drawn between the two. The cclenterate mouth
is different from either pores or oscula. Indeed it is disputed whether the
collared cells are entoderm. Nearly all sponges possess a skeleton secreted
by special mesoderm cells, and this skeleton affords the means, according
as it is composed of calcic carbonate or of silica, of dividing the sponges
into two classes. Besides, there are two groups, the Ceraospongiz and the
Myxospongiv, in which the skeleton is respectively of horny substance or
spongin or is lacking entirely. These, however, seem to be descendants of
the silicious forms.

Order I. Calcispongie.

The cale sponges are exclusively marine and mostly live in shal-
low water. They are grayish or white in color, of small size,
rarely exceeding an inch or soin length. The skeletal spicules
which arise in the mesoderm usually project through the epithelium
and form, especially in the neighborhood of the osculum, silky
crowns. One-, three-, and four-rayed spicules are recognized, these
ground forms presenting by unequal development a great variety
of shapes (fig. 170).

Sub Order I. ASCONES. Sponges with thin porose walls and a cen-
tral ‘stomach’ (figs. 161, 167).  Leacosolenia.*

Sub Order II, SYCONES. A cloaca present surrounded by ampulle
radially arranged (fig. 163). @rantia,* Sycon,* Sycandra.*
226 PORIFERA.

Sub Order III, LEUCONES. A complicated system of branching in-
current and excurrent canals in the thick walls connects the ampulle
with the outer surface and the cloacal cavity (figs. 164, 168). Leucetta,
Leucortis.

 

Fia. 170.—Sponge spicules. (From Lang.)

Order II. Silicispongie.

The siliceous sponges are richest in species and occur at all
depths of the sea, being frequently noticeable from their size (up
to a yard) and their bright colors. They are subdivided into
Triaxonia and Tetraxonia. In the Triaxonia the spicules com-
posing the skeleton—appearing as if of spun glass (hence
Hyalospongia, or glass sponges)—have three crossed axes (six
threads radiating from a common point)—hence Hexactinellida.
The mesoderm is scanty and in consequence the afferent and
efferent canals are loose-meshed lacunar spaces and the ampulle
large and barrel-formed. In the Tetraxonia, on the other hand,
the mesoderm is usually abundant and the canal system well
developed. The four-axial spicules of the Tetractinellide must
be regarded as the fundamental skeletal type. From this are
derived the compact agglutinated frameworks of the Lithistide
and the monaxial spicules of the Monactinellide.

In both groups the spicules may be united by secondary deposits of
silica to an extensive framework; or the union is effected by spongin,
which, if the spicules disappear, forms the whole skeleton (horny
sponges), or, as in slime-sponges, the whole skeleton may be lost.

Sub Order I. TRIAXONTA. The HEXACTINELLIDE belonging here live
chiefly in the deep sea, and for a long time only a few species were known:
Euplectella aspergillum, Venus’ flower-basket, tubular, consisting of fine
spicules. Hyalonema. Apparently the horny sponges Aplysina and
Aplysilla, as well as the slime-sponges, Halisarca,* have descended from
this group.

Sub Order IT. TETRAXONTA. Typical representatives are the largely
extinet Lrruistmpz: (of which some genera—Discodermia—persist in deep
SUMMARY OF IMPORTANT FACTS. 297

seas) and the TETRACTINELLIDH: Geodia.* Near here apparently belongs
Oscarella,* without a skeleton (Myxospongia).

In the MONACTINELLIDA the spicules are united by spongin (Corna-
cuspongia), and can even be entirely replaced by that substance. Numer-
ous marine forms, among them Chalina,* and also the fresh-water
SPONGILLIDE (Spongilla,* Hphydatia *), widely distributed as encrusting
masses on submerged sticks and stones. The natural color is light gray,
but they are usually colored green by Alga. They are distinguished
from most marine relatives by the formation of gemmule or statoblasts.
At times the protoplasm divides into round bodies, as large as the head of
a pin and these become surrounded by a firm membrane strengthened in
many forms by collar-button-like spicules, the amphidiscs. These stato-
blasts remain entangled in the skeleton and survive times of freezing or
drought. On return of good conditions the contents escape and form
small Spongille, often utilizing the old skeleton. This process recalls
encystment among the Protozoa.

When the spicules entirely disappear and nothing but the spongin
fibres remain we have the horny sponges or CERAOSPONGLE. The skeleton
consists of a horny substance, spongin, which differs chemically from the
substances of true horn—keratin. This spongin is always laid down in
long fibres by peculiar cells, the spongioblasts, and it always consists of
concentric layers. The fibres interlace, branch, and unite into a skeleton.

The best known horny sponges are the bath sponges, Huspongia
officinalis,* occurring in the Mediterranean, West Indies, Florida, and
other seas in many varieties. Best of all are the Levant sponges (var.
mollissima). Sponges of commerce consist only of the skeleton, the ani-
mal parts being killed and, after decay, washed away with fresh water.
Less valuable are Huspongia zimocca and Hippospongia equina,* the
horse-sponge, while the Cacospongie are useless.

Summary of Important Facts.

1. The sponge body is largely a mass of connective tissue cov-
ered externally with pavement epithelium (meso-ectoderm) and
penetrated by canals.

2. An entoderm of collared flagellate cells occurs only in the
ampulle or flagellate chambers which are intercalated between
incurrent and excurrent canals (in ascons in the central cavity).

3. The animals receive nourishment through fine pores in the
body wall; indigestible bodies are cast out through one or several
oscula.

4, Since nerves, muscles, and sense organs are lacking or very
weakly developed, the animals show the most inconspicuous move-
ments.

5. Sponges are divided into Calcispongiw and Silicispongie
according to the character of the skeleton.
CQ@LENTERATA.

bo
bo
CO

PHYLUM III. C@LENTERATA (CNIDARIA, NEMATO-
PHORA).

The animals belonging to the celenterates were formerly called
Zoophyta (plant-animals). They were united by Cuvier with the
Echinoderma to form the type Radiata, a union which Leuckart, the
father of the name Coelenterata, set aside because a special intes-
tine and a special body cavity occur in the Echinoderma, while in
the Celenterata there is but a single system of cavities in the
body. Kach of the three names indicates certain important
characters of the group.

(1) The name Zoophyta was selected with regard to the gen-
eral appearance. Most coelenterates, like the plants, are fixed
and form bush-like or mossy colonies by incomplete budding. This
resemblance is but superficial, for in any accurate investigation
there cannot be the slightest doubt of the animal nature of any
Celenterate. The name therefore must not be understood to
imply that these are doubtful forms which stand on the border
between plants and animals. Besides, there are not only fixed but
free-moving forms which swim in the water with great ease.

(2) Most Colenterata are radially symmetrical. There is a
main body axis one end of which passes through the mouth and
the other through the blind end of the digestive tract, and the
organs of the body are radially arranged around this so that the
body may be divided into symmetrical halves by numerous planes.
In the higher Coelenterata this may be replaced by a biradial
symmetry or even by bilaterality (Ctenophora, many Anthozoa).

(3) The term Celenterata is given these animals because they
contain a single continuous celenteron or gastrovascular cavity.
Jn the simplest species this is a wide-mouthed sac into which food
passes for digestion. The single opening into it serves at once as
mouth and anus; the sac itself is the alimentary tract. Frequently
lateral diverticula or branched canals are given off from the central
sac which distribute the nourishment to the peripheral parts of the
body, and thus functionally replace the vascular system of higher
forms.

Since this gastrovascular system is primarily for nourishment,
it is an error to call it a body cavity and to say that the celenterates
are stomachless. On the other hand, the term ‘ ceelenteron,’ that
isa cavity which is at once gastric and celomic (p. 158), is perfectly
defensible, since in many higher animals which possess a true body
CQ@LENTERATA. 229

cavity this is seen in development to arise as diverticula from
the primitive stomach (enteron). Since such diverticula occur in
celenterates without becoming independent, one can say that the
gastrovascular system consists not only of intestinal portions but,
in potentia, of the celom as well.

To even a superficial observation the Celenterata are more
clearly animals than are the sponges. The single animals, though
often united in colonies, and fixed to some support, are capable of
quick and energetic motion. These movements are most striking
in the tentacles—long tactile threads, in the neighborhood of the
mouth, which have the functions of feeling for food, grasping it,
and conveying it to the mouth. The means of killing the prey are
the cnide, nematocysts, or nettle cells (fig. 171), which with rare

 

Fic. 171.—Nettle cells of Celenterata. (After Hertwig, Lendenfeld, and Hamann.)

exceptions in Protozoa, Turbellaria, and molluscs occur in no
other group. These structures, of great systematic importance,
are oval vesicles with fluid contents and firm membrane. Each is
drawn out at one end into a long tube, so delicate as to appear as
a thread (hence an additional name, thread cells). This thread is
sometimes armed throughout its length with retrorse hooks, or it
may have only a few stronger hooks on its basal portion, which is
thicker than the rest. In the resting stage the thread is spirally
coiled inside the cell. On stimulation the thread is quickly
extended (‘explosion of cell’) and produces a wound into which
passes the irritating fluid contents. Some ce@lenterates (¢9.
Physalia) can produce in this way very painful nettling even in man.

The nettle capsule arises asa plasma product inside a cell.
When fully developed the nettle cell extends to the surface and
ends with a tactile process (cnidocill) which upon contact stimu-
lates the protoplasm and causes the explosion. The cell itself is
frequently enclosed by a muscular sheath or a network of muscle
fibres.

Among the ccelenterates both sexual and asexual reproduction
230 COHLENTERATA.

may occur, the latter usually by budding, more rarely by division.
Sexual and asexual types of reproduction can be combined in the
same species, producing an alternation of generations,

In comparison with the sponges the Celenterata may be called
epithelial organisms. A mesoderm (‘ mesoglea’) may be entirely
lacking or may have but a subordinate development. The ectoderm
and entoderm, on the other hand, are the important tissues—pro-
ducing muscles, nerves, sense organs, sexual products and cnide.
Hence the group is often called Diploblastica—two-layered animals.

Class I. Hydrozoa (Hydromeduse).

According to varying standpoints the Hydrozoa can be placed
either higher or lower than the Anthozoa in the system, since in the
former group two forms are frequently introduced into the life
history, one agreeing well in structure with the Anthozoa, the
other standing on a higher grade. The first is the sessile and
usually colonial polyp, the second the free-swimming medusa, well
provided with sense organs. These are usually related to each
other by an alternation of generations. The polyp is asexual and
by budding produces meduse; the medusa, on the other hand, is
the sexual stage, and from its eggs polyps arise.

The polyp of the Hydrozoa is the hydropolyp, forming in the
branch of celenterates an important archetype from which all
other conditions—medusex, seyphopolyp, and even the coral polyp—
may be derived. Our best example of this is the fresh-water
Hydra, so common in pools and streams. The body (fig. 172) isa
sac, the hinder closed end of which, the pedal disc, is used for
attachment. The other end bears the mouth which leads to the
internal gastrovascular (digestive) cavity. Around the mouth is a
circle of tentacles used in capturing food (mostly small erustacea).
These are outgrowths of the body wall; the circle dividing the
body into a peristome inside the circle and a column constituting
the rest of the outer wall.

Hydra has but two body layers (fig. 173), an entoderm of
flagellate cells lining the gastrovascular space, and the ectoderm
covering the outer surface. Between the two is the supporting
layer (mesogl@a), a structureless membrane without cells and hence
not a body layer. Both layers consist of epithelial muscular cells
(cf. p. 92), the basal ends of which are produced into smooth
muscle fibres, those of the ectoderm running lengthwise, those of
the entoderm around the body. The ectoderm further contains
ganglion, nettle and sex cells. The nettle cells on the tentacles
HYDROZOA, 231

are crowded into small ridges or batteries. The sex cells (at cer-
tain times) produce swellings on the column; a circle of male
swellings close beneath the tentacles, the female cells farther down
the column (fig. 172). Individuals reproducing by budding are
more common than the sexually mature (fig. 90). Small eleva-

    
 

b

    

en s eke
Fig. 172. Fig. 173.
Fic. 172.—Hydra viridis,* testes above; ovarian enlargement below.

Fic. 173.—Body layers of Hydra. (After Schulze, from Hatschek.) c¢, cuticula; cn,
nettle cells; ek, ectoderm; en, entoderm; s, supporting layer.

tions appear on the column, enlarge, form tentacles, and at last a

mouth, after which they may separate from the parent.

In the sea are numerous hydroid polyps which, while agreeing in
the main with Hydra, are distinguished from it in two important
respects: (1) they do not directly produce sexual organs; (2) they
reproduce asexually, and by incomplete budding form persistent
colonies. In this formation of colonies a series of parts have
arisen which require special designations (fig. 174). The separate
animals are the hydranths, and are connected together by a system
of tubes, the cwnosarc, which, like the hydranths, consist of ecto-
derm, entoderm, and mesog]lwa, and since the gastrovascular space
continues in them, these effect a distribution of food throughout
the colony. The cenosarcal tubes may creep over some support
(stone, alga, snail-shell, etc.) and form a network, the hydrorhiza,
or it may stand erect and tree-like, forming ahydrocaulus. Usually
both hydrorhiza and hydrocaulus occur in the same colony.
232 C@LENTERATA.

 

Fic. 174.—Campanularia johnstoni. (After Allman.) a, hydranth with hydrotheca;
b, retracted: d, hydrocaulus; f, gonotheca, with blastostyle and medusa buds;
g, free medusa.

 

Wie. 175. Section of Hudendrium ramosum, ek, ectoderm; en, entoderm; p, perisarc;
s, Supporting layer.
HYDROZOA. 233

Usually the colony is strengthened and protected by the perisarc,
a cuticular tubular secretion of the ectoderm. In some (fig. 175)
the perisarc stops at the base of the hydranth; in others (fig. 176)
it expands distally into a wide-mouthed bell, the hydrotheca, into
which the hydranth may retract at times of danger. In rare cases

 

Fie. 178.—Campanularia geniculata. ek, ectoderm; en, entoderm; p, perisarc, ex-
panded around hydranth to 1 hydrotheca; s, supporting layer.

this perisarc may be greatly increased and calcified, forming large

coral-like masses with openings from which the hydranths may

protrude (fig. 177).

 

Fic. 177.—A bit of Millepora alcicornis, enlarged. (After Agassiz.)

The lack of sexual organs, which distinguishes the marine
species from the fresh-water Hydra, is explained by the fact that
sexual individuals of special form are produced from the colony
23 CUILENTERATA.

by budding. These, the meduse, may separate early from the
colony and swim freely. A medusa (figs. 178, 179) has the form

 

 

Fia. 178.—Rhopalonema velatum. c, ring canal; e, exumbrella; g, gonads; h, otocysts:
m, stomach; n, nerve ring; 0, mouth; s, subumbrella; t’, t’’, tentacles of first
and second order; v, velum.

of a dome-like or disc-like bell and consists chiefly of an ex-

traordinarily watery jelly. The bell or umbrella of the medusa is

covered on both its surfaces—the concave or subumbrella, the con-
HYDROZOA. 235

9

vex or exumbrella—with ectodermal epithelium. At the margin
of the bell the sub- and exumbrellar ectoderm is produced into a
two-layered sheet with a central opening, the velum or craspedon
(fig. 178, B, v) of systematic importance, since these medusa are
often spoken of as Craspedota. Tentacles (usually 4, 8, or
multiples in number) also arise from the edge of the bell just
outside the velum.

 

Fie. 179.—Tiara pileata, (After Haeckel, from Hatschek.)

Comparable to the tongue of the bell or the handle of the
umbrella is the manubrium, hanging from the highest point of the
subumbrella and bearing the mouth at its tip. It contains the
chief digestive space from which radial canals run on the sub-
umbrellar surface to a ring cana] in the margin of the umbrella.
The radial canals are usually four in number, but in some species
the number is increased during growth even to a hundred or more.
Manubrium and canals are lined by entoderm, which also extends
into the tentacles and forms their axes.
236 CQ@LENTERATA.

All other important organs arise from the ectoderm. Gonads
arise in many species (fig. 179) from the ectoderm of the manu-
brium; in others from the same layer covering the subumbrellar
surface of the radial canals (fig. 178), forming in either case con-
spicuous, often orange or red, thickenings. Longitudinal ectoder-
mal muscles move the tentacles in a snaky fashion, whence the
name medusa. Circular striped muscles run on the subumbrellar
side of bell and velum, causing the characteristic motion. By this
contraction the bell becomes more arched and narrowed, while the

 

Fia. 180.—Otocysts of Meduse.—A, Cunina ; B, Rhopalonema :C. Carmarina (Trachy-
meduse); PD. Octorchis (Leptomedusan), a, epithelium: h, auditory cells ; hf,
origin of hairs; hh, auditory hairs ; hp, auditory cushion ; ©, otoliths 3 n, audi-
tory nerve ; nr, nerve ring.

velum (which hangs down when at rest—fig. 178, 4) contracts

like a diaphragm across the mouth of the bell (fig. 178, B). Since

water is thus forced out through the opening the medusa is forced
forward by the reaction.

The circular muscles of umbrella and velum are separated by
the nerve ring, with which are connected the sensory organs—
eyes of the simplest type; red pigment spots with or without a
lens (fig. 81); and open or closed auditory vesicles (otocysts).
Tactile hairs are abundant on the tentacles.
HYDROZOA. 237

The auditory organs are of two types, both beginning as free organs
and receiving their highest development as closed vesicles (otocysts). One
type, the tentacular organs, occur in the Trachymeduse, the other, or
velar organ, in the Leptomeduse. The tentacular organs are modified
tentacles, the entodermal axis of which forms the otoliths and the
ectodermal covering the sense cells. In the Aiginidse (Fig. 180, A) the
club-like tentacles, seated on an auditory cushion, project freely into the
water; in the Trachynemidew (Fig. 180, B) they are partially transformed
into vesicles by the upgrowth of epithelium, and in the Geryonide (Fig.
180, C) they are completely enclosed and are sunk in the jelly of the bell.
The velar organs of the Leptomeduse are placed on the subumbrellar sur-
face of the velum. They may be either simple pits (Fig. 180, B), or the
mouths of the pits may close (Fig. 180, D). In these both sense cells and
otoliths are ectodermal. Eyes and otocysts occur in different forms, a
fact which formerly lead to a division of medusze into ocellate and vesicu-
late groups.

While polyps and medusa apparently differ so greatly from each
other, their morphology shows that the meduse are only highly
modified polyps adapted to a swimming life. The long axis of
the polyp has been greatly shortened (fig. 151) and the cylindrical

 

Fic. 181.—Diagram of sections of (A) a polyp and (B) a medusa. ek, ectoderm; ck’, of
exumbrella; ek?, of subumbrella; ek3, of manubrium; el, endoderm (cathamnal)
layer arising from obliteration of digestive space; en, entoderm ; 1, ring canal;
s, subumbrella; t, tentacles; v, velum; .v, supporting layer (gelatinous in B).

body developed into a disc; the mesoglea of column and dise thick-
ened to a conspicuous layer of jelly; while manubrial cavity,
radial and ring canals are to be interpreted as remnants of the
large gastrovascular space of the polyp, obliterated in part by the
pressure of the mesoglea. To the parts thus formed only the
velum and sense organs are added.

This comparison of medusa with polyp is of importance in
understanding the development, which usually is complicated by
an alternation of generations. From the eggs of the medusw a
small ciliated embryo (planula) escapes, which becomes attached,
238 CQ@LENTERATA.

develops mouth and tentacles, and, by budding, produces a
hydroid colony. This hydroid colony lacks sexual organs. It
produces by budding the sexual individuals, the meduse, which
separate and swim freely. Since polyp and medusz are morpho-
logically comparable, there is a time before the escape of the
meduse when the colony is polymorphic, consisting of asexual
individuals (hydranths) which reproduce only asexually and of
others which have taken over the sexual reproduction (medusz),
Hence we conclude that the alternation of generations here has
arisen from a division of labor or polymorphism of individuals
originally of equivalent value, in which some individuals (the
sexual) have separated and acquired a peculiar structure.

While alternation of generation has arisen from polymorphism,
it can again produce it. This occurs when the medusa, instead
of separating, remain permanently attached to the colony. They
then degenerate into ‘ sporosacs,’ which always lack mouth, tentacles,
and velum (fig. 182), often also radial and ring canals, so that at last

 

Fie. 182.—Comparison of a medusa, and a sporosac (orig.). 4, fully developed medusa;

, medusa with the manubrium closed, still attached to the blastostyle; C,

medusa reduced toa simple manubrium (sporosac); D, last stage, eggs being pro-
duced in the body wall (Hydra).

there remains only the manubrium (‘spadix’) and the sexual
organs, the latter enveloped by the rudiments of the umbrella.
Since medusee and sporosac replace each other in closely allied
species, a common name, gonophore, has been applied to both.
This developmental history may be modified in two ways:
either the polypoid or the medusan generation may be suppressed.
In the first case we have polyps which reproduce both sexually and
asexually, in the other medusxe whose eggs develop directly into
other meduse. (A few meduse may produce new meduse by
budding.) Thus we can have four conditions: (1) Polyps which
produce sometimes asexually, sometimes sexually, but always
HYDROZOA. 239

polpys; (2) Medusa which always produce medusew; (3) Polyps
and meduse in alternating generations; (4) Polyps and sessile
medusz (sporosacs) united in a polymorphic colony.

The Hydrozoa are almost exclusively marine. The colonial forms occur
mostly on rocky coasts down to a depth of 100 fathoms, but have been
found in water 4000 fathoms deep. The medusz belong to the pelagic
fauna. For a long time the only fresh-water species known belonged to
the cosmopolitan genus Hydra, but more recently both hydroid (Proto-
hydra ryderi,* America ; Polypodium hydriforme, Russia) and medusan
forms (Limnocodium sowerbyi, Brazil; Limnocnida tanganyice, Africa ;
Halomises lacustris, Trinidad) have been found. Cordylophora lucustris*
occurs in the brackish waters of Europe and America.

The Hydrozoa may be classified according to characters, derived either
from the hydroid or the medusan stage. The former basis gives us four
groups:

(1) Hydraria. Polyps with asexual and sexual reproduction ; no per-
sistent colonies, no perisarc, no gonophores (fig. 172).

(2) Tubularize. Mostly colonial, with perisarc but without hydrothece.
Reproduction by gonophores (medusz or sporosacs, figs. 91, 175).

(3) Campanulariz. Colonial, with perisarc and hydrotheca. Repro-
duction by gonophores arising in special perisarcal envelopes, the gonotheca
(figs. 174, 176).

(4) Hydrocorallina. Colonial, with massive, calcified perisarc, resem-
bling coral. Reproduction by sporosacs or short-lived meduse.

 

Frq. 183.—American Trachy and Narcomeduse. 4, Liriope scutigera. (After Fewkes.)
B, Cunocantha octunaria. (After Brooks.)

The characters derived from the meduse also give five groups :

(1) Anthomeduse (Ocellate), Gonads on the manubrium ; no audi-
tory organs ; eyes usually present ; polyp generation present.

(2) Leptomedusee. Gonads on radial canals ; usually velar auditory
organs ; polyp generation present.

(3) Trachymeduse. Gonads on the radial canals ; tentacular auditory
organs; develop directly to medusze (fig. 183, A.)

(4) Narcomeduse, Gonads on the manubrium or gastral pouches ;
tentacular auditory organs ; no polypoid stage (fig. 183, B.)
240 CQ@LENTERATA.

(5) Siphonophora. Polymorphic, free-swimming colonies of Anthome-
dusz ; no polyp generation,

From this it is seen that there are medusee without polyp stages and
polyps without medusz, so that a true system must take into account both
these features. When this is done and life histories are traced it is seen
that the Anthomedusz and the Tubulariz are connected by an alternation
of generations, and the same holds good for Leptomeduse and Campanu-
lariz. There are three groups—Trachymeduse, Narcomeduse, and
Siphonophora—without a hydroid stage, and two in which the polyp
plays the chief rdle, the medusa being rudimentary in the Hydrocoralline,
lacking in the Hydraria. The hydroid polyps are usually but a few

cae

i

if
oS ¢

 

Fia. 184.—American hydrozoan meduse. (Mostly after Fewkes.) A, Eutima gracilis;
B, Hydrichthys mirabilis; C, Obelia; D, Euchilota ventricularis; E, Lizzia grata; F,
Turritopsis metricula; G, Dipurena strangulata.

millimetres or fractions of a millimetre in size, but the huge Jfonocaulis

imperator, of the deep seas, a yard in length, forms an exception. The

colonies are usually only a few inches in extent. The meduse have bells
varying between a millimetre and a few inches in diameter, reaching in

Aiquoria forskalea a diameter of sixteen inches.

Order I. Hydraria.

Until recently only the cosmopolitan species of Hydra were known.
During most of the year they reproduce by budding (fig. 90), only occa-
sionally developing gonads (fig. 172). The eggs remain in connexion
with the mother during segmentation, and later form an embryonal shell,
protecting them from drought or cold. In this ‘eneysted stage’ they
can be distributed by wind or water birds. These animals formed the
basis of the celebrated researches of Trembley on regeneration. He
showed that small portions when they contained both body layers could
I. HYDROZOA: HYDROCORALLINE. 241

regenerate the whole animal. His experiments upon turning the animals
inside out have not been fully confirmed ; for in such cases the layers
resume their normal positions. Hydra grisea * (fusca), large brown
species ; H. viridis,* green, from the presence of symbiotic alge. Pro-
tohydra ryderi,* without tentacles. Polypodium hydriforme, parasitic
on sturgeon eggs in Russia, needs more study. The marine Haleremita
cumulans may belong here.

Order II. Hydrocoralline.

Exclusively marine, forming colonies of thousands of individuals whose
calcareous skeletons so closely resemble true corals that they were asso-
ciated with them until the animals were studied. Jfillepora alcicornis,*
stag-horn coral, in Florida. The rosy Stylasters occur in tropical seas.

Order III. Tubularie = Anthomeduse (Gymnoblastea).

As a rule these colonial forms with perisare but without hydrotheca
produce anthozoan meduse, but there are forms like Clava* (pink, on

de Moe,
See
we =

wt
g

dy
1b

WONG

  

Fia. 185.—American Tubularian hydroids. 4, Myriothelia phrygiana (after Danielssen
and Koren); B. Sarsia rosaria(atter Fewkes); C, Monocaulis pendula (atter Agassiz),
D, Clava leptostyla; E, Parypha crocea; F, Podocoryne mirabilis (after Agassiz).

rockweed) and Hydractinia * (on shells inhabited by hermit crabs)
which have sporosacs. Indeed the genera Corymorpha * and Jfono-
caulis * are only differentiated by the existence of meduse in the former
and of sporosacs in the latter. In the forms with alternation of genera-
tions different names are applied to the hydroid and medusan stages as
follows :

H[yDROID. MEDUSA.
Pennaria. Globiceps.
Syncoryne. Sarsia.
Bougainvillea, Hippocrene, Margelis.
Gemmelaria, Gemmaria,

Podocoryne. Dysmorphosa.
242 CO@LENTERATA.,

‘Other common genera in American waters are, of hydroids, besides
those mentioned, Hudendrium, Tubularia, and Thamnocnida, of medusa,
Tiaris, Turritopsis, Dipurena, Lizzia, Nemopsis, and Hydrichthis.

Order IV. Campanularie — Leptomeduse (Calyphoblastea).

These forms are readily distinguished from the last by the fact that
they are always colonial and possess hydrothecz, the meduse always being
Leptomeduse (p. 239). A peculiarity of the group is the existence of
gonothecee, closed perisarcal envelopes, inside which the gonophores arise
from the blastostyle, a specialized polyp, without mouth or tentacles
(fig. 174, 7). The typical Campanularie produce meduse, while some
forms, like Thaumantia* and siquoria* have no hydroid stage, and
on the other hand Sertwlaria* and Plumularia* have no medusa stage.

 

Fig. 186.—American Campanularians. (After Verrill.) | 4. Clytia noliformis ; B,
Calycella syringa; C, Obelia dichotoma ; D, Opercularella pumila.

Other common genera, Clytia,* Diphasia,* and Aglaophenia* among
hydroids ; Obelia,* Tima,* Rheymatodes* among medusze. Possibly the
fossil group of GRAPTOLITES belongs near here. Only the perisare is
known, and this is composed of hydrothecie, in which it is supposed the
hydranths occurred.

Order V. Trachymeduse.

These meduse, mostly from warmer seas, have no hydroid stage. The
characters are given on p. 239, Trachynema, Liriope * (fig. 183), and Cam-
panella in our own waters, Geryonia, ete., in Europe.

Order VI. Narcomeduse.
In addition to the characters on p. 239 may be added the fact that the
tentacles arise from the outside above the rim of the bell. Cunocantha *
(fig. 183), and Cunina * in our warmer waters, <Zyina in Europe.
I, HYDROZOA: SIPHONOPHORA.

bo
ese
ew

Order VII. Siphonophora.

The Siphonophora are among the most beautiful of pelagic
animals, some transparent, some brightly colored. Each (fig.
187) consists of a colony of individ-
uals springing from a common co-
nosarcal tube which is strongly mus-
cular and contains a central canal
lined with entoderm by which the
members of the colony receive their
nourishment. At one end the tube
is usually closed by a float filled with
air, the pneumataphore, which acts
as a hydrostatic apparatus, and keeps
the colony vertical in the water.

The individuals, springing from
the ccenosarcal axis, perform differ-
ent functions and hence have differ-
ent structures. Close behind the f
float commonly come several swim- *
ming bells (nectocalyces) which re-
tain of medusal structures only those
(bell, velum) necessary for swimming
and those (ring and radial canals)
for the distribution of nourishment
received from the common tube.
Then come, scattered through the
colony, the covering scales, for pro-
tection, firm gelatinous plates which
have lost the ring canal, the muscles,
and the bell shape of the meduse.
Food is taken by wide-mouthed feed-
ing tubes (ky) which may be com-

sb

 

 

Y Fia. 187.—Diagram of Siphonophore.
pared to polyps (fig. 57) or the ma-" (From Lang). 1-H, groups of dit
jf es r oe ferent individuals; ds, covering
Thev digest scales; yt, gonophores + hy, feeding

nubrium of a medusa. sly.
the food by means of large masses of ae ae hel Gee
glands (‘liver bands’) and convey it CMY%)s sh stk.

by the central tube to all the members of the colony. At the
base are long muscular tentacles (4) from which small lateral
threads depend, each ending in a brightly colored swelling, the
nettle head, composed of large, closely packed nettle cells. These
are the cause of the nettling produced by the siphonophores, which
in many is so severe as to be feared by man. The ‘feelers’ (p)
244 COILENTERATA.

recall mouthless polyps and manubria; they are very sensitive and
mobile and, while tactile, apparently in some cases are digestive
organs. Latest to develop in the colony are the sexual bells.
They are usually brightly colored and resemble small mouthless

 

F1a. 188.—Stephalia coronata. (After Haeckel, from Lang.) A, in section; au, canal
to float; ka, canal system of stalk; 0, mouth; other letters as in fig. 188.
Anthomeduse without tentacles. They but rarely (Chrysomitra)
separate from the colony, but usually persist as more or less reduced

sporosacs.

From this it follows that the Siphonophora afford fine examples
of division of labor and of the consequent polymorphism of indi-
viduals. This can indeed be carried so far that many convey the
impression of being individuals witha multiplicity of organs. The
Siphonophora are all marine, and occur most abundantly in trop-
ical seas.

Sub Order TI. PHYSOPHOR# (Physonecte). Float present, but
small; next a large series of swimming bells, and then the other members
of the colony. Physophora, Agalmia, Nanomia* (fig. 189).

Sub Order IT. CALYCOPHORAE (Calyeonectie). Float lacking ; one
or two large swimming bells ; the other individuals in groups which fre-
quently separate before becoming mature, and were once regarded, under
the name Hudowia, as distinct animals. Praya, Diphyes* (tig. 189), in
warmer seas.

Sub Order III, CYSTONECTUE. Float greatly enlarged ; the ceno-
sareal tube reduced, the individuals (no covering scales nor swimming
II, SCYPHOZOA. 245

bells) being attached to the under side of the float. Physalia, the Portu-
guese man-of-war, occurs as far north as New England. It is brightly
colored, and, sitting high on the water, is driven about by the wind. It
stings very severely.

Sub Order TV. DISCONANTH.®. Float a flattened dise with con-
centric air chambers; the manubrium projects from the centre of the lower

 

Fic. 189.—American siphonophores. A, Nanomia cara. (After A. Agassiz.) B. Velella
meridionalis, (After Fewkes.) C, Diphyes praya. (After Fewkes.)

surface of the float. Porpita,* with circular disc. Velella * (fig. 189), the
paper sailor, has a triangular ‘sail’ on the disc. Both are tropical and
subtropical.
Class II. Scyphozoa (Scyphomedusz).
The Scyphozoa parallel the Hydrozoa in that they frequently
have an alternation of generations. The asexual generation is the

 

Fia. 190. Fia. 191.
Fia. 190.—Scyphostoma of Aurelia aurita. (From Korschelt-Heider.) k, perisarc cup;
pb, proboscis; s, stalk; t, gastral folds; tr, ectodermal tunnels. |
Tie. 191.—Section of Scyphostoma. (From Hatschek.) g7, gastric pouches; s, gas-
tric folds; sm, muscles.

scyphopolyp or scyphostoma, the sexual an acraspedote medusa.
In contrast to the Hydrozoa the asexual stage plays a subordinate
246 CUMLENTERATA.

role; it is closely similar, even in the most different species, and
can even be lost (Pelagia), while the meduse are always well devel-
oped and present great variety of form.

The scyphostoma (figs. 190, 191) recalls superficially Hydra,
but is distinguished externally by a small perisarcal cup in which
the aboral end is placed. Internally there are four longitudinal
folds projecting into the gastral cavity and extending from the
margin of the mouth to the opposite pole. These septa or teniole
appear in cross-section as small folds of entoderm supported by ¢
process of the supporting layer. They are important morpholog-
ically, since in budding they produce the gastral tentacles ( phacellw)
of the meduse. Further, they are the first appearance of the septal
system, so strongly developed in the Anthozoa.

The acraspedote meduse are large forms (four inches to four
feet or more in diameter) with an
arched umbrella often of almost
cartilaginous consistency. They
are distinguished from the craspe-
dotes externally by notches in the
margin of the umbrella, dividing
the periphery into lobes. In the
common forms at least eight lobes
occur (figs. 192, 193), each notched
at its tip, and in the notch the
sensory pedicels bearing both ears
and eyes and covered by a lappet.
Fia. 192.—Ephyra of Cotylorhiza. In some (fig. 193, J, JZ) the sen-

(After Claus.) gt, gastral tentacles :
(phacellee); rk, marginal (sensory) SOrY lobes follow each other, but in

: others the intermediate region is
also notched, the sensory pedicels then being found only on careful
search (fig. 194). Tentacles, when present, spring from the
notches of the intermediate region.

The sensory pedicels predicate the position of eight principal
radii, of which four are called the perradii, the four alternating
with them the interradii. Adradii are radii lying between the
principal radii.

The lobing of the umbrella influences all other structures.
There is no velum (hence these are called Acraspedia), its place
being taken by a thick muscular mass (fig. 86, 22) on the sub-
umbrellar surface. Instead of a nerve ring there are eight nerve
centres connected with the sensory pedicels. Each of these
pedicels (fig. 195) is a modified tentacle with an entodermal

 
II, SCYPHOZOA. 247

   

pS ES)

Fic. 193.—Ulmaris prototypus. (From Hatschek ) J, radii of first order (perradii);
I, radii of second order (interradii); /, marginal lobes; 0 oral lobes (cut away on
right side); t, tentacles (adradial); the gonads (right side) are interradial.

 

Fra. 194.—Polyclonia frondosa* and one of its branching oral Jobes, showing the
closed grooves (s). (After Agassiz )
248 CQ@LENTERATA.

axis and an outer layer of ectoderm. The entoderm forms an
otolith sac at the tip, while the ectoderm furnishes a nervous cushion
of ganglion cells and fibres and usually a simple eye spot. Less
evident is the effect of the lobing on the internal organs. The
gastrovascular system begins with a quadrate or X-shaped mouth
(fig. 193). The perradial angles of the mouth are usually produced
into Jong curtain-like oral tentacles of great use in the capture of
food. The ‘stomach,’ which begins just inside the mouth, gives off
four interradial (i.e., alternating with the corners of the mouth)

 

Fre. 195.—Sense organs of Awrelia aurita. (After Schewiakoff.) ec, ectoderm; en,
entoderm; gv, gastrovascular space; m, supporting layer ; 0, cup eye; oc, pigment
eye; ot, otolith sac 3; rg, olfactory groove.

pouches, the gastrogenital pockets. The epithelium of these
pouches produces on the one hand a group of small gastral tentacles
(phacellz), each extremely mobile and supported by an axis of
mesoglea; on the other plaited folds of the gonads, these being, in
contrast to the Iydrozoa, of entodermal origin. In this the
Scyphomeduse show relationships to the Anthozoa. From the
central digestive sac arise the peripheral portions. These consist
in the larval medusz (Ephyvra stage, fig. 192) of eight radial canals
to the sensory pedicels, and in most adult meduse of these same
pouches and eight others, adradial in position, to the tentacles. In
some this primitive arrangement is complicated by an extensive
network of tubes (fig. 193).

In the species with an alternation of generations the egg pro-
duces a ciliated larva (fig. 196) which attaches itself and develops
into a scyphostoma. This scyphostoma is always capable of ter-
Il, SCY PHOZOA. 249

minal, and often of lateral, budding. The lateral buds always
produce new scyphostome, the terminal, meduse. In the latter
the scyphostoma develops into a strobila, becoming divided by
circular constrictions into a series of saucer-like discs, the young
jelly-fish. As the successive discs become ready they separate
from the pile and swim away as ‘ephyre.’ At first the ephyre
(fig. 192) have only four gastral tentacles, parts of the gastral
folds of the scyphostoma (p. 216); they lack marginal tentacles,

 

Fie. 196.—Development of Aurelia aurita. (From Hatschek.) First row, growth of
planula to scyphostoma; below, strobilation (separation of ephyre): left, oral view
of scyphostoma; right, two ephyre.

but have the eight lobes and the corresponding sense pedicels.
Since the ephyre differ markedly from the adult medusez and only
gradually change into the sexual form, the alternation of genera-
tions is complicated by a metamorphosis. This metamorphosis
persists in some cases (Pelagia noctiluca) where the alternation of
generations is suppressed; the egg develops directly into an
ephyra, which becomes transformed into the adult jelly-fish. On
the other hand no case is known where the medusa generation is
dropped out and the scyphostoma give rise sexually to other scy-
phostome.
250 COSLENTERATA.

Some forms differ from the foregoing description in structure and ap-
parently in development. Some have only four sen-
sory bodies, the places of the other four being taken
by tentacles. In these cases the sensory organs
lie (Peromedusa) in the same radii (?.e., interradii)
as the sexual organs or (Cubomeduse) the sense
organs are perradial. Lastly, some have no sensory
organs, their place being either taken by tentacles or
left vacant (Stauromeduse). This shows that tenta-
cles can replace sensory pedicels, and since they have
essentially the same structure, they, like the cordylii
of the Trachymeduse, are modified tentacles.

Order I. Stauromedusz (Calycozoa).

 

The best known forms are the Lucernarize S
(fig. 198), whose exumbrellar surface is drawn out P!S,,19 07 Haluclustus
into a stalk by which the animals are attached. The Clark.)
disc is drawn out into eight lobes, each with a cluster of knobbed tentacles.
Several species, dark green in color, occur in New England waters. The
Tesseride (unknown in America) are free-swimming.

Order II. Peromeduse.

Cup-shaped medusee with four interradial sense bodies. Mostly high
sea forms. Pertcolpa, Periphylla in the Gulf Stream.

Order III. Cubomeduse.
Sense organs perradial in position. Occurring in tropical and semi-
tropical seas. Charybdea (fig. 198). Development
unknown.
Order IV. Discomeduse.

These are the most abundant and richest in spe-
cies of Seyphomedusee and hence have served as the
basis of the foregoing account. The order is subdi-
vided according to the characters of the mouth.
(1) CANNOSTOM, mouth triangular without oral
tentacles; shape and other features of the ephyra
retained in the adult. MNausithoé albida (fig. 86) of
Europe is noticeable because its seyphopolyp, de-
scribed as Stephanocyphus mirabilis, is parasitic in
sponges. Linerges and Atolla in the Gulf Stream.
(2) SEM&osToM.E, mouth X-shaped with long fringed
and folded arms at the corners. Avrelia flavidula*
and Cyanea arctica* common in New England, the
latter, the ‘blue jelly,’ often very large; dise 7 feet in
diameter, tentacles extending a hundred feet or
more. Pelagia * in our warmer waters. (3) RHIZzO-

STOME.E, four oral arms, these branched dichotomously.

Fie. 198. — Charubdet The mouth and grooves on the arms closed by union
marsupialis.® — (erom = 7

Hatschek.) of their edges so that many small sucking stomata

 
ITI. ANTHOZOA. 251

remain through which nourishment is taken. Stomolophus* and Polycio-
nia frondosa* (fig. 194) on coral banks in our warmer seas.

Class III. Anthozoa (Actinozoa).

The Actinozoa, including the sea anemones, sea pens, and corals,
are exclusively marine. With few exceptions they are sessile and
form colonies, often of enormous size. In this as in appearance
(fig. 199) they resemble the hydroid polyps. They have a pedal

 

FiG. 199.—Autheomorpha elegans. s, s, sagittal plane.

disc, column, tentacles, and peristome with central mouth. They
are distinguished by their greater completeness in histological and
organological differentiation. The Anthozoan polyp has a well-
developed mesoglea, the supporting layer of the hydroid being
here a layer of connective tissue with numerous cells, giving the
animals a tough fleshy consistency. Still more important as points
of distinction are the presence of an esophagus and septe bearing
mesenterial filaments and gonads.

The mouth lies in the centre of the peristome, and in shape is
usually oval or slit-like. Hence there is a biradial symmetry—of
importance in the architectonic of the polyp—for there is a sagittal
axis (fig. 199, s, s) passing in the long axis of the mouth and a
transverse axis at right angles to it. From the mouth the wesopha-
gus hangs down into the body as a flattened tube and opens at its
lower end into the wide gastrovascular cavity. In its development
this esophagus is an inflected part of the peristome and hence lined
with ectoderm, and its lower end alone can be compared with the
mouth of the hydrozoan (fig. 200).

The esophagus is held in position by radial partitions, the
septa, which stretch from base, column, and peristome to the
252 CQ@LENTERATA.

csophagus, dividing the peripheral part of the gastral space into
small pockets, the radial chambers, connected below the end of
the esophagus with the central part. Above, these chambers con-
tinue into the tentacles. The tentacles therefore are outgrowths
from the radial chambers and usually are equal in number to them.
Besides the complete or primary septa which reach the csoph-

        
 

=
CoE

i
ae

  

   
    
 

 

een eer

Fia. 200.—Stereogram of an Anthozoan (orig.). In the cut edges the ectoderm white,
the entoderm blocked, the supporting layer black. The septa show the septal
muscles, and the communication of the interseptal chambers with the csophagus

is seen.
agus, there may be others incomplete in that they do not reach
the esophagus and belonging to secondary, tertiary or other series
(fig. 203).

The septa support a number of important organs: the mesen-
terial filaments, gonads, and muscles. The mesenterial filaments
are thick strands of epithelium rich in glands and nettle cells,
fastened like a hem on the edge of the septa. Since they are
much longer than the peristomial-pedal length of the septa, they
cause these latter to wrinkle and fold, thus strikingly resembling
ITI, ANTHOZOA.

bo

538

the mesenteries of the mammals. Lower down, in some species,
the filaments become free and form long threads, acontia, rich in
nettle cells which are protruded for defence either through the
mouth or pores (cinclides) in the column. The gonads—only
exceptionally hermaphroditic—lie inside the mesenterial threads
as band-like folded thickenings (fig. 201, 4°). They arise as in
the Scyphomeduse from the entoderm, but early migrate into the

 

Fia. 201. F1a. 202

Fic. 201.—Sections of Cereus spinosus, showing complete and incomplete septa.
a, acontia; b, mesenterial filament; ¢, septal stoma; g, gonads; h*, septa of first
order with gonads; h?—h4, incomplete septa of second to fourth order; t!—14,

corresponding tentacles. ;
FG. 202.—Section of septum of Edwardsia tuberculata. ek, ectoderm; en, entoderm;

me, supporting layer; m/f, septal muscle; 0, ovary; v, mesenterial filament.

mesoglea of the septum (fig. 202, 0). The eggs, when ripe,
escape into the gastrovascular cavity by dehiscence. The young
leave the parent at various stages of development, sometimes as
planule (fig. 206, A), sometimes as young with tentacles.

The muscles are very important, morphologically. Muscles
and nerves occur in both ectoderm and entoderm; but while the
nerves are best developed in the ectoderm, forming especially a
254 CQILENTERATA.

thick subepithelial sheet of fibres and ganglion cells in the peri-
stome, the muscles of the entoderm are weakly developed and are
confined to the peristome and the tentacles. The entodermal
musculature is much stronger. At the oral end of the column is
usually a strong circular (sphincter) muscle which by its contrac-
tion can draw the top of the column over the peristome. The
septa also bear muscles, on one side running transversely, on the
other longitudinally, the latter alone being strongly developed and
producing marked ridges (fig. 202) on the septa.

In the Hexacoralla the septa are arranged in pairs, not only in being
close to each other, but in having similar faces turned towards each other.
The rule is (fig. 203) that in each pair the sides bearing muscle ridges are
turned towards each other, but in two pairs lying in the sagittal axis these
muscles are turned outward. From these relations these septa are called
directives. It is however to be noted that in our common anemone, Me-
tridium, occasionaily three, more frequently but one pair of directives
occur. The paired condition of the septa allows the recognition of two
kinds of radial chambers; between the two of a pair is an intraseptal,

 

Fia. 203.—Transverse section of actinian (Adamsia diaphanc) AB, plane of symme-
try, a second lies at right angles. I-IV, septa of four orders.

between two pairs an interseptal chamber. New septa only appear in’
the interseptal chambers. At one time all Hexactinians have but six septa,
a pair of directives and, right and left, four lateral septa. With growth,
other septa of asecondary order may appear in the interseptal areas, giving
six of these. And so with septa of the tertiary order. Irregularities how-
ever occur, and forms are found which have abandoned this sexfold plan
III, ANTHOZOA, 255

and have assumed a plan of four or ten, but without altering the primitive
conditions,

In the Octocoralla (fig. 204) the conditions
are simpler, only eight septa being developed.
These are disposed equally on either side of the
esophagus and may have (most octocorallans)
all their muscles towards one end, or (Edwardsia,
fig. 205, IV) may have the muscles of one pair
reversed. It is to be noted that hexactinians
pass through an Bdwardsia stage. In Cerianthus
new septa are always added at one end of the sag- Fic. aa bea ees oe
ittal axis (fig 205, II), while in the extinct Tetra- tion of an Octocorallan

= (Aleyonium), x, siphono-
coralla (fig. 205, I), so far as one may judge from glyphe; 1-4, septa of one
the hard parts, the septa have an arrangement Pe ene

with four as the basis. van those of the other
side.

   

ie

VV

    

Frac, 205.—Arrangement of septa in various Actinozoa. I, Tetracoralla; II, Cerian-
thus; III, Octocoralla; 1V, Edwardsia.

By far the greater part of the Anthozoa reproduce by budding
as well as by eggs. Only rarely do the buds separate, but generally
they remain connected with the mother to form a colony of hun-
dreds or thousands of individuals. These are connected by an
extensive coonenchym or cenosarc, consisting largely of mesogloa,
but having an outer coat of ectoderm and penetrated by a system
of branching and anastomosing entodermal canals (fig. 206). On
disturbance the polyps can quickly retract themselves into the
coonosare.

The colonial Anthozoa have almost invariably a skeleton,
secreted by the ectoderm and consisting either of calcic carbonate
or of an organic horn-like substance. Sometimes the horn and
lime alternate. One recognizes an axial and a cortical substance.
The axial skeleton is confined to the deeper portions of the cwnosarc,
while the cortical portions are formed by the polyps themselves
and to a large extent (figs. 207, 208) repeat their complicated
structure. Except ina few forms (Fungia) a theca is present;
this is a calcareous cup, and from this usually extend inward
calcareous partitions called, in distinction to the fleshy- or sarco-
septa, the sclerosepta.
256 CQ@{LENTERATA.

The theca arises by a fusion of sclerosepta. If this fusion takes place
some distance inside the peripheral ends of the sclerosepta, the distal ends

 

Fia. 206.—Corallinm rubrum, red coral. (After Lacaze Duthiers.) 4, ciliated young;
B, young colony; C, part of colony with polyps in extension (a) and _contrac-
tion (¢); d, cenosarc; DP, stereogram of a eae b,c, partly and completely re-
tracted polyps; d, coanosarc; e, skeletal axis exposed; f’, f, larger and smaller
conosarcal canals; m, mesenterial filaments; s, esophagus; t, retracted tentacles;
A, greatly, B, C, P, slightly enlarged.

of these project on the outer surface as costa. Still outside these may be
asecond cup, the epitheca. In the centre may occur a large calcareous
column or several smaller ones, the colwmella (fig. 208). Pali are small
Ill. ANTHOZOA. 257

free particles between the inner ends of the sclerosepta and the columella,
while synapticule are small projections connecting the septa. As the
polyps grow they build the thece higher and higher and consequently draw

 

Fie. 207. Fia. 208,
Fic. 207.—Sclerophyllia margariticola. (After Klunzinger.)
Fie. 208.—Section of coral of Caryophyllia cyathus. (After Koch.) Outside the theca,
septa (I-XII) of first and second order, their pali and, in centre, columella.
out from the deeper portions, which may become cut off by horizontal parti-
tions, the tabula. Such tabulee occur in some Madreporaria, Octocorallans,
and Millepores (p. 241) which were formerly united in a group Tabulate.

 

 

Fia. 209. Fia. 210.
Fia. 209.—Diagrammatic section of the flesh and coral of a hexacorallan ; above the
line the section passes through the cesophagus, s; below the line it is lower down;
r, directives; coral black.
Fia. 210.—Diagram of the relations of the coral to the polyp. (After Koch.) Ectoderm
lined, mesogl@a black, entoderm dotted, coral white. a, theca; b, mesenteries;
c, costee; d, basal plate; e, external wall; f, sclerosepta.

It was once thought that the coral was a calcified portion of the soft
parts and hence that sclerosepta were hardened sarcosepta ete. This has
been disproved. The sclerosepta are formed in the radial chambers between
258 CQILENTERATA.

the sarcosepta, and the theca inside and at some distance from the col-
umn, the outer surface of which secretes only the inconstant epitheca
(fig. 209). From the above it would appear that the sclerosepta correspond
in number to the sarcosepta, but this is not always the case. Thus the
Helioporids, which on the grounds of the skeleton were regarded as Hex-
acoralla, are shown by the soft parts to be undoubted Octocoralla.

By means of their skeletons the Anthozoa produce large accumulations
of carbonate of lime, the well-known coral reefs, on the bottom of the sea.
These are formed by many species, the Madreporaria playing the most
important réle. When the reef reaches the surface it produces an island,
the most noteworthy form being the atoll, a ring-like structure with a
central lagoon. The origin of these atolls, as well as that of fringing and
barrier reefs, was for a long time explained by Darwin’s and Dana’s theory
of coral reefs. Later investigations, notably those of Mr. Agassiz, afford
another explanation.

Order I. Tetracoralla (Rugosa).

Extinct forms from the paleozoic rocks with the parts arranged
in fours (fig. 211). The present tendency is to regard them as modi-
fied Hexacoralla.

Order II. Octocoralla (Alcyonaria).

These forms, which have eight single septa, are externally re-

cognizable by their feathered tentacles, eight in number (fig. 206).

 

Fia. 211. Fre. 212

Fic. 211.—Diagram of septa in a tetracorallan. (Orig.)

Fig. 212.—Three stages in development of Renilla reniformis. (After Wilson.)
A, cleavage of egg; B, planula; C, development of cesophagus; ec, ectoderm ;
en, entoderm; m, mesoglea ; 0, esophagus.

They occur in all seas from near the shore to great depths. In
development there is a planula (fig. 212) in which the cesophagus
arises as a solid ingrowth which becomes perforated later. The
eight septa arise simultaneously. Usually colonies are formed by
budding and a polymorphism may occur, some individuals which
have reduced septa and lack tentacles, taking in water for the
colony. Many are phosphorescent.
ITI, ANTHOZOA: HEXACORALLA. 259

In the Atcyonup& (Alcyonium,* Anthomastus) an axial skeleton is
lacking, but the flesh contains numerous calcareous particles, the scleroder-
mites. The sea pens, PENNATULIDZ, have the basal part buried in the
mud, the rest, expanded like a disc or feather, bears the polyps. An axial
skeleton usually occurs in the stalk. Pennatula,* colder waters; Renilla,*
warmer seas. The GORGONUD (sea fans, sea whips) have an axis of more
firmness, which may be calcareous, and the colony may branch and the
branches anastomose. Here belong, besides many tropical genera whose
names end in ‘ gorgia,’ Primnoa* of our colder waters; Jsts of tropical
seas, with skeleton of alternating calcareous and horny parts, and the pre-
cious coral (Corallium rubrum, fig. 206) of the Mediterranean, the fishing
for which at Naples amounts yearly to half a million dollars. In the TuBI-
PORID#, or organ-pipe corals, the separate polyps are enclosed in parallel
tubes united at intervals by horizontal plates. The Heliopore were long
regarded as Hexacoralla because of their massive skeletons with six sclero-
septa. The paleozoic Syringopora belongs near Tubipora, while the
FavositTip& resemble the Alcyonlide.

Order III. Hexacoralla (Zoantharia).

The simple tubular tentacles are highly characteristic of the
Hexacoralla, as is the arrangement of the paired septa in sixes as
described above. Yet there are exceptions to this rule. On the
one hand is Hdwardsia (common in our colder waters), in which
there are sixteen or more tentacles and only eight septa (fig. 205),
but which exhibits a condition through which the young actinians
pass; on the other hand in the Zoantharia, Cerianthie, and
Antipatharia the rule of six has undergone extensive modification.

Sub Order I. ACTINARIA (Malacoderma), The sea-anemones are
mostly solitary, without skeleton; with numerous septa and tentacles.
They occur in all seas from tide marks
to the greatest depth. A few are
free, but most are sessile. Except the
colonial Zoanthee all can creep by
the pedal disc. Represented in our
seas by Metridium, Bunodes, Sagar-
tia, Bicidium (parasitic on Cyanea),
Halcampa, ete. The Zoanthew have
two kinds of alternating mesenteries
and the individuals of the colonies
are usually inerusted with foreign
matter. Epizoanthus lives symbi-
otically with hermit crabs (fig. 118).

Sub Order I]. ANTIPATHARIA.
Six pairs of septa and six (Antipathes)

. or twenty-four (@erardia) simple ten-
TS ee ane ah end ce eer ouiear: tacles; colony with a black horny axis
son, B, Bicidium purasiticum (after and no calcareous skeleton. Simus

Verrill), C, Bunodes stella (after Ver- ; :
rill). late the Gorgonids.

 
260 CAILENTERATA.

Sub Order III. MADREPORARIA. This group, the richest in species of
any, is characterized by the great development of the skeleton. Theca,
septa, and usually columella and synapticuli are present, and frequently
cost as well. Solitary forms are few. Usually they form colonies, fre-
quently of thousands of individuals, bound together by a ccenenchym
extending from polyp to polyp over the surface of the coral. A colony

 

Fig. 215.

Fie. 214.—Astrangia danae*, five polyps in various stages of expansion.
Fig. 215.—Celoria arabica. (After Klunzinger.)

  

Fie. 216, Fie, 217.

Fie. 216.—Cladocora cespitosa. (After Heider.) Relations of coral and flesh.
Fie. 217.—Favia cavernosa. (After Klunzinger.)

arises from a single animal by continued fission or budding. When the
division is not complete the animals may form long series with numerous
mouths but with the other parts united, the result being that the surface
of the coral is marked by long winding grooves—incompletely separated
theca—with sclerosepta, as in the brain corals (fig. 215).

Since but little is known of the soft parts, the classification of the Mad-
reporaria is based upon thecoral. Three sections of the sub order are recog-
nized. (1) Aporosa, with compact skeleton. Some, like Caryophyllia
IV. CTENOPHORA. 261

(fig. 208) and Sclerophylia (fig. 207) are solitary. Others, like Oculina,*
branch, and still others form compact masses. Astrangia danae (fig. 214),
the only true coral in New England; Astrea, the brain corals (Celoria,
fig. 215, Diploria, Manicina); Cladocora (fig. 216), Favia (fig. 217).

 

Fig. 218.—Madrepora erythrea. (After Klunzinger.)

(2) FUNGIACEA, or mushroom corals, with no outer wall to the coral. Some
are colonial, others (Fungia) are solitary. A sort of strobilation in de-
velopment. (8) Porosa, with skeleton porous like a fine sponge. Madre-
pora,* deer’s-horn coral (fig. 218), Porites, Astroides,

Class IV. Ctenophora.

The Ctenophores excel all marine animals, even the meduse,
in transparency and delicacy of tissues; many are so soft that a
strong current tears them, and no attempts to preserve them have
been successful. The body is almost always biradially symmetrical ;
i.e., is divided by both sagittal and transverse planes into sym-
metrical halves. Since the longitudinal axis is usually longer than
the others, which are generally equal, the body is usually oval or
pear-shaped. In Cestum the sagittal axis is greatly longer, giving
the animal the form of a band, whence the name ‘ Venus girdle.’

The bulk of the animal is composed of a soft jelly with con-
nective-tissue cells, penetrated in every direction by polynucleate
muscle cells branched at their ends and apparently innervated by
special nerve cells. On the outer surface is a layer of ectoderm,
while in the interior is a system of branched entodermal canals.

At the bottom of a depression (fig. 221B, p) at the aboral
pole is a thickened patch of ectoderm, the sense body, which has
considerable resemblance to an otocyst (fig. 222). The thick
sensory epithelium forms a shallow groove, strong hairs which rise
from the edge of the groove arch over it, enclosing a space to be
compared to an incomplete vesicle. In the centre is a spherical
262 CQ@LENTERATA.

Z

   

SNe
\ we

\ A : Fic. 220.
Fie. 219, Ag

 

Fig. 221B. Fig. 221C.

Fie. 219.—Swimming plate and epithelial cushion. (After Chun.)

Fia. 220.—Hormiphora plumosa, (After Chun.)

Fic. 221.—-Pleurobrachia rhododactyla. (After Chun.)
view. MM, sagittal axis; 77, transverse i
radial v ; cpr, right and left gastrov:
vessel; yg, subcostal vessel; m, ‘stomach’; mg, paragastric canals; n, ciliated
grooves; uc, sense body 30, mouth; ov, ovary ; p, pole-plate ; r.7%, rows of combs ;
sch, tentacular pouch ; schs, its aperture; sp, testes; tb, basis of tentacle; tg. sch,
tentacular canal; tr, funnel; trg, funnel canal.

A, aboral pole: R, front, C, side
axis: cadr, radial vessel: c.cr, inter:
cular trunks; ec, opening of funnel

         
  

 

 

 
IV. CTENOPHORA. 263

mass of otoliths, supported on four bundles of S-shaped agglutinate
cilia. From these bundles of cilia eight bands of thickened epi-
thelium, at first in pairs (fig. 223, ws), later diverging, pass to the
oral pole (fig. 221, r). These meridional bands (so called from
their course) consist in part of ciliated epithelium, in part of the
characteristic ‘combs’ which are the locomotor organs, and which
must be regarded as transverse rows of long agglutinated cilia.
The combs (fig. 219) arise from thick epithelial ridges, transverse
to the meridional bands, and are so far apart that the free edges

wa 40

A to

   

Fic. 222. Fig. 223.

Fia. 222.—Section of sense body of Callianira, A, through the centre; B,excentric ;
d, roof of sensory groove; f, support of otoliths, oy p, pigment cell; se, sensory

cells.

Fia. 223.—Ahoral pole of Callianira biadlata. (From Lang.) f, supports of otoliths,
0; pp, pole plate; sk, sense body; fo’’, openings of yastral funnels; ts, ciliated
bands.

of one comb overlap the base of the next like shingles. In conse-
quence of their fibrous structure the combs are strongly iridescent
and in motion cause a beautiful play of metallic red, blue, and green
over the meridional bands. These combs act like oars and row
the body about. Since the combs begin some distance from the
aboral pole, they are connected with it hy means of ciliated grooves
following the line of the meridional bands. Experiment shows
that the sense body is an organ of equilibration and of correlating
the action of the different rows of conibs.

The ectoderm gives origin to two more important organs, two
pole fields and two tentacles. The pole fields (fig. 221, p3 223, py)
are two epithelial patches extending a short distance in the
sagittal axis from the sense body and possibly are olfactory or
taste organs. The tentacles arise, in the transverse axis, from the
264 CQ(LENTERATA.

bottom of deep tentacular sacs, from which they project as long
cords with numerous lateral branches, and into which they may be
retracted. Tentacles and branches contain an axial muscle, while
the ectodermal coating consists largely of adhesive cells. These
are spherical bodies (fig. 224) covered with a very sticky granular
EP secretion, and, like a Vorticella, supported on the

end of a spiral stalk muscle. These are used in
capturing prey.

The ectoderm also forms part of the gastrovascu-
lar system. It turns inward at the mouth—situated
at the lower end of the chief axis—and lines the
large space commonly called stomach (fig. 221,
m) but which corresponds to the esophagus of the
Actinozoa. At the aboral end of this stomach
begin the true entodermal portions, the so-called
Fig. 24—Adhesiye funnels, and from them the canals distributed

Coogan (A en®, through the jelly to the various organs. Two

Samassa.) (rarely four) funnel canals run to the aboral pole
and empty (fig. 225, 0) near the sense body; a second pair, the
paragastric canals (fig. 221B, mg), which run parallel to the
esophagus, end blindly. The perradial canals (c.pr) proceed out-
ward from the funnel, and besides giving off a canal to the tentacle
(tg) each divides dichotomously twice, first into interradial and
then into adradial canals, each of these last connecting with a
meridional vessel running just beneath a row of combs, nourishing
them as well as the gonads. The gonads consist of two bands, one
male, the other female, running in that wall of the meridional ves-
sel nearest to the combs. In spite of their position they are
apparently ectodermal in origin.

These gonads are regular in distribution, those of two vessels
which are nearest each other being of the same sex. The eggs and
sperm pass out through the gastrovascular system.

  

The few species of the group are divided into the TENTACULATA,
with tentacles, and the NUDA, without. To the first belong the CyprP-
PID, with pear-shaped bodies (Plewrobrachia * on our coast, fig. 222), and
Hormiphora (fig. 221); the Loparx (Mnemiopsis,* Bolina*), with lobes ;
and the band-like Crsripas (Cestem, the Venus girdle) of the warmer
seas. The Brroma (Beroe, Idyfa*), with wide mouth, belong to the
Nuda, The small ereeping forms, Cwloplana and Ctenoplana, are supposed
by some to form a transition to the Turbellaria.
SUMMARY OF IMPORTANT FACTS. 265

Summary of Important Facts.

1. The CHLENTERATA (together with the Echinoderma)
were formerly called Radiata because in most a radial form of
structure is present; in the higher groups this can be transformed
into biradial or even bilateral symmetry.

2. The Celenterata are sometimes called Zoophyta (animal
plants), from their appearance and their attachment. In many
the resemblance is heightened by their formation of plant-like
colonies by fission and budding.

3. The name Coelenterata was chosen because they have but
one system of cavities, a simple or ramified digestive sac, repre-
senting at once the alimentary tract and the as yet undifferen-
tiated body cavity.

4, This celenteric apparatus is called the gastrovascular sys-
tem because its branches distribute nourishment to all parts and so
perform the function of blood vessels.

5. The reproduction is either sexual or asexual, very frequently
cyclical (alternation of generations).

6. The animals are provided with nerves, muscles, and sense
organs and possess marked sensibility and mobility. »

7. Especially characteristic are the tentacles and small nettling
organs, the cnide, in special cells.

8. Nearly all histological differentiation proceeds from ectoderm
or entoderm, since the mesoderm (mesoglea) plays but a subordi-
nate role and usually functions only as a support.

9. Four classes—Hydrozoa, Scyphozoa, Anthozoa, and Cteno-
phora are recognized.

10. In Hyprozoa and Scypwozoa there are usually two
alternating generations, the sessile asexual polyp and the free-
swimming sexual medusa.

11. The hydroid polyp and the craspedote medusa are charac-
teristic of the Ilyprozoa.

12. The hydroid polyp is a two-layered sac of ectoderm and
entoderm, a supporting layer and a circle of tentacles. In the
colonial forms there is usually a cuticular envelope, the perisarc,
secreted by the ectoderm.

13. The craspedote medusa is bell-shaped, with smooth bell
margin, its aperture partially closed by a diaphragm-like velum;

 

the gonads are ectodermal.
14. The meduse arise by lateral budding from the hydroid.
15. If the medusa remain attached to the parent as a sporosac,
266 COLENTERATA.

alternation of generations is replaced by polymorphism; it can
entirely disappear with the total loss of either hydroid or medusa.

16. The scyphostoma and the acraspedote medusa are typical
of the ScyPHozoa.

17. The scyphostoma differs markedly from the hydroid polyp
in the presence of four longitudinal gastric folds or septa (teeniole).

18. The acraspedote medusa lacks a velum, has a lobed umbrella
edge, gastral tentacles (phacelle), and entodermal gonads.

19. The medusa arises from the polyp by terminal budding
(strobilation).

20. Alternation of generations rarely is lost, and then only by
suppression of the scyphostoma,

21. The Anrio0zoa have only one form, the coral polyp; it is
distinguished from the hydroid polyp by the ectodermal esophagus,
the radial septa reaching the msophagus; the well-developed
mesoglea and the gonads which, arising from the entoderm, early
migrate into the mesoglaa.

22. Most Anthozoa are colonial and produce a skeleton usually
of calcic carbonate, but sometimes of ‘horny’ substance.

23. The skeleton may be either axial or it may extend over the
individual polyps (cortical skeleton).

24. The living Anthozona are divided according to the number
of septa into Octocoralla and Ilexacoralla. To these are added
the fossil Tetracoralla.

25. The Heracoralla have numerous tubular tentacles and six,
or a multiple of six, pairs of septa.

26. The Octocoralla have eight single septa and eight feathered
tentacles.

27. The CrenopHora are always free-swimming and have a
large mesoderm with numerous muscle cells.

28. Nettle cells are absent, and are replaced by adhesive cells.

29. Most characteristic are the eight meridional rows of
‘combs’ whose motions are controlled by a common organ, the
sense body, constructed like an otocyst,

30. The digestive tract consists of an ectodermal c@sophagus
and a branching system of entodermal vessels.
PLATHUELMINTHES. 267

PHYLUM IV. PLATHELMINTHES (FLATWORMS).

This group is well characterized by the name. With few
exceptions (rhabdoceles, many trematodes) the nearly flat ventral
surface and the slightly arched back are closely approximate and
pass with a more or less sharp margin into each other. In many
cases the ventral surface is distinguished by its lighter color. In
all the bilaterally symmetrical body is composed of a solid paren-
chyma, a mass of connective tissue traversed by muscle fibres, in
which the various organs—alimentary tract, nerves, excretory and
reproductive organs—are imbedded. In the lower forms the di-
gestive system is markedly like that of the cwlenterates (Actinozoa,
Ctenophora) in that there is but a single opening and this leads by
an ectodermal csophagus (stomodzeum) to the interior. In para-
sites the digestive tract may be lost. The skin is a one-layered
epithelium, sometimes ciliated, sometimes protected by a thick

cuticula. Inside this comes a muscular layer (fig. 225) in which

 

 

  

 

 

Fig. 22 ion (right half) of a Planarian. d, vit. sllaria 2 ae. dorso-
ventral muscle fibr: e, ectodermal epithelium with cilia; y, gastric diverticula;
h, testicular follicles ; Un, longitudinal muscles (dots, in section) ; 1, lateral nerve
cord,

longitudinal muscles are always present, and in addition frequently
circular and oblique muscles, as well as those passing from dorsal
to ventral surfaces. The nervous system (fig. 228) consists of a
pair of ganglia (brain *) in front of (/.e., above) the @sophagus
and longitudinal nerves leading backwards from it. The excretory
organs (fig. 226) are composed of a series of tubes, the protone-
phridia or ‘water-vascular system,’ which branch and ramify the
parenchyma. In most the sexes are united in one individual and
the reproductive organs take up considerable space. There isa
small paired or unpaired ovary and vilellaria, usually paired and
branched. The eggs arise in the ovary, and to these are added
nourishment in the shape of cells (abortive ova) rich in yolk from
268 PLATHELMINTHES.

the vitellaria. At the point where oviducts and yolk ducts unite
a single egg cell together with several yolk cells are united into
an oval body—the compound egg—protected by a shell secreted
by special glands (fig. 227, A). This forms only an apparent
exception to the rule that the egg is but a single cell, for the
development shows that only the egg cell takes a direct part in the

 

Fig. 226. Fie. 227.

FIG. 226.—Excretory system of Cercaria. (After Albert Lang.) hb, limb of bladder ;
Wy’, same with urinary concretions; ec, collecting canal; cs, canals of ventral
sucker; cv, collecting vacuole; e, eye; ep, excretory pore; 1, lumen of tail; os, oral
sucker ; vs, ventral sucker.

Fia. 227.—Eggs of Distomum nodulosum. (After Schauinsland.) A, before develop-
ment; B, later, the yolk broken down. d, yolk cells; ei, egg cell; ek, ectoderm ;
en, entoderm; p, pigment spot.

formation of the embryo and is the true ovum, while the yolk cells

break down and furnish food to the growing embryo (fig. 227, B).

Class I. Turbellaria.

The Turbellaria are small, only a few being measured by inches,
while many are almost microscopic in size. The name Turbellaria
has reference to the currents produced by the ciliated ectoderm
which covers the body, the cilia arising from the single layer of
columnar epithelial cells (fig. 58). This ectoderm serves at once
for motion and for respiration. Most species are aquatic (fresh
water or marine), only a few land planarians living in moist earth.
In the water they cither creep slowly over stones or plants on
their ventral surface, or they swim freely. In swimming the larger
species progress by undulations of the body, the smaller by means
of the cilia.
Lf. TURBELLARLIA. 269

The alimentary canal (fig. 228) consists only of esophagus
(pharynx) and mesenteron, the latter terminating blindly since no
intestine or anus is present. The mouth is on the lower surface,
at some distance from the anterior end, being occasionally in the
middle or even behind the middle of the body (fig. 231). It
leads into the muscular esophagus, which is frequently enclosed
in a special sheath and then can be protruded like a proboscis.

 

 

 

 

 

Fig, 228. Fie. 229.

Fic. 228.—Digestive and nervous systems of Syncelidium pellucidum, (After
Wheeler.) a, alimentary tract; 6, brain; ln, longitudinal (ventral) nerves; m,
marginal nerve; pl, longitudinal nerve of pharynx; pv, ring nerve of pharynx;
tn, transverse nerve; wu, uterine ostium.

Fi. 229.—Pulycherus caudatus. (After Mark.)

The mesenteron, of entodermal origin, varies greatly in shape, its
modifications being made the basis of division of the class into
orders. In the Polycladidea there is a central portion from which
numerous branched ceca are given off; in the Tricladidea there
are three main trunks, each with lateral cecal diverticula; while in
the Rhabdocelida the digestive tract is a simple rod-like sac, in
some cases (Accla) without internal cavity. The supra-cesophageal
ganglia always lie at the anterior end of the body, which is most
sensitive, and may be produced into feeler-like processes. This
270 PLATHELMINTHES.

region usually bears two or more simple eyes, and in a few a single
otocyst.

In many Turbellaria nettle cells, like those of the Coelenterata,
occur in the skin. Much more common are the rhabdites, small
rods which arise in epithelial cells which sometimes project like
glands into the mesoderm. Those rhabdites occur in the shiny
tracks which the animals leave in creeping.

The hermaphroditic sexual organs (fig. 73) and the excretory
system vary considerably in the separate orders and families. The
eggs are usually very large and are fastened by a stalk to water
plants. Many species form a sort of cocoon, containing a few eggs
and numerous yolk cells. In the marine species a free-swimming
larva (fig. 230) with lobe-like processes may hatch from the egg.

 

Fie. 230.—Larva of Stylochus pilidium. (From Korschelt-Heider, after Gétte.) D,
enteron ; En, remains of entoderm cells; S, esophagus.

This larva, by a metamorphosis, is converted into the creeping
adult. Not infrequently besides the sexual asexual reproduction
occurs. The Microstomide and some Planurie are capable of
transverse division, and, when well nourished, by rapidly repeated
divisions will form chains of individuals arranged ina row, separa-
tion taking place gradually. For each posterior individual a new
brain and a new cesophagus are formed (fig. 58). The Turbellaria
possess the power, to a marked degree, of reproducing lost parts,
which makes them favorites for regeneration experiments.

In a few Turbellaria there is a noteworthy condition of the digestive
organs. The pharynx connects with an entodermal syncitium, a proto-
plasmic mass, without lumen, containing nuclei in which, as in the pro-
toplasm of a protozoan, the food is digested. This entoderm is hardly
If, TREMATODA. Oa.

marked off from the mesoderm, but it is a question whether these ‘Accela’
are primitive or degenerate.
Order I. Polycladidea.
Marine species of considerable size, in which the digestive ceca, spring
from a central chamber. Species of Leptoplana and
Stylochus on our shores. Thysanozoon, Europe.

Order II. Tricladidea.

Alimentary canal with three main trunks, an anterior
unpaired and a pair of posterior branches, arising from
the pharynx. These trunks bear lateral cecal branches.
Among the marine genera are Bdellowra* and Synce-
lidium * (fig. 229) (parasitic on Limulus), Gunda,* Poly-
cherus * (fig. 229); in fresh-water occur Dendrocelum,*
Planaria,* and Polyscelis.* Phagocata * with divided
pharynx. The land planarians (Bipaliwm,* 10 or 12
inches long) are tropical, but have been introduced
into greenhouses in various parts of the country.

Order III. Rhabdoceelida.

 

Small, even microscopic, in size, and recalling in F16.231,—Gunda loba-

habits and appearance the Infusoria; alimentary canal Leen
rod-like, without branches. Monops* and Monoscelis,*
marine. The fresh-water MIcRosTOMID® reproduce
almost exclusively by fission, so that sexual individuals
are rare.

g, cerebral ganglia,
with eye-spots;

0, mouth (entrance
to long pharynx);
p, genital pore with

 

male organs be-
hind, female in
front.

Class II. Trematoda.

These are exclusively parasitic, some living on the skin or
gills (ectoparasites) or in the interior of other animals (entopara-
sites). In structure they are closest to the triclad Turbellaria,
from which they are especially distinguished by characters the
direct result of their parasitic life. Thus they have lost the cilia
or these only appear in the aquatic larval stages. On the other
hand they are armed with structures derived from the skin—
suckers and hooks—for adhesion to the host. The suckers are
shallow pits of columnar epithelium, lined with cuticle and fur-
nished with a thick muscular layer which by its contraction increases
the lumen of the sucker, the edges of which are closely applied to
the host. At least one such sucker is present; if but one or two
(entoparasites), one is at the anterior end (oral sucker) and sur-
rounds the mouth, while a second larger sucker may occur near the
mouth (fig. 232), but may be (Amphistomum) at the posterior end.
In the ectoparasites there are a pair of anterior suckers near the
mouth; at the posterior end a single sucker, or a number of suck-
ers or hooks or both on a sucking dise (fig. 254).
272 PLATHELMINTHES.

Other results of parasitism are the weak development of sense
organs and brain and a tendency to development of accessory
ganglia near the adhesive organs. Hye spots (two to four) occur
occasionally in the ectoparasitic species and in the larve of the
entoparasitic, rarely in their adult condition. The alimentary tract
is forked (fig. 233) and occasionally (fig. 232) has dendritic blind
sacs. To parasitism may also be attributed the great development
of the sexual organs, which at maturity fill a great part of the body.
Its features may be seen in fig. 233. Two vasa deferentia pass for-

 

 

 

 

 

 

FIG. 232. FIG. 233.
FIG. 232.—Distomum hepaticum, liver fluke. (From Boas.) m, ceca of ta, limbs of
digestive tract; s,s,, anterior and posterior suckers.
Fia. 233.—Distomum lanceolatum. c, cirrus, beneath it the opening of the oviduct; d,
vitellaria, the ducts leading to the shell gland; g, ganglion; h, testes with ducts

  

to cirrus; /, Laurer’s canal; 0, ovary, the shell gland behind it; s’, s’’. anterior
and median suckers, the pharynx and the bifurcated digestive tract leading
from s’; u, uterus; w, terminal vesicle of water-vascular (excretory) system.

 

ward from the testes (2), unite and form a seminal vesicle. The
terminal portion of the united ducts can be protruded as a penis
or cirrus, armed with retrorse hooks. It is usually enclosed ina
‘cirrus pouch.’ The ovary (0) is very small and produces small
eggs, deficient in yolk; hence the vitellaria (d) are well developed.
The ducts from these unite with the oviduct, producing the uterus
(w), which receives the eggs, is much convoluted, and empties beside
(in some species in a common antrum with) the male sexual
opening. The first part of the uterus is called the ootype because
here the eggs and yolk cells are formed into eggs (fig. 227) and
II. TREMATODA: POLYSTOMEA. 273

enclosed in a shell with a lid or cover. A second duct—Laurer’s
canal—goes from the oviduct to the dorsal surface. In many
Polystomee the canal is double (fig. 254, sw) and is connected
with copulation, but in the Distomes it is rudimentary or lacking
and the opportunities are present for
self-impregnation. Many Trematoda
have a third canal—the vitello-intes-
tinal duct—leading to the digestive
tract.

The Trematodes fall into two great
groups, the Polystomex, largely ecto-
parasites, and the Distomee, exclu-
sively entoparasitic, the distinctions in
parasitism being correlated with differ-
ences in structure.

Order I. Polystomeze
(Monogenea, Heterocotylea).

Most Polystomes live on aquatic
animals—usually fish, rarely crustacea,
where they attach themselves especially
to the thin-skinned and richly vascular
gills. Since as ectoparasites they are
exposed to more dangers, their adhesive
organs are stronger than in the ento-
parasites. So while the anterior suckers
are weakly developed, in some cases
absent, the hinder end bears sometimes
only a single sucker, but usually a large
adhesive disc armed with many suckers
and hooks. The transfer of Polystomes
from one host to another presents few P16. 234.— Polystomum  integer-

rimum. (After Zeller.) Above

difficulties and hence the life history is two individuals in copulation ;
: i below a single animal enlarged.
without complications. The stalked 4, digestive tract, distended
with blood; dg,*yolk duct: dst,
egos are attached near the mother and _ vitellarium; gp, genital pore; h,
2S . ; testicular vesicles; m, mouth;
produce larve, which soon after hatch- ph, pharynx; ov, ovary; str,
ei e openings of the paired vagine ;
ing have the adult form (hence the vw, uterus; v, vagine; rd, vas
is ‘ deferens; 2, vitello-intestinal
name Monogenea). canal.

 

The American species have been scarcely touched, hence most of our
knowledge is of European species. Gyrodactylus, parasitic on the gills of
the carp, is interesting, since it brings forth living young which, even
before birth, produce a new generation in their interior. More striking is
Diplozoon paradoxum (gills of Cyprinoids), which owes its name to the
274 PLATHELMINTHES.

fact that, at the time of sexual maturity, two individuals become fused
like Siamese twins (fig. 109). The young, described under the name
Diporpa, escape from the eggs and only unite later. Each has a ventral
sucker and a dorsal papilla. When they unite each of the pair seizes the
papilla of the other with the sucker, and then the two grow together so
that the male opening of one comes opposite the female opening of the
other. Polystomum integerrimum of the frog (fig. 234) affords a transition

 

Fic. 235.—A, Polystomum hassalli* (after Goto), from bladder of mud-turtle.
B, Acanthocotyle verrille* (after Goto), from skate. C, Gyrodactylus elegans (after
von Nordmann).
to entoparasitism. At first it lives on gills of the tadpole, but at the time
of metamorphosis it is forced to leave this place and pass, by way of
the alimentary canal, to the urinary bladder. The TEMNOCEPHALIDZ of
warmer regions are partially ciliated, and have from four to twelve
anterior tentacles and a posterior sucker. They are parasitic on crustacea,
molluscs, and turtles, and are regarded by some as a distinct order.
American genera of Polystomew are Epibdella, Polystomum, Tristoma,
Sphyranura, Microcotyle.

Order II. Distomee (Digenea).

The entoparasitic Trematodes oceur largely in the digestive
tract and its appendages; more rarely in blood-vessels, urogenital
organs, and celom of vertebrates and other animals. As inhabitants
of the dark they have, with few exceptions, lost the eyes, which
II, TREMATODA: DISTOME A. 275

appear in larval life, and not always then. Since not exposed to
danger of being pulled from the host, they possess either the oral
sucker alone (Monostomum) or this and a second ventral sucker,

pk

 

Wetnagey ss ‘

Fia. 286.—Development of Distomum hepaticum. (From Korschelt-Heider after Leuck-
art.) A, young larva; b, sporocyst from the lung of Limnaa; C, older sporocyst
with rediw; D, redia which has produced redie internally ; E, redia with cer-
carie;, Fy cercaria , G, encysted Distomum. A, eye spot; D, digestive tract; Dr,
glands; £r, ciliated lobules and main trunks of excretory system ; G, birth open-
ing; Az, germ cells; N, nervous system. sie

     

and only rarely other attaching apparatus. They are markedly
separated from the Polystomes by their life history. The alterna-
tion of hosts necessitated by the endoparasitic life is complicated
by an alternation of generations (better heterogony, p. 145) with
276 PLATHELMINTHES.

metamorphosis. To illustrate this the history of Distomum
hepaticum of the sheep is chosen (fig. 236).

The eggs leave the maternal uterus before embryonic develop-
ment is begun, pass down the bile ducts and thence by the intes-
tine to the exterior. They must come into water and remain here
awhile before the ciliated larva (‘miracidium,’ 4) escapes by a
lifting of the lid of the shell. This larva bores its way into a
small snail (sp. of Zimnea), where it grows into a ‘ sporocyst’ (ZB).
The sporocyst, a muscular sac with protonephridia but lacking all
other organs, produces in its interior eggs which develop into a
second reproductive sac, the ‘redia’ (PD). These are distinguished
from the sporocysts by the possession of pharynx anda tubular
intestine as well as a birth-opening for the escape of the young pro-
duced inside. According to the season these young are either
‘cercarie’ (#’), or several generations of rediz may follow before
the cercarie appear. The cercarie are adapted for an aquatic hfe,
since each has, besides the characteristic organs of a Distomum
(genitalia excepted), a strongly vibratile tail. The cercarie escape
from the snail, swim about in the water until the tail drops off,
when they encyst on water plants. When these encysted young
are eaten by sheep along with the vegetation, infection follows.

In general it can only be said of the life history of other Trematoda
that the miracidia must penetrate a mollusc, and that the different species
have many modifications : (1) Ordinarily development begins in the ma-
ternal uterus. (2) Many miracidia are naked or only partly ciliated. (8)
In many species the miracidia only hatch when the egg is taken into the
stomach of a snail along with food. (4) Very frequently the cercaria
passes from the water into a new host (mollusc, arthropod, or vertebrate)
and becomes encysted here. In such cases there are three hosts in the
cycle. (5) On the other hand the history may be simplified, as when the
sporocyst in the snail produces directly ‘ cercariz without tails’ (/.e., small
Distoma), which only need to be eaten by the definitive host to reach the
sexually mature condition. (6) It is doubtful if the sporocyst may be
omitted and the miracidia develop directly into redia.

As the adjacent scheme shows, the typical development is distributed
among three hosts by the intercalation of a second aquatic interval. It
consists of two generations ; one extends from the fertilized egg to the
sporocyst, the second begins with the unfertilized egg of the latter and de-
velops, through the cerearia and the encysted D/stomran, into the sexually
mature individual. There is no sexual reproduction by fission or budding,
rather an alternation of sexual and parthogenetic generations or heter-
ogony. Columns @ and ¢ show how the history may be simplified and com-
plicated.

Best known of the Distomev are the following: Distomam (Fasciolaria)
hepaticum, the liver fluke (fig. 282), about the size and shape of a pump-
kin-seed.

(twenty known cases) of man.

IT. TREMATODA: DISTOMEA.

(a) Simple

 

 

277

It lives in the bile-ducts of sheep, cows, pigs, etc., and rarely
It stops up the ducts and causes a disease

DEVELOPMENT OF DISTOME.
(b) Ordinary

 

 

(c) Complicated

 

 

 

 

 

 

 

 

 

 

= [| Hf a
et | Larva | Water | z | Larva Water | § | | Larva Water
soil | Sr) | a
ai 1 ae | #4
é | Host I || &}|Sporecyst, | post r |} 3 | Host I
=!) Sporocyst | OS B perhaps maar, | a | | Sporocyst ji ‘
© |) | Molluse)) © also redia Mollusc |) 9 Molluse
2 | Cel Sl | ook |
eee Sa 3
| | £ |
| = Redia ‘

| | ee

f ot ||
ql | Cercaria Water [ Cercaria Water
- | |
sh aa Fall mil
= Encysted s Encysted Z eg | Encysted
=} Distomum | Host I | S | Distomum | Host II a J Distomum | Host Il
3 || | aa 3
|| 5 5 |
o | | Sexually | ee | Sexually 5 || Sexually

Mature Host IL || ~ || Mature Host IIl}| 6 Mature Host III
|| Distomum | | | Distomum | | Distomum
| I | !

 

known as ‘liver rot,’ generally resulting
described above shows why sheep pastured in
the disease, and why wet seasons are times
rainy year of 1830 about one and a half mil-
lions of sheep were killed in England; in
1812, 300,000 in the neighborhood of Arles,
Pranee: This species is frequently accom-
panied by D. lanceolatum, less than half an
inch in length (fig. 233).
Bitharziahematobia is a human parasite,
most common in hot climates, and especially
so among the Fellahin of Egypt. The sexes
are separate. The male, half an inch long, by
inrolling of the ventral side (fig. 287) forms
an incomplete canal (canalis gynzecophorus)
in which the more slender female usually lies.
These united worms occur in the portal vein
and connected vessels. They follow these
vessels in either direction and lay their eggs
in the mucous membrane of the ureters and
urinary bladder, as well as in liver and intes-

in death. The history as
moist places are subject to
of epidemics. Thus in the

 

tine. The suppurative sores of the urinary
tract cause albuminuri r, by emor-

veh . - é a oe °9 hemor 1G. 237.—Bilharzia haematobia.
hage, hematuria. Diagnostic of the dis- ~ Wemale in the gynecophoral

7

canal (c) of the male; s’, 8”,

ease is the presence of the eggs, each with a fava Ot and posterior gages’
in the urine. Several other species

in man, among them D. carnoswm*

spine,

occur and D. wwestermanni* in
278

Oo

PLATHELMINTHES.

America, Otherspecies occur encysted in man, two (D. ophthalmobius and
Monostomum lentis) in the capsule of the lens and in the lens itself. The
genus Amphistomum is common in the intestine of Ungulates, one species,
A, hominis, occurring in man. With few exceptions the adult stages of
all Distomes occur in vertebrates, the larval stages in molluscs. Aquatic
birds are very apt to be infested with them, and ‘‘it may be of interest to
gourmets to know that the trail of a woodcock largely consists of distomic
Trematodes.”

Class III. Cestoda.

The majority of the cestodes, and especially those of the human
intestine, are distinguished from the similarly entoparasitic trema-
todes in a striking manner. But the boundaries between the two
groups disappear in certain forms like Archigetes, Caryophylleus,
and Amphilina, parasitic in lower vertebrates or invertebrates and
which are now assigned to the trematodes, now to the cestodes.
The most important character of the cestodes is that as a result of
their parasitic life they have lost the last traces of an alimentary
canal, and are nourished by the juices or the partially digested
food of the host, since the fluid nourishment is taken in through
the skin into the body parenchyma. It is a disputed question
whether the cuticula of the surface is penetrated with pores for
this purpose.

Two other characters are so striking that they are among the
first thought of. (1) The differentiation of two developmental
stages, the bladder worm, or cysticercus, living chiefly in paren-
chymatous organs (muscles, liver, brain), and the sexually mature
animal, living as a parasite in the alimentary tract; (2) the division
of the body of the adult into different parts, the head or scolex,
and following this a series of joints or proglottids. Since this last
feature holds for all human tapeworms and hence for the best
known species, the following description begins with these typical
forms.

The sexually mature tapeworm or strobila (fig. 238) consists of
a single scolex in front, and behind this follow in a single row the
proglottids. The number of these last varies from smaller forms
(Tenia echinococcus, fig. 252) with three or four to several hun-
dreds or even several thousands, a fact which speaks for the enor-
mous size of some species. The proglottids are derivatives of the
scolex, from the hinder end of which they become separated by a
kind of budding. This explains the well-known fact that the body
is not rid of the tapeworm, so long as the head remains in the
host. It also explains the peculiar shape of the worm, which is
UI. CESTODA. 279

almost thread-like in front, increasing posteriorly to a broad band,
whence the common name. At first the proglottids are small;
they increase by individual nourishment to considerable size, and

 
      

  
  

   

Mi

eee 0 rae

aI

CELCPEEE TTC terete
Ne

MINING

Fia. 238. Lig. 280.
Fig. 238.—Tenia saginata. (From Boas, after Leuckart.) Head with series of pro
glottids taken from various regions of the strobila.
Fic. 239.—Nervous system of Monezia. (After Tower.) a, suckers; e, excretory tubes;
g, cerebral ganglia. Nerves black.
separate from the hinder end of the chain and live separately when
a certain measure of development is reached. For example, the

young proglottids of the human tapeworm, Tenia solium, are 0.5
280 PLATHELMINTHES.

mm. broad and 0.01 mm. long; the ripe proglottids at the end are
elongate oval, 5 mm. broad and 12 mm. (half an inch) long.

Head and proglottids have certain common characters. Their
connective-tissue parenchyma contains numerous spherical con-
cretions of lime, and consists of cortical and medullary sub-
stance. The first contains to a marked degree the muscles, the
latter the other organs. Nerves and water-vascular system extend
through the whole length of the worm. In the head is the paired
cerebral ganglion of the flatworms (fig. 239), sometimes fused to a
single mass by the great development of the commissure or partially
concealed by accessory parts connected with attachment (fig.
242). From the brain two principal nerves run backwards, usually
near the edges of the proglottids (fig. 244, ’). The water-vascu-
lar (excretory) system begins with a capillary network richly
provided with flame cells. It extends through head and proglottids;
usually four main trunks are present, two being less developed and
it is possible are sometimes absent. The two chief trunks are fre-
quently connected by a cross-trunk on the hinder margin of each
proglottid (fig. 244). The system opens on the posterior edge of
the last proglottid, but accessory mouths may occur on other
proglottids.

The scolex and proglottids are distinguished by the facts that
the proglottids contain the sexual organs, while the scolex bears
the anchoring apparatus, for the latter has, besides producing
proglottids, to fasten the worm in the intestines. Most important
of the adhesive organs are the suckers (acetabula); less important
are the hooks, which, in numbers, are either arranged ina circle or
are borne on protrusible and retractile probosces (fig. 240-242).

When a circle of hooks is present it is on the anterior end and is moved
by a special apparatus, the rostellum. This is a plug of complexly arranged
muscles (fig. 242) which can arch and flatten the central area. In many
species the arching is increased by a muscular sheath, the flattening by
retractors. Each hook has its point outwards and its base with two roots,
one of which rests on the rostellum; the protrusion of the rostellum forces
the points outwards into the mucous membrane of the intestine. In some
Tania without the circle of hooks (LZ. sayinata) the rostellum is replaced
by a sucker-like depression. Since the rostellum arises in development
from a similar cup, it may be a modified apical sucker, but it is doubtful
how far comparisons may be made with the oral sucker or the alimentary
tract of the trematodes.

The sexual organs are hermaphroditic and are present in num-
bers equal to those of the proglottids, so that these were formerly
ITI, CESTODA. 281

regarded as sexual individuals of a colony, each with its own
reproductive apparatus. Two types must be recognized. In the
one the presence of vitellaria and the separate openings of uterus
and vagina recall the conditions in trematodes, while in the second

 

Fig. 240. Fig, 241. Fig. 242.

Fie. 240.—Apical view of head of Tenia solium. (From Hatschek.)

Fie. 241.—Head of YVetrarhynchus viridis. (After Wagner.) Dissected to snow the
internal parts of the proboscides (0) and the ganglion (q).

Fie. 242.—Schema of action of rostellum. On the right the hooks are exserted for
adhesion, on the left retracted. 7, rostellum; s, sheath; /, longitudinal muscles.

 

 

   

S|

Wvack sd dgu ov at fe

Fie. 243.—Proglottis of Bothriocephalus latus. (After Sommer.) Right only vitel-
larium, left only testes, shown. cb, cirrus sheath opening with the vagina; dq,
vitelline duct; dt, vitellarium; h, testes; od, oviduct; ov, ovary; sd, shell gland;
u, uterus; va, Vagina; vd, vas deferens (dark-lined); w, excretory canal.

Ss

the uterus ends blindly and the vitellaria are modified into a small
albumen gland. Since vagina and vas deferens almost always open
together, self-impregnation is possible. Besides cross-fertilization
of separate proglottids has been seen. The general features of the
two types may be made out from figures 243 and 244, reference
      

 

 

PLATHELMINTHES.

 

 

 

 

 

282

B43
ea
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Say
ASS,
DS Prise
n
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333
HHP
oop
aun
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He
B59
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ee
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aR
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aog
ons
o-
AA,
I
3
$s
s
a
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bo

k,

ov, ovary; rs, receptaculum seminis;

deferens.

dt, vitellarium ;

Fig. 244.—Proglottid of Tania saginata
sheath

 

Bos
ce
gs
a3
2s
or
gS
S a
ct
S:
=
0

oO

enlarg
Ff, Distomum hepaticum; g, Dist. lanceolatum

caris lumbricoides; b,e, Oxyuris vermicularis
lium, i, T. saginata,; k, Bothriocephalus latus.

urasites from the human intestine,
e, Dochmius duodenalis

ae
as

st se
os
ant
ss
igss
So28
“HSS
S

es

&
ITI, CESTODA. 283

being made to the description of the organs in the trematodes (p.
272).

The difference in the sexual apparatus has its influence on the
peculiarities of the egg (fig. 245). In Bothriocephalus it is large
(%), has a tough shell with a lid, and encloses a small egg cell
with numerous yolk cells. The eggs of Tenia (h, i) are small,
with a layer of albumen and a delicate shell which is lost early. It
is replaced by an embryonic shell, a radially striped envelope
secreted by the embryo in a somewhat advanced stage. It is in
this condition that one usually sees Tenia eggs.

A further consequence is a difference in development. In most
Bothriocephalidw, as in the Trematoda, the egg must enter the
water for its further development. Here a ciliated oval larva
escapes which contains a six-hooked larva (oncosphera, fig. 246).

 

Fig. 246.—Development of Bothriocephalus latus. (From Leuckart.) A, ciliated
larva; B, same with escaping six-hooked larva; C, young encysted Bothrio-
cephalus.

The ciliated envelope is temporary and is cast off like the ciliated
coat of the trematode larva. The six-hooked larva in some
unknown way enters a fish, becomes encysted (pleurocercoid) in
muscles or viscera, and changes directly into the head of a Both-
riocephalus. This on being taken, in feeding, into the intestine
of the proper host develops into the adult.

The longer and better known history of the Tenias differs con-
siderably. The distinctions are early recognizable, since the six-
hooked larva lacks the ciliated envelope but is enclosed in its
homologue, the embryonic shell already alluded to. Since this
envelope cannot open of itself, the young must be freed from
it by its digestion in the stomach of the proper intermediate host.
Thus the eggs of Tenia soliwn must pass into the stomach of the
pig (they are taken by admixture of the food with embryos con-
tained in fecal matter) and after being freed from their shell in
284 PLATHELMINTHES.

the stomach the larve with their six hooks bore through the intes-
tinal wall and migrate, using the blood-vessels in their course, into
the muscles, or more rarely other organs. Here they develop
into bladder worms (cysticerci). In this they become oval and
secrete a cyst to which, as a foreign body, the pig adds an envelope
of connective tissue. The cysticercus blastema grows through

 

Fig. 247.—Structure and development of the cysticercus (C. cellulose of Tenia
solium), a,measly meat, natural size; below an escaped cysticercus; b, cysticer-
cus, with exserted scolex, enlarged; c-e, development of the scolex, more en-
larged; c, young cysticercus with blastema of scolex (above) and water-vascular
net; eS different stages of scolex in receptaculum, the cysticercal wall mostly
removed.

increase of cells, but more by the infiltration of serous fluid, so
that it becomes distended into a delicate translucent vesicle. So
abundant can this be that in 7. soliwm the microscopically small
embryo can grow in three or four months to the size of a bean or
pea; in other species as large as a hen’s egg. By invagination the
wall of the bladder produces the blastema of the scolex (fig. 247, c).
This has at first a sac-like shape, but soon increases in length, its
growth being confined by an envelope, the receptaculwn (d), so
that it is bent.

At the apex of this blind sac arises the characteristic armature
of the scolex which makes it possible to say what tapeworm will
come from the cysticercus. Thus in 7. sodiwn there are four
suckers and a crown of hooks. These parts are at first inverted
III. CESTODA. 285

and only come to their definitive position on the outside of the
scolex when the latter is protruded as one would turn out the
finger of a glove. The further development follows when the
cysticercus is taken into the stomach of the new host. When
man, for instance, eats infected (‘measly’) pork, the cysticerci are
freed by action of the digestive juices and later the scolex is
everted. The embryo passes to the intestine, becomes attached
and, surrounded by nourishment, begins to grow, the bladder
remaining attached to the hinder end, and soon the formation of
proglottids begins in the middle piece connecting the bladder with
the scolex. So rapid is the growth that in ten or twelve weeks
Tenia solium begins to set proglottids free.

In cases where the bladder reaches a considerable size it has the power
of producing more than asingle scolex. The bladder of Cenurus cerebralis,
which lives in the brain of sheep, produces hundreds of scolices. The num-
ber is even greater in Tenia echinococcus, in which the bladder increases
by budding for some time, and by the formation of numerous daughter
bladders produces marked tumors in the liver of man and domestic animals,
before the formation of scolices begins. In the interior of each daughter
vesicle appear a number of brood vesicles, each of which produces numbers
of scolices, so that from a single six-hooked embryo thousands of scolices
can arise (fig. 253). This extreme case stands in contrast to others which
connect with the development of Bothriocephalus, in which the cysticercus
is replaced by a cysticercoid (fig. 248). Here there is no infiltration and
the scolex is closely enclosed by an envelope comparable to the bladder
wall.

All of this is of importance in the correct conception of the development
of a tapeworm, which was earlier believed to be a complicated alternation
of generations; the bladder to be a stage which by endogenous budding
produced scolices; the scolex, in turn, a stage which by terminal budding
produced the sexual animals, the proglottids, and the tapeworm itself a
chain of individuals, a strobila. This view, so easy to learn, so easily ex-
plaining the development, contains two errors. The bladder is not an inde-
pendent generation, but only the precocious hinder end of the scolex. The
tapeworm is not a colony, but a single animal; the proglottids are not in-
dividuals, but specialized parts of a single whole. This view is confirmed
by a comparison with other forms. The Caryophylleidie (fig. 249) are
single bodies, the anterior end elongate and taking the place of the scolex,
while the broader hinder part contains a single hermaphroditic apparatus.
In the Ligulidew the body is still unjointed, but has increased in length and
contains numerous sets of sexual organs. This duplication of the repro-
ductive apparatus explains the appearance of proglottids.

Family 1. CARYOPHYLLZIDH (Cestodaria). Cestodes without ace-
tabula, simple sexual apparatus, scolex and proglottis not differentiated.
Distinguished from trematodes by absence of digestive tract. Larval stages
in invertebrates, adults nearly always in fishes. Caryophylleus (fig. 249)
286 PLATHELMINTIES.

in the intestine of cyprinoids; Amphilina in body cavity of sturgeon;
Archigetes in annelids (Senwris).

Family 2. Licunim. No acetabula; numerous sexual organs, but no
proglottids. The immature stages in the body cavity of fishes, the adults
in the intestine of birds. Ligula.

 

Fie. 248

  

Fig. 250. Fie. 249,

Fia. 248 —Cysticercoid in invaginated andjfextended condition from <Arion ater.
(From Hatschek.)

Fia. 249.—Caryophylleus mutabilis. (After M. Schultze.) df, vas deferens; dv, vitelline
duct; k, scolex; ov, ovaries; ps, penis; vs, vagina with receptaculum seminis; ¢,
testes; ut, uterus; vi, vitellarium ; vs, vesicula seminalis. The connexion of
vagina with the crossing point of genital duct, vitelline duct, and uterus is
lacking in the figure.

Fie. 250.— Tapeworms of fishes. (After Linton.) 4, Echinobothrium variabile*; By,
Rynchobothrium bisulcatum * ; C, Letrabothriun.*

 
   

Family 3. TETRARHYNCHIDA. With scolex and proglottids, the head
with four protrusible hooked probosces (fig. 241). Immature and mature
stages in fishes. Tetrarhynchus, Rynchobothrium.*

Family 4. TETRAPHYLLID.Z. Head with four very mobile suckers, often
armed with hooks. Zchinobothriwm * (fig. 250), Acanthobothrium.*
UI. CESTODA. 287

Family 5. BoTHRIOCEPHALIDE, Scolex and proglottids present; head
spatulate with two sucking groves on the narrower sides. Most interesting
is Bothriocephalus latus (fig. 251), the largest tapeworm which occurs in
the human intestine (also dogs and cats), and which may reach a length of
forty feet and consist of over four thousand proglottids. As has been out-
lined above, the pleurocercoid occurs in fishes, and man acquires the parasite
by eating uncooked fish. It is especially abundant in Russia, the eastern
provinces of Prussia, andin Switzerland. It is rare in America and occurs
most frequently in immigrants. Other species occur in man in Greenland
(B. cordatus) and China (B. mansont).

 

Fia. 251.—Head and ripe proglottids of Bothriocephalus latus, the head showing the
sucker at the angle, the proglottids the marking produced by the uterus.

Family 6. Tzn1ap&. With scolex and separable proglottids; the scolex
always bears four suckers and in many a ro- A
stellum with a circle of hooks (fig. 252). In
the proglottids the vitellarium is replaced by
an albumen gland; the uterus is cecal, and the
genital pore occurs usually laterally in the
proglottids, alternating right and left, rarely
only on one side (Hymenolepis, Anoplocepha-
lus). It is rarely doubled in a proglottid
(Dipylidium, Moniezia). Intermediate stage a
cysticercus or cysticercoid. The human tape-
worms are grouped here together, but are sub-
divided accordingly as the sexual animal or
the cysticercus has been found in man.

A, Tenie sexually mature in the human
intestine. Most noticeable are Teenta soltiwm
and J. saginata, the differences between
which are shown in fig. 252 and the follow-
ing table. It is to be noticed that, in spite of
the lack of hooks, the stronger suckers render
T. saginata more difficult to expel. Tenia Fie, 252.—Head and ripe pro-

sane 3 . glottid of (A) Tania saginata
solium, as the table shows, is not rare in the and (B) 1. sulium.
cysticercus stage in man and occurs sometimes in places, like the brain
and eyes, where it causes severe injury. These cases are in part explained
by lack of cleanliness in the food, which may contain eggs, but it is possible
through internal infection ; pieces of the worm passing the pylorus and
entering the stomach, where they are digested, setting the embryos free.

 

   
   
   
 

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sy
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iL
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288 PLATHELMINTHES.
wn
os Length (a) of
8b the worm Character | Occurrence
Head 2s Uterus and (h) of the oO )
E i ripe Cysticercus | Cysticercus
Ze proglottids
2 ie Ea In pigs, occa-
5 te Tpstel- sionally in
5 circle of S Each side a. 10 feet, 6-20 mm., muscles,
B hooks  |with7-9large| bh. 9-11 mm. with brain, and
3 | 6 in 2 rows): S branched long, 6-7mm.! abundant eyes of man,
2 4-weak || @ pouches broad fluid nace iy in
othe
& suckers | tiamimals
rtIn, 4 Each side _ |a. 20 to 25 feet |
3 3 pa ea copa : s with 20-30 and more, 4-8 mm.,
as 4 ‘Strate " | delicate little | b. 18-20mm. | tough, with | Cattle
a2 eer eae Ss branched long, 5-7 little fluid |
a iN pouches mm. broad |

 

Many other Tenice, which are common to other mammals, occur occa-
sionally in the human intestine. In mice and rats occur T. (Hymenolepis)
murina and T. diminuta (= leptocephala). The first (identical with 7.
nana) has recently been very abundant in human intestines in Italy. The
worm, an inch or two long, may occur in thousands and cause severe in-
jury. This species may develop without an intermediate host; the eggs
taken into the stomach pass the cysticercoid stage in its walls and thence
to the intestine to become adult. 7. diminuta (= flavopunctata), which
has insects for its intermediate host, has been described from man. Other
species occur in the tropics.

B. Forms passing the cysticercus stage in man. Besides the eysticercus
cellulose of TZ. soliwm that of T. acanthotrias
(possibly identical with TZ. soliwm) has been
found in man, More frequent and of more im-
portance to the physician is the cysticercus of
Tenia echinococcus (fig. 253), which lives as an
adult in the dog, and is easily overlooked on ac-
count of its size. It is at most 5 mm. (4 inch) long
and consists of a scolex and three or four pro-
glottids. The scolex bears four suckers and hooks
on the rostellum. When the eggs are taken into
the human stomach, as may easily happen by
stroking and kissing infected dogs, the embryos
are set free and wander into liver, lungs, brain,
or other organs and produce here tumors which,
in the case of the liver, may weigh ten or even
thirty pounds. This extraordinary size is ex-

plained by the formation of daughter bladders
eee and their sco- (eghinococeus) deseribed above. Echinococei are
more common in cattle, sheep, and swine than in man.

Common Tani of domestic animals are in the horse Anoplocephala
plicata (4 to 80 inches), A. perfoliata ($ to 8 inches), A. mamillana (4 to

 

Fia. 253.—Tenia _echinococ-
cus. (After Leuckart.)
Right sexually mature;
left a part of an echino-
coccus with two brood
IV. NEMERTINI. 289

2 inches); in ruminants, J/oniezia expansa (usually 7 feet, sometimes 30
feet or more), often fatal, JL denticulata (1 to 5 feet), the most common
tapeworm of cows; in dogs, Tenia marginata (cysticercus in sheep and
swine), 7’. serrata (cysticercus in rabbits), 7. echinococcus (above), T. ca-
nurus (cysticercus in brain of sheep, causing the disease called ‘stag-
gers’), Dipylidium cucumerina (most common, larva in the dog-louse,
Trichodectes); in the cat, Tenia crassicollis (cysticercus in mice). Several
species occur in domestic birds, one (Drepanidotenta infundibuliformis),
causing epidemics among chickens,

Class IV. Nemertini.

Most nemerteans are of appreciable size, some reaching a length
of a yard or more (Lineus longissimus 90 feet !), and yet they are
so contractile that a specimen of our Cerebratulus lacteus, which
can extend itself to fifteen feet, can retract to two. Nemerteans
are rare in fresh water or moist earth, but are most abundant in the
sea, where they burrow through the mud or lie rolled up beneath
stones. Many are noticeable for their bright colors. Their system-
atic position is a problem. Frequently they are included in the
Plathelminthes, but the presence of an anus, of distinct vascular
system, and the higher organization in other respects renders such
a position doubtful.

Like some flatworms they have a solid parenchyma bounded
externally by a ciliated ectoderm rich in mucus cells, and inside
this at least two muscular layers, which, when but two are pres-
ent, are an outer circular and an inner longitudinal layer. They
differ from all other Plathelminthes in having a complete

 

 

 

 

 

 

 

 

 

 

Fie. 254.—Diagram of Nemertean (orig.). b, brain; ¢, ciliated pit; d, dorsal nerve
trunk; di, dorsal blood-vessel; gc, gastric cw#ca; i, intestine; 1, lateral nerve
trunk; Iv, lateral blood-vessel; », proboscis retracted ; pnt, proboscis muscles ;
pn, protonephridial tube; po, its opening ; ps, cavity of proboscis sheath.

   

alimentary tract, beginning with a ventral anterior mouth and
continuing as a straight tube, with, usually, paired diverticula, to
the vent at the posterior end of the body (fig. 254).

Especially diagnostic is the proboscis, which hes dorsal to the
alimentary tract and usually opens separate from the mouth. The
290 PLATHELMINTHES.

proboscis is a muscular tube closed at
one end and at rest is infolded like
the finger of a glove inside a closed sac,
the proboscis sheath, which extends far
back in the body. Its tip is bound to
the posterior end of the sheath by
retractor muscle. By contraction of
the sheath the proboscis is everted,
while it may be retracted again by the
muscle. Nettle cells are not uncom-
mon in the proboscis wall, while in some
forms (the older Enopla) the effective-
ness of the organ is increased by the
presence of a dart-like stylet at the
tip (reserve stylets occur on either side,
fig. 255), and at the base of the stylet
is the opening of a poison sac.

The blood-vascular system consists
of a pair of lateral tubes connected by
transverse loops, and in most forms a
third tube is present lying between the
intestine and the proboscis sheath.
The excretory system consists of two
tubes lying close beside the lateral blood-

 

 

 

  

Fig. 255 bid. 2ov.

Fie, 255.—Young Tetrastemma obscurum. (From Hatschek, after M. Schultze.) a, anus;
cc, dorsal commissure; ey, cerebral ganglia; f, ciliated grooves; i, digestive tract;
lv, lateral, mv, dorsal hlood- vessel; neph, Ww ater- vascular tubes; nl, lateral nerve;
ac, eye spot; or, proboscis pore; 7, ‘probosci is; r,, glandular hinder por

is; rm, retractor of proboscis: st, sty lets: *, opening of excretory

—Pilidium larva. (From Lang, after Salensky.) es, invaginations which

late r give rise to the nemertine skin; im, oral lobes; md, archenteron; rn, ring

nerve; sp, apical plate; st, esophagus; my ciliated band.

 
    
 
IV. NEMERTINI. 291

vessels and connecting with branches terminating in flame cells,
while they open separately to the exterior by one or several open-
ings.

The central nervous system (in some forms still in the ectoderm)
consists of a supracesophageal brain of a paired ganglia, from which
nerves run to the proboscis and two lateral cords united on the
ventral side by numerous transverse commissures. Connected
with the brain, either directly or by means of a short nerve, are the
cerebral organs or ciliated grooves, pits placed on the sides of the
head. These, formerly regarded as respiratory, are now considered
sense organs. ‘Tactile organs and simple eyes are widely distrib-
uted; otocysts are very rare.

As arule the nemertines are dicecious, the gonads forming a row
of lateral sacs, alternating with the intestinal blind sacs and open-
ing dorsally. The development is sometimes direct, but usually
a metamorphosis occurs in which a larva, the pilidium (or a
reduced form of it, Desor’s larva), appears. The pilidium is a
gelatinous helmet-shaped larva with right and left below a pair of
lappets (fig. 256). The margins of lappets and helmet are ciliated,
while at the top a bundle of longer cilia project from a thick-
ened patch of ectoderm, the apical plate, which apparently func-
tions as a central nervous organ. Inside is the simple cxcal arch-
enteron, the mouth (blastopore) opening between the lappets.
By a complicated process of growth and infolding this mesenteron
becomes enclosed in its own skin, produced from four inpushings
(es); an anus is formed, and at the time of metamorphosis the
worm thus produced escapes from the rest of the pilidium, which
quickly dies.

Order I. Protonemertini.
Nervous system outside the muscles; no stylets in the proboscis; mouth
behind brain. Carinella.*
Order II. Mesonemertini.
Nervous system in the muscles; mouth behind brain; no stylets.
Cephalothrix.*
Order III. Metanemertini.

Nervous system in the parenchyma inside the muscles, mouth in
front of brain; proboscis as a rule with stylets. Geonemertes * and some
species of Zetrastemma,* terrestrial. Amphiporus* (numerous eyes),
Nectonemertes.*  DMalacobdella,* leech-like with posterior sucker, parasitic
in lamellibranchs.
292 PLATHELMINTHES.

Order IV. Heteronemertini.

Body wall with several muscular layers, the nervous system in the
muscles; mouth behind brain; proboscis unarmed. Linews,* Micrura,* and
Cerebratulus * (Meckelia) on our coast, with cerebral organs. Zupolia.

Summary of Important Facts.

1. The PLATHELMINTHES are bilateral animals of flattened
form whose nervous system consists of a supracesophageal ganglion
and lateral nerve trunks; the excretory system of branched water-
vascular tubes (protonephridia).

2. The TURBELLARIA are the most primitive; the Trematoda
and Cestoda have descended from them.

3. The Turbellaria are ciliated externally. They have no anus
and no circulatory system. The digestive tract consists of ectoder-
mal pharynx and entodermal stomach, the latter many-branched
in the Polyclads, with three main branches in the 7iclads, and
rod-like in the Rhabdoceles.

4. Polyclads and Triclads are often united under the name
Dendrocela.

5. In the parasitic TREMATODA the cilia are entirely lost or
confined to the larval stages. Jlooks and suckers are present for
attachment to the host; several in the ectoparasitic forms; only
one or two suckers in the internal parasites.

6. In the Distomige there occur heterogony and alternation of
hosts. From the egg arises a sporocyst, always parasitic in mol-
luses, from the parthenogenetic eggs of which develop cercarix
which become encysted Distomiz in the second host, sexual Di-
stomiz in the third.

7. Best known of the Distoma are D. hepaticum and D.
lanceolatum (rare in man, common in sheep) and D. hematobium
in the portal vein of man in warm climates.

8. The Cestopa are characterized by the entire absence of
digestive tract, and usually by the existence of scolex and pro-
glottids.

9. The scolex is the organ of attachment, and as such is pro-
vided with suckers and frequently with hooks. It also produces
the proglottids by terminal budding.

10. The proglottids contain an hermaphroditic sexual apparatus.

11. The eggs produce a six-hooked embyro which must pass
into an intermediate host. This is accomplished either by taking
the eggs in passively with the food, or the embryo must pass into
the water, where it infects fishes.
ROTIFERA. 298

oo

12. The embryo, in the intermediate host, becomes encysted
and changes directly to a scolex (pleurocercoid) or into a bladder
‘worm (cysticercus) which produces internally one or more scolices.

13. The scolex is freed from its cyst when taken along with
food into the stomach of the proper host, and then acquires the
capacity of development into a tapeworm.

14. In man occur as cysticerci Tenia echinococcus (adult in
dog) and 7. solium; as adults Tenia solium (cysticercus in pigs),
T. saginata (eysticercus in cattle), and Bothriocephalus latus
(pleurocercoid in fish).

15. The NemMERTINI are distinguished by a complete alimentary
canal with anus, and a proboscis dorsal to the digestive tract.

PHYLUM V. ROTIFERA (ROTATORIA).

The aquatic wheel animalcules, or Rotatoria, are among the
smallest Metazoa, and can be distinguished from the Infusoria,
which they resemble in habits, only by the microscope. The body
is divisible into three regions, head, trunk, and tail. The trunk
is covered by a tough cuticle into which head and tail can be

 

Fic. 257.—Diagram of rotifer. (After Delage et Herouard.) }, brain; fc, flame cell;
qy, gastric gland; i, intestine: m, mastax; ov, ovary; py, pedal gland; pr, pulsat-
ing vesicle of excretory system; s, stomach.

retracted. The tail or ‘foot’ is often composed of rings which can
be telescoped into each other and which by their superficial resem-
blance to segmentation formerly led to the association of the roti-
fers with the Arthropoda. The last tail ring often bears a pair of
pincer-like stylets which together with adhesive glands enable the
animal to adhere to objects. The head has the most delicate
cuticle and is expanded in front to a trochal disc, an apparatus of
varying appearance, which is surrounded by a ring of cilia of use in
swimming and also in directing food to the ventral mouth. The
204 ROTIFERA.

alimentary canal consists of @sophagus, mastax (chewing stomach),
glandular stomach, and intestine; all except the mastax cilated.
The mastax bears two chitinous jaws (trophi), which in life are in
constant motion and comminute the food. The cerebral ganglion
lies above the wsophagus, with which simple eyes and peculiar sense
organs, the cervical tentacles, are frequently connected. The
usually single ovary and the paired protonephridia empty into the
posterior part of the alimentary canal, which thus becomes cloacal
in character. For a long time males were unknown until Dal-
rymple discovered that these are much rarer and smaller ‘dwarf

 

Fie. 258.—Brachionus urceolaris. A,female with four eggs in various stages; B, male;
C, ‘flame’ from protonephridia, greatly enlarged; b, urinary bladder; ¢, cloacal
opening; d, gastric glands; g, ganglion, with eye; h, testis; k, mastax: m,
stomach; 0, ovary; p, penis; t, tentacle; w, protonephridia.

males,’ and that they have a much simpler structure (fig. 258, /).
Usually the alimentary tract is reduced to a solid cord in which the
testes are imbedded.

The Rotifers have two kinds of eggs, large winter eggs enclosed
ina thick shell and smaller thin-shelled summer eggs. ‘The latter
develop parthenogenetically and by their numbers and rapid
growth subserve the distribution of the species. The winter eggs
require fertilization, and have a long resting period, thus serving
to tide over periods of cold or drought. The adult animals can
withstand a certain amount of desiccation; and they are often
found in damp moss or in eave troughs in a sort of sleep from
which they are awakened by water.
CQ@HLUELMINTHES. 295

In structure the Rotifiers are much like the trochophore type of
embryo of annelids and molluscs to be described later. They must hence
be regarded as extremely primitive forms, connected at once with the
ancestors of these groups, and, as shown by nervous system and excre-
tory organs, with the flatworms as well. Most species are cosmopolitan
and inhabitants of fresh water. Many species in America. Near the
Rotifera may be placed the fresh-water GASTROTRICHA (Ichthydium,
Chetonotus) and the marine ECHINODERID A, forms which are little
understood.

PHYLUM VI. CQHHLHELMINTHES.

The Colhelminthes are distinguished from all the forms which
have gone before by the presence of a body cavity, separating the
outer body wall from the intestine. This cavity is the celom, but
whether it be homologous in different groups, e.g. nematodes and
annelids, is not settled. The body muscles are developed from the

 

Fie. 259.—Section of Ascaris lumbricordes through the pharyngeal bulb; beside ita
bit of the body wall more enlarged. c¢, cuticle; d, dorsal line; h, hypodermis; m,
longitudinal muscle; n, nucleus of muscle cell; p, muscle cell; s, lateral line;
v, ventral line; w, excretory canal.

outer (parietal) epithelial wall of the celom and hence are ‘ epi-
thelial musele cells’ (figs. 259, 260). The excretory organs con-
nect the body cavity with the outer world and hence are nephridia
(earlier called segmental organs, cf. fig. 69). Internally they begin
with a ciliated funnel, the nephrostome, and continue as long coiled
tubes expanding just before the outer end to a kind of bladder.
The sexual apparatus is simple. The gonads (fig. 260, 0) are
296 CQ@LHELMINTHES.

specialized parts of the cwlomic epithelium and their products are
usually carried to the exterior by the nephridia (more rarely by
special ducts), so that here, as in vertebrates, we can speak of a
urogenital system. A closed blood system is now present, now

 

   
    
    
  
    

   

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UNUM EustAt hae:

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Fia. 260.—Transverse section of Sagitta bipunctata and a bit of the body wall more
enlarged. (After O. Hertwig.) c, celom; ed, entoderm; df, splanchnic meso-
thelium; ¢, epidermis; m, somatic mesoderm (muscles and epithelium); 0, ovary.

absent. Little in general can be said of the nervous system:
details will be given in connexion with the separate classes.

Class I. Chetognathi.

These marine forms, a half to two inches long, perfectly trans-
parent, are well adapted to serve as an introduction to the celomate
worms. They live at the surface of the sea, preying on other ani-
mals, and from their shapes and rapid motions deserve the name
Sagitta —arrow—given some forms. The animals swim by means
of horizontal fins, one surrounding the tail and one or two pairs
on the sides of the trunk (fig. 261). On either side of the mouth
is a lobe bearing strong bristles used in seizing prey (Cheto-
gnathi, bristle-jaw). Internally the body is divided into three
segments, head, trunk, and tail, by transverse septa which divide
the celom into corresponding parts. Each segment of the ce-
Jom again is divided into right and left halves by a mesentery
(fig. 260), supporting the straight intestine, running lengthwise
through it. The intestine terminates at the anus at the end of the
trunk segment.

The nervous system is entirely ectodermal. In the head is a
I. CHETOGNATH. 297

pair of fused cerebral ganglia (fig. 262), in the trunk segment a
large ventral ganglion, and these are connected
by long esophageal commissures. Of interest,
because characteristic of nematodes and many
annelids, are the relations of the musculature,
which consists of longitudinal fibres alone.
The body cavity is lined with epithelium (fig.
260), which, so far as it abuts against the ali-
mentary tract, is called splanchnic (or visceral)
mesoderm; that on the side of the celom to-
wards the ectoderm is the somatic mesoderm.
The muscles arise from the latter layer and are
divided into four fields, right and left dorsal,
right and left ventral. The sex cells also arise
from the epithelium of the celom, the eggs
in the trunk segment, the sperm in the tail.
The eggs are carried to the exterior by special
ducts. The sperm-forming cells early lose
their connexion with the epithelium, fall into
the celom, where they develop the spermato-
zoa. These are carried out by canals which
by their relations to the celom recall the ne-
phridia of the annelids.

 

  

Fia. 261. Fig, 262.

Fie. 261.—Sagitta heraptera, ventral view. (After O. Hertwig.) a, anus; bg, ventral
ganglion; d, intestine; fl, fin; ho, testes; m, mouth; ov, ovary; ovd, oviduct: sh,
seminal vesicle; sc, cesophageal commissure; s/l, tail fin; sl, sperm; wo, female
opening. 7 ;

Fia. 262.—Head of Sagitta bipunctata, dorsal view. (After O. Hertwig.) an, nerve
to au, eye: g, brain; gh, bristles; rn, nerves to ro, olfactory organs; sc, esophageal
commissure.

The development of Sagitta is significant from two points of view.
The archenteron (fig. 108) is divided by lateral folds into an unpaired
middle portion and two paired lateral chambers ; the first is the defini-
tive digestive tract, the latter the anlagen of the cwlomic diverticula.
298 CQ@LHELMINTHES.

In other words the ccelom is an outgrowth from the archenteron, @.e. is an
enteroceele. Second: The gonads are derived from a pair of cells in the
primitive entoderm, which later are carried into the ccelomic walls. Hence
each divides into anterior and posterior cells, the anterior developing
into the ovary, the posterior into testes. Hence here the male and female
sex cells are beyond doubt descendants of a common mother cell.

The few species of Cheetognathi are arranged in two or three genera,
of which Sagitta, represented on our coasts by S. elegans,* is best known.
Spadella.

Class II. Nemathelminthes.

Like the flatworms, the roundworms are characterized by their
shape, they being thread-like or cylindrical animals whose form is
the result of the existence of a body cavity in which the viscera are
so loosely held that on cutting through the muscular body wall
they will fall out (fig. 259). Since the Nemathelminthes share this
celom with most annelids, the distinction between the two rests
largely upon negative characters, the roundworms lacking the
segmentation of the body cavity and the corresponding ringing or
annulation of the body wall. To the Nemathelminthes belong
three orders, much alike in habits and appearance but differing
considerably in structure. Of these the most important are the

nematodes.
Order I. Nematoda.

The nematoda contain numerous species of thread-shaped
worms varying from 0.001 to 1.0 metre in length, many of
which, through their wide distribution as parasites in plants,
animals, and man, possess special interest. The outer surface is
covered by a tough cuticle secreted by the underlying hypodermis
(fig. 259), a layer corresponding to epithelium and cutis, which in
cross-section shows, median and lateral, four thickenings, the
dorsal, ventral, and lateral lines. In the lateral lines run the excre-
tory vessels, two longitudinal canals which are united near the head
by a transverse vessel opening on the ventral surface by an unpaired
porus excretorius to the exterior. They are related to the celom
by two giant cells on either side which send processes into the
body cavity. These lateral and median lines divide the muscles
(here only longitudinal) into four fields, asin Chetognaths. These
muscles are parts of the somatic epithelium, a layer of vesicular
cells which by their size (fig. 259) so encroach upon the celom
that scarce space is left for the alimentary canal and reproductive
organs. <A splanchnic mesoderm is lacking.

The alimentary canal begins with a terminal mouth and ends
with the anus, which is ventral and in front of the end of the body.
II, NEMATHELMINTHES : NEMATODA. 299

The mouth connects with the muscular esophagus, which, for
sucking purposes, is expanded posteriorly to a pharyngeal bulb and
is lined throughout with a cuticle. From this point to the anus
the stomach-intestine is usually uniform (fig. 263), The esophagus
is surrounded by a nervous ring which sends forward and back a

 

 

Fic. 263. Fig. 264.
Fic. 263.—Structure of young female Ascaris (based on a drawing by Leuckart).
d, intestine; v, ovary; s, lateral line; v, ventral line: va, vagina.
Fig. 264.—Diagram of nervous system of a nematode. (After Biitschli.) ce, commis-
sures; d, dorsal nerve; /, infracesophageal, s, supracesophageal part of nerve
ring; v, ventral nerve.

large number of nerves, those in the mid-dorsal and ventral lines
being strongest. At points on these nerves are collections of
ganglion cells, but a formation of ganglia, as in the annelids, does
not occur (fig. 264).

The sexual organs of these rarely hermaphroditic forms are
very simple. Males and females are easily distinguished not only
by the copulatory organs, but by the openings of the genital ducts.
These, in the male (fig. 265), are in the end of the alimentary
canal, which hence is a cloaca. In the female (fig. 263) there is a
special genital opening on the ventral surface between mouth and
anus, the position varying with the species. In general the struc-
300 C@LUELMINTHES.

ture of the reproductive organs is alike in both sexes. In both,
on account of the great fertility, these are long tubes coiled for-
ward and back and ending in fine threads which produce eggs or
sperm (ovaries, testes), while the rest serves as seminal vesicle, or
receptaculum seminis, and ducts. In the male the genital tube is
always single; in the female it is usually double, the right and
left halves uniting a little before the external opening (fig. 263,
va). Most common of copulatory organs in the male are spicula,
bent spines, which lie ina sheath behind the vent and can be
protruded through the cloacal opening, appropriate muscles caus-
ing them to retract. Besides there may be valves to right and left
to clasp the male, or, as in Trichina, the whole cloaca is pro-
trusible.

Since there is copulation, the eggs are fertilized in the uterus,
after which they are either laid or retained for more or less of their
development, many, like Jrichina, being viviparous. The post-
embryonic development depends largely upon the mode of life.
Free-living species grow by repeated molts without much change
of form. In many Anguillulide, which show how free life can be
transformed into parasitic, there is an alternation of generations
(heterogony) from an hermaphroditic entoparasitic to a free
dicecious generation. The occasional suppression of the free gen-
eration which occurs in many Anguillulids leads to the Strongylida,
where the offspring of the parasitic generation can live free for a
time (rhabditis larve), but must return to parasitism to undergo a
metamorphosis and become sexually mature. The free life is
shortened again in the Ascaridw, where the eggs must pass to the
exterior for a longer or shorter time, but the embryos only escape
when the eggs are taken into another host. Lastly, there are
species like Trichina where the free life is entirely suppressed and
transportation from host to host takes place in the encysted con-
dition passively by food.

Family 1. ANGUILLULIDZ ; small thread-like nematodes with double
pharyngeal swelling which live in mud, organic fluids or plants, rarely in
animals ; male with two spicula. <Angwéllula aceti, vinegar eel, 2 mm.
long, in vinegar and stale paste. Rhabditis (Rhabdonema) nigrovenosa,
not 1 mm. long, lives in mud and stands in heterogony with a second
form which lives in the lung of frogs. Strongyloides intestinalis of the
tropics, but which has recently appeared in southern Europe, has a some-
what similar history, the adult stage being reached in the human intes-
tine. Here also belong numerous plant parasites of which Tylenchus
tritict and Heterodera schachti demand notice, the first doing great dam-

age to wheat, the second to turnips in Europe. Zylenchus devastatrix
attacks rye and hyacinths.
II. NEMATHELMINTHES: NEMATODA. 301

Family 2. Ascarip#, Mouth surrounded by three lips, one dorsal,
two ventral ; males with two spicules. Besides numerous species in lower
vertebrates two of the most common parasites of man, the human round-
worm and the pinworm, belong here. The former, Ascaris lumbricoides,*

as ae az ag

 

Fie. 265.—Dorsal, end, and ventral views of head and hinder end of male Ascaris
lumbricoides. (From Hatschek.)

inhabits the small intestine, often in enormous numbers. The females
average about 5-6 inches, the males 4 inches, in length. The ani-
mals are enormously prolific, a female containing about 60,000,000 eggs.
The eggs (fig. 246, a) are easily recognized. Shortly after fertilization
the eggs pass out of the intestine with the feces, but develop without
intermediate host if in the course of two or three months, when the embryo
has developed, they are taken into the human intestine. The development
of the pinworm, Oxyuris vermicularis,* is somewhat similar except that
the embryos are developed in the egg at the time of oviposition, and hence
after a shorter stay outside the body are capable of infection. The white
worm, not half an inch long, lives in the rectum, especially of children,
and in crawling outside the anus causes intolerable itching. Ascaris
mystaz * occurs in dogs and cats (occasionally in man). A. megalo-
cephala * (a favorite animal for cytological researches) and Oxyuris equé in
the horse. These do little harm. On the other hand Heterakis maculosa
often destroys whole flocks of pigeons.

Family 3. STRONGYLIDZ. These are readily recognized by the bursa
of the male, a broadening of the hinder end of the body by two ring-like

 

Fic, 266.—Anterior end of Ankylostomum duodenale, dorsal and side views. a, inner,
b, outer ventral teeth; c, dorsal tooth of m, mouth capsule ; d, stylet; e, ventral
ridge ; 9e, esophagus.

processes, Which contains two spicula. Frequent but not constant is a
widened capsule surrounded by papille at the mouth. A number of
species of Strongylus occur in domestic animals. Syngamus trachea-
302 COLHELMINTHES.

lis,* half to three quarters of an inch in length, the male and female
always in pairs, cause the disease known as ‘gapes’ in fowl. <Ankylo-
stomum (Dochmius) duodenale (fig. 266), about two fifths of an inch iu
length, lives in the small intestine of man, causing severe loss of blood
and the disease known as Egyptian chlorosis, The eggs develop in mud
and moist earth, and hence people who drink muddy water (Fellahin of
Egypt) or work much with clay (potters and brick-makers) are especially
subject to infection, It was first known in Egypt, caused considerable
trouble during the building of the St. Gotthard tunnel in Switzerland,
and now is common in Germany. Recently it has been thought that the
; Ankylostoma larve obtain entrance to
man through the skin, as in bathing,
ete.

Family 4. TRICHOTRACHELID. These
owe their common name of ‘ hair necks’
to the fact that the part of the body
which contains the pharynx is hair-like
and elongate, while the pharynx itself
traverses a peculiar cord of cells. Long-
est known of the family is Trichocephalus
dispar * of man (fig. 267), about an inch
or an inch and a half in length, which
lives with its neck burrowed like a cork-
screw in the wall of the intestine near the
cecum. Since it does not move, it causes
little injury. Its presence can be recog-
nized by the oval brown double-shelled
eggs (fig. 246, d) in the feces.

A second species, Trichina spiralis *
(figs. 268, 269), is much smaller, but much
more dangerous. Two stages are to be
distinguished, the encysted muscle Tri-
china and the sexually mature intestinal
Trichina, The first was discovered ina
human body in 1835; the latter was not
known until much later, its history being
worked out by Leuekart, Virchow, and
: Zenker. In the encysted stage it occurs

Fic. 269. Fia. 268. in the muscles of pigs, rats, mice, man,
Pia. 267, Trichocephalus dispca, male rabbits, guinea pigs, dogs, ete. (never in
investing wall, (From Leuck- }irds), enclosed in an oval capsule about
Fie. 268. — Trichina spiralis, male. 0.4 to 0.6 mm, long and hence recogniz-

(From Hatschek). cl cloacs: % anle by a practised observer with the
Via. 269.—Trichina in muscle. (From pjaked eye. They are more easily seen

Eons when they are partially calcified and
have a whitish color. Certainty in their recognition demands a low power
of the microscope. In the capsule is coiled up the worm, about 1 mm.
long, which is not yet sexually mature, although furnished with the

 

    
II, NEMATHELMINTHES : NEMATODA. 203

anlagen of sexual organs. To attain this it must be transported into the
intestine of another host. ‘hen, for instance, man eats trichinosed pork
the worms are freed from the muscle and capsules by the digestive
fluids and, entering the small intestine, become sexually mature ina few
days. The female (3-4 mm. long, the male 1.5 mm.) penetrates into the
superficial layer of the intestinal villi and in course of a month gives birth
to 1500 (some say 10,000) living young, after which she dies. The young,
on the other hand, penetrate the lymph vessels, and by way of the
thoracic duct are carried into the blood-vessels, and wander from the
capillaries into the muscles, especially those which are much worked, like
the diaphragm, eye muscles, and muscles of the neck, and which conse-
quently have arich blood supply. They enter the sarcolemma of the
muscle, destroy the muscle substance, and finally become enclosed by a
capsule secreted by the host. The wandering takes place about the second
or third week after infection, the encystment in about three months. A
slight infection causes disagreeable symptoms ; but where large numbers
obtain entrance the cases are frequently fatal. The worst epidemic known
was in Emmersleben, Saxony, in 1884, where 57 died in four weeks from
infection from one pig.

 

Fig. 270.—Transverse section of young Gordius. (After von Linstow.) a, hypoder-
>), muscular layer; c, cuticle; d, parenchyma; e, s, muscles and mesenteries: g,
aentary canal; 4, nervous system.

  
 

Family 5. Finarmp#®. These are extremely elongate, hair-like worms.
Their best-known representative is Dracunculus medinensis, the guinea
worm (the female about a yard long, and about as large as stout packing
twine), which produces a sickness known to the Greeks as dracontiasis. It
forms abscesses beneath the skin in which the worm is coiled up. The em-
bryos break through the wall of the mother and must enter the water and
penetrate a small crustacean, Cyclops. It is apparently introduced into
the human system by swallowing the crustacea with drinking water. The
worm has recently been found in the tropics of America.

A second species is Filaria sanguinis hominis, the adults of which—
3 to 6 inches long—live in the lymphatic glands of man, while the young
escape into the blood, often in immense numbers. They often escape
804 CHLHELMINTHES.

through the kidneys, where they produce serious disturbance (albuminuria,
hematuria). There is possibly a connexion between them and elephanti-
asis. The intermediate host is apparently the mosquito. As yet they are
known only in the tropics. Other species occur in man and other animals.

Family 6. MerMITHID«%. Elongate nematodes with six oral papille.
They live in the body cavity of insects and pass into damp earth, where they
become sexually mature. They share with the Gordiacea the common
name ‘ hairworms.’? Jfermis.*

Order II. Gordiacea.

The hairworms resemble the nematodes in general appearance, but
differ greatly in structure. The body cavity
has both splanchnic and somatic epithelium ;
the intestine is supported by mesenteries; there
is an csophageal nerve ring and unpaired ven-
tral nerve cord, and the female genitalia enter
the cloaca. The adults live in water, where
they lay their eggs; the larve live in insects,
there being in some cases at least an alterna-
tion of hosts. These (and the Mermithide) are
popularly believed to be horse hairs changed
into worms. Gordius,* Chordodes.*

Near the Gordiacea must be mentioned the
marine WVectonema,* the young stages of which
are apparently passed in the mosquito.

Order III. Acanthocephala.

The species of spine-headed worms live in the
alimentary canal of vertebrates. In appearance
they resemble the Ascaride (p. 301), but are easily
distinguished by the proboscis, which may be re-
tracted by muscles and exserted by contraction of
the muscular body wall. This proboscis bores into
the intestinal wall and is held in place by numer-
ous retrorse hooks (fig. 271). In internal anatomy
the entire absence of an alimentary canal marks
them off from Nematodes and Gordiacea, as also the
peculiar structure of the reproductive organs and
a closed vascular system in the body wall which
extends into two sacs, the lemnisci, lying beside
the proboscis sheath. The unpaired ganglion lies
on the proboscis sheath between the lemnisci. An
Fra. 271.—-Male Echinorhyn- Intermediate host occurs in development, the larva

rect at aeons see, living in an arthropod. Thus the larva of Echin-
de, seminal vesicle; dr, orhynchus (Gigantorhychus) gigas* of the pig
ene en lives in the larva of the ‘June bug’ (Jfelolontha),
Se s oe * that of 2. proteus of European fresh-water fishes
r, proboscis; rs, proboscis in crustacea. One species, E. hominis, is ex-

sheath; ©. testes; vd, Vas >
Ee tal tremely rare in man.

 

 

 

 
III, ANNELIDA. 305

Class III. Annelida.

The segmentation of the body, which was shown ina slight de-
gree in the Chetognathi, reaches its highest development in the

 

Fic, 272.—Diagram of annelid somites (orig.). am, acicular muscles; c, ccelom: cm,
circular muscles ; cv, circular blood-vessels; d, dorsal blood-vessel; i, intestine ;
dn, longitudinal muscles; m, mesentery; x, nerve cord; na, nephridium; ne, nv,
neuro- and notopodia, forming parapodium; s, septum; so, somatopleure; sp,
splanchnopleure; t, typhlosole.

Annelids, where it appears both in the outer ringing of the body
and in the arrangement of the most important systems of organs—
metameric arrangement of excretory organs, nervous system, blood-
WS |i

A

a

   

 

Fig. 273.—Trochophore (Loven’s larva) of Polygordius. (From Hatschek.) 4, anus;
dLM, dorsal muscles; £D, hind gut: J, stomach; Jj, intestine: Msfr, mesoderma
band; 1, nerves; Neph, protonephridia; O, mouth; Oe, esophagus; oeLM, cesopha-
geal muscle; SP, apical plate; vLM, ventral muscle; vLN, lateral nerve; Wkr, wkr,
pre- and post-oral zones of cilia; WS, apical cilia; wz, adoral cilia.

vessels—internal segmentation. To this is added an extraordinary
increase in number of body segments (somites, metameres), which
can far exceed a hundred. We can thus define the Annelids as
306 C@LUELMINTHES.

worms with colom and with external and internal segmentation.
In the development there frequently occurs a type of larva, the
trochophore, which must be referred to here, since it is of great
morphological significance, resembling in structure the rotifers
and recalling the larva of the molluscs and to a less extent that of
the echinoderms. It is (fig. 273) a gelatinous body traversed by an
alimentary canal with fore-, mid-, and hind-gut regions. At first
it is everywhere ciliated, but with advance of development the
cilia become restricted to certain thickened tracts of epithelium,
the ciliated bands. One of these, the preoral band, is especially
constant. It runs circularly (]4r) around the body, surrounding
a circular prestomial area, in the centre of which is the anlage of
the cerebral ganglia, a thickened patch (apical plate) of ectoderm,
often bearing a tuft of cilia. Other ciliated bands (post-oral,
perianal) often occur. Of internal organs, besides numerous
muscle fibres, the most noticeable are the excretory organs, true
protonephridia, which open to the exterior either side below. The
trochophore in some respects resembles the larve of some Turbel-
laria (fig. 231) and Nemertines (fig. 256), showing that the annelids
are related to these groups.

The above account applies most closely to the Chetopoda and
the closely related Archianellida. In other forms one or more
features may be lacking—in the Gephyraa segmentation of the
body; in the Hirudinei most of the celom and the trochophore.
Yet these are so closely related that they must be included under
the common head; the missing
characters have been lost during
evolution. ,

Sub Class I. Chetopoda.

These, like the Nematoda, are
cylindrical worms, but are at once
distinguished by the segmenta-
tion. Deep circular constrictions
(fig. 274) bound the somites ex-
ternally. Internally the cclom
is divided by the septa—delicate

double membranes which extend
eae ptr in er end from the ectoderm to the alimen-

 

and Jung.) 1, first segment with Baie VA = ae ‘ r ahe
mouth and prostomium; 15, male tary canal—into as many cham
sexual opening; 33-37, clitellum. bers as there are metameres, while

a longitudinal mesentery, also double, separates the ccelomic
Tf, ANNELIDA: CHA?TOPODA. 807

pouches of the right side from those of the left (figs. 275 and
272). The alimentary canal also shows distinctions; for while it
differs greatly in the various species, it has constantly a terminal
anus, while the mouth is ventral and is overhung by the preoral
segment, the prostomium.

Nervous system, blood-vessels, and excretory organs are influ-
enced by the segmentation. The nervous system is built on the
ladder plan. It begins with asupracesophageal ganglion (‘ brain ’)
lying in the prostomium, from which the esophageal commissures
pass around the esophagus to form the ventral chain, which con-
sists of as many pairs of ganglia, united by longitudinal commis-

  

   

oN
OT
THI]

HH

# |
“oA \\\\I ETT
A HLL

 
  
  

Fic. 275.—Anterior end of Nais elinguis. h, cerebrum, connected by commissure with
ventral chain, n; dy, contractile dorsal, vg, ventral blood-vessel; m, muscular
mee A of skin; db, vb, dorsal and ventral chet; d, septa; k, prostomium; 0,
mouth.

sures, as there are somites present. These ganglia of the ventral
chain are closely similar, since the segmentation of the body is
homonymous. There is but the slightest division of labor among
the somites, and hence they differ but slightly among themselves.
The prostomium always bears tactile organs and frequently eyes,
which in many marine forms are highly developed, with lens,
vitreous body, and retina. Otocysts are rare, but occur in diverse
species. Ciliated pits (olfactory) occur on the head, goblet organs
(taste) on head and trunk, and, lastly, lateral organs, sensory struc-
tures of unknown function, may be metamerically arranged.

The blood-vessels are most frequently represented by two main
trunks which frequently (as in earthworms) contain blood colored
red by hemoglobin. One trunk runs dorsal, the other ventral,
to the intestine, the two being connected by vessels (figs. 272, 276)
in each segment. The blood passes forward in the dorsal vessel,
backwards in the ventral. It is propelled by contractile portions
of the vessels; usually the dorsal vessel pulsates, but, as in the earth-
worms, certain of the circular vessels in the anterior part of the
308 C@LHELMINTHES.

body may function as hearts (fig. 276, c). Rarely, as in the
Capitellidx, circulatory organs may be lacking.

The excretory organs (nephridia) were formerly known as
‘segmental organs,’ since they occur in pairs in each segment.
These supplant the embryonic protonephridia; each consists of an
internal ciliated funnel, the nephrostome, a more or less conyo-

¢ oe dg ly “ pr st ae

 

 

Fic. 276.—Anterior end of Pontodrilus marionis. (After Perrier.) a, vasculararches;
b, ventral nerve chain; ¢, ‘hearts’; co, esophageal commissure; dg, dorsal blood-
vessel; ds, septa; gc, cerebrum; l, retractors of pharynx; lg, lateral blood-vessel:
0, Ovary; 0€, @sophagus; Dp, receptacula seminis; ph, pharynx; pt, ciliated
funnels of vas deferens; s, nephridia; vd, vas deferens,

es Teepe
ou 4

m up bm

Fie. 277.—Schematic cross-section of an annelid. (After Lang.) ac, aciculum; ),

cheetee ; bm, ventral nerve cord; dc, dorsal cirrus: dp, notopodium,; ky, gill: Im,

longitudinal muscles; md, digestive tract; mp, nephridium; ov, ovary; rm, circu-

lar muscles; tm, transverse muscles; tr, nephrostome; vc, ventral cirrus; vd, vv,
dorsal and ventral blood-vessels; vp, neuropodium.

     

luted tube, and the external opening (fig. 69). In many instances
(Oligochetes, some Polychvtes) the nephrostome is in one somite,
the external opening in the succeeding. The nephridia also usually
serve as genital ducts, carrying away the reproductive cells, which
III, ANNELIDA: CHA#TOPODA. 309

always arise from the ccelomic epithelium. In the Oligocheta
(fig. 286), besides the nephridia in the genital segments special
oviducts and vasa deferentia occur which are usually regarded as
modified nephridia.

Of the many modifications of nephridia only a few can be noticed here.
Occasionally there may be more than one pair in a somite, or they may
have more than one nephrostome. Again, they may be lacking from more
or fewer somites. In many Oligochztes they may empty into the anterior
or posterior ends of the digestive tract. In many (@lycera, Hesione,
Nephthys, Gontada) the internal ends of the nephridia are branched,
the branches being closed by ‘ solenocytes,’ tubular cells bearing an internal
bundle of cilia.

In many marine annelids there occurs a metamorphosis in which
pelagic larve occur. These, in spite of their many modifications,
are comparable with the ‘ Loven’s larva,’ the trochophore already

mes

 

mes a

Pra. 278.—4, larva of Polygordius; B, same changing to segmented worm. (After
Hatschek.) a, anus; kn, excretory organ; mes, segmented mesoderm.

described (p. 306). The differences largely consist of modifica-
tions of the ciliary apparatus; the number of bands may be
increased (polytroche larva), or a single band may occur at the
middle (mesotroche) or at the end (telotroche) of the body. The
larva becomes a segmented worm by the hinder end of the larva
growing out and dividing into segments (fig. 278). In this
310 COHLUELMINTHES.

jointed portion the celom arises as a new formation, divided from
the first into separate chambers. The nephridia also arise de
novo, independent of the protonephridial system, which is often
called head kidney because the chief part of the trochophore forms
the head of the adult.

The fresh-water annelids develop directly, but the embryos pos-
sess a reminiscence of an earlier larval life in that the head lobes are
very apparent and contain protonephridia, which leads to the con-
clusion that these animals earlier hada metamorphosis. From the
resemblance of the trochophore to the Rotifera the farther conclu-
sion is drawn that the annelids have descended from Rotifer-lke
ancestors, the body cavity, nephridia, blood-vessels, and veutral
nerve chain being new formations.

Besides sexual reproduction many fresh-water and marine
species may reproduce asexually, this being rendered possible by
the great homonymy of the segmentation. By rapid growth at
the hinder end as well as at a more anterior budding zone numer-
ous somites are formed which separate in groups from the parent
to form young worms. In some cases the formation of new somites

 

Fig. 279.—Budding in Myrianida. (After Milne-Edwards.) The sequence of letters
shows the ages of the individuals.

may take place more rapidly than the separation, the result being

chains of worms (fig. 279).

By a combination of sexual and asexual reproduction a typical alter-
nation of generations occurs, the origin of which receives light from the
following facts: In many polychewtes which reproduce exclusively by the
sexual process the sexless slowly-moving young (atoke) at sexual ma-
turity becomes so altered in appearance as to have been described under
another name. It becomes very active in its movements, and the hinder
WI. ANNELIDA: CHA#TOPODA. 311

somites, which contain the sexual organs, develop special bristles and para-
podia (fig. 284, 4). Thus many species of Wereis pass into the ‘ Hetero-
nereis’ stage. In other Polychzetes the sexual part (epitoke) separates
from the sexless atoke portion and swims freely, while the atoke produces
new epitokes. In the Samoan Islands Hunice viridis reproduces in this
way, the epitokes coming to the surface at certain times in incredible
numbers, forming the ‘palolo worm,’ a delicacy in the Samoan diet. In
still other species the epitoke regenerates the head and thus becomes an
independent generation. Syllis and Heterosyllis are thus related. The
Autolytide furnish the most complication. Here the atoke, by budding
as in Myrianida, fig. 279) forms chains of dimorphic individuals which
later separate. The individuals of male chains were formerly described as
‘ Polybostrichus,’ the females as ‘ Sacconereis.’ This same homonymy ex-
plains the regenerative powers of many worms. Thus if certain earth-
worms be cut in two, they will continue to live and will reproduce the lost
parts.

Another important character of the Chetopoda is the posses-
sion of bristles or chat. These arise in special follicles, simgly
or several in a bunch, of which usually there are four—right and
left, dorsal or lateral and ventral—in each somite. Each follicle
(fig. 280) is a sac of epithelum opening on the surface and having
at the base a special cell for the development of each bristle. The
developed bristles project from the follicle and, moved by appro-

 

 

Fig. 280.—Arrangement of a bristle in an Oligochete. (After Vejdowski.) ¢, epithe-
lium; rm, Um, circular and longitudinal muscles; m, muscle of the follicle; by,,
cheta follicle, its chwta in function; b., follicle for replacement, the formative
cell at its base.

priate muscles, form small levers of use in locomotion. Their
numbers, shape, and support are of much systematic importance.
Order I. Polychete.

The Polychxtw owe their name to the fact that each group of
bristles contains many chete; but more important is that the
312 CALUELMINTIES.

bristles of each side are supported by a fleshy outgrowth of the
somite, the parapodium, in which two portions corresponding to the
bunches of bristles—dorsal, notopodium; ventral, neuropodium
—may be recognized (fig. 281). This is the first appearance of

 

Fig. 281.—A, head with protruded pharynx; 8, parapodium of Nereis versipedata.
(After Ehlers.) ¢, cirri; k, jaws; l, head with eyes; p, palpi; t, tentacles.

true appendages, but they differ from those of Arthropoda in
that they are not jointed to the body nor jointed in themselves.
On the dorsal surface may occur diverse outgrowths, known, accord-

 

Fig, 282.—Amphitrite ornata.* (From Verrill.)

ing to position or function, as cirri, elytra, gills, ete.; onthe head,
palpi and tentacles. The cirri are long processes on the parapodia,
and like palpi are tactile (fig. 281). Elytra are thin lamelle
which cover the back like shingles and thus protect the body.
Ill, ANNELIDA: CHATOPODA. 3138

Nearly all Polycheetes are dicecious and undergo a more or less pro-
nounced metamorphosis; with few exceptions (Manyunkia*-from the
Schuylkill, Weveis * in California) they are marine. They are usually
divided according to their habits into fixed (Sedentaria) and free forms
(Errantia), but this classification lacks a morphological basis. The Seden-
taria feed on vegetable matter, usually form tubes of leathery organic
substances, in which foreign matter may be incorporated or which may
be calcified. The worms project their anterior segments from the tubes.
The Errantia often secrete gelatinous tubes in which they can hide, but
they never lose their powers of locomotion, and from time to time leave their
retreats and swim about preying on other animals. Correlated with habits
are differences in structure. In the Errantia the head and trunk are not
very different ; the anterior part of the alimentary tract can be protruded
ag a proboscis, and, corresponding to their predaceous habits, is often
armed with strong jaws (fig. 281, A). In the Sedentaria there are no such
pharyngeal teeth, but, on the other hand, there is a greater difference
between anterior and posterior somites. In the
latter the parapodia are weakly developed, and
this part resembles the Oligochetes in ap-
pearance. The head and beginning of the
trunk (thorax) are richly provided with gills
and tentacles for respiration and the taking of
food (fig. 282).

Sub Order I. ERRANTIA. Predaceous
annelids with strongly armed pharynx. The
EUNICID£, mostly represented on our shores by
small species, contains some species a yard in
length. Diopatra,* Nothria.* The ALCIOPIDE
are transparent pelagic forms with large, highly

Fie. 283.—Head of Polynoe
developed eyes. The SyLuIp# usually have © spinifera (After Ehlers.)
three long tentacles ; Autolytus,* Myrianida*  Faek entirely covered win
(p. 210). The PoLynoip& * (Lepidonotus,* Poly- projecting at the sides.
noe* (fig. 284), Aphrodite aculeata,* the sea
mouse, 6 inches long) are bottom forms with elytra covering the back.
NEREID ; Nereis virens,* the clam worm of all northern seas.

Sub Order II. SEDENTARIA (Tubicola, Cryptocephala). These forms
cannot wander about at will, but live in their tubes. In the SABELLI-
D& the tube is membranous and there is a crown of gills ; Myxicola,*
Chone,* Manyunkia.* In the SERPULID& the tube is calcified and is
closed by an operculum on one of the gills. Hydroides;* NSpirorbis,*
forming coiled tubes on seaweed ; Protula.* The ARENICOLIDA,* which
burrow in sand, have gills on the sides of the body. The MaALDANip#®
(Clymene,* Azxiothea*) have extremely long segments and build tubes
of sand. The TEREBELLIDA (Terevbella,* Amphitrite (fig. 282), Thelepus *)
have numerous filiform tentacles and branched gills on the anterior end.

 

The ARCHIANELLID.E, which lack bristles and parapodia,
must be placed near the Polychxte and are usually regarded as very
3814 CQ@ZLHELMINTHES.

primitive forms which in structure and development (fig. 250) are
of importance in the phvlogenesis of the Annelids. Polygordius.*

 

Fia. 284.—New England Annelids. A, male Autolytus ; B, Sternaspis fossor ; C, Cise
tenides gouldii; D, Clymene torquata. (From Emerton and Verrill.)

Order II. Oligochete.

The Oligochetes are almost as preeminently fresh-water and
terrestrial forms as the Polychetes are marine. They are in
most respects simpler than their marine relatives, apparently the
result of degeneration, which has followed from the more simple
conditions of existence. Eyes are rudimentary or lacking, they
have no palpi, cirri, or tentacles; gills are rare, but most striking
is the absence of parapodia, the bristles projecting directly from
the body wall (fig. 280). The chet may be regularly distributed
around each somite (Pericheta) or gathered on the sides (Jfegas-
cole) or arranged in four bunches so that in the animal four
longitudinal rows occur. The animals are hermaphroditic, testes
and ovaries lying in different somites. Usually the integument in
the neighborhood of the sexual openings is thickened by the
ITI, ANNELIDA: CILHTOPODA. 315

presence of numerous glands, forming a clitellum (fig. 274), which
secretes the egg cocoons. The clitellum also functions in copula-
tion, secreting bands which hold the animals together so that
the sperm from one passes into the receptaculum seminis of the
other. After impregation the eggs are usually enclosed in
cocoons.

Sub Order I. LIMICOLA (Microdrili.) Mostly fresh-water forms.
The TUBIFICIDA, in consequence of the red blood, when present in large
numbers color the bottom red. They quickly retract into the tubes which

 

FIG. 285.—Aulophorus vagus, in tube of Pectinatella statoblasts. (After Leidy.)
they form in the mud. Z'ubifea,* Petoscolex ; Clitellio inoratus * common
on our seashores. The NaIpID# are transparent forms living on water
plants which reproduce asexually throughout nearly the whole year.

 

 

 

 

 

 

 

 

Of ow by

Fia. 286.—Sexual organs of Lumbricus herculeus (after Vogt et Jung); the seminal
vesicles of the right side cut away. bm, ventral nerve cord; bl, bv, lateral and ven-
tral rows of sete; di, septa; hi, he, testes, enclosed in sperm reservoir; 0, ovaries ;
ov, oviducts; sbu, sperm reservoir; sh,, 9, 3, Sperm sacs (seminal vesicles) : Lost
seminal receptacles; t,, tz, seminal funnels connected with the vasa deferentia;
to, funnels of oviducts; vd, vas deferens.

Dero* and Aulophorus* have gills around the anus. ENCHYTR#IDA;

Distichopus, Pachydrilus. The DiscopRitips# (Bdellodrilus, Myzobdella)

are parasitic.

Sub Order II. TERRICOLA (Macrodrili). Here belong the terrestrial
forms, the earthworms, our species of moderate size, in the tropics large
316 CHLHELMINTHES.

species (Megascolex australis four feet long). Our species belong to
Lumbricus,* Allobophora*; Pericheta* has been introduced from the
tropics; Diplocardia * with double dorsal blood-vessel. Most species agree
in habits; they burrow through the earth, swallowing the humus and
casting the indigestible portions on the surface. They loosen the soil and
are continually bringing the deeper parts to the surface, and thus do great
good. Contrary to oft-repeated statements, earthworms occurred in our
prairies and plains when first broken up by the plow. Details of the
reproductive organs of one species are shown in fig. 286. These vary
greatly and are largely used in classification.

Sub Class II. Gephyrea.

The exclusively marine Gephyrea are distinguished at the first
glance from the Chetopoda by the
entire absence of segmentation. The
body is oval or spindle-shaped, circular
in section. The mouth, at the ex-
treme anterior end, is either surround-
ed by a circle of tentacles (fig. 287)
and is retracted together with the
anterior end of the body by internal
retractor muscles, or is overhung by a

  
    
  
   

 

Fia, 287. Fig. 288.

Fie. 287.—Anatomy of Phascalosoma gouldi (orig.). a, anus; a, anterior retractors;
d, digestive tract; g, gonads; m, mouth; », nephridia; nce, ventral nerve cord;
pr, posterior retractors.

Fie. 288.—Larva (trocophore) of Echiurus. (After Hatschek.) a, anus; d, intestine;
hw, postoral cilia; kn, protonephridia; m, mouth; mes, mesoderm bands with indi-
cation of segments; 7”, ventral nerve cord; sc, esophageal commissure; sp, apical
plate; vw, preoral ciliated band.
III. ANNELIDA: GEPHYRAA. 317

dorsal spatulate preoral lobe or ‘ proboscis’ which may be several
times the length of the body and forked at its tip (fig. 289).

Internal segmentation is also lost, septa being entirely lacking.
The nephridia are also reduced in number, at most but three pairs
being present, and in some but a single unpaired organ. They
are sexual ducts, and in the Chetiferi there are special excretory
organs (fig. 289, g) covered with branching canals opening to the
body cavity by nephrostomes and emptying into the intestine. These
resemble somewhat the branchial trees of the holothurians (infra),
and hence these animals were formerly supposed to bridge the gap
between holothurians and annelids, whence the name (ye@upa,
bridge) Gephyrea. The vascular and nervous systems are more
like those of other annelids. The vascular system consists of a
dorsal and usually a ventral longitudinal trunk; the nervous
system of a brain, esophageal collar, and ventral cord, the latter
without division into ganglia. The relations of the Gephyrea
to the Chetopoda are shown by the development. In some
(Chetiferi) there is a trochophore (fig. 288) from which the worm
arises, as in the Chetopoda, by growth at the hinder end; this at
first has a segmented ceelom and nervous system, the metamerism
being lost later.

Order I. Chetiferi (Armata, Echiuroidea).

With spatulate preoral lobe, often forked at the tip; at least a pair of
ventral sete; a trochophore in development. Hchiurus pallasti* in our
northern waters, Thalassema* farther south. In Bonellia there is a
marked sexual dimorphism (fig. 389). The female is 2 to 3 inches long and
has a proboscis 8 to 12 inches long. The male is only 1 mm. long, totally
different in appearance, and lives parasitically in the cesophagus of the
female (fig. 289, B).

Order II. Inermes (Acheta, Sipunculoidea).

Distinguished by lack of chzete, the mouth surrounded by tentacles,
and the dorsal and anterior position of the anus. The larva is a modified
trochophore without preoral ciliated band and without segmentation; only
two, sometimes but one, nephridia. Phascalosoma* common on our
shores. Phascolion strombi* builds tubes in deserted snail shells. Sipun-
culus.*

Order III. Priapuloidea.

No tentacles, mouth with chitinous teeth, terminal anus, no nephridia;
two protonephridia united with the sexual organs and opening either side
of the vent. Development unknown. Priapulus, Halicryptus.
318 C@LHELMINTHES.

Sub Class III. Hirudinet (Discophort).

Three points of external structure clearly distinguish the leeches
from the chetopods. First, the absence of bristles (except in
Acanthobdella) and the presence of two suckers, one of which occurs
on the posterior ventral surface and is used only for attachment
and locomotion, the other, sometimes scarcely differentiated,

 

Fia. 289.—Bonellia viridis. A, female (after Huxley); B, male (after Spengel). c,
cloaca; d, rudimentary intestine; g, excretory organ; 7, intestine; m, muscles sup-
porting intestine; s, balls of spermatozoa in B, in A, proboscis (preoral lobe); u,
single segmented organ, functioning as oviduct; vd, nephridium with ciliated
funnel serving as vas deferens.

surrounds the mouth and is used in sucking the food. In locomo-

tion anterior and posterior suckers are alternately attached, the

body being looped up and extended like that of a ‘span worm.’

The animals can also swim well by a snake-like motion of the

whole body.

A second point is the fine ringing of the body, there being
usually many more rings than somites, the primitive segment rings
being divided by secondary constructions, there being three, five,
or even eleven rings toasegment. The middle or one of the
anterior rings is often distinguished by bearing strongly developed
sense organs. Asinthe earthworms, certain of the somites at the
time of reproduction may develop into a clitellum which secretes the
egg cocoons.
TI, ANNELIDA: HIRUDINEI. 319

A third character is the marked flattening of the body in the
dorsoventral direction (except in Ichthyobdellide and a few other
forms), the animals thus recalling the flatworms. This may be
the result of the very sight development of the celom. In most
leeches there is a body parenchyma, traversed by longi-
tudinal transverse and dorsoventral muscles in which
the organs are immediately imbedded (fig. 290).

The alimentary tract is provided with paired
diverticula (fig. 291), varying in number in different
species. Between the last and largest pair of these
sacs is the intestine, which opens dorsal to the pos-
terior sucker. The jawed and jawless leeches show
considerable differences in the pharyngeal region.

 

  

Fie, 290.

Fic. 290.—Transverse section of Hirudo medicinalis. (From Lang.) dm, lm, rm, dorso-
ventral, longitudinal, and circular muscles; vl, vd, vv, lateral, dorsal, and ventral
blood-vessels, the latter surrounding the ventral nerve cord, ni; h, testes; vd, vas
deferens; md, midgut; np, nephridial tubule; enp, urinary bladder.

Fig. 21.—Digestive tract of Hirudo medicinalis. (From Lang.) a, oesophagus; b, in-
testine; d,, dy, gastric diverticula.

In the first there are three jaws in the phaynx, semicircular chitin-

ous plates, the free edge of each armed with numerous calcified

teeth (fig. 292). To these are attached two muscles, one to retract
them, when not in use, into pockets, while the other exserts them
and rotates them, causing a triradiate wound from which the blood
flows. This bleeding is difficult to staunch, since glands on the
lips and between the jaws produce a secretion which hinders the
coagulation of the blood, In the jawless leeches a sharp conical
process arising from the pharynx can be protruded from the
mouth, and serves for wounding and sucking. The vascular
system usually contains red blood, and consists, in the Gnatho-

bdellidw, of four longitudinal trunks, a dorsal, two lateral, and a
320 C@LHELMINTHES.

ventral, the latter surrounding the ventral nerve cord. These are
connected by a complicated system of capillaries.

The nervous system consists of brain and ventral cord, the Jat-
ter containing frequently twenty-three ganglia (the first of five
fused, the last of seven). Nerves from the brain go to the eyes.
Right and left of the ventral cord are the hermaphroditic sexual
organs. In Hirudo medicinalis (fig. 293) there are nine pairs of

 

Fia. 292. Fic. 293.

Fie. 292.—Hirudo medicinalis, medicinal leech. (After Leuckart.) a, anterior end
with three jaws (k); b, a single jaw with its muscles.

Fig. 293.—Nervous system, blood-vessels, sexual organs, and nephridia of a leech,
ventral view. h, testes; hb, urinary bladder; /g, lateral blood-vessel; n, ventral
nerve cord; nh, epididymis; 0v, ovary; p, penis; sc, nephridia; uw, uterus and
vagina; vd, vas deferens; vg, ventral blood-vessel.

testes (h), the ducts of which unite to form a vas deferens on
either side (vd). These pass forward, form by coiling a so-called
epididymis (xk) and empty into the median unpaired penis (p).
In the space between the epididymis and the first pair of testes
are the ovaries (ov) and oviducts and an unpaired vagina (wz). The
nephridia (17 pairs in this species) are complicated and are pro-
vided with bladder-like expansions.

That the Hirudinei are true annelids and not segmented Plathelminthes
is based upon the view that their ccelom is reduced by ingrowth of paren-
chyma and altered to canals connected with the vascular system. At
any rate the ventral and lateral vessels are to be regarded as remnants of
acelom. In Clepsine there are the dorsal and ventral blood-vessels of
the Cheetopoda and besides four longitudinal ccelomic sinuses connected
by transverse anastomoses. The dorsal sinus encloses the dorsal blood-
vessel, the ventral many of the viscera, among them the ventral nerve
cord, This is also to be regarded as ewelomic, since the nephrostomes con-
nect with it. In most Hirudinei a canal system filled with blood has
arisen from the calom and blood-vessel, and in Nephelis is in part lacunar
in character. Further relations are shown by Acanthobdella peledina,
parasitic on fishes. This has both blood-vessels of the Oligochetes, a
IV. POLYZOA. 321

body cavity divided by septa and chete. On the other hand it is leech-
like in other features; two suckers and sexual apparatus on the Hiru-
dinean pattern.

Order I, Gnathobdellide.

The jawed leeches include Hirudo medicinalis, once extensively used
for blood-letting but now little employed. Ha@madipsa includes land
leeches, one of the terrors of travelers in the tropics. In Wephelis* the
jaws are soft. Dacrobdella * includes our largest native species.

Order II. Rhynchobdellide.

Without jaws. The CLEpPsINIDH mostly feed on snails and fishes.
Clepsine* in our waters. Hementaria officinalis of Mexico is used for
blood-letting ; H. ghiliant of South America is poisonous. The IcHTHyo-
BDELLID#, cylindrical, occur in salt and fresh water, parasitic on fishes.
Ichthyobdella,* Pontobdella,* marine ; Piscicola, fresh water,

Class IV. Polyzoa (Bryozoa).

In external appearance the Polyzoa closely resemble the
hydroids, so that the inexperienced have difficulty in distinguishing
them. Like them by budding they form colonies which are either
gelatinous or calcareous incrusting sheets or assume a more bush-
like character. Further they have a crown of ciliated tentacles
which can be spread out or quickly retracted. In internal charac-
ters the two groups are greatly different. The Polyzoa have a
complete alimentary canal, with its three divisions, which is bent
upon itself so that the anus lies near the mouth. The central nerv-
ous system lies between mouth and anus, and the single pair of
nephridia empty by a common opening. Beyond this it is diffi-
cult to go, since the two groups of Polyzoa—Endoprocta and Ecto-
procta—differ so widely that one may doubt whether they belong
together. The Entoprocta have no celom, and resemble in this
respect the Rotifera; the Ectoprocta are true Celhelminthes and
by way of Phoronis show resemblances to the Sipunculoida and so
to the Annelida.

Sub Class I. Entoprocta.

The single individuals of the Entoprocta (fig. 294) are shaped
like a wine-glass and are placed on stalks which rise from (usually)
creeping stolons. The circle of tentacles, arising from the edge
of the cup, enclose the peristomial area in which are both mouth
and anus, and between these the excretory and reproductive organs
322 CQVLHELMINTHES.

open. The Bpaee between the horseshoe-shaped intestine and the
body surface is completely filled by a pa-
repchyma containing muscle cells, and
correspondingly the excretory organs are
protonephridia. In our fresh-water Urna-
tilla gracilis * these organs are branched
and begin with flame all Pedicellina*
and Loxosoma, marine.

Sub Class II. Ectoprocta.

In the Ectoprocta there is a spacious,
often ciliated, celom between the alimen-
tary canal and skin, so that these are
separated and have a certain amount of

independence (fig. 295). On this account

MIG fer Mitsche) Singlewn, has arisen a peculiar method (wholly
aoe gs ea indefensible morphologically) of treating
J, intestine; 7, tentacles; Ys them as two individuals, polypid, the in-
testine and tentacles; cystid the rest, especially the body wall and

skeleton.

 

 

 

Fig. 295.—Flustra membranacea (after Nitsche), asingle animal. a,anus; ea, ectocyst;
en, entocyst; f, funiculus; g, ganglion; hk, collar which permits complete retrac-
tion; m, stomach, aiso dermal muscular sac; 0, esophagus. A, avicularium; B,
vibracularium of Bugula. (After Clapardde.)

The cystid is cup-shaped or saceular. It consists of an endo-
cyst—the body wall—and an ectocyst—a cuticular skeleton, usually

strongly calcified, secreted by the ectoderm. The surface of the

 
IV. POLYZOA: ECTOPROCTA. 328

entocyst is always covered by the ectocyst only on the basis and side
walls: the peripheral end remains soft and forms a sort of collar
into which the tentacles and adjacent parts of the cystid can
be retracted. In the ectocyst there is, as will be seen, a larger or
smaller opening which in many species (Chilostomata) can be
closed by a lid (operculum). The circle of tentacles surrounds
the mouth alone, while the anus is outside near the collar. The
strongly bent alimentary canal extends into the cystid and is bound
at its hinder end by a cord, the funiculus, to the base of the cystid.
Ganglion and nephridia lie between the mouth and anus. The
gonads arise from the epithelium of the celom, the testes usually
on the funiculus, the ovaries on the wall of the cystid.

Hundreds and thousands of individuals form colonies (fig. 297)
in which cystid abuts against cystid. The ceelom of adjacent cystids
may be distinct or a wide communication may exist. The colonies
grow by budding; in the Gymnolemata a part of a cystid becomes
cut off as a daughter cystid in which the polypid—alimentary tract
and tentacles—arises by new formation; or (Phylactolemata) the bud
anlage of the polypid arises before the first appearance of the cystid.

Division of labor or polymorphism is common. Besides the
animals already described, which are primarily for nourishment,
three other individuals may occur, ovicells, vibracularia, and avic-
ularia. All three are cystids which have lost the polypid. The
ovicells are round capsules which serve as receptacles for the
fertilized eggs. The vibracularia (fig. 295, B) are long tactile
threads; the avicularia (fig. 295, A) are grasping structures of
uncertain function. They have been seen to seize small animals
and hold them until decay set in. It is possible that the fragments
serve as food for the polypids. The avicularia have the form of a
bird’s head, the movable lower jaw being a modified operculum.

Under unfavorable conditions a polypid in a cystid may break down
and be lacking for some time until better relations cause its new forma-
tion. Besides in the depopulated cystids there may appear statoblasts,
lens-shaped many-celled internal buds enveloped in a firm envelope which
form a resting stage for the preservation and distribution of the species.
Each statoblast is surrounded by a girdle of chambers which by drying
become filled with air, causing the statoblast to float when it again comes
into water. From the statoblast a smaller polyzoon escapes which de-
velops a new colony. The statoblasts are adaptations to the conditions
of fresh-water life and occur only in the Phylactoleemata.

Order I. Gymnolemata (Stelmatapoda).

The tentacles in a ring around the mouth. The numerous species are

almost exclusively marine and are abundant on every coast. In the
3824 CQ@LHELMINTHES.

CHILOSTOMATA the cystids can be closed by an operculum: Gemmel-
laria,* Cellularia,* Bugula,* Flustra* (fig. 295), Eschara.* The
CYCLOSTOMATA have tubular cystids without an operculum. Crisia,*

   

Fie. 296.—American gymnolematous Polyzoa. (After Busk, Hincks, Norman, and
Verrill.) A, Uubulipora flabellaris, young; B, Flustrella hispida; C, Eucratea
chelata ; D, Gemellaria loricata ; E, Kinetoskias smitti; F, Membranipora spini-
Sera; G, Porella levis; H, Lepralia americana: I, Cribillina puncturata,

Tubulipora,* Hornera.* In the CTENOSTOMATA the cystid is more cal-
careous and the opening is closed by a folded membrane. Alcyonidiwim,*
Vesicularia, Valkeria,* Paludicella * (fresh-water).
Order II. Phylactolemata (Lophopoda).
Tentacles borne on a horseshoe-shaped lophophore extending on either

 

Fie. 297.—Small colony of Lophopus crystallinus (after Kraepelin), with young and
old, some extended, others more or less retracted. 0, statoblasts.

side of the mouth, the tentacles on its margins. All are fresh-water
species. Pectinatella,* Lophopus (fig. 227), Plumatella.*
V. PHORONIDEA. VI. BRACHIOPODA. 825

Class V. Phoronidea.

The single genus Phoronis* occurs on our eastern shores.
The animal was first placed among the Chetopoda on account of
its worm-like body situated in a chitinous tube like many sedentary
annelids. Then it was placed in the Polyzoa, with which it is
more nearly related. The young, described as Actinotrocha, is a
modified trochophore with the mouth overhung by a large hood
and the postoral band of cilia drawn out into a series of fingers
which become the tentacles of the adult; the anus is terminal.
At the time of metamorphosis this larva becomes doubled on itself
by a complicated process, so that the alimentary canal is U-shaped
and the anus is near the mouth, while the tentacles are borne on a
horseshoe-shaped basis around the mouth.

Class VI. Brachiopoda.

On account of the bivalve calcareous shells the Brachiopoda
were long regarded as molluscs, but later the fact that the valves
are not paired as in the lamellibranchs, but are dorsal and ventral,
that the nervous system, the excretory and reproductive organs,
the body cavity, and the development resemble those of the annelids
rather than those of the molluscs, led to their recognition as a dis-
tinct class allied to the former group.

The body has a greatly shortened long axis (fig. 298) and in
consequence a transversely oval visceral sac. In most a stalk (st) for

 

Fic. 298.—Anatomy of Rhynchonellu psittucea, (After Hancock.) a’, left, a%, right
arm; a, opening into the cavity of the arm: d, intestine; ¢, blind end of the intes-
tine; g, stomach with liver; m, adductors and divaricators of shell; 0, esophagus;
p’,p®, dorsal and ventral mantle lobes; st, stalk; 1,2, first and second septum, on
the second a nephrostome.

attachment arises from the posterior end. From the anterior side
two folds, the mantle lobes, extend forwards (jp), one ventral, the
326 CH@LUELMINTHES.

other dorsal, their free edges bearing bristles. Each mantle
secretes a shell largely composed of carbonate and phosphate of
lime. In a few the dorsal and ventral shells are similar, but
usually the ventral valve (in Crania attached directly without the

 

Fig. 299.—Waldheimia flavescens. (From Zittel.) Shell with arms and muscles. a, arm
with fringed border (h): c, c’, divaricators; d, adductors; D, hinge process (the
vertical line shows position of hinge).

intervention of a stalk) is more strongly arched and has an opening

at the posterior end for the passage of the stalk (figs. 299, 300).

The flatter dorsal valve frequently bears a characteristic feature in

the skeleton of the arms (fig. 300) which, when present, has greatly

 

Fic. 300.—Waldheimia flavescens. (From Zittel.) A, dorsal, B, ventral valve: a, b,c,
impressions of muscular insertions; a, adductors; 0’’, adjustors (stalk muscles):
c, ce’, divaricators; s, hinge groove of upper valve in which the tooth (ft) of the
lower valve passes; /, support of arms; d, deltidium; /, foramen for stalk.

 

different expression. Its basis consists of two calcareous rods
which, bilaterally symmetrical, project downwards from the dorsal
valve. These may be connected by a curved transverse band, and
from their ends a spiral process may extend on either side. This
apparatus supports the spiral arms. When closed the valves com-
pletely enclose the body. When they open the gape is anterior,
VI, BRACIIOPODA. BOT

the posterior parts remaining in contact. At this part, except in
the Ecardines, a hinge is developed just in front of the posterior
margin, consisting of projections (teeth) in the ventral valve which
fit into corresponding grooves in the dorsal. Opening and closing
the valves are, contrary to what occurs in Lamellibranchs, active
processes, accomplished by appropriate divaricator and adductor
muscles (fig. 299). These produce scars on the shell, important in
the study of fossil forms.

The usually spirally coiled arms, which lie right and left of the
mouth and which give the name to the class, fill most of the shell.
On the outer side of the spiral axis runs a longitudinal groove
which reaches to the tip of the arms and is bounded by a row of
small tentacles. By means of cilia on tentacles and groove food is
brought to the mouth, These arms strongly resemble the lopho-
phore of a phylactolemate Polyzoan, which only needs extension
and coiling to produce this condition. In development the arms
of the Brachiopod pass through a lophophore stage.

In the body there is a body cavity which extends into both
mantle folds. It encloses alimentary tract, gonads, and liver, and
is divided into right and left halves by a dorsal mesentery support-
ing the intestine. Each half in turn is divided by incomplete
septa into anterior, middle, and posterior divisions recalling those
of Sagitta (p. 296). If the arrangement of the septa is not so
clear as in that form, it is to be explained by the shortening of the
long axis and the twisting of the alimentary tract. This latter
consists of esophagus, stomach, which receives the liver ducts, and
intestine, which in some species terminates blindly.

The gonads are chiefly in the mantle lobes. The sexual cells
pass outwards through the nephridia, which begin in one cwlomic
pouch with a wide nephrostome, perforate the septum, and open
to the exterior in the next somite. Since usually there are two
septa, two pairs of nephridia may occur, hut one is usually degen-
erate. The nervous system consists of an esophageal ring with
weak dorsal ganglion, which sends nerves into the arms, and a
stronger ventral mass representing the ventral chain. The heart
lies dorsal to the stomach.

In development the brachiopods recall both Sagitta and the Annelida.
They resemble Sugitta in thatin Argiope the colom arises by out-
growths from the archenteron, divided by septa into three pairs of pouches.
They are annelid-like in the form of larva and in the presence of chetez
which are formed in separate follicles. In an earlier period of the earth
brachiopods were so numerous in species and individuals that they are
among the most important fossils in the determination of geologic horizons.
328 C@LHELMINTHES.

Now there are but few species, some inhabitants of the greatest depths of
the sea.

 

Fic. 301.—Development of brachiopod. (After Kowalevsky.) A, gastrula with early
enterocceelic pouches; B, closure of blastopore; C, celom separated, body annu-
lated; D, cephalic disc and mantle developing, the latter with long sete; E, at-
tached embryo, the mantle lobes folded over cephalic disc (seta omitted). ¢,
ae disc; d, dorsal lobe of mantle; e, enteroccele; m, mantle; v, ventral man-

e lobe.

Order I. Ecardines.

Hinge absent; valves similar when the stalk passes out between them
(Lingula *), or unequal when the ventral is perfo-
vated by the stalk (Disctna) or when the animal is
directly attached by the shell (Crania).

Order II. Testicardines.

Hinge present, valves unequal, the ventral
perforated by the stalk; anusdegenerate. Rhyn-
chonella,* Terebratulina* in our colder waters.
Fic. fae: Soo: Spirifer, Orthis, Pentamerus, Atryna, important

tentrionculis.* fossil gener.

 

Summary of Important Facts.

(1) The CELHELMINTHES are characterized by a well-developed body
cavity (ecelom).

(2) The CH&TOGNATHI are hermaphroditic worms with three pairs of
celemic pouches, with fins, and bristle-like jaws.

(8) The NeEMaATopA are mostly di@cious, usually parasitic elongate
worms, with cylindrical unsegmented body, with nerve ring (no ganglia),
paired excretory organs, and tubular gonads.

(4) The most important species parasitic in man are Ascaris lumbri-
coides in the small intestine, Oxvyuris vermicularis in the large intestine,
the blood-sucking AnAylostoma duodenalis, and the notorious Trichina
spiralis. In hot climates occur Filaria sanguinis hominis and Dracun-
culus medinensis.

(5) The GoRDIACEA have mesenteries and splanchnic mesoderm; they
are parasitic in insects.

(6) The ACANTHOCEPHALI lack alimentary tract, have a spiny proboscis
and a very complicated reproductive apparatus. The adults are parasitic
in vertebrates, the young in arthropods,
ECHINODERMA. 329

(7) The CH&TOPOoD ANNELIDS have segmented bodies, the segmentation
showing itself in ringing of the body wall and in the separation of the
ceelcem into a series of pouches by transverse septa and the metameric
arrangements of blood-vessels, ganglia, and excretory organs.

(8) The CHa&TopopDA are distinguished from other annelids by the
cheete (usually four bunches in a somite) arising in special follicles. The
chvetee are few in the hermaphroditic Oligochetz, numerous and borne on
special parapodia in the Polychete.

(9) The GEPHYR#A are closely related to the Cheetopoda. They are
saccular, with a crown of tentacles or well-developed preoral lobe. They
have largely or entirely lost the segmentation. Evidence of segmentation
appears in some cases in development and in the ventral nerve cord and
nephridia,

(10) The HrrupINer are hermaphroditic Annelida which lack chet
but have sucking discs. Their flattened bodies, rudimentary ccelom, and
rich body parenchyma give them a certain similarity to the Plathelmin-
thes.

(11) The Hirudinei have either a protrusible pharynx (Rhynchobdella)
or three toothed jaws (Gnathobdella). To the latter belongs the medici-
nal leech (Hirudo medicinalis).

(12) The Potyzoa are like the Hydrozoa in being colonial and having a
cireumoral crown of tentacles. They are distinguished by the complete
alimentary canal, the large celom, and the ganglionic nervous system.

(13) The PHoroniDEa are closely like the Polyzoa.

(14) The Braculopopa have a bivalve shell, the valves being dorsal and
ventral.

(15) The body cavity is divided by two septa into three (paired) cham-
bers, of which one, rarely two, are provided with nephridia.

(16) Most brachiopods are attached by means of a stalk. They are
divided into Ecardines, without a hinge and with anus, and Testicardines,
with a hinge and no anus.

PHYLUM V. ECHINODERMA.

The Echinoderma are separated from most other animals by
their radial symmetry, but recall in this respect the C@lenterata,
a fact which led to their inclusion by Cuvier in the group
‘Radiata,’ a view of their relationships which was set aside by
Leuckart on account of their different structure, especially the
presence of ace@lom. In fact the radial symmetry of the echino-
derms has a different value, for while in the Coelenterata the
number four or six (apparently derived from four) is fundamental,
Echinoderma are, with few exceptions, five-radiate. Further, the
radial symmetry of the Cclenterata is primitive, that of the
Echinoderma, as development shows, is derived from the bilateral
330 ECIINODERMA

type. In other words, the echinoderms have descended from
bilateral, possibly worm-lke, ancestors.

The structure of the integument gives these animals a charac-
teristic appearance. In the mesoderm under the
epithelium calcareous plates arise, forming a body
armor or test, and since these are usually pro-
duced into spines, they have given the name
Echinoderma, spine skin, to the group. This
mesodermal skeleton at times becomes degenerate,
as in the Holothurians (it rarely entirely disappears
as in Pelagothuria), but even then shows itself
as spicules and ‘wheels’ of lime. The spheridia

and pedicellaria (fig. 303)—not always present—

PIG. a close eg are characteristic appendages of the integument.

open The first are sense organs; the latter are usually

stalked forceps-like grasping structures with calcareous skeleton.

In life they are active and apparently either clean the skin or are
defensive.

 

Certain plates possess a morphological interest since they appear early
in many larve, and in the adults of different classes can be recognized in
similar positions. In the neighborhood of the arms are five basalia, inter-
radial in position, farther five radialia (‘apical skeleton’) and five inter-
radial ‘ oralia’ around the mouth.

Not less characteristic than the skeleton is the ambulacral (or
water-vascular) system (fig. 304). This begins usually externally
and then ordinarily by a calcareous
plate, the madreporite, which is
perforated with fine pores and
serves for the entrance of sea water.
The water passes into a canal
which, on account of its calcified
walls in the starfish (fig. 305), is
called the stone canal and leads

 

Fa. 304. Fia. 305.

Fig, 304.—Water-vascular system of starfish (orig.). a, ampulle ; ab, ambulacra; ¢,
radial canal; m, madreporite; n, radial nerve; p, Polian vesicle; 7, ring canal,
beneath it the nerve ring; s, stone canal; t, racemose vesicle.

Fic. 305.—Transverse section of stone canal of Astropecten aurantiacus. (After
Ludwig.)
ECHINODERMA. 351

orally to a ring canal around the mouth. The ring canal bears
usually several (up to five pairs) Polian vesicles, which, with
Tiedemann’s vesicles of the starfishes, are now regarded as appen-
dages which, like lymph glands, produce the leucocytes. From
the ring canal radiate five radial canals which give off right and
left in pairs the ambulacral canals. These in turn connect with
the ambulacra and ampulle, the highly characteristic locomotor
organs of the echinoderms. An ambulacrum is a muscular sac
which can be distended and lengthened by forcing in fluid from
the ambulacral vessels, on the other hand can be retracted and
shortened by its muscles. The ampulla is a sac connected with
the ambulacrum and projecting into the body cavity. In locomo-
tion the animal extends its ambulacra, anchors them by the suck-
ing dise at the tips, and then pulls the body along by contraction of
the ambulacral muscle. In the sessile crinoids and the ophiuroids
(which move by their snake-like arms) the ambulacra are not
locomotor but tactile in function, lacking suckers and ampulle.
So among the holothurians and sea urchins the ambulacra are in
many places replaced by tentacles. Frequently each radial canal
ends in an unpaired tentacle with olfactory functions.

The arrangement of the ambulacral system conditions the
arrangement of other organs. Alongside the stone canal is a
saccular organ formerly called the ‘heart,’ but now regarded as a
lymphoid gland (ovoid gland, paraxon gland). Ring and radial
canals are accompanied by corresponding blood canals, with which
are often associated two vessels to the alimentary canal. There is
a similar nerve ring and radial nerve, frequently in the ectoderm,
to which may be added an enteroccelic or apical nervous system,
possibly of peritoneal origin.

The courses of the radial vessels and nerves mark out five chief
lines in the animal, the radii; between them come the secondary
radii or interradii. The stone canal, madreporite, and lymphoid
gland are interradial in position, as are the gonads, usually five
single or five pairs of racemose glands; in some cases but one is
present. The gonads are supported in the spacious ccelom by
special bands, while mesenteries support the alimentary tract and
its derivatives.

Respiratory organs are represented by very various structures: branchie,
or thin-walled outpushings of the ccelom, either around the mouth, as in
Echinoidea, or on the aboral surface, as in the Asteroidea, the burse of the
Ophiuroidea, the branchial trees of the Holothuroidea and the various
parts of the ambulacral system.
332 ECHINODERMA.

The Echinoderma are exclusively marine, occurring in large
numbers even in the deepest seas. Many groups, like the Crinoids,
are largely bathybial, others frequent rocky coasts. At the period
of reproduction the urchins, starfish, and holothurians frequent
the shallow waters, passing their sexual cells into the sea, where
fertilization occurs. In some, however, the young are carried
about in brood cases until the earlier developmental stages are past.

 

Fie. 306.—Echinoderm larve. (After J. Miiller.) a, anus; m, mouth; the black line:
the course of the ciliated bands. J, form common to all: JJ, IJ, developmental
stages of auricularia (Holothurian); IV, V, stages of the Asteroid bipinnaria;
VI, pluteus of a spatangoid; VIJ, larva (Brachiolaria) of Asterias (orig.). m,
mouth ; v, vent.

Where there is no brood pouch the young escape from the egg
as larvee which swim at the surface, and are distinguishable from
the adults (fig. 306, 7) by their soft consistency, transparency, and
bilateral symmetry. By the development of lobe-like processes
and slender arms supported by calcareous rods the larve assume
the most different and bizarre shapes (plutei of echinoids and
ophiuroids, brachiolaria and bipinnaria of asteroids, auricularia of
holothurians), all of which can be referred back to a common type
with tri-regional alimentary tract and a ciliated band around the
mouth, strikingly resembling tornaria, the larva of Balanoglossus.
The different appearances of the larve are due to the drawing out
of the ciliated band into lobes and arms, and also to its becuming
broken into parts which unite themselves into complete rings
(fig. 306, JV).

The metamorphosis of the bilateral larva into the radial adult is very
complicated, It begins early with the formation of outgrowths from the
archenteron (fig. 307), which become separated and form the anlagen of
I, ASTEROIDEA. 333

the celom and ambulacral system. This becomes divided, and one por-
tion develops itself as a ring around the cesophagus, the future ring
canal, and from this five outgrowths, the radial canals, arise. Since these
canals, as they grow out, carry the body walls before them, the arms in the
starfishes, which show the process most clearly, arise as outgrowths which
recall buds (fig. 308). This has given rise to one view which regards the
arms as individuals, the whole body (and hence that of all echinoderms) as
a colony of five individuals. According to this the development would be
a kind of alternation of generations, the larva being the asexual genera-

 

F1a. 307. Fia. 308.

F 1a. 307.—Formation of the celom in Echinus. (From Korschelt and Heider.) 4,
first anlage of ccelom; B, later stage; C, complete constriction of cc@elom (vaso-
peritoneal vesicle) from archenteron

Fra. 308.—Formation of Ophiuran from the pluteus larva. (After Miiller, from Kor-
shelt-Heider.)

tion which by budding produces the colony, Yet this view does not agree
with the actual relations, since it draws an untenable contrast between the
larva and the perfect echinoderm. The most important organs of the
former are carried over into the latter, and the echinoderm brings the anla-
gen to further development. In the insects many features which are lack-
ing or incompletely developed in the larva are developed in the course of
the metamorphosis. There is a metamorphosis in the echinoderms as in
insects. It is a question as to which group of Echinoderma is the most
primitive, but ease of treatment makes it best to begin with the Asteroidea.

Class I. Asteroidea (Starfish).

Two parts can be recognized in the body of a starfish, a
central disc and the arms, usually five in number, which radiate
from it (fig. 316). The relations in which these parts stand to
each other vary between two extremes. In many starfish the
arms play the chief réle and the disc appears as only their united
proximal ends (figs. 309, 310). On the other hand the disc may
334 ECHINODERMA.

increase at the expense of the arms, absorbing these in its growth so
that they form merely the angles of a pentagonal disc (fig. 311).

In both arms and disc two surfaces are recognized, oral and
aboral, which pass into each other, usually without a sharp margin.
In the normal position the oral side is downwards and has in the

 

Fria. 309.—Comet form of Linckia multiflora. (From Korschelt-Heider.) One of the
arms is producing a new animal by budding.

  
   

    

Cie

ea a ell

   

Fia. 310. Fig. 311.

F1G. 310.—Ophidiaster ehrenbergi. (After Haeckel). Comet form: one of the original
arms shown only in part.

Fic. 311.—Culcita pentanguluris, aboral view. (From Ludwig.) a, madreporite; }, re-
flexed end of ambulacral grooves.

 

centre the mouth and radiating from it to the tips of the arms the
five ambulacral grooves. On the aboral surface is the anus (when
not degenerate) near the centre, and excentric from it in an inter-
radius is the madreporite (in many armed species two to sixteen
radii may have madreporites).

A line passing through the madreporite and the opposite arm divides the
body into symmetrical halves. This ray is frequently spoken of as anterior,
since in the irregular sea urchins (Spatangoids) the homologous arm is
clearly anterior, while the madreporic interradius is posterior, This plane
of symmetry does not correspond with that of the larva. The two rays on
either side of the madreporite form the bivium, the three others the
trivium.

The skin is everywhere protected by large and small plates
jointed together. These make a dry starfish hard and stiff, but
in life it is extremely flexible, the arms can be bent in any direc-
£)
ov

I. ASTHEROIDEA.

eo
Ot

tion, and the animal can work its way through narrow openings.
Of the skeletal pieces the ambulacral plates need special mention.
B

 

Fic. 312.—A, cross-section of starfish arm (orig.). a, adambulacral plates; am, ambu-
lacra; ap, ambulacral plates; 6, branchi; c, ceelom: h, hepatic ceca; ¢, inter-
ambulacral plates; », radial nerve; p, ampulla; 7», radial canal; v, radial blood-
pone B, ambulacral plates, ventral view, showing the ambulacral pores

etween.

These form the roofs of the ambulacral grooves, and between them
are openings, the ambulacral pores, through which connexion is
made between the ambulacra and ampullew. In each arm the pairs
of ambulacral plates meet above the groove like the rafters of a

 

Fie. 313.—Asteriscus verruculatus, aboral surface removed. (After Gegenbaur.) g
gonads; h, hepatic ceca; /, stomach with anus.

roof. Laterally each ambulacral plate abuts against a small inter-
ambulacral plate, bearing usually movable spines. Beyond these
comes the adambulacral plates, and then those of the aboral sur-
face. Each ambulacral area terminates at the tip of the arm with
an unpaired (ocular) plate.
336 ECHINODERMA.

The organs lie in part in the celom, in part in the ambulacral
grooves. The alimentary tract is in the celom and extends
straight upward from the mouth to the aboral surface, where it
ends with an anus or is entirely closed (figs. 313, 314). By a

 

Fig. 314.—Section through ray and opposite interradium of a starfish (orig.). B,
branchie; C, cardiac pouch of stomach; EF, eye spot; G, gonad; 4H, ‘liver’; M,
mouth; N, radial nerve; P, pyloric part of stomach; RC, ring canal; RD, radial
canal of water-vascular system; S, stone canal.

constriction it is divided into a larger, lower cardiac portion and

asmaller, upper pyloric division. From the latter arise five hepatic

ducts which connect with five pairs of hepatic glands lying in the
arms. The cardiac division gives origin to five gastric pouches
which can be protruded from the mouth or retracted by appro-
priate muscles. The gonads are five pairs of racemose glands lying
in the basis of the arms and opening interradially between the
arms. Lastly, the stone canal, extending from the aboral madre-
porite to the ring canal, and the lymphoid gland lie in the celom.

The radial nerve, canal, and blood-vessel lie in the roof of the
ambulacral groove between the ambulacra. The nerve ends at the

 

Fia. 315.—Longitudinal section of eye of Asterias. (Orig.)

tip of the arm in a compound eye spot colored with red or orange
pigment which experiment shows is sensitive to light. A second
nerve has been described lying in the celom of the arm. The
ambulacral system corresponds with the foregoing description
II. OPHIUROIDEA. 837

(p. 330), the ampulle as well as the five or more Polian and
Tiedemann’s (racemose) vesicles projecting into the cwlom.

Since the arms contain nearly all important organs, the physiological
independence of these is easily understood. Arms broken off not only
live, but regenerate first the disc and then
new arms which appear at first like small
buds (comet form, figs. 309, 310). This
separation of arms may occur through
accident, or it may be, and not infre-
quently is, produced by the animal itself.

Examples of species with well-devel-
oped arms and ambulacra in four rows
are furnished by the ASTERID#&, repre-
sented on our shores by the five-finger
Asterias* and Leptasterias,* and Heli-
aster * with numerous arms. In the
SOLASTERIDZ the ambulacra are two-
rowed; arms sometimes numerous. Py-
thonaster (fig. 316). In the ASTERINIDE&
the arms are short or the body is pentag-
onal, no large plates on the margins of
the arms. Asteriscus (fig. 313). In other
forms (Culcita,* fig. 317, Hippasteria,*
Ctenodiscus*) the body is more or less Fie. 316.— Pythonaster  murrayi.

: (After Sladen.) Aboral view
pentangular, the margin being covered showing ambulacral grooves.
with large plates.

 

Class II. Ophiuroidea (Brittle Stars).

In these the animal consists, as in the Asteroidea, of disc and
arms, the latter sometimes branched, but the internal anatomy is
different. The ambulacral plates
have been drawn inside the arm
and each pair fused to a large
‘vertebra’ (fig. 317). As a result
the celom of the arms is greatly
reduced, the hepatic ceca are lack-
ing, and the alimentary canal, which
lacks an anus, is confined to the
disc. By the ingrowth of ven-
Fic. 317.—Section of Ophiuroid arm tral plates the ambulacral grooves

a, ambulacrum; }, blood ves- z
; ¢, celom; m, muscles of arm; 7, are converted into tubes, and the
bean sodieeeey eA luisapal clstes) ambulacra, which lack sucking
discs, are tactile organs, locomotion being effected by the snake-
like motion of the arms. The madreporite is on the ventral sur-

  
     
 

  
 

VIN

SW

  
338 ECHINODERMA.

face. Also on the ventral surface are five slits which connect
with as many burs, thin-walled respiratory sacs into which the
sexual organs open.

In many brittle stars (Ophioenida, Ophiothelia, Ophiocoma), especially
in young specimens, there is a kind of asexual generation (schizogony), the
animal dividing through the disc, the halves regenerating the missing parts.
The classification is based largely on small details. In the majority the
arms are unbranched (Ophiopholis * (fig. 818), Ophioglypha,* Amphiura*),
but in the EuRYALID&, or basket fish, the arms are branched (Astrophyton,*
fig. 319), but not, as usually stated, dichotomously.

  

Fie. 318.

FG. 318.—Ophiopholis aculeata.* (From Morse.)
Fig. 319.—Astrophyton arborescens, basket fish. (From Ludwig.)

Class III. Crinoidea (Pelmatozoa).

The crinoids or sea lilies are on the road to extinction. In
early times, especially in the paleozoic, they were very abundant,
but to-day there are but few genera and species, these mostly
restricted to the greater depths of the ocean, only the Comatulide
occurring near the shore. The crinoids are attached to the sea
bottom by a long stalk which contains a central canal (fig. 320).
This stalk is composed of cylindrical dises and often bears five rows
of outgrowths, the cirri. In the Comatulide (fig. 821) the adult
is not thus attached, swimming about in the water with the arms
or moving about on the tang. In these earlier stages these animals
have a stalk (fig. 322), passing through a Pentacrinus stage, a
proof that the fixed condition was the primitive one. In these
forms, when the separation takes place, one joint of the stalk with
its cirri remains attached to the animal, as the centrodorsal united
with the lowest cup plates, the infrabasals (fig. 321).

On the upper joint of the stalk is a cup-shaped body (theca)
the edges of which bear five or ten (usually branched) arms. The
III. CRINOIDEA. 339

 

 

 

 

 

 

 

Aha
=
Sy
a a ey
aa? : PS
4 =
yo} =
i’
be ie 23
tbs
EAE
JC ay
y nt
7h
Te sh
2 i oo
4 AN

(After Wyville Thompson.)

 

Fig, 321. Fig, 322,
Fic, 321.—Adult of Antedon macronema, (After Carpenter.)

Fig. 322.— Different Pentacrinus stages (4, ),¢) of Autedon rosacea, 1,arms; 2, cirri;
3, stalk.
340 ECHINODERMA.

walls of the theca are covered with polygonal calcareous plates.
Usually the stalk bears five plates, the basalia, and then come five
radialia, alternating in order with the basalia (fig. 323). In some

 

Fie. 323.—Hyocrinus bethleyanus. A, upper end of stalk with cup, and the bases of
the arms; b, basalia; br, brachialia; r, radialia. B, oral surface of cup with
mouth, five oralia, and the bases of the arms.

there is a circle of infrabasalia in a line with the radialia. Fre-

quently the elements of the arm, the brachialia, are directly attached

to the radials (fig. 823). But often the arm branches once or
several times dichotomously, and the first branching takes place at
the base, so that the arms seem to spring from the theca. In
these cases the first brachialia are considered as part of the theca
and are called radialia distichalia (figs. 320, 321). From the arms
arise, right and left, a row of pinnule, lancet-shaped processes
supported by calcareous bodies in which the sexual products ripen

until freed by dehiscence (fig. 325).

The mouth opening, in the middle of the oral dise which closes
the theca, is frequently surrounded by five radial plates, the oralia.
The mouth, which in contrast to other echinoderms is directed
upwards, connects with a spacious digestive tract in which wsopha-
gus, stomach, and intestine can be distinguished. The anus is
interradial and near the mouth (fig. 324). Five ambulacral
grooves begin at the mouth and extend out on the arms, branching
with them and extending to the tips of the pinnule. These are
III. CRINOIDEA. 341

ciliated and serve as conduits to bring food to the mouth. Nervous,
ambulacral, and blood systems begin with a circumoral ring. They
follow the ambulacral grooves as in the asteroids, but the ambulacra

 

Fia. 324. Fia. 325.

Fre. 324.—Oral area of crinoid (Antedon), showing by dotted lines the course of the in-
testine from the mouth (m) to the anus («); gy, ciliated grooves leading from the
arms to the mouth (orig.).

Fig. 325 —Cross-section of pinnula of Antedon. (After Ludwig.) a, axial nerve
cord; c, ciliated cups; c, c, celiac canal; g, gonad; s, sacculi; sc, subtentacular
canal; ¢, tentacles.

here have no suckers nor ampulle and are merely tactile tentacles.
A typical stone canal is also lacking; in its place are five or several
hundred tubules leading from the ring canal to the cwlom. Oppo-
site their celomic mouths are fine pores in the oral disc through
which water enters to pass through the tubules into the ambulacral
system. The ambulacral nervous system is weakly developed.
The enterocele system, on the other hand, is well developed and
forms the axial cord running through the brachialia and radialia
to unite ina ring in the centrodorsal. A problematical organ,
the so-called dorsal organ, also begins in the centrodorsal and
extends up through the axis of the theca to the oral disc. It is
possibly a lymphoid gland, possibly a structure for the transfer of
nutriment, and is apparently homologous with the ‘heart’ of the
starfish.
349 ECHINODERMA.

Sub Class I. Eucrinoidea,

The foregoing account applies entirely to the Encrinoidea, which may
be divided into two groups :

Order I. TESSELLATA (Paleocrinoidea). Theca with its side walls
composed of immovably united thin plates ; the ambulacral grooves usu-
ally completely covered by calcareous plates. Exclusively paleozoic.

Order II. ARTICULATA (Neocrinoidea). Ambulacral grooves open,
theca with compact, in part movably articulated, plates. This order left
the other in the mesozoic age, and some families have persisted until now.
Rhizocrinus * (fig. 323) and Pentacrinus (fig. 320), deep seas ; the Coma-
TULID# are fixed in the young, free in the adult. Antedon* (fig. 321).

Sub Class 11. Edrioasteroidea (Agelacrinotdea).

Theca of irregular plates ; arms unbranched and lying on the theca.
Possibly the ancestors of the noncrinoid echinoderms. Paleozoic. Ageda-
crinus.

Sub Class III. Cystidea.

Exclusively paleozoic ; body spherical, composed of polygonal plates.
Stalk and arm structures rudimentary, sometimes lacking. The AMPHo-
RIDA are by some regarded as ancestral of all echinoderms. Holocystites,
Echinospherites (fig. 326).

  

Fie. 326. Fie. 3827.

Fig. 326.—Kchinospheerites aurantium. (From Zittel )
Fia. 327.—Pentremites florealis. (From Zittel). Lateral, oral, and aboral views.

Sub Class IV. Blastoidea,
Arms lacking ; the mouth surrounded by five petal-like ambulacral
areas. The group appears at end of Silurian and dies out with the earbon-
iferous. Pentremites (fig. 327).
IV. ECHINOIDEA. 348

Class IV. Echinoidea (Sea Urchins).

The structure of the sea urchins is best understood in the
spherical forms (figs. 328, 330).
Mouth and anus lie at opposite
poles of the main axis, each open-
ing immediately surrounded by
areas covered by calcareous plates,
the arrangement of which varies
with the family. Around the anus
is the periproct, around the mouth
the peristome, the latter bearing
spheridia and in the Echinoids five
pairs of interambulacral gills. Be-
tween peristome and periproct the He see—Ceepieirin anidaniae teas
body wall is composed of calcareous Agassiz.) Aboral ‘view, the spines

: ‘ a removed to show the ambulacral (a)
plates, which, except in the Echino- and () interambulacral areas, end-

ee . 3 ing respectively in the ocular and
thuride, are immovably united. genital plates: in the centre the four
Aside from the extinct Palechei- P!tes of the periproct.

noidea the plates are arranged in twenty meridional rows, or, more
accurately, in ten double rows, two rows being always intimately

associated together. Five of these double rows are ambulacral,

  

 

Fia. 329. Fig. 330.

Fia@. 329.—Clypeaster subdepressus. (After Agassiz.) Aboral view, showing the peta-
loid ends of the ambulacral areas. ; j

Fria, 330.—Diagrammatic longitudinal section through a sea urchin.

the alternating five interambulacral. Both bear small hemispheri-
cal articular surfaces on which are situated the spines, either long
and pointed or swollen to spherical plates. These spines are ex-
tremely mobile and are moved by muscles so that they serve both as
protecting and locomotor structures. The ambulacral plates are
344 HCHINODERMA.

distinguished from the interambulacral by the ambulacral pores
by which the ambulacra on the surface are connected with the
internal ampulle. In most sea urchins the paired grouping of
the pores results from the fact that a double canal extends from
ampulla to ambulacrum.

In the arrangement of the ambulacra two modifications, the band form
and the petaloid, occur. In the first the ambulacra are equally developed
from peristome to periproct (fig. 328). In the second oral and aboral
regions may be distinguished (fig. 329). In the oral region alone are loco-
motor feet always present, but these are so irregularly arranged that no
striking figure results. In the aboral area the ambulacra are tentacular
in character and are regularly arranged, their pores bounding five petal-
like figures around the periproct, very distinct after removal of the spines.
In the Echinoids, the Cidarids excepted, the interambulacral plates around
the peristome show five pairs of notches for the gills, five pairs of thin-
walled branching extensions of the body cavity.

Ambulacral and interambulacral fields both end at the periproct
with an unpaired plate, the five ambulacral plates (radialia of mor-
phology) being called ocular plates, since they often bear pigment
spots formerly regarded as eyes. They are perforated by the end
of the radial canal and nerve, the latter here uniting with the epi-
thelium of the skin. The five interambulacral plates are called
genital plates, since they usually contain the openings of the genital
ducts: One is often madreporite as well.

The interior of the body is occupied by a spacious ceelom, to

 

I V A st

Fra. 331.—Sea urchin opened around the equator. 4, ambulacral area; J, interam-
bulacral area; L, lantern; d, intestine; ed, anal end of intestine; g, gonads; nd,
siphon; oe, cesophagus; p, p’, ring canal and Polian vesicles; st, stone canal.

the walls of which the thin-walled alimentary tract is fastened by

a mesentery. In the Clypeastroids this tract forms a simple spiral,
IV. ECHINOIDEA. 3845

but elsewhere it is a double spiral. It ascends from the mouth,
turning once, and then, bending on itself, coils
in the reverse direction to the anus (fig. 331).
Usually the first portion of the canal is accom-
panied by a siphon, an accessory tube opening
into the main tube at either end. Except in
the Spatangoids the mouth is surrounded by hy
five sharp-pointed calcareous plates, the teeth, ric. 332. — Aristotle’s
which in the Echinoids are supported by a [anfern of simongine

centratus lividus. (Af-

complicated system of levers, fulcra, and mus- a aa meee ae
cles, the ‘lantern of Aristotle’ (fig. 332). cr

The ring canal and the ring of the blood system lie on the

lantern, the stone canal and ovoid gland (‘heart’) extending

upwards from them (fig. 330). The blood-vascular ring gives off

two blood-vessels which run along the alimentary canal, while from

the ring canal arise five ambulacral or radial canais which run on

 

 

Fig. 333.—Oral (A) and aboral (B) surfaces of the sand dollar, Echinarachnius parma.
a, anus; g, genital pores; 7, ambulacral areas ; m, madreporite ; 0, mouth.

the inner side of the test accompanied by nerves which radiate

from a nerve ring. The gonads are five (rarely four or two)

unpaired organs in the aboral half of the test, opening through the

genital plates, that is, interradially as in the starfish.

Order I. Palwechinoidea.

Paleozoic forms with five ambulacral areas, the interambulacral areas
containing more than two rows of plates, J/elonites.

Order II. Cidaridea (Regulares).

Ambulacral areas band-like, body more or less spherical, mouth and
anus polar. Here belong the common urchins, represented on our coasts
by Toxopneustes,* Strongylocentrotus,* Arbacia,* Calopleurus* (fig. 828).
B46 ECHINODERMA.

Order III. Clypeastroidea.

Trregular flattened echinoids with central mouth and teeth; anus out-
side the periproct in the posterior interradius, sometimes marginal ; five
petaloid ambulacral areas. Clypeaster (tropical), Echinarachnius* (sand
dollar, fig. 833), Ifellita,* with holes through the test.

Order IV. Spatangoidea.

Bilateral flattened forms more or less
heart-shaped ; mouth and anus excentric,
no teeth; usually five petaloid ambulacral
areas and four genital plates. From the
forward position of the mouth it follows
that only two ambulacral areas (bivium,
p. _) are upon the lower surface. Warmer
seas. Spatangus,* Echinocardium, Brissus.

Class V. Holothuroidea.

The sea cucumbers are most re-
Fic. 334. Young Spatangus pur- 7 : 0 ic
ice ee moved of any group from the typical
removed, oral surface. In front, echinoderm appearance. At the first
the slit-like mouth ; behind, the :
anus. The bivium without tu- glance the skin appears naked and the
ercies, . . 7
characteristic plates absent. Yet these
are imbedded in the skin in the shape of plates, wheels, and anchors

(fig. 335). The integment is tough, leathery, and muscular with

 

      

PIORS9

BPO)

LIN

Fig. 335.—Dermal plates of Holothurians. A, Myriotrochus rinkii. (After Daniels-
sen.) B, Thyone briareus; C, Synapta giraraii (orig.).

 

longitudinal and circular fibres. The saccular body gives these
forms a worm-like appearance, strengthened by its elongation in
the main axis, and with the mouth and anus at the poles. Unlike
other echinoderms these move with the main axis parallel to the
ground, a condition which, to a greater or less extent, leads to a
replacement of radial by bilateral symmetry. One surface (trivium)
becomes ventral, the bivium dorsal, and in many the trivial ambu-
V. HOLOTHUROIDEA. 347

 

Fia, 336.—Anatomy of Caudina arenata. (After Kingsley.) a, anastomoses of dorsal
blood-vessel ; b, branchial tree; d, dorsal blood-vessel; f. mesenterial filaments;
g, genital opening; /, alimentary canal; /, longitudinal muscles; m, mouth: 0,
genital duct; p, pharyngeal ring; 7, gonads, cut away on right side; f, ampulle
348 ECHINODERMA.

lacra alone are locomotor, those of the bivium being tactile or
wholly absent.

In the body cavity (fig. 336) lies the alimentary canal, which
(except in Synapta) is coiled in a uniform manner, although many
minor conyvolutions may obscure this. It passes backwards in the
median dorsal interradius foi ward in the left ventral interradins, and
then back in the right dorsal interradius to the anus. It is held in

position by mesenteries (fig. 337), and
aS near the anus by numerous muscular
filaments. Into the terminal portion
one or two branchial trees may empty.
These are tubular sacs with small
branched outgrewths which are filled
with water. The similarity of these to
: A the excretory organs of some Gephy-
“Int 7? yea (p. 317%) was one ground for re-
Fig. 337.—Transverse section of garding those forms as intermediate
Holothuria tubulosa. (After Lud- :
wig.) d, digestive tract; db, dor- between worms and echinoderms.
sal blood-vessel; g, gonad duct; ,,, E 2
h, skin; Im, longitudinal muscles: They are to be regarded as respiratory,
betiesse sae eihtiocralcompiex since they are periodically filled with
or anwiam fambulacra} vessel) fresh water. In many species ‘ Cuvie-
ee eee rian organs’ occur; these are morpho-
logically specially modified portions of the branchial tree and are
either connected with them or separately with the cloaca. Many
zoologists regard them as defensive structures because of their
sticky nature and because they can be cast out through the anus.

The esophagus is usually surrounded by a ring of five radial

and five interradial plates which serve as points of attachment for

   

rommhk +

the longitudinal muscles. Just behind it He the ring canal, ring
nerve, and the ring of the blood system, each giving off a radial
branch which here runs inside the muscular sac of the body. From
the beginning of the radial canals (rarely, as in Synapta, from the
ring canal) tubes extend outward to form the extremely sensitive
retractile tentacles which surround the mouth, and which either
branch (Dendrochirotw) or bear frilled shield-shaped extremities
(Aspidochirotw). A single Polian vesicle is usually present, and
the stone canal (except in the Elasipoda) connects with the
celom. Blood-vessels going from the vascular ring form rich
anastomoses on the alimentary canal. Only a single gonad (or é
pair of united gonads) occurs. This consists of numerous tubules
which open usually interradially near the mouth.
V. HOLOTHUROIDEA. 349

The regenerative powers of these animals are of interest. In unfavor-
able conditions (hence in preserving the animals in alcohol without nar-
cotization with chloral) they void the whole viscera and yet may live and
reproduce the lost parts. In certain species are found a few parasites.
One or two harbor a small fish (Fierasfer) in their cloaca and branchial
trees. A parasitic snail, Hntoconcha mirabilis, lives in one species of
Synapta, and a mussel, Hntovalva mirabilis, in another.

Order I. Actinopoda.
Radial canals present, sending branches to the tentacles and am-

bulacra when present. Divided into Pedata, with ambulacra, and Apoda,
without. The PEDATA include the Holothuride with peltate tentacles.

 

Fia, 338.—Cucumaria frondosa, sea cucumber. (From Emerton.)
Holothuria* in warmer waters, one species furnishing the trepang of
Chinese markets. The Cucumarim. represented in our waters by Cucu-
maria* (Pentacta) with regular rows of ambulacra, Thyone* with them
scattered, and Psolus,* scaly with a creeping disc. The deep-sea Eva-
sIPODA belong to the Pedata. The APODA are represented by Caudina *
(fig. 336) and Molpadia.*

Order II. Paractinopoda.

No radial canals nor ambulacra. Tentacular canals arising from ring
canal, Myriotrochus,* Synapta,* Oligotrochus* (fig. 339).
350 ECHINODERMA.

Summary of Important Facts.

1. The ECHINODERMA share the radiate structure with the
Coelenterata, but differ from them (a) in the numerical] basis of
the symmetry (five); (4) in that, as embryology shows, they have
descended from bilateral forms.

2. Farther characters are the existence of a ccelom, the
ambulacral system, and the mesodermal spiny skeleton, which has
given the name to the phylum.

3. Theambulacral system is locomotor and occurs nowhere else.
It consists of a sieve-like plate, the madreporite (not always pres-
ent), which passes water to the stone canal, and from this to the

 

Fie 339.—Oligotrochus vitreus.* (After Danielssen and Koren.)

ring canal and the radial canals to fill the ampulle and ambulacra.
Lateral branches supply the tentacles and cause their extension.

4. Blood-vessels and nerve cords run in the same radii as the
radial canals of the ambulacral system; stone canal, madreporite,
ovoid gland, and genital ducts are interradial.

5. The Echinoderma are divided into five classes: (1) Aster-
oidea, (2) Ophiuroidea, (3) Crinoidea, (4) Echinoidea, and (5)
Holothuroidea.

6. The ASTEROIDEA have a dise and (usually) five arms into
which the gastric pouches and hepatic ceca extend. The ambu-
lacral groove open.

%. The Opnrvurorpea also have dise and arms, but the ambu-
lacral groove is closed and the hepatic ceca absent.

8. The CRINOIDEA have a cup-shaped body bearing arms,
usually branching, with pinnule, and a stalk, usually with cirri.
They are either temporarily or permanently attached. The
Crinoidea are subdivided into Eucrinoidea, Edrioasteroidea,
Cystidea, and Blastoidea.

9, The EcHINOIDEA are usually spherical or oval, armored with
calcareous plates which extend as meridional bands from peristome
to periproct, five pairs of ambulacral and five of interambulacral.

10. The ambulacral plates end at the periproct with a single
ocular plate; the interambulacral with a similar genital plate.
The madreporite is fused with one of the genital plates. :
MOLLUSCA. 351

11. The regular sea urchins have the anus in the centre of the
periproct, the mouth in the peristome; the ambulacral areas are
band-like.

12. The Clypeastroidea have a central mouth, the anus outside
the periproct in the posterior interradius; the ambulacral areas
petaloid.

13. The Spatangoidea are markedly bilateral, the mouth an-
terior, the anus posterior; ambulacral areas petaloid.

14. The Honornurorpea are elongate and worm-like; the
skeletal system greatly reduced; they are more or less bilaterally
symmetrical and have usually a single gonad and two branchial
trees. They are divided into Actinopoda, with radial canals, and
Paractinopoda, without.

PHYLUM VI. MOLLUSCA.

At the first glance the molluscs, like the flatworms and leeches,
give the impression of parenchymatous animals. A spacious celom
is absent; what was formerly regarded as a body cavity is a system
of sinuses surrounding the viscera and connected with the blood
system, and is especially developed in the Acephala. More recently
the view has gained ground that the molluscs have descended
from celomate animals, and from forms in which, by encroach-
ments of a connective tissue and muscular parenchyma, the eelom
has been reduced to the inconspicuous remnants of the pericardium
and the lumen of the gonads.

Where the molluscan organization is well developed, as in the
snails, four parts may be recognized in the body (fig. 340). The

‘visceral sac forms the chief mass of the body; it is less rich in
muscles than the rest because it is reduced to a thin peripheral
layer by the alimentary canal, liver, nephridia, and gonads. In
front it is continuous with the head, which, according to the group,
is more or less marked off by a neck, and bears, besides the mouth,
the tentacles and eyes, the most important sense organs. Below,
the visceral sac passes into a muscular mass, usually used for loco-
motion, the foot. From the back extends the pallium or mantle,
a dermal fold which envelops a goodly part of the body. The
Acephala (fig. 340, C’) have a double mantle, right and left, both
halves springing from the dorsal line and extending down over the
visceral sac and foot. The cephalopods (fig. 340, 1) and the snails
(fig. 340, 2), on the other hand, have an unpaired mantle which
arises from about the central part of the back and either extends
352 MOLLUSCA.

down on all sides or, like a cowl, covers either the anterior or
posterior parts of the body. The mantle is of importance in two
ways: its outer surface is covered with epithelium which, like that
of the adjacent surface, has the power of secreting shell, a thick
cuticular layer of organic matter (conchiolin) largely impregnated
with calcic carbonate. The inner surface of the mantle, together

 

Fic. 340.—Diagrams of three molluscan classes. 4A.a cephalopod (Sepia); B, a gas-
teropod (Helix); C, an acephal (Anodonta), a, anus; c, cerebral ganglion; fu, foot;
m, mantle chamber; sch, shell; ¢, siphon; v, visceral ganglion. Visceral sac
dotted; mantle lined, shell black.
with the outer surface of the body, bounds a space, the mantle cavity,
which, from its most important function, is also called the dran-
chial chamber. Since most molluscs are aquatic, special vascular
processes of the body, the gills or branchix, le in this space; in,
the terrestrial forms its walls serve as lungs and thus are respiratory.
From the foregoing it will be seen that the character of the
mantle must exert an influence on the shape of the shell and on
the respiratory organs. Paired mantle folds necessitate two valves,
right and left, to the shell; a right and Jeft branchial chamber,
and right and left gills. With an unpaired mantle the shell is
MOLLUSCA. 353

always unpaired, while the gills may retain their primitive paired
condition.

The gills in the mantle cavity are called ctenidia, from their resem-
blauce to combs with two rows of teeth. Each consists of an axial portion
(back of the comb), containing the chief blood-vessels and two rows of
branchial leaves. The whole is united to the wall of the branchial cavity
by the axis (fig. 885). In many aquatic forms the ctenidia are lacking,
and then the respiration is either diffuse by the skin or by accessory gills
which by structure (usually outside the mantle cavity) are distinguished
from the ctenidia.

Those parts of the surface of the mollusc which are not covered
by the shell have a columnar epithelium which is frequently ciliated
and which contains unicellular mucus glands, especially abundant
on the edge of the mantle. These give these animals the soft slip-
pery skin which is implied in the name Mollusca (iollis, soft).
Many-celled glands, like the byssus gland of the Acephala, the
pedal gland of many snails, occur.

Although the existence of head, foot, and mantle is very char-
acteristic of the molluscs, they are not always present. In the:
Acephala there is no distinct head region; many gasteropods lack
the mantle and hence the shell; in the Cephalopoda the foot is
converted into other appendages, the siphon and arms. These
modifications are to be explained by degeneration and evolution.
In the nervous system are also some highly characteristic features.
As a rule it consists of three pairs of ganglia associated with
important sense organs and connected by nerve cords. One pair
lies dorsal to the esophagus and corresponds to the suprawesophageal
ganglion of the worms; it is the brain (cerebrum) and supplies

  
   
 

  

RNG pl. . £ EB wal Ly UE ee
* ; ae i >» Semmes q \ par
&
‘pe. aN re. at pe
A B Cc

¥ 1g. 341.—Nervous systems of Molluscs. A, most gasteropods; B, acephals; C, cepha-
lopods and pulmonates. c, cerebral; pu, parietal, pe, pedal, pl, pleural, and v,
visceral ganglia.
the tentacles and eyes. A second pair lies ventral to the alimentary
tract on the front part of the muscle mass of the foot; these are
the pedal gangha which are connected with the otocysts. The
third pair, the visceral ganglia, are also ventral, and near them
are the third sense organs, which are widely distributeed through
the Mollusea, and which from position and structure are regarded
354 MOLLUSCA.

as organs of smell (osphradia). They are thickened patches of
ciliated epithelia extending into the mantle cavity. Pedal and
visceral ganglia are united to the cerebrum by nerve cords, the
cerebropedal and cerebrovisceral connectives respectively. Accord-
ingly as these connectives are long or short the ganglia are wide
apart or united into a nerve mass around the wsophagus.

Primitive Mollusca (Amphineura) have a simpler condition. The
cerebral ganglia lie dorsal to the cesophagus and are united by a cord
around the cesophagus (fig. 844). From it are given off two pairs of lat-
eral nerve tracts, the ventral or pedal cords, and lateral or pleural cords,
the latter united by a loop dorsal to the anus. By a concentration of
ganglion cells the pedal cords give rise to the pedal ganglia, and similarly
the pleural cords form three pairs of ganglia, the pleural and the parietal,
as well as the visceral already mentioned, of the cerebrovisceral cord (fig.
341, 4). Thepleural ganglia are connected with the pedal by nerve cords;
the parietal innervates the osphradium. When farther concentration takes
place the pleural may unite with the cerebral, and the parietal with the
visceral (fig. 841, B), or both may fuse with the visceral (C). In the latter
case the visceral ganglion (in the wider sense) is associated with the pedal
by the pleuropedal connective ; while in the other the connective is appa-
rently absent because fused with the cerebropedal. Although the otocyst
receives its name from the pedal ganglion, the centre of innervation lies ia
the cerebrum.

The heart, which lies dorsally, consists of auricles and ven-
tricles. The ventricle is always unpaired, but there are two auricles
where two gills exist from which the blood flows to the heart, but
with the loss of one gill one auricle may disappear. Distinct arteries
and veins occur; capillaries are found only in the Cephalopoda,
while in the lower molluscs, and especially in the Acephala, the
smaller arteries open into lacunar spaces which were formerly
regarded as the body cavity. A completely closed vascular system
does not exist even in the Cephalopoda.

The heart is enclosed in a spacious sac or pericardium, which,
with few exceptions, is connected with the nephridia by a ciliated
canal, and in many molluses (Cephalopoda and some Acephala) is
also related to the gonads. These facts support the view, already
mentioned, that the pericardium and the lumen of the gonads are
the remnants of the ca@lom; for here, as in the annelids, the
nephridia open by ciliated nephrostomes into the cclom, and the
sexual cells arise either from the celomic walls or from sacs cut
off from them. Even more important for this view would be
confirmation of the disputed statement that in Paludina vivipara
the celom (enterocawle) arises as diverticula from the archenteron.
MOLLUSCA. 355

Nephridia and sexual organs are primitively paired, but fre-
quently are single by the degeneration of the structures of one side.
The animals are either hermaphroditic or dicecious, but the gonads
are always very large. Even more room in the visceral sac is
demanded by the digestive tract in which wsophagus, stomach, a
coiled intestine, a voluminous liver, and frequently salivary glands
may be recognized. The radula or lingual ribbon is also a char-
acteristic organ, and its absence from the Acephala is probably to
be explained by degeneration. It is a plate or band armed with
teeth which lies on the floor of the pharynx on a ventral ridge,
the tongue, and is used for the communication of food (figs. 366,
367).

Reproduction is exclusively sexual; budding, fission, or parthen-
ogenesis have not yet been observed. The eggs, united in large
numbers, are usually enveloped in jelly and are either rich in
deutoplasm or are enveloped in a nourishing albumen. A few
molluses (¢.9., Paludina vivipara) are viviparous. A metamor-

Sp oS

ws.

Ce

  
    
   
 
      

     
   
   
     

  
  

  

   

UK AHANY \ i my °
. a ful | \ p—We
0 ANOH i \E

LAM,

   
 

YN,
WLLL iu
as

fp ARS.

Fic. 342.—Veliger larva (trochophore) of Teredo navalis. (From Hatschek.) A, anus;
J, stomach; J;, intestine; L, liver; LM.d, LM.v, dorsal and ventral longitudinal
muscles; Mes, primitive mesoderm cells; MP, teloblast ; Neph, protonephros; 0,
mouth; Oe, esophagus; R, rectum ; S, shell; Schl, hinge; SM.h, SM.v, posterior
and anterior adductors; Sp, apical plate; Whkr, wkr, pre- and postoral ciliated
bands; ws, cilia of apical plate.

phosis is of wide occurrence. In such cases a ‘ veliger’ larva
escapes from the egg (fig. 342); in this can be recognized head,
foot, and mantle, even in those cases where one or the other of
these is lacking in the adult. This shows that the absence of
356 MOLLUSCA.

mantle, shell, or head, which occur in large groups of molluscs,
is not a primitive condition, but can only be explained by degen-
eration. The name veliger arises from the velum, a strong circle
of cilia, which surrounds a frontal or velar field in front of the
mouth, and which serves as a locomotor organ for the larva. In
some cases (fig. 343. B) it is lobed like the trochus of a Rotifer.

B

  

Fie. 343.—Veliger stages, 4, of a snail; B, of a Pteropod. (From Gegenbaur.)
o, shell; op, operculum ; p, foot; t, tentacle: v, velum.

The veliger recalls the annelid trochophore and serves for the
distribution of the species; it is therefore of great importance for
animals which, like most molluscs, are sedentary or slow-moving.
In cases without metamorphosis (Cephalopoda, Pulmonata, etc.)
the veliger stage is frequently indicated during embryonic devel-
opment by a ridge of cells surrounding a preoral velar field.

Class I. Amphineura.

These forms, some of which appear in the Silurian, are clearly
the most primitive of molluscs, and are distinguished by a marked
bilateral symmetry. The nervous system already described (p.
354) consists of pleural and pedal cords with scattered ganglion
cells and no ganglia, these cords being connected by numerous
commissures (fig. 344, B).

Sub Class I. Placophora (Chitonide).

The chitons were formerly included among the gasteropods
becanse of the presence of a creeping foot and a radula. They
are at a glance distinguished from them by the rudimentary con-
dition of the head and the shell. This last is unique among mol-
luses; it consists of eight transverse plates overlapping like shingles,
which allows the animal to roll itself into a ball. The edge of the
~e
od

oo

ID AMPHINEURA.

mantle extends beyond the shell and is covered with spines, while
in the mantle cavity beneath are, right and left, a series of ctenidia.
Nerves enter the shell and end with noticeable sense organs (2s-

me)

“

ib

aS
fy

ry

  

Fic. 344.—Chiton squamosus, dorsal view. (After Haller.) 4, the entire animal; B,
after removal of shell and viscera. a, anus; C, brain; K, ctenidia; 0, mouth;
P, pedal nerve cord; pl, pleurovisceral nerve cord.

thetes and, in some, eyes, fig. 345). The symmetry of the body is

also expressed in the viscera. The anus is medial, and right and

 

Tie. 345.—Eye and esthetes of Acunthopleura spiniger. (After Moseley.) a, macrees-
thete; 6, micra f, calcareous cornea; g, lens; fh, iris; k, pigmented cap-
sule; n, p, nerves; 7, retina.

left of it are the openings of the nephridia and sexual organs. The

sexes are separate, the gonads unpaired, while corresponding to the

paired arrangement of the gills there are two auricles to the heart.
The Chitons are represented on our northeastern coast by several
small species (Lrachydermon,* Amicula*); farther south and on the

Pacific shores are larger species (Cryptochiton *).
358 MOLLUSCA.

Sub Class II. Solenogastres (Aplacophora).

Worm-like forms without shell; the
foot rudimentary and at the bottom of
a ventral groove. The radula is also
reduced; in Chetoderma it bears but a
single tooth. The gills are either small
or wanting. The usually hermaphrodite
‘ animals have the gonads emptying into
4..p an unpaired chamber (pericardium?) and
Fic. 346.—Neomenia carinata, thence to the exterior by the paired

ce ee ae nephridia. Chetoderma in New Eng-
serlon ye, ventraleroove: land; Neomenita, Dondersia.

   

Class II. Acephala (Lamellibranchiata, Pelecypoda).

These have, among the molluscs, the least powers of locomo-
tion. Some are fixed, the majority burrow slowly through sand
or mud; only a few spring by means of the foot or swim by open-
ing or closing the shells. Hence it is that they need more pro-
tection than other species, and this is afforded by the strong shells
in which the body can usually be completely enclosed. This shell
recalls that of the brachiopod in that it consists of two halves or
valves, but these valves are right and left rather than dorsal and
ventral, and hence are usually symmetrical in shape. Only when
the animal rests permanently on the right or left side is this sym-
metry lost, and then the symmetry of the soft parts is affected.

The two lobes of the mantle which secrete the shell on their
outer surface arise from the back of the animal and grow down-
wards, forwards, and backwards, so that they envelop the whole
(fig. 352). Hence the oldest and the most thickened part of the
shell, the umbo, occurs near the back (fig, 347). Around _ this
the lines of growth are arranged concentrically, lines which show
how, by gradual growth of the mantle, the shell has increased in
size. On the back the valves approach each other, and in the
majority are movably connected by a hinge, which consists of
projections (‘teeth’) in one valve fitting into depressions in the
other. In the Brachiopoda the valves are opened by appropriate
muscles; in the Acephala by an elastic hinge ligament usually placed
dorsal to and behind the hinge. ‘he shell is closed by adductor
muscles which extend through the body from shell to shell, leav-
ing their impressions or scars on the inner surface (fig. 347).
IT, ACEHEPHALA. 359

Usually there occur an anterior and a posterior adductor equally
well developed (Dimyaria); less frequently the anterior is rudi-
mentary (Heteromyaria) or entirely disappears (Monomyaria).

iN
hn

 

Tig. 348.

Fic, 347.—Left valve of Crassatella plumbea, inner and outer surfaces. (From Zittel.)
The outer surface showing lines of growth; no pallial sinus.

Fic. 348.—Right valve of Mactra stultorum, with pallial sinus. (From Ludwig-Leunis.)
Letters for both figures: a’, anterior; «’’, posterior adductor scar; ¢, hinge; lL
internal ligamental groove; m, pallial line; s, pallial sinus.

y

When the muscles are relaxed (as always occurs at death) the elastic
ligament opens the valves.

The heterodont hinge is the typical form (fig. 348); each valve bears a
group of teeth near the umbo, those of the left alternating with those of
the right. Besides these ‘ cardinal teeth’ there are in front and behind
‘lateral teeth,’ often produced into ridges. The ligament lies behind the
hinge and is usually visible from the outside (external ligament), but is
occasionally transferred to the interior (internal ligament, fig. 347). The so-
called schizodont and desmodont hinges are modifications of the hetero-
dont. Then there are Acephala of apparently primitive character which
either lack the hinge (dysodont), or have one composed of numerous teeth
in a series symmetrical to the umbo(taxodont), or of two strong teeth like-
wise symmetrical to the umbo (isodont). In these cases the ligament is
developed in front of as well as behind the umbo, and may be either
external or internal.

Since the secretion of shell takes place most rapidly at the edge
of the mantle, both are closely united, the union being strength-
ened by small muscles. So the edge of the shell has a different
appearance from the rest, this part being marked off by a pallial
line parallel to the margin (fig. 547). Tn many species, the Sinu-
360 MOLLUSCA,

palliata, the line at the hinder end makes a large bay (pailial
sinus) (fig. 348, s). Since the mantle folds are membranes with
free margins, it follows that when the shell is closed these edges are
pressed together, which would prevent the free entrance and exit
of water. To accommodate this each mantle has its margin exca-
vated at the posterior end, so that when brought together two
openings, an upper and a lower, result (fig. 349, C). The lower

 

Fic. 349.—Ventral views of siphonate and asiphonate acephals. 4, Anodonta cygnea ;
B, Lsocardia cor ; C, Lutraria elliptica. a, anal siphon ; >, branchial siphon; fs, foot;
k’, outer, 4’’, inner gill lamella; m, mantle; s, shell.

of these is the branchial opening by which fresh water passes into
the mantle (branchial) chamber; it flows out after passing over
the gills, along with the feces, through the upper or cloacal open-
ing. In many bivalves the free edges of the mantle grow together,

 

 

 

 

 

 

Re

 

Fig. 350.—Section of shell of Anodonta. c, cuticula; p, prismatic layer; 1, nacreous
layer.

leaving three openings, one for the protrusion of the foot, the
others the two just described, which are now called the ineurrent
(branchial) and excurrent (cloacal) siphons (fig. 349, 2B). By
further development the margins of these openings are drawn out
Il, ACEPHALA. 361

into two long conjoined tubes (fig. 349, 4), which for their retrac-
tion need special muscles, which are attached to the valves and
thus cause the pallial sinus referred to above (fig. 348).

In the shell three layers may be distinguished (fig. 850): on the outside
a thin organic cuticula and below two layers largely of calcie carbonate.
In many these two layers are distinguished as the prismatic layer and the
nacreous layer, the first consisting of closely packed prisms; the nacreous
layer of thin lamelle generally parallel to the surface. These by their free
edges produce diffraction spectra and so the iridescent appearance of the
shell; the finer the lines thus formed the more beautiful the play of colors.
This is especially noticeable in the mother-of-pearl shells Meleagrina and
Margaritina margaritifera. When foreign substances get between mantle
and shell they stimulate a greater secretion of nacreous substance and
become surrounded by layers of it. In this way pearls are formed.

2

 

K* Ks K' ml gf

Fig. 351.—Anatomy of Anodonta, the mantle, gill, and liver of the right side
removed, the pericardium opened. 1, ?, anterior and posterior adductors; I, II,
IIT, cerebral, pedal, and visceral ganglia; a, anus; |}, b?, upper and lower limbs
of organs of Bojanus,; br, branchial siphon; «, intestine; ¢, nephridial opening ;
fu, foot; g, gonad; hj, h®, ventricle and auricle of heart; k’, insertion of both
lamelle of right gill; Kk, k4, inner and outer lamelle of left gill; 1, left liver < l 5;
its opening in m, stomach; ml, pallial line; 7, anterior, r?, posterior retractor
muscle: sp, nephrostome ; v, labial palpus. The arrows show the planes of sec-
tion of fig. 352.

The gills lie between the mantle and the body and from their
lamellar character have given rise to the name Lamellibranchiata
(figs. 851, 352). Two gill-leaves occur on either side, although
occasionally the outer or both may degenerate. Frequently the
gills of the two sides unite behind the body and produce a parti-
tion which separates the mantle cavity into a small dorsal cloacal
362 MOLLUSCA.

chamber and the larger lower respiratory cavity. Into the cloaca
empty the anus and the water which has passed over the gills; it
opens to the exterior through the excurrent siphon. The incurrent
siphon leads into the branchial chamber. In front of the gills
are two more pairs of leaf-like lobes, the Jabial palpi, between
which is the mouth.

The gills are variously developed. The Nuculide—the most primitive
of living Acephala—have true ctenidia consisting of an axis grown to
the body and an inner and an outer row of gill leaves (fig. 355). From
this the filibranch type is easily derived. The gill leaves grow out into

 

 

Fie. 352.—Projection of sections shown by the arrows in fig. 351. b1, b2, upper and
lower limbs of nephridium (organ of Bojanus); @, intestine; e, nephridiopore;
fu, foot; g, gonad; h}, ventricle surrounding the intestine ; 22, auricle: k!, k2,inner
and outer gill lamelle; /, hinge ligament; m, mantle; n, cerebro-visceral com-
missure ; sp, hephrostome; v, venous sinus.

long filaments, each bent on itself so that it presents two limbs, a descend-
ing and an ascending. These branchial threads are so matted together
that they give the impression of a continuous leaf. In the true lamellar
gill the threads of the filibranch grow together at intervals, leaving open-
ings, the gill slits. Since there is an ascending anda descending limb, it
follows that each gill consists of an inner and an outer leaf (fig. 852), leav-
ing a space between into which the gill slits open. This internal space in
some serves to contain the young.

The complete enclosure of the body in the mantle folds has
led to a degeneration of the head and its normal appendages
IT, ACEPHALA. 38638

(Acephala). Hence there are only two divisions in the body,
dorsally the visceral sac and ventrally the foot. The foot, degener-
ate in many, has a broad sole only in Pectunculus and the Nuculi-
de; usually it is hatchet-shaped (Pelecypoda), that is, compressed
with a rounded ventral margin. It may be enormously expanded
and contracted again. This expansion is often explained by the
taking of water into the blood, but now it is generally accepted
that it is accomplished hy forcing blood from other regions into
it. While the foot by this extensibility can serve as a locomotor
organ, it also functions in many as an organ of attachment.
Inside is a large byssus gland which can secrete silky threads, the
byssus (fig. 853), one end of which is fastened to foreign objects by

 

Fig. 353.—Mytilus edulis. (After Blanchard.) a, edge of mantle; b, spinning finger of
foot; c, byssus; d,e, retractors of foot; f, mouth; gy, labial palpi; h, mantle; i, j,
inner and outer gills.

means of a finger-like process of the foot, while the other end

remains in connection with the foot. Molluscs which have a byssal

gland are found anchored by a thick bunch of byssal threads to
stones, etc.

The heart, surrounded by a pericardium, usually occupies the
most dorsal part of the visceral sac. It consists of a ventricle and
a pair of auricles (figs. 351, 352, h', 4°). The auricles receive the
blood direct from the gills; the ventricle forces it out through
anterior and posterior aorte (fig. 351), the latter lacking in many
species.

The excretory organs (organs of Bojanus) lie immediately
below the pericardium. The organs of the two sides touch in the
364 MOLLUSCA.

middle line. Each consists of a dorsal smooth-walled chamber
and a lower portion traversed by threads, both connected behind
but separated elsewhere by a thin partition. The lower chamber
is connected in front with the pericardium by a ciliated canal, the
nephrostome, while the upper opens to the outside by a short
canal, the ureter, the external opening being in the region of the
inner cavity of the inner gill. In this way a connexion is estab-
lished from the pericardium to the exterior, the apparatus being
apparently a true nephridium. In many it serves also as genital
duct, but usually the genital and reproductive ducts are separate.
The animals are usually diwcious, the gonads being acinose
glands.

The digestive tract (fig. 351) begins with a short oesophagus,
widens out to alarge stomach from which a slender intestine leads,
with many convolutions, to the anus. In the majority of Acephals
the terminal portion enters the pericardium in front and below,
passes through the ventricle and out through the upper posterior
wall of the pericardium. In its course the alimentary tract is
enveloped by the gonads and the voluminous liver, the secretion
of the latter emptying by two ducts into the stomach. Usually
the stomach has a blind sac, in which lies the ‘crystalline style,’ a
rod-like structure of uncertain significance.

The three typical molluscan ganglia (p. 355) are uncommonly
wide apart. The two brain ganglia (cerebropleural ganglia) lie
either side of the mouth at the base of the labial palpi and ventral
to the anterior adductor. They are very small, since cephalic
sense organs are lacking, and are united by a transverse supra-
cesophageal commissure. The posterior ganglia, composed of the
united parietal and pedal ganglia, He near the anus ventral to the
posterior adductor. The pedal ganglia, rather far forward in
the muscles of the foot, are closely approximate. Of the higher
sense organs only the otocysts near the foot are constant. The
labial palpi are also highly sensory, while two small osphradia occur
at the basis of the gills. When eyes occur they are, as in the seal-
lops (Pectinide), arranged in a row like pearls on the margin of
the mantle. Small tentacles with sensory powers may oceur both
on the margin of the mantle and on the tip of the siphon.

Veligers (fig. 842) are very common in development. When this stage
is lacking the history may ecntain a metamorphosis as in the fresh-water
Anodonta, The young which grow in the maternal gills are known as
Glochidia, which are distinguished from the adult by a byssus thread,
by only a single adductor, and by a hook or tooth on the free margin of
IT, ACEPHALA: PROTOCHONCHLE. 365

the shell (fig. 354). After escape from the gills they swim about by
opening and closing the shells, and by
means of the hooks attach themselves
to passing fish. They produce an ulcer
in the skin of the fish in which they,
grow, and by renewal of the shell and “
the adductor muscles attain the de-
finitive condition. After this metamor-
phosis they fall to the bottom, to live
henceforth half buried in the mud.

Structure of gills, hinge, edge of Fic. eae oe aes rae:
mantle, and adductor muscles have been (From Balfour.) ad, adductor ; by,
used as basis of classification, the usual DeSsHe? ©, Sense tains sh Shell,
divisions being founded on characters derived from only one of these
organs.

 

Order I. Protochonchie.

The primitive character of these forms is shown by the struc-
ture of the gills, which are either ctenidia (Protobranchiata) or

 

ct ob P

Fic, 355.—Anatomy of Nucula. (After Drew.) aa, anterior adductor; b, byssal gland;
c, cerebral ganglion; ct, ctenidium; f, foot; lt, heart; 1, labial palpus; 0, otocyst;
p, pedal ganglion; pa, posterior adductor; s, stomach ; t, appendage of palpus;
v, Visceral ganglion.

 

filamentary (Filibranchiata), yet here and there, as in the scal-
lops and oysters (Pseudolamellibranchiata), the fusion of gill fila-
ments is already begun. ITlinge and ligament are symmetrical
with regard to the umbo, or vary little from symmetry. The hinge
may be Jacking, and the ligament is wholly or in part internal.
The mantle edges are free, and rarely is there the first trace of fusion.
366 | MOLLUSCA.

 

Fic. 356.—Yoldia limatula.* (From Binney-Gould.)

 

    
   
    

Lig ae i“
ee,
HARTER)

| KO

 
    

Fia. 357.—A, Modiola plicutula*; RB. Pecten irradians*; C, Mytilus edulis.* (From
Binney-Gould.)
il, ACEPHALA: HETEROCONCHLE. 367

Sub Order I. DIMYARIA. Two equally developed adductors. The
taxodont NucULID& have ctenidia, a broad foot, pleural and cerebral gan-
glia separate, and gonads emptying through the nephridia, all points which
show them extremely primitive. Mecwla,* Leda,* Yoldia.* The ARcID#
are also taxadont, but filibranch. Scapharea,* Argina.* SOLEMYIDA,

Sub Order I]. ANISOMYARIA. Anterior adductor rudimentary
(Heteromyaria) or wanting (Monomyaria). With the exception of the
isodont SPONDYLID#, all the families lack a hinge (dysodont). To the
Heteromyaria belong the MYLILIDA, or mussels, with strong byssus and
shells pointed anteriorly. Modiola,* Pinna,* Mytilus edulis, abundant on
our mud flats; eaten in Europe, but occasionally poisonous. Dreissenia
polymorpha, a brackish and fresh-water species, has spread from the
Caspian through central Europe. Zthodomus* bores into stone. The
AVICULIDA of warm seas have wing-like projections either side of the umbo,
The pearl oysters of the East and West Indies (Meleagrina) belong here.
The OsTR#ZID& and the PECTINIDZ are monomyarian, The Ostreide, or
oysters, usually become attached by the right valve. Our American
Ostrea virginiana differs from the European species in having the sexes
separate. The Pectinide, or scallops, are free-swimming and are well
known for their highly developed green eyes on the edge of the mantle.

Order II. Heteroconchie.

Gills always lamellar, their outer surface frequently plaited.
Hinge—in rare cases (Anodonta) lost by degeneration—is hetero-
dont or modified from a heterodont condition. The mantle edges
but rarely free in their whole extent; siphons usually present, but
in some so small (Integripalliata) as to cause no sinus in the pallial
line; in others (Sinupalliata) large, the pallial line having a marked
sinus. Anterior and posterior adductors equally developed.

Sub Order I. INTEGRIPALLIATA. The UNIONID# (Naiadee) include
the fresh-water mussels, of which hundreds of species occur in the Missis-
sippi basin, some of which are markedly iridescent and afford material
for pearl buttons. In some pearls of value are occasionally found, Unio,
Anodonta. The tropical TripaentpZ, with small siphons, includes the

 

Fic. 358.—A, Saricara arctica; B, Astarte suleata; C. Siliqua costata. (From Binney-
Gould.)

largest Acephala, Tridacna gigas, the shell of which may be four feet
long and weigh three hundred pounds. The heart shells (CaRDIIDeE—
368

MOLLUSCA.

Cardium*, Serripes*) and ASTARTIDA, marine, and the fresh-water CycLa-
DIDE (Cyclas, Pisidium*), about the size of peas, belong here, as probably
do the extinct RupisTip# of the cretaceous.

Sub Order I]. SINUPALLIATA. The VENERID# with swollen shells,
represented by the quahog, Venus mercenaria on our east coast and by

 

Fig. 359.— Te-
redo neavatis,
ship worm in
its tube, the
siphons (a,
anal; b, bran-
chial) drawn
out of the
tube (r); ky,
shell. B,teeth
of the shell
enlarged.

brightly colored species in the tropics; the MACTRID& or
hen clams, and the flattened delicate TELLINIDE (Tellina*,
Macoma*), have short siphons. In others the siphons are
so large that they cannot be entirely retracted within the
shell. This is the case in the MyIp#, represented in all
northern seas by the long clam, Mya arenaria, and in the
razor clams (SOLENIDE; Solen Ensatella*). The allied Sax1-
cAVID# have burrowing species. These forms connect
with others in which the united siphons far exceed the rest
of the body in length, giving the animal a worm-like ap-
pearance (fig. 359). Since the valves do not
cover the whole shell, they are supplemented
by accessory shells, or the worm-like body
secretes a tube in which the rudimentary
valves are imbedded (fig. 360). The PHo-
LADIDH, some of which are phosphorescent,
burrow in wood, clay, or stone. The shell is
well developed. In the ship worms (TERE-
pIp#) the shells, on the other hand, are small,
while in some species the burrows made by
these animals in wood are lined by calcareous
deposits. The several species of Teredo* by
their boring habits do much damage to wood
in the sea, especially in the tropics. The
GASTROCHENID also form tubular shells, the
valves being imbedded in the tube (fig. 360);
at the smaller anterior end the tube is open,
but the other end is closed by a perforated
plate, giving these animals the name of
‘sprinkling-pot’ shells.
Lastly, there should be mentioned the
me: ae eran. little-known Septibranchiata, in which the
a gills have the shape of a septum perforated
Ludwig-Leu- by gill slits separating the branchial and clo-
nis.) a, shell. ah ‘ , ;
acal chambers. Silenta, Cuspidaria.

 
Il, SCAPHOPODA. IV. GASTEROPODA., 369

Class III. Scaphopoda (Solenoconche),

The tooth shells are primitive forms which have some resem-
blances to the Acephala in the paired
liver and nephridia and in structure of the
nervous system (with the exception that a
buccal ganglion is present and the pleural
ganglia are distinct from the cerebral).
In some points they are primitive (persis-
tence of jaws and radula), but in others
they are considerably modified. They lack
gills, have unpaired diwcious gonads,
rudimentary heart (no auricle), and have
two bunches of thread-like tentacles either
side of the mouth. The mantle lobes,
which are paired in the larva, unite below,
forming a sac open at either end, and this pig 361 —pentarium Soman:
secretes a shell shaped like the tusk of an — fiw. tooth shell; left the
elephant, from the larger end of which ae i ee
protrudes the long three-lobed foot used
for boring in the sand. Dentalium (fig. 361), £ntalis *.

 

Class IV. Gasteropoda.

Although more highly organized than the Acephala, the snails
are in some respects more primitive. The regions of the body—
foot, visceral sac, head, and mantle—occur in all orders, although
in each one or more forms may occur in which one or another part
is lost.

As a rule the foot is flattened ventrally to a creeping sole. In
it may be distinguished anterior and posterior processes, the pro-
podium and metapodium, a sharp lateral margin, the parapodium,
and, above these, appendages or ridges, the epipodia. Inside the
foot is usually a pedal gland.

The head bears (1) the tentacles, a pair of muscular lobes or
hollow retractile processes; (2) a pair of primitive vesicular eyes,
which usually lie at the basis of the tentacles, but may rise even
to their tips. In many snails the eyes are on special stalks which,
as in the stylommatophorous Pulmonata, form a second pair of
tentacles. The protrusion of the tentacles is caused by an inflow
of blood, their retraction by muscles attached to the tip which
draw them in like a finger of a glove.
370 MOLLUSCA.

The mantle begins on the back and extends thence forward
over the body to near the beginning of the head. It covers the
mantle cavity, a spacious chamber, which in the water-breathing
Prosobranchiata, etc., contains the gills (ctenidia) and opens
outward by a large aperture under the margin of the mantle.
The edge of the mantle may be produced into a long groove-like
siphon, conveying water to and from the branchial chamber,
which is of importance in determining the shape of the shell.
When, by degeneration of the gill, the animals become air-breath-
ing, the mantle cavity becomes a lung, and the opening, by growth
of the mantle edges to the body, becomes a small spiraculum,
closed by muscles.

The visceral sac, by the great development of the gonads and
liver, becomes very large. Since growth downwards is prevented
by the muscular foot, the organs press towards the back, carrying
before them the dorsal wall at the origin of the mantle folds, the
line of least resistance. Some organs, like nephridia and heart,
may be pressed into the mantle cavity. When the visceral sac, as
often occurs, becomes enormous, it does not stand directly upwards,
but coils from left to right in a spiral. The older the animal
the more the spiral coils and the larger the last or body whorl.
The visceral spiral therefore begins at the tip with narrow whorls
which increase in size with approach to the rest of the body.

From the foregoing the shape of the shell is easily understood.
As a secretion of the mantle it takes the form which the mantle
assumes under the influence of the visceral sac. With slight devel-
opment of the visceral sac it forms a flattened cone (fig. 362, 1),
or is slightly coiled at the apex, as in the abalone (B). When the
visceral sac is greatly elongate the shell is correspondingly an
elongate cone. It is rarely irregularly coiled (Vermetida, fig.
362, C). It is usually coiled like a watch spring in one plane, or
like a spiral staircase; in the latter case the shell is more or less
conical (fig. 362, D, #) and one can speak of its apex and base.
In the middle of the base is usually a depression, the umbilicus.
Sometimes the coils are loose and do not touch in the axis con-
necting umbilicus and apex, so that one can look into the space,
but usually the coils fuse together into a calcareous pillar, the
columella, around which the whorls pass (fig. 362, #, c).

The shell increases to a certain size by additions from the mantle
edge; and since this determines the aperture, the shell is marked
with parallel lines of growth. The pigment is elaborated on the
edge of the mantle, and in the formation of the shell passes into
IV. GASTEROPODA. 871

it, causing its color pattern. When the siphon is present the
shell shows a corresponding process. Thus are distinguished
holostomate shells with smooth mouths (fig. 362, D) and siphono-

 

F 1G. 362. Various forms of shells. (After Schmarda, Bronn, and Clessin.) A, Patella
costata ; B, Haliotis tuberculata ; C, Vermetus dentiferus ; D, Lithoglyphus naticoides ;
E, shell of Murex opened to show c, columella; 0, siphon.

stome shells, in which the anterior margin is drawn out in a
groove (fig. 362, £).

A simple conical shell without further evidence is not proof of primi-
tive structure. It may arise from the spiral form by degeneration, if the
visceral sac be reduced. Thus the shells of Fisswrella and Patella are to be
explained, for the viscera here show the results of an earlier spiral twist.

In most places the union between shell and soft parts is not very firm,
but the connexion at the aperture is more intimate, while a muscle is at-
tached to the columella (musculus columellaris) at about the middle point
of its height, the other end being inserted in the foot. It is for the retrac-
tion of the animal within the shell, first the anterior part with the head
and then the rest with the metapodium. In this the metapodium is folded
so that its dorsal surface lies towards the aperture. Hence in many species
this surface secretes a door, or operculum, which closes the aperture when
the body retracts. Since the aperture increases in size with growth, the
operculum must also enlarge, which is accomplished in a spiral manner
(fig, 362, D), the process sometimes showing in a spiral line on the out-
side. So-called eye stones are the opercula of small Trochidee and Tur-
binidee. Land snails are usually without opercula, but at certain times,
872 MOLLUSCA,

as in hibernation, they can close the shell by a calcareous plate, the epi-
phragm. In the spring this separates from the shell and is lost.

In most gasteropods the shell is coiled to the right, but in some species
(fig. 363) the whorls are constantly turned to the left,
while reversed specimens occasionally occur in many
species which are normally dextral.

In the shell there are at most two layers, an inner
lamellar layer (not always present), which sometimes
is highly iridescent, and an outer porcellanous layer,
FIG. 363.—Sinistral shell “hich is opaque and contains the pigment. In rare

of Lanistes carinatus. cases the mantle and consequently the shell are lack-

(From Ludwig-Leu- . ‘ : 4 a

nis.) ing, or the mantle is present but the shell is rudi-
mentary and not visible externally because the mantle folds have grown
over it. In these cases the visceral sac is not prominent. Since the shell-
less forms possess a mantle and shell in the young, the adult conditions
are explained by degeneration.

 

Only a few gasteropods are like the Amphineura and Acephala
in being bilaterally symmetrical. Usually the spiral twist of the
visceral sac has resulted in a torsion of other parts from left to
right, in which alimentary tract, nephridia, gills, heart, and nerv-
ous system take part. The intestine is bent in this way, the anus
opening into the mantle chamber on the right side, or the twisting
may be continued so far as to double the intestine on itself, the
anus being in the middle line in front, near the head. Nephridia,

  

Fig. 3€4.—Three diagrams illustrating the torsion of the body and the twisting of the
nervous system in gasteropods. (After Lang.) 4, bilateral, B, asymmetrical,
C, streptoneurous condition. The reference letters are placed upon the organs
ot the primitive left side. a,anus: ¢c, cerebral ganglion: g, ctenidium; 7, auri-
cle; m, mouth; 2, nephridial opening; o, osphradium; pa, parietal ganglion;
pe, pedal ganglion; p/, pleural ganglion; v, ventricle. ;

 

gills (with them the osphradia), and heart wander in company, so
that the organs primitively belonging on the left side may be trans-
2

IV. GASTEROPODA. 373

e

ferred to the right and vice versa. With this there is a tendency
to asymmetry and the loss of the organs (usually of the primitively
left side). When the nervous system takes part in the twisting
a notable crossing of the cerebrovisceral commissures takes place,
known as streptoneury or chiastoneury.

The alimentary canal begins with a muscular region which in
some groups is developed into a large protrusible proboscis (fig.
365). The pharynx, which follows, contains the tongue, a ventral
ridge supported by one or more cartilages and covered by a cutic-
ular layer, the radula or lingual ribbon (odondophore). The upper
surface of the radula is armed
with sharp, backwardly di-
rected teeth (fig. 366) which
are usually arranged in trans-
verse and longitudinal rows,
but which vary so in num-
ber, form, size, and arrange-
ment that they are of value in
classification. Although the
radula covers the tongue, it is

 
 
 
 
 
 
 
 
 
   
  

Fic, 365. Fic, 366.

Fia. 365.—Pyrula tuba, male. (After Souleyet.) The mantle has been cut on the
right side and turned to the left, reversing the pallial organs. a, anus; ¢, ctenid-
jum; em, columellar muscle; f, foot; h, heart in pericardium ; i, intestine ; 1,
liver; m, mantle; mf, floor of mantle cavity; 1", nephridium, ns, opening ot
nephridium; 0, osphradium ; p, proboscis; pe, penis; t, testes; v, vas deferens
cut in two. . : ; i

Fig. 366.—Pharyngeal region of Helix pomatia. A, side view ; B, section. m, muscle;
oc, @sophagus ; 7’, radula; 7s, radula sac ; sp, salivary duct; 2, lingual cartilage.

formed in the radula sac, which lies behind the tongue. From
this it grows forward like a nail over its bed as fast as it is worn
out in front. It is opposed in eating by a single median or a
pair of lateral jaws (lacking in carnivorous forms).
The rest of the alimentary canal is convoluted, the anus being
3B¢4 MOLLUSCA.

usually on the right side in front, in or beside the mantle chamber,
tarely it empties in the middle line behind (figs. 365, 370, 371).

(Esophagus, stomach, and intestine are slightly marked off
from each other. The convolutions of the intestine are enveloped

 

Fic. 867.—Row of teeth from the radula of Trochus cinerarius. (After Schmarda.)

by the liver, which by its large size forms the chief part of the
visceral suc. <A pair of salivary glands empty into the pharynx,
these in the Doliide secreting free sulphuric acid.

The nervous system usually differs from that of other molluscs
in that the pleural and parietal ganglia are free (p. 353). If the
commissures be short, the ganglia are collected near the pharynx
and, thus freed from the body torsion, are symmetrical (orthoneu-
rous, fig. 368, //). If the cerebrovisceral commissure be longer,
the result is almost always streptoneury (chiastoneury). Pleural
and visceral ganglia hold their place, but the right parietal ganglion
crosses above the intestine to the left side (hence called supra-
intestinal), while the left passes under the intestine to the right
side (subintestinal), the cerebrovisceral commissure being twisted
like the figure 8. The strong development of the pharynx is ac-
companied by buccal ganglia. The existence of streptoneurous
forms among the orthoneurous Opisthobranchs (Acteon) and Pul-
monata (Chilina) shows that orthoneury in these groups has arisen
from streptoneury.

Gills, heart, and nephridia are best treated together. Certain
genera (Haliotis, Fissurella) recall the Acephala in having these
organs in pairs, while the intestine passes through the heart. As
a rule the asymmetry induced by the torsion of the body has
resulted in the loss of the ctenidium, osphradium, nephridium,
and auricle of one (the primitively left) side. Prosobranchs and
Opisthobranchs are recognized accordingly as the gills are on the
anterior or posterior part of the body. In the Opisthobranchs
(fig. 369) the ctenidia have been lost and are replaced by secondary
gills on the back. Here the heart is in front of the gills; it receives
blood from behind and forces it forward to the head by an aorta.
In the Prosobranchs the heart has been twisted about ninety
IV. GASTEROPODA. 375

degrees, so that the auricle is in front and the ctenidium in front
of this (fig. 570), while the aorta leads backwards. The nephrid-
ium, which communicates with the pericardium by a nephro-
stome, is rarely a racemose gland; usually it is saccular, the lumen

   

   
     

c\
mt
O

 

T'te. 368. Fie. 369.

Fic. 368.—I, streptoneurous nervous system of Paludina. (After Hering, from
Gegenbaur.) JI, orthoneurous system of Limncea, (After Lacaze-Duthiers.)
A, visceral; B, buccal; (, cerebral; p, pedal; Pl, pleural; sb, sp, sub- and supra-
intestinal ganglia: 1, olfactory nerve; 0, otocyst.

Fig. 369.—Diagram of circulation in Doris. (After Leuckart.) a, auricle; c, gills
eres anus; t, tentacle; v, ventricle; 2, vessels returning venous blood from the

ody.

bearing gland cells and concretions; its duct either empties into
the mantle cavity or beside the anus.

The sexual organs in some forms (Cyclobranchs and many
Zy gobranchs) empty into the nephridia. They show two extremes.
On the one hand are completely diccious species, on the other
there may be complete hermaphroditism (many Tectibranchs,
Pteropoda), in which the male and female organs are united
throughout their extent. Intermediate stages occur; those of
the pulmonates are described below.
376 MOLLUSCA.

In the Helicide there is a hermaphrodite gonad which lies together
with the liver in one of the first whorls of the shell (fig. 371, 2). A coiled
genital duct follows which widens to a thick-walled ‘uterus’ (2) along

 

Fic. 370.—Anatomy of Cyprea tiqnis. (After Quoy et Gaimard.) br, ctenidium; c,
heart; df, vas deferens; hi, liver; 7, stomach; N, cerebral ganglion ; oc, eye: pe,
penis ; ph, pharynx, the radula drawn out; 7, rectum ; re, nephridium; ¢, testes.

which a second seminal canal appears to lie. Actually in the interior
there is but a single lumen, the different appearances being due to glands
in the walls. A separation into vas deferens and vagina occurs at the
end of the vagina. The vas deferens (vd) proceeds as a small coiled
canal to the genital pore. Here it enlarges to a protrusible penis (p)
with which is connected a retractor muscle and an appendage, the flagel-
lum (fl). The vagina is broader and goes straight to the genital pore,
where it meets the penis. Connected with the female genitalia are the
large albumen gland (é¢) at the beginning of the uterus and a receptac-
ulum seminis (7); a round vesicle connects with the vagina by a long
duct, and (not always present) two ‘ finger-form glands.’ Lastly, the dart
sac (ps) of the vaginal wall, which secretes a calcareous stylet, the ‘ love
dart,’ which in copulation acts as a stimulus to the male genitalia. In
spite of hermaphroditism a copulation lasting for days may oceur, con-
nected with which is the fact that in many species the male cells are first
matured, then the female (proterogyny); or the reverse may occur
(proterandry).

The sexual opening is almost always on the right side, beside
the anus or in front of it on the head. Its position may be rec-
ognized in hermaphroditic species and in dimcious males by the
IV. GASTEROPODA. 377

grooved dermal fold, the penis (fig. 370, pe). Occasionally this is
separated from the genital pore, but is connected with it by a cili-
ated groove.

The terrestrial snails lay their large tough-shelled eggs in damp
earth; in the aquatic forms the eggs are laid in masses, usually

55

 

Fig. 371.—Anatomy of Helix pomatea, the roof of the pulmonary sac cut at the left
side and turned to theright; the pericardium and visceral sac opened and the
viscera separated. «a, anus; ¢c, columellar muscle; d, intestine; ¢i, albumen
gland; f, finger-form gland; fl, flagellum; fu, foot; y, cerebral ganglion; h, heart ;
l, liver; lu, lung; m, stomach; n, nephridium; 7’, its opening; p, penis; ps, dart
sac: r, receptaculum seminis; s, pharynx; sp, salivary gland; wu, uterus; v,
vagina ; vd, vas deferens ; z, hermaphrodite gonad.

gelatinous, each egg with a layer of albumen and a firm shell.

Occasionally there is a kind of nest, as is the case with /anthina

which carry the mass of eggs, attached to the foot, about with

them. <A few gasteropods are viviparous.

In the development the great constancy with which the veliger
stage (figs. 342, 345) appears is noticeable. Most marine larve
swim by their velum (often divided) at the surface before creeping
at the bottom. But in those cases where the snail leaves the egg
378 MOLLUSCA.

in the definitive condition the velum is usually developed in
embryonic life, sometimes so strongly that the embryo rotates in
the surrounding fluid.

Order I. Prosobranchia.

The Prosobranchs, like most gasteropods, have the twisting of
the visceral complex from left posterior to right anterior, so that
the anus lies on the right side near the head, the nervous com-
missures are twisted into an 8, and the nephridia of the right
side have been carried to the left, where they le far forward.
This has twisted the heart so that it receives branchial blood from
in front and sends it backwards through the aorta, ‘The sexes
are separate and the shell and mantle are usually well developed.
Accordingly as the mantle is drawn out in a siphon or not, the
shells are siphonostomate or holostomate (p. 371). Certain
Prosobranchs are near the primitive Amphineura in the reten-
tion of both ctenidia, both auricles, and both nephridia, but in the
great majority only one gill (the primitive right) is present and the
corresponding auricle alone is well developed, although the other
may exist in a rudimentary condition.

Sub Order I. ASPIDOBRANCHIA (Diotocardia, Scutibranchia).

Ctenidium bipectinate (fig. 372) or absent. There are usually two
auricles and two nephridia. DOCOGLOSSA (limpets), auricle single ;

 

Fig. 372. Fie. 373.
Fia. 372.—Fissurellu pataugonica, ventral view. (From Bronn.) br, the paired gills;

p, foot.
F ia. 373.—Acmeea testudinalis, limpet. (From Binney-Gould.)

one or no ctenidium ; intestine not passing through heart, shell conical.
ACM@ID# with ctenidium. <Acmea* (fig. 373). PaTELLID®, ctenidia
lacking, replaced by aring-like mantle gill. Pavella (fig. 362, 4). ZYGO-
IV. GASTEROPODA: PROSOBRANCHIA. 379

BRANCHIA. Two ctenidia (fig. 872), shell with marginal slit or with
holes corresponding to an anal notch in the mantle; auricles and ne-
phridia paired; heart traversed by intestine. FIssuRELLID&, keyhole lim-
pets; shell conical, with apical opening. HawioTip#, abalones; shell
weakly spiral, flat, with a series of holes. Haliotis* (tig. 362, B). AZYGO-

Cc

 

Fic. 374.—American Pectinibranch gasteropods. (From Binney-Gould.) A, Crepidula
fornicata; B, Lacuna vincta; C, Ilyanassa_ obsoleta ; D, Littorina palliata; E, L.
litorea; I’, Urosalpinx cinerea ; G, Purpura lapillus ; H. Buccinum undatum; I,
Lunatia heros.

BRANCHIA. One ctenidium, but two auricles. TROCHID®, operculum
horny ; Trochus, Margarita.* TURBINID&, top shells; operculum calca-
reous. Turbo, Phasianella.

Sub Order Il. PECTINIBRANCHIA (Monotocardia, Ctenobranchia).
Ctenidium unipectinate, osphradium well differentiated (fig. 365), intestine
380 MOLLUSCA.

not passing through the heart. Many groups are recognized, based upon
the structure of the lingual ribbon. Of the thousands of species only a
few groups can be included here. RHACIHIGLOSSA ; siphonostomate,
predatory. Muricipas (Murex, Purpura,* Urosalpinw*) have an anal
gland secreting a substance first colorless, turning to purple by exposure
to air. The Tyrian purple was produced by Murex truncilus. Urosalpina
cinereus * drills into oysters. Buccinip&, whelks, VOLUTIDZ, and OLIVIDE
belong here. TOXIGLOSSA ; Conipm, with large cesophageal pcison
gland, some species producing severe wounds. Conds, tropical; Bele.*
TANIOGLOSSA ; Naticipa, Neverita,* and Luwnatia,* common snail of
Atlantic coast, their egg-masses being the familiar sand saucers, Lit-
TORINID# ; periwinkles. CYPR#ID&, cowries; Cypraa moneta of India is
used as money in Africa. AMPULLARID#; amphibious, part of branchial
cavity acting as lung, part containing ctenidium. PALUDINID®, fresh
water. CYCLOSTOMID®, tropical terrestrial forms, the mantle cavity a
lung.

HETEROPODA. In all details of gills, genitalia, heart, and nervous
system these are true Pectinibranchs, but from an exclusively pelagic life
have acquired peculiar modifications. As in most pelagic animals the
body is gelatinous and transparent. The head is elongate, and the body is
enlarged so that usually it cannot be retracted into the shell. Most char-
acteristic is the division of the foot into pro- and metapodium (fig. 375), the

 

Fig. 375.—Carinaria mediterranea (after Gegenbaur), shell removed. A, metapodium ;
a, anus ; ar, aorta; B, visceral sac: br, branchiw, the heart above: df, vas def-
erens; 0, mouth; oc, eye with tentacle; w, @sophagus; p, propodium ; ps, penis;
f, UW, WT, cerebral, pedal, and visceral ganglia. :

latter forming a tail-like elongation of the body. The propodium is verti-
cally flattened and by its undulations serves as a swimming organ. The
Heteropoda are predaceous and extremely voracious ; they swim back
downwards. The ATLANTID.® can completely withdraw into the shell and
close it with an operculum ; the CARINARIID.® (fig. 375) have a shell which
scarcely covers the visceral complex ; the PreroTRaCHEID.& have no shells.
LV. GASTHROPODA: OPISTHOBRANCHIA. 381

Order II. Opisthobranchia.

The Opisthobranchia have not varied from the primitive sym-
metry to such an extent as have Prosobranchs and Pulmonates.
The anus is in the plane of symmetry or only slightly removed
from it, although it may be placed far forwards. The nervous
system is orthonenrous, the twist being straightened (except in
Actwonide). The heart also retains its primitive position, receiy-
ing blood from behind and forcing it forward to the body through
the aorta (fig. 369). In rare cases a (right) ctenidium, a poorly
developed mantle, and a thin shell enveloped in the latter occur.
Usually these have been lost and the place of the ctenidium is
taken by accessory gills of various forms or a dermal respiration

a

   

Bie, 56, Hyaken complinats from a ectatie, de, cesonlagus? ter neplirldiua yw
stomach; IJ, pedal ganglion and otocyst.
occurs. It isinteresting to note that the larve have well-developed
mantle and shell. Also important from the systematic standpoint
is the existence of hermaphroditism, the genital duct opening
on the right side. Many of the Opisthobranchs afford fine
examples, in form and coloration, of protective resemblance. All
are marine.
Sub Order I. TECTIBRANCHIA. Mantle and usually a shell and

etenidium present, parapodial processes often present. Scaphander,*
Bulla,® Philine,* Aplysta.
382 MOLLUSCA.

Sub Order II. PTEROPODA. Pelagic forms which in most points of
structure agree with the Tectibranchs. The head and usually eyes and
tentacles are lacking, while the fins (in reality greatly developed para-
podia) are highly characteristic, giving the name ‘ wing-footed’ to these
forms. Like the Tectibranchs they are hermaphroditic, orthoneurous,
have a single ctenidium and a posterior auricle. The THECASOMATA

 

Fig. 377.—A, Clione papilionacea ; B, Hyalea tridentatu. (After Verrill.)

have shells, those of LIMACINID& (spiral) and HYALEID2 (pyramidal) being
calcareous. The CyMBULID& have transparent gelatinous pseudo-shells
formed by the subepithelial connective tissue. The long nearly cylindrical
shells of the CAVOLINID® make up much of the ‘pterpod ooze’ of the deep
seas. GYMNOSOMATA ; shelllacking. Pnewmodermon, with suckers like
those of cephalopods on the proboscis. Clione,* arctic.

Sub Order II. NUDIBRANCHIA. Shell, ctenidia, and osphradia
lacking ; most possessing accessory gills (or cerata) of varying form and

hee? Y p22,

8 AE

ee

Fia. 378. Fia. 379.
Fig. Doris bilamellata*
Fig. 379.—-Holidia papillosa. (From Ludwig-Leunis.)

  
  

  

distribution. In the Doripiipz they form a cluster of retractile bushes
around the anus (fig. 378). In the Trironupa they are in two rows, right
and left (often branched) upon the back. The <KoLIpDx have several rows
(Dendronotus*), while in the Etysupas cerata are lacking. The olide
are noteworthy in having nematocysts like those of the coelenterates
(p. 229).
IV. GASTEROPODA: PULMONATA.

co
a
co

Order III. Pulmonata

In several respects the Pulmonata are intermediate between the
Prosobranchs and Opisthobranchs. Like the latter they are
orthoneurous and hermaphroditic (p. 376). On the other hand
the respiratory organ is far forward near the head, with the result
here, as in the Prosobranchs, that the auricle is forward, the aorta
behind. The opisthopneumous Testacellide have the lungs at
the posterior end of the body. Here and there streptoneurous
conditions occur (Chilina).

The lung, the most characteristic feature of the order, is a
spacious sac arising from the mantle cavity along with the degen-
eration of the ctenidium. It begins on the right side and like a
half-moon stretches some distance on the left. On the right side
is a small opening, the spiracle, with a sphincter muscle, and in its
margin the anus and sometimes the ureter. The roof of the lang
is occupied by a rich network of blood vessels (fig. 371, /w) which
draw the blood from a marginal vein, collect
it In a main trunk and carry it to the heart.

Many pulmonates are aquatic, but since they have
no gills they must occasionally come to the surface to
fill the lung sac with air. This is the case with many
pond snails of the Limnzide, but some, which live
at great depths, as in the lakes of Geneva and Con-
stance, and consequently cannot reach the surface,
use the skin and to some extent the lung for
water-breathing. Several genera (Planorbis, Pulmo-
branchia, Siphonaria) have given rise to secondary
gills,

Sub Order I. STYLOMMATOPHORA, = Four re-
tractile tentacles, the eyes being borne at the tips of
the second and longer pair. The HELIcIDa have a
well-developed shell, closed by an epiphragm (p. 372)
during hibernation. Heléav,* many hundred species
distributed among many sub genera. Pupa,* Acha-
tina, Bulimus, many tropical species, Limacip#. :

c A : : 5 Fie. 380.—Limae cine-
Shell reduced, completely concealed in the mantle. © jens” (Atter Ludwig-
Limax,* Arion,* Ariolimax.* Leunis.) s, spiracle.

Sub Order II. BASSOMATOPHORA. Only onepair of uon-retractile
tentacles, the eyes at their hase. LimNasipz2, pond snails, living in shallow
ponds and brooks. Limnea,* Planorbis,*

 
MOLLUSCA,

ce
Qn
ies

Class V. Cephalopoda.

The Cephalopoda are distinguished among the molluscs by their
size and their high organization. The majority measure, includ-
ing the arms, from eight inches to three feet in length, a few are
smaller (two to seven inches), while especially rare are the huge
giants, some of which may be over forty feet in length. These
large species for a long time were only known from the tales of
sailors, who said that the animals had grasped vessels with their
large muscular arms and had drawn them into the sea. In the last
half-century some of these forms, belonging to the genus Archi-

 

Fig. 381. Fig. 382.

Fie. 381.—Octopus tonganus from the side. (After Hoyle.) Funnel and mantle fold to
the right; back and eyes on the left.

Fig. 382.—Loligo kobiensis, ventral view. (After Hoyle.)

teuthis, have been stranded by storms on the coasts of Newfound-
land and Japan. One of these Newfoundland specimens had a
body twenty feet long from head to tail, and one of the arms was
thirty-five feet in length. Since these arms are composed entirely
of muscle, it is easily conceivable that they might swamp a small
vessel.

The body of a cephalopod is divided by a constriction into
head and trunk. At the extremity of the head is the mouth, and
around this a circle of arms or tentacles. Each tentacle is taper-
ing and bears on its oral surface rows of suckers (in some species
altered to hooks). The Octopoda have cight of these arms, all
V. CEPHALOPODA.

co

85

equal in size (fig. 381), four on the right side, four on the left.
The Decapoda (fig. 382) have in addition two longer arms which
bear suckers only on the enlarged tips and can be retracted into
special pouches. This additional pair come between the third
and fourth of the Octopoda, counting from the dorsal side.
Behind the crown of tentacles are, right and left, the pair of
large eyes which superticially closely resemble those of the verte-
brates, since they have a transparent cornea and a large pupil sur-
rounded by an iris. Internally the resemblance is not less pro-
nounced (fig. 383). Behind the iris is a lens and a vitreous body,

 

Fie, 383. Fia. 384.

Fie. 383.—Diagrammatic section of Cephalopod eye. (After Gegenbaur.) ae, argentea
(choroid) ; C, cornea; ci, ciliary process; go, optic ganglion; ik, iris; hk. cartilages;
L, lens; p, pigment layer; Re, cellular layer of retina; Ri, rod layer of retina;
w, white body.
Fig. 384.—Schematie section of eye of Nautilus. (From Balfour.) A, aperture of
optic cup; Jnt, iris-like fold of integument; N.op, optic nerve; R, retina.
the latter being bounded by the retina and this in turn by a pig-
mented silvery layer, the argentea or choroid, which contains
cartilages recalling the sclerotic coat. Two striking peculiarities
separate these eyes from those of the vertebrates and show that
they have arisen independently and have an entirely different de-
velopmental history. (1) he cornea in many species has an
opening by which water enters the anterior chamber; (2) the layer
of rods in the retina abuts against the vitreous body and the gan-
glionic layer lies behind, while in the vertebrates the reverse is

the case.
386 MOLLUSCA.

The foregoing description applies to but part of the Cephalop-
oda. The highly different Nautilide have a large number of
lobe-lke processes on the head, these without suckers. The eyes
are deep pits, opening to the exterior by a smull aperture, the base
of the pit being occupied by the retina, while lens, vitreous body,
iris, and cornea are lacking (fig. 384). It is to be noticed that the
other cephalopod eyes pass through a Nautilus stage.

In the trunk anterior and posterior sides are distinguishable,
the two passing into each other on the sides. he anterior side
(which corresponds only in part to the ventral side of other mol-
luses) is wholly covered by the mantle, a strong muscular fold,
which takes its origin from the periphery of the body, often
encroaching upon the back and always terminating with free mar-
gins at the head. On opening the mantle by a ventral incision (fig.
385) the two ctenidia (four in Neatizus) are seen on either side.

 

Fra. 385.—Sepia officinalis, the mantle and left nephridial sac opened to show the
vena cava leading to the branchial heart. a,anus: b, d, lock of siphon and mantle;
yg, genital opening; A, head: hk, ctenidium ; 1, nephridial sac ; 1’, nephridial open-
ing; sp, nephrostome; t, ink sac; 77, siphon.

 

setween them in the middle line is the anus, and right and left
of this and a little behind are the nephridial openings (four in Vau-
Zilus, which also has osphradia). More laterally are the sexual
openings, of which one (usually the right) is commonly absent.
1D

V. CEPHALOPODA. 387

Q

At the head the mantle opens by a slit to the exterior, but it can
be closed and fastened by various locking contrivances (in Sepia,
Loligo, etc., by button-like projections (fig. 385, @) which fit into
corresponding sockets (/) on the trunk). When thus closed the
communication with the exterior is by a special conical muscular
tube, the funnel or siphon, which is fastened to the body and has
uw wide mantle aperture. Since the cephalopods, by contraction
of the mantle wall, can drive the water from the mantle cavity
through the siphon with great force, they can swim very rapidly by
the reaction. Here, too, Nautilus is peculiar in that the siphon is
throughout life composed of two overlapped folds, which is sig-
nificant since in the embryos of other forms the siphon (fig. 396)
arises as two separate folds which later unite to produce the defini-
tive condition. A typical foot is lacking, but comparative mor-
phology shows that the siphon is composed of a pair of epipodia,
while the arms are differentiations of fused foot and head.

  

Head and trunk are covered with a thin mucous skin, which shows in
a marked degree the power of changing color. oligo will pass from a
dark red to a translucent white ; Octopus has an even greater gamut of
color. These color changes are possible since in the cutis there is a silvery

 

Fic. 386.—Female Nautilus, the shell bisected. (From Ludwig-Leunis.) Ay mantle; ?,
dorsal lobes; 3, tentacles: 4, head fold; 5, eye; 6, siphon; 7, position of nid-
amental gland; 5, shell muscle; 9, living chamber; 1, partitions between
chambers; 11, siphuncle.

layer over which are numerous different-colored pigment cells or chroma-

tophores, in which radial muscle fibres are inserted. On contraction of

these the chromatophores are flattened and thus influence the color ; when
the fibres relax the pigment cells contract to small spots. In deep-sea

cephalopods phosphorescent organs have been observed.

Notwithstanding the soft bodies a well-developed shell occurs
in living cephalopods only in Nautilus and Argonauta (figs. 386,
383 MOLLUSCA,

398). Externally the shell of the former, coiled in a plane,
resembles that of certain snails like Planorbis; but on section it

    
 

oN Wee

Fie. 387.—Spirula, with internal shell. (After Owen.)
is seen to be divided by partitions into numerous chambers which
increase in size towards the aperture. Only in the last is the
animal situated: the others are filled with air. Each partition
has a small opening, and through these runs a strand of tissue, the

siphuncle. Among the fossil cephalopods many forms—the Nau-

 

tl cAALVAANT

  

Fia, 388.--Diagram of shells, etc. of various cephalopods. (After Lang.)
B, Belosepia; C, Belemnites ; D, Oxtracoteuthis ; By Ommastrephes.
posterior ; ph, phragmocone ; pr, proostracum ;

tiloids and Ammonites

A, Sepia;
a, anterior: Pp,
r, rostrum; s, siphon.

 

have a similar chambered shell; but in
other recent forms and in many extinct species the shell has
undergone a more or less complete degeneration. In Spirula
perontt (the animals of which are extremely rare, the dead shells
common) there is a similar chambered shell, buried in the mantle
(fig. 387). In the Decapoda the equivalent of the shell is com-
pletely concealed in the back of the animal. In the Sep/as it is
a lamellar calcareous structure, the well-known cuttle bone; in the
V. CEPHALOPODA. 389

Loliginide it forms a ‘pen’ of purely organic nature (fig. 340, A).
Like true shell these dorsal structures are products of the external
epithelium, only the epithelium which forms them, the shell
gland, has become folded in and the walls have united over it.

The shell of Argonauta (fig. 398) is different. It occurs only in the
female, is thin as paper, spirally coiled at the tip, and is only in part a
secretion of the body, for a part of it is formed by two tentacles which are
expanded for this purpose. Internal partitions are lacking, and this shell
serves as a nest for the eggs. A word or two may be added to correllate
the recent and fossil shells of the Dibranchiata, which are always internal
and more or less rudimentary. The fossil Belemnites (fig. 388, ¢) had a
chambered shell (‘ phragmocone’) perforated for the siphuncle. In front
this is prolonged yentrally into a thin broad plate, the proostracum, while
behind it is inserted in a calcareous sheath, the guard or rostrum. From
this, by comparison with the fossil Belosepia (B), it is seen that the cuttle
bone as it appears in commerce (A) is the anterior part of the chambered
shell, its laminze being the partitions, while in the animal the rostrum and
siphuncle are in part retained. On the other hand, comparison with
the fossil Ostracoteuthis (D) shows that in Ommastrephes (EZ) we have but
a remnant of the phragmocone, while the bulk of the pen is proostracum,
In Loliyo the phragmocone is entirely lacking.

The mouth, situated in an oval buccal mass, lies between two
horny jaws, like the beak of a parrot
(fig. 389); then follows a pharynx
with a radula, and in turn a long
wsophagus, often with a crop-like dila-
tation. The @sophagus opens into a
wider pouch, the stomach, with which
is connected a blind sac, frequently P16-—Jows of Sepeo optcmnatie.
coiled. Here the tract doubles on itself and goes straight to the
anus, or makes one or two convolutions in its course (fig. 390).
One or two salivary glands (upper and lower, the latter poisonous
in Octopus) open into the esophagus, and a pair of liver sacs
(frequently fused) open by two bile ducts into the gastric blind
sac. These ducts may bear racemose glands called the pancreas.
Lastly, the ink sae opens into the intestine near the anus. This
gland, which has a duct of varying length, secretes in its interior
a brownish or blackish pigment. When alarmed the animal ejects
this secretion and clouds the water so that it can escape unseen.
This organ is best developed in Sepia officinalis, and its secre-
tion forms the basis of the well-known color, sepia.  Mautilus
has no ink sac.

Just behind the buccal mass are the closely united chief gan-

 
390

  
 

 
    
       

      

if x 4 anf
ally ©, Oe
My, Qa, OX, ©
Sgt © |i ¢
Ly, ©, O10]

MOLLUSCA.

  
  
   

  

  
 
    

 

Fic. 390.—Anatomy of Octopus vulgaris. «a, anus; ao, aorta; cv, vena cava with ne-
phridial appendage; d, intestine; go, optic ganglion; ”, systemic heart; i, crop;
K, head ; k, ctenidia ; kh, branchial heart ; kn, cartilage; J, l’, liver and gall duct,
the liver indicated by dotted line; M, mantle; 0, ovary ; od, oviduct; p, pedal

ganglion; s, buccal mas
and sympatt

S$ with salivary glands; st, stellate ganglion ; sy, stomach

hetic ganglion ; T, basis of tentacles; t, ink sac; v, visceral ganglion:

vk, auricle of systemic heart ; *, spiral blind sac.

 

Fie. 391.—Nervous system
of Sepia officinalis from
the side. ghi, inferior
buccal ganglion; ghs, su-
perior buccal ganglion;
gc, cerebral ganglion ; gp,
pedal ganglion; gv, vis-
ceral ganglion ; mb, buc-
cal mass; @, oesophagus ;
op, optic ganglion.

glia of the nervous system (fig. 391). A
single dorsal mass represents the cerebral
ganglia; connected with this by broad com-
missures, the pedal and visceral (viscero-
pleuro-parietal) ganglia lie close together
ventrally. With these parts are associated
upper and lower buccal ganglia. The large
optic ganglia, developed in the optic nerve
arising from the cerebrum, are especially
characteristic of the Cephalopoda, as are the
ganglia stellata, right and left at the anterior
edge of the mantle (fig. 390), which owe
their name to the radiation of fibres to inner-
vate the mantle. An unpaired sympathetic
ganglion lies at the junction of stomach and
blind sac. Cerebral, pedal, visceral and optic
ganglia are enclosed in the cephalic cartilage,
VY. CHPHALOPODA. B91

which has the shape of a ring with wing-like processes. The
otocysts lie in the ventral arch of the ring. Two pits opening
behind the eye are regarded as olfactory, while Nautilus has,
besides osphradia, two pairs of ciliated optic tentacles.

Most noticeable of the circulatory structures is the presence of
two kinds of hearts (fig. 390). The systemic heart consists of two
(four in Nautilus) auricles receiving the blood from the gills, and
a median ventricle from which arise anterior and posterior aorte.
Then there is a branchial heart at the base of each ctenidium
which receives the blood from the vena cava and pumps it into the
gill. Of venze cave there are an anterior unpaired and two pos-
terior paired trunks, the former dividing and sending a branch to

 

Fia. 392.—Male sexual organs of Sepia officinalis. (After Grobben.) b, celomic sac
passing to the left and above into the pericardium; ¢, cwlomic canal to the
vas deferens; d, vas deferens; d’, its opening to celom; /, portions of ccelom; 7,
Needham’s pocket; n’, its mouth; p’, p?, prostates; t, testis; t’, its opening to
ce@lom.

each branchial heart. These trunks are of importance in con-
nexion with the nephridia. The nephridial openings (p. 386) lead
to two spacious sacs through which the veins pass obliquely, this
part of the blood vessels being enclosed by diverticula of the
lumen, covered with epithelial excretory cells. Near its mouth each
nephridial sac communicates by a nephrostome with the usually
large celom (pericardium, gonads, etc.).

In the Octopoda the cwlom is reduced to the gonads and narrow
canals leading from the nephrostome to the gonads and branchial
hearts, but elsewhere there is a well-developed system of connected cavi-
ties (in Nautilus opening by two pores into the mantle cavity), consisting
389 MOLLUSCA.

bo

of the pericardium around the systemic and branchial hearts and the thin-
walled genital sac, one wall of which bears the genital ducts, while on the
other the sexual cells arise or the ducts of a separate sexual gland open
(fig. 392).

The gonads of the always dicecious Cephalopoda are unpaired
and lie far back in the visceral sac. The ducts in the female
Octopoda (rarely in the males) and in some Decapoda (Oigopsida)
are paired. In Nautilus only the right duct is. functional in
either sex, although the left is well developed. Elsewhere there
is only the left duct. The oviducts are saccular with glandular
walls; independently of them two pairs of glands open to the
exterior, the accessory glands and the large nidamental glands.
The vas deferens (fig. 392) is more complicated. It has swellings
known as seminal vesicle, prostate, and Needham’s sac, in which
the spermatophores are stored. These have such a complicated

 
 

i
a

F 1a. 393.—Spermatophore of Sepia. (From Hatschek, after Milne Edwards.) «a, dis-
charging apparatus, b, packet of spermatozoa ; c, envelope.

structure and show such motions when swollen with water that
they were long regarded as parasitic worms (fig. 393).

 

Fic. 394.—Male of Argonauta argo. (After Miiller, from Hatschek.) 1-4, arms of right
e; J.-4, arms of left side; 3, hectocotylised arm, at the left in its sac, at the

s
right protruded.

 

 

The spermatophores are conveyed to the female by means of
more or less modified (hectocotylised) arms of the male. In a few
genera the proper tentacle becomes a ‘ Ilectocotylus’ (fig. 394).

 
V. CHPHALOPODA. 393

It swells at its base to a sac in which the peripheral end is enclosed.
This part contains a canal for the spermatophores, cuts loose from
the male, and can creep about for days in the mantle chamber of
the female. Since it appears as if an independent animal, it was
first described as a parasitic worm under the name /fectocotylus.
Later it was regarded as a rudimentary male cephalopod.

The eggs are either fastened singly to aquatic plants or are laid in large
gelatinous masses. They are rich in yolk, and in consequence undergo par-

 

Fic. 395.—T wo stages of the germinal area of Sepia. (From Balfour, after Kélliker.)

an, anus; br, ctenidia; f, siphon folds; m, mouth; mt, mantle with shell gland;
oe, eye ; p, head lobes; 1-5 arms.

  

Fia. 396.—Embryos of Loligo pealei (orig.). a, arms, ¢, eyes; t, fin ; y, ctenidia ; h, ot-
ocyst; ne mantle ; s, siphonal folds and siphon ; v, anus; y, yolk sac.
tial discoidal segmentation (fig. 103). The blastoderm, on the end of the

oval egg, forms the anlagen of the separate organs (eyes, arms, siphon, and
394 MOLLUSCA.

shell gland) as flattened projections beside each other (fig. 395). Later
the embryonic body becomes distinct from the yolk, which, enclosed in a
cellular envelope, remains attached to the rest, near the mouth, until its
substance is absorbed in the growth of the young and the animal is ready
for hatching (fig. 396).

The Cephalopoda are exclusively marine. Some inhabit rocky shores,
others the high seas. All are carnivorous and in turn are preyed upon by
fishes, etc. Classification is based upon the number of gills and number
and character of the arms.

Order I. Tetrabranchia.

With four gills, four auricles, and four nephridia; numerous tentacles
without suckers, a well-developed chambered shell, siphon of two separate
parapodia, and simple eyes (fig. 384). Only four living species known, all
belonging to the genus Nautilus. The animals, which live in the Malay-
sian regions, are rare, but their shells are abundantly cast up by the sea.
In past time the tetrabranchs were very abundant. The NavrTiuipa,
with straight (Orthoceras) or coiled shells (Goniatites, etc.) flourished in
paleozoic times. They had simple septa. The AMMONITID with folded
septa were largely mesozoic. Since no living forms exist, their pertinence
to the tetrabranchiates is assumed from the character of the shell.

Order II. Dibranchia,

With two nephridia, two gills, and two auricles; eight or ten arms
with suckers ; highly organized eyes ; shell rudimentary or absent.

SAS
y

   

Fig. 397.—Octopus bairdii.* (From Verrill.) A hectocotylised arm on the right side.

Sub Order I. DECAPODA. Ten arms, with lateral fins to the body.
Shell usually present. SprruLip#, with internal chambered loose-coiled
shell. Spirula (fig. 387). O1gorsipa, pelagic, with perforated cornea (p. 385)
V. CHPHALOPODA: SUMMARY OF IMPORTANT FACTS. 395

and two oviducts. Ommastrephes common in New England; A7chi-
teuthis,* the giant squid (p. 384). Myopsipa. Oviduct single (left) ; cornea
unperforated. Loliyo,* common squid ; Rossia*; Sepia, cuttle fish, fur-
nishing the ‘cuttle bone’ once used in medicine, now fed to cage birds,
and the pigment sepia.

Sub Order II. OCTOPODA. Eight arms webbed at their base; shell
very rudimentary, sometimes fragmentary or wanting ; oviducts paired,

 

FIG. 398.—Argonauta argo, paper sailor, female. (After Rymer Jones.)

OcToPpoDIDz. Octopus * (fig. 397), Alloposus,* ARGONAUTIDA, female with
boat-like shell (fig. 398), males much smaller and without shell. Argonauta
argo, paper nautilus. In the Argonautides and PHILONEXID& the hecto-
cotylus separates of itself.

Summary of Important Facts.

1. The MOLLUSCA are parenchymatous animals with re-
duced celom. They consist of head, visceral sac, mantle, and foot.

2. The head bears eyes and tentacles.

3. The foot is an unpaired muscular mass used in Jocomotion.

4. The mantle bounds the mantle cavity which is connected
with respiration; it either functions as a lung or covers the gills
(ctenidia). It secretes the shell from its outer surface.

5. Foot, head, mantle, and with the latter the shell, may be
lost in many groups.

6. The molluscs, without exception, agree in the nervous

system.
396 MOLLUSCA.

7. Three pairs of ganglia with which three pairs of sense
organs are connected almost always occur: a, cerebral ganglia
and eyes; 4, pedal ganglia and otocysts; c, visceral ganglia and
osphradia (olfactory).

8. The heart is dorsal and arterial; it is enclosed in a peri-
cardium (reduced celom) which connects with the nephridia by
nephrostomes.

9. There is always a single ventricle and, according to the
number of respiratory organs, one, two, or four auricles.

10. The alimentary canal is well developed; the liver large;
salivary glands usually present. In most there is a pharynx or
buccal mass with radula and jaws.

11. A veliger stage is common in development.

12. The Mollusca are divided according to the respiratory organs
and appendages of the body into five classes: (1) Amphineura; (2)
Acephala; (3) Scaphopoda; (4) Gasteropoda; (5) Cephalopoda.

13. The AmMpHINEURA have an extremely simple nervous sys-
tem in which the three pairs of typical ganglia are replaced by
nerve tracts.

14. The AcEpuata, or Lamellibranchia, lack head and ceph-
alic appendages.

15. They are bilaterally symmetrical and have paired organs:
mantle folds, bivalve shell, nephridia, and gonads.

16. In many Acephala (Asiphonia) the mantle folds are com-
pletely separated ventrally.

17. In the Siphonata the lower edges of the mantle are united,
leaving three openings: (1) in front for the foot; (2) behind and
below, the branchial siphon for the ingress of water and nourish-
ment; (3) behind and above, the anal or excurrent siphon for the
water used by the gills and the feces.

18. There are two pairs of gills, which may be comb-like (true
ctenidia), filiform, or most commonly lamellar.

19, Correspondingly the heart has two auricles; the aianed
ventricle is usually traversed by the rectum.

20. The foot is a compressed muscular mass frequently pro-
vided with a byssus gland.

21. The shell consists of cuticular, prismatic layer and nacreous
layer. It is closed by two adductors and opened by an elastic
ligament.

22. Some Acephals (Profeconchi) are very primitive in their
gill and hinge structure; others (Heteroconcha) are more highly
developed.
V. CEPHALOPODA: SUMMARY OF IMPORTANT FACTS. 397

23, The ScAPHOPODA are primitive forms with tubular shells.

24. The Gasreropopa (Cephalophora, or snails) have a distinct
head bearing eyes and tentacles; a creeping foot, an unpaired
mantle (occasionally absent), and a univalve shell.

*5. The mantle cavity contains one or less frequently two
ctenidia, or these may be degenerate and a Jung may occur.

26. Nephridia and auricles are rarely paired (with paired gills) ;
the gonads, always unpaired, are hermaphroditic or diwcious.

27. The shell is always unpaired; it is usually coiled ina (right-
hand) spiral, and is frequently closed with an operculum.

28. According to characters derived from nervous system,
sexual organs, heart, and respiratory organs the Gasteropods are
divided into (1) Prosobranchia; (2) Opisthobranchia: and (3)
Pulmonata.

29. The Opisthobranchia are hermaphroditic; orthoneurous;
have gills of various kinds (or none), and have the auricle always
behind the ventricle; shell and mantle reduced or absent.

30. The Pteropoda are pelagic Opisthobranchs with wing-
like processes of the foot and frequently reduced shell or none.

31. The Prosobranchia have the gills (etenidia—occasionally
paired) far in front, and in consequence the auricle in front of
the ventricle; they are streptoneurous and diccious; the mantle
and shell well developed.

32. The Heteropoda are pelagic Prosobranchia with foot
divided into fin and tail, shell rudimentary, or naked.

33. The Pulmonata are in some respects (orthoneurous and
hermaphroditic) Opisthobranch-like; in other respects—as in posi-
tion of heart, development of shell and mantle—like the Proso-
branchs; the mantle cavity functions as a lung.

34, The CepHaLopopa have no true foot; but its homologues
are to found in the siphon and in the tentacles, usually provided
with suckers, on the head; they have an unpaired mantle and a
single shell or none.

35. The unpaired mantle cavity contains one or two pairs of
ctenidia. The water is forced from the mantle cavity through
the siphon.

36. The number of auricles corresponds with the number of
ctenidia; besides the systemic heart there are one or two pairs of
branchial hearts, elsewhere unknown in molluscs.

37. The sexes are separate.

38. The ink sac is peculiar to Cephalopoda.

39. The eye is (usually) highly developed (with retina, choroid,
398 ARTHROPODA.

iris, cornea, vitreous body, and lens), as is the nervous system,
which has, in addition to the usual centres, optic, sympathetic,
and stellate ganglia.

40. The eggs have a discoidal segmentation.

41. The Cephalopoda are divided into Tetrabranchia and
Dibranchia.

42. The Tetrabranchia (extinct save for Nautilus) have four
gills, a chambered shell, primitive eyes, and finger-like cephalic
lobes in place of tentacles.

43. The Dibranchia have two gills, eight or ten tentacles with
suckers, and the shell is reduced or absent.

PHYLUM VII. ARTHROPODA.

Under the term Arthropoda are included the spiders, crabs,
insects, and myriapods, which, together with the annelids, were
united by Cuvier to form his sub-kingdom Articulata. Annelids
and arthropods agree in many features. They are, as the term
articulates implies, segmented animals, and they differ from the
vertebrates, which are also segmented, in the extension of the seg-
mentation, the ringing of the body, to the external surface. The
boundaries between the successive segments, which cannot be rec-
ognized in the skin of the fish or other vertebrate, are marked in
the articulates by a constriction of the body wall, whence the old
names évrouia, Insecta, applied to these forms. The articulates
are further characterized by a ladder-like nervous system in which
the brain, present in most invertebrates, is supplemented by a
ventral chain composed of ganglia metamerically arranged. The
most evident distinctions between the annelids and the arthropods
are (1) the character of the segmentation and (2) the presence of
jointed appendages.

In superficial appearance the lines between the segments are
constricted more deeply in the arthropods than in the annelids.
The cause of this lies in the character of the integument (fig. 25, f),
which is developed as a hard armor, in which two layers are rec-
ognizable, the epidermis (often called hypodermis) and the chitin-
ous layer. The epidermis is a thin cubical or pavement epithelium,
while the chitinous layer is of greater thickness and, since it is
secreted by the epidermis, is stratified parallel to the surface. — Its
firmness is due to the chitin, which is unlike most organic substances
in its resistance to acids and alkalis; only under the action of
sulphuric acid and heat is it broken up into sugar and ammonia.
ARTHROPODA. 399

A firm chitinous armor would render the animal incapable of
motion were there not joints between the parts. While the seg-
ments themselves are heavily armored, the
cuticle between them is reduced to a delicate
articular skin, and this is so protected by :
kind of telescoping of the segments that
injury in these softer regions is nearly im-
possible (fig. 399).

Since the ringing of the body is connected with
this armoring, it disappears with the need for
such protection. The hermit crabs (fig. 480) are
instructive illustrations of this. These animals live Fie. 399.—Diagram of Ar-
with the abdomen inserted in a snail shell. That eae oe or
part of the body which projects from the shell is ane
armored, while the abdomen is soft-skinned and membranes, the mus-

: eh cles indicated by dotted
without traces of external ringing. lines. (After Graber.)

The hardened cuticula causes the periodic molting (ecdysis or exuvia-
tion). When once hardened it is incapable of distension and so would
prevent farther growth. Hence when the body has completely filled the
armor, the latter splits in definite places and the animal crawls out of the
old ‘skin’ (exuvia) and rapidly increases in size while the new cuticula is
yet soft and extensible.

Another result of the cuticula is seen in the peculiar relations of both
ordinary and sense hairs. These are cuticular structures, each usually
secreted by a single epidermal cell and renewed after each molt. Each
hair has a ball-like head situate in a socket in the surrounding chitin, and
hence is movable ; it is traversed by a canal in which is a process of the
underlying matrix cell. In the case of sensory hairs these structures are
connected with a nerve (fig. 77). The sense cell, like a bipolar ganglion
cell, has two processes ; one peripheral, which enters the axis of the hair,
the other central, which runs as a nerve fibre to the central nervous sys-
tem. The cell itself may be in the epithelium or situated deeper and
interpolated as a ganglion cell in the sensory nerve.

 

Another important character is the heteronomous segmentation,
which, in the lowest forms (Peripatus and Myriapods), is little
pronounced, but elsewhere leads to a marked inequality of the
divisions of the body and to a greater centralization of structure.
Different body regions may be distinguished. A few segments at
the anterior end always fuse and form a head (fig. 400, C’); behind
this there is usually a second segment complex, the thorax (7), and
then a third, the abdomen (4). An apparent reduction of regions
can occur when the head and thorax unite (fig. 401, C?) to form
a cephalothorax; or again the number of regions may be increased
(fig. 402) by a division of the abdomen into abdomen proper (A)
400 ARTHROPODA.

 

Fie. 400. Fie. 401.
Fia. 400.—Campodea staphylinus. (From Huxley.) 4, abdomen: C, head; 7, thorax.
Fic. 401.—Palemon serratus. (From Ludwig-Leunis.) 4, abdomen; Ct, cephalo-
thorax.

 

Fia. 402. FG. 403,

Fie. 402.—Androctonus australis. (From Blanchard.) A,abdomen ; Ct, cephalothorax;
P, post-abdomen ; 1, chelicere; 2, pedipalpi ; 3-6, legs.
Fic. 403.—Gammasus coleoptratorum. (From Taschenberg.)
ARTHROPODA. 401

and post-abdomen (P). Finally, in many arthropods (ey., the
mites or acarina, fig. £03) it is impossible to recognize regions or
somites because internal fusion of parts has obliterated the exter-
nal evidences of segmentation.

In order clearly to understand what is meant by head, thorax,
etc., requires a consideration of the second character distinguish-
ing the arthropods from the annelids, the jomted appendages,
which give the name to the former group. The arthropodan
appendages are highly developed parapodia, differing in being
jointed to the body, in consisting of a series of joints themselves,
and in having their intrinsic musculature. As was first pointed
out by Savigny, there is but a pair of appendages to a somite, and
this belongs to the ventral surface. Hence it follows (Savigny’s
law) that if any region shows no external signs of segmentation, but
bears more than one pair of appendages, we conclude that the
region is a complex of at least as many somites as there are pairs
of appendages. Thus the unsegmented head of an insect consists
of four somites, the cephalothorax of a lobster of thirteen, for the
one bears four, the other thirteen, pairs of appendages. Ontogeny
supports this, for in the embryo the somites are clearly visible.*
It is not necessary that each somite in the adult should bear ap-
pendages, since these may disappear in growth without leaving
a trace.

The appendages subserve many functions (fig. 404). Their
primary purpose is locomotion. Locomotor appendages ( pereio-
poda, feet or legs) are long and consist of a number of well-de-
veloped joints which may form flattened oars or may be provided
with claws for creeping (8). Besides locomotor appendages there
are tactile appendages or antenne (1), chewing appendages (jaws,
mandibles, maxilla, 2-4), false feet or pleopoda (9) of varying
functions, and forms—maxillipeds (5—7)—transitional between jaws
and legs.

Aside from their tactile function, antenns are characterized
by position and innervation. They ara always placed in front
of the mouth and receive their nerve supply from the supra-
@sophageal ganglion, while all other appendages are innervated
from the ventral chain.

The form of the jaws is strikingly modified. One or two
basal joints serve for the comminution of food, and these parts

* This statement is not exactly correct, for in certain insects and in the
lobster there is one somite which is entirely lost in the adult.
402 ARTHROPODA.

are strong and are covered especially on the medial side with a hard,
toothed chitin (figs. 404, 2; 410, 7/, V;
507). The other joints may entirely
disappear. When they persist they form
a more or less leg-like appendage, the
palpus. Since several appendages may
be modified into jaws, the first are called
mandibles, the next maxille, and second
maxilla may follow. The maxillipeds
may have more the appearance of jaws,
at other times are more Jeg-like (fig. 404,
5-7).

The false feet (pleopoda) are small
and inconspicuous appendages which may
have various functions: they may serve
as gills or supports for the gills, as places
for the attachment of eggs, as organs for
the transfer of sperm, or as swimming or
creeping organs.

These appendages have constant po-
Vira) Wie Rig paeidapuetot the sitions in the body. First on the head

crayfish. 1, first antenne ;~ come the antenne and then, in the region

mandible; 3. 4, first and :

second maxille; 4, 6,7,maxil- of the mouth, the jaws and, so far as

lipeds:; $8, walking leg; 9, ? ae ‘

pleopod. they are present, the maxillipeds. Third
come the true feet, and lastly, when they exist, the false feet.
Those somites which bear antennex or jaws belong to the head, those
bearing walking feet to the thorax, while the somites of the abdo-
men bear either false feet or lack appendages. As a sequence the
cephalothorax is that region of the body which bears, besides an-
tenne and jaws, legs as well.

 

The extremities of Arthropoda have given rise to various disputes.
Many zoologists speak of a pre-antennal somite and a pre-antennal ap-
pendage, referring to the eye stalk of a part of the Crustacea, which, how-
ever, differs markedly in its development from the true appendages.
Those who accept an ocular somite must add one to the number of somites
as stated in this volume. A second theory regards the antenne as ventral
appendages innervated from the ventral chain which secondarily become
dorsal and receive their nerves from the brain. This view is firmly
grounded for the second antennew of the Crustacea. Other questions are
as to the possible loss of segments and appendages.

The concentration or fusion of somites to body regions has had
an influence upon the internal structure and especially upon the
nervous system (fig. 405). A ladder-like nervous system consists,
ARTHROPODA. 403

as was pointed out (p. 124), of a dorsal brain (supracesophageal
ganglia) and a ventral chain of ganglia, all connected by longi-
tudinal nerve cords, the brain being connected with the rest by
cords or commissures passing on either side of the wsophagus.
The ventral chain should contain as many pairs of ganglia as there
are somites, but this is not the case except in the embryo. The
tendency is rather towards a fusion of ganglia, especially of those
somites which unite or fuse. This fusion of ganglia occurs to a

 

 

 

Fic. 405.—Different degrees of concentration of the ventral cord of Arthropods.
(From Gegenbaur.) A, Termite (after Lespés) ; B, water beetle (after Blanchard);
C, fly (after Blanchard); D, Thelyphonid (after Blanchard). a, abdomen; g’, g3,
ganglia of ventral cord; gi, infracesophageal ganglion; gs, supracesophageal
ganglion ; 0, eye; p’-p/’, walking feet ; tr, lung books; 1, chelicere ; 2, pedipalpus.

yarying extent in different species, the extreme being reached in
the spiders and crabs (fig. 441), where the whole ventral chain
forms a large ganglionic mass. In all cases, however, the brain
remains distinct from the rest, its position dorsal to the esophagus
precluding its fusion with the ventral chain.

Of the sense organs the best known are the eyes, of which two
types are recognized, the simple (ocellus, stemma) and the com-
pound (faceted). The ocelli are very small. In their highest
development, as in spiders (fig. 406), they are composed of lens,
vitreous body, and retina. The lens is formed by the cuticula, the
rest from the epidermis. The lens differs from the rest of the
cuticle in being transparent, and is usually thickened to a biconcave
body (7) which converges the light upon the retina. Behind the
lens comes a layer of transparent cells, the vitreous body (2), and
404 ARTHROPODA.

behind this, in turn, the retina, consisting of cells which, at the
one end, bear ‘rods’ (4 and 7), at the other pass into nerve fibres.
The retina and vitreous body, surrounded by pigment, form a

IETS

NS
mY
LAS

 

4 & 6 7
Fie. 406.—Diagrammatic section through anterior (A) and posterior (B) eyes of Epeira

diademata. (After Grenacher.) The hinder eye shows the inverted retina; 1,

lens; 2, vitreous body ; 3, epidermis, outside this, chitinous layer; 4, rhabdomes;

6, retinal cells; 6, capsule of eye; 7, rhabdomes of inverted eye.
spherical thickening sharply marked off from the rest of the epi-
thelium. These eyes, like those of vertebrates, must form inverted
images.

In many spider eyes there is an inversion recalling that of the verte-
brates (fig. 406, B), the rhabdome lying behind the nuclear portion of the
cell, Behind the rhabdomes comes a layer of strongly iridescent cells, the
tapetum lucidum.

Hh NS

Y

HF
ROA
AE
Sy a f

Fia. 407.—Head of drone bee_ (After Swammerdam, from Hatschek.) Showing the
large faceted eyes and between them three ocelli.

The compound eyes are much larger. They owe their name
‘faceted eyes’ to the fact that the cuticle over them is divided
into polygonal (usually hexagonal) areas or facets (fig. 407). Each

 
ARTHROPODA.

405

facet corresponds to a small chitinous lens (the number of which
varies, in different species, between a dozen and several thousand),

 

1238 4 6

Fic. 408.—Section of compound eye of Forficula.

(After Carriére, from Hatschek.)

1, cuticula, producing the cornea of many lenses over the eye; 2, epidermis,
which in the eye forms the ommatidia; 3, basal membrane; 4, reentrant chitin-

ous fold ( sclerotic’); 4, rudimentary larval eye.

and bounds the eye externally, whence this layer is called the

cornea (fig. 408). The part of the eye be-
neath the cornea consists of radially arranged
prismatic parts or ommatidia which corre-
spond in number and position to the facets,
their broader ends being placed beneath the
facets, their narrower internal ends con-
necting with fibres of the optic nerve which
Each ommatidium (fig.
409) has essentially the structure of an
ocellus: (1) the lens (/) with its epi-
thelium; (2) the vitreous body (dz); (3) the
retinula (7z). The vitreous body is usually
composed of four cells which in the so-called
euconous eyes surround a transparent body,
the crystalline cone (/:), secreted by these cells.
The retinular cells are almost always seven in
number, each bearing on its inner surface a
rhabdome (7), the seven rhabdomes frequently
fusing into a common mass. Each omma-
tidium is surrounded by a pigment sheath,
isolating it optically from its fellows.

From this it appears that the compound
eye may be regarded as a complex of ocelli.

go to the brain.

 

Fira. 409.—A single om-
matidium (with sec-
tions) of a compound
eye. k, crystalline
cone; kz, cone cells; 1,
lens with hypodermis;
r. rhabdomes; re-
tinular cell.

Wes

This anatomical conception must not, however, obscure the physio-
406 ARTHROPCDA.

logical. As Johannes Miller first pointed out, the whole compound
eye forms but a single erect picture composed of separate images
of small area formed by the separate ommatidia. This ‘mosaic
theory’ has completely replaced the view that each ommatidium
formed a complete inverted picture.

While the number of ocelli varies, the compound eyes are almost
always two in number. Where, apparently, as in the Daphnida,
there is but one, there is in reality a fusion. There is also con-
stantly present a large optic ganglion where the optic nerve enters,
but lying outside the eye itself.

The tactile organs, consisting of tactile hairs (fig. 77), are uni-
form in structure. On the other hand the senses of hearing, taste,
and smell are subserved by varying organs. It is to be regretted
that we know but little of these senses in arthropods, although
beyond question they are frequently well developed. The sense of
smell resides chiefly in the antenne and in the palpi of the jaws.
The organs are olfactory cones (modified hairs) which frequently
lie in pits in the skin. Similar organs in the mouth are probably
connected with taste. As organs of hearing (? equilibration) besides
the otocysts of the Podophthalmata and the tympanal organs of
the Orthoptera, the widely distributed ‘ chordotonal’ nerve ends
of insects are to be mentioned,

Concerning the alimentary canal it need only be said that the
larger proportion of it is formed of ectodermal stomodeum and
proctodeum, while the entodermal portion (mesenteron) forms on
an average but one third of the total length. At ecdysis the chitin-
ous lining of these parts, including the large chewing stomach, is
cast with the rest of the integument. The entire absence of cili-
ated epithelium is noteworthy. Ciliated cells have never been
found in arthropods.

The most constant portion of the circulatory system is the heart,
which usually lies immediately beneath the back and is enclosed
in a more or less distinct sac which, although called pericardium,
is not a part of the celom. From the pericardium blood passes
into the heart by openings right and left, the ostia. Since the
margins of the ostia project far into the lumen of the heart and so
form folds functioning as valves, the heart itself may be divided
into a series of chambers, especially distinctly separated from each
other by the progressive contraction of the wall (fig. 66). The
chambers disappear when, with reduction of the body, the heart
shrinks to a sac. In small arthropods the heart together with
the whole vascular system may be lost. Since the Annelida have
ARTHROPODA. 407

a well-developed circulatory system, this loss in these animals
must be regarded as secondary rather than as primitive, and is
explained by the fact that with reduction in size the organization
is simplified.

The blood may pass from the large arteries either directly into
the large blood sinuses of the body, erroneously called the body
cavity, or by a more complicated course through capillaries and
veins as wellas through the respiratory organs. There is, on this
account, the greatest difference in the development of the vascular
system, but even in the highest forms the system is not entirely
closed, the blood passing to the sinuses of the body (hemoccele,
p. 110) and thence to the pericardium (probably arising from the
coalescence of veins and certainly not cwlomic), from which it is
sucked through the ostia into the heart.

The variations in the circulation depend upon the modifica-
tions of the respiratory organs, which can be described adequately
only in connexion with the various groups. In general it can
only be said that the more respiration is localized in regions and
organs the more nearly complete is the circulation, while with
respiration diffused over or through the whole body, the vascular
system, including even the heart, may be reduced.

The various spaces in the body are frequently encroached upon
by a fat body, a kind of connective tissue whose cells, richly laden
with fat, serve as a store of nourishment for the animal. Besides,
urinary products, like uric acid, have been found in it, leading to
the conclusion that the fat body acts as a reservoir for excretory
substances before their elimination by the excretory organs. These
latter vary greatly in the different groups: true nephridia in Peri-
putus, shell glands and antennal (green) glands in the crustacea,
and tubules (Malpighian tubules) connected with the intestine in
arachnids and insects.

The sexual organs, which empty through ducts which are
apparently modified nephridia, are only rarely hermaphroditic.
In the bisexual species one can usually distinguish males and
females by external characters, such as coloration, size or form of
appendages, especially those used in copulation. The eggs are
usually large and rich in yolk, and consequently but rarely undergo
total segmentation. In most eggs occurs that type of partial
segmentation called superficial (fig. 104).. While the surface of
the egg divides into the cells which form the blastoderm, the
central yolk long remains undivided—a condition of systematic
interest since it is not known to occur outside the Arthropoda.
408 ARTHROPODA.

The cases of discoidal and unequal segmentation are upparently
derived from the superficial.

In accordance with their high organization, reproduction by
fission or budding never occurs, but parthenogenesis and padogen-
esis do. In some parthenogenesis has a certain relationship to
the life history. In lower crustacea and in Aphides (plant lice)
it allows the species to spread rapidly in large numbers over suit-
able feeding grounds. Among the bees parthenogenesis has a
relation to the sexes, since males are only produced from unfer-
tilized eggs. Along with parthenogenesi
ceptions—sexual reproduction occurs, so that not rarely asexual
alternates with sexual generation (heterogony), though not in such
a pronounced manner as in the worms.

   

s—there may be rare ex-

The French entomologist Latreille divided the Arthropods into four
classes: Crustacea, Myriapoda, Arachnida, and Insecta. Later the dis-
covery, by Moseley, that Peripatus possesses trache led to the creation of
anew class, Protracheata, and the grouping of all arthropods into branchi-
ate and tracheate divisions, the branchiates including the crustacea alone.
Later researches have shown that these divisions are not natural and that
tracheze have had different origins, the spiders being nearer to the crus-
tacea than to the insects, and that crustacea and insecta have come from
the annelids through different lines. Similarly the myriapods have been
divided, one group, the chilopods, being closely related to the true insects,
the other (diplopods) being very uncertain in position,

Class I. Crustacea.

The Crustacea owe their name to the fact that their chitinous
cuticle is usually rendered hard and firm by deposits of carbonate
and phosphate of lime and, in contrast to that of other arthropods,
has lost much of its elasticity and has become ‘crusty.’ Another
important characteristic is the habitat of the group; the Crustacea
are typically aquatic and hence breathe by means of gills. This
branchial respiration persists, as in the case of crayfish, when the
animals are taken from the water, for they retain water in the gill
chamber and hence for a long time the gills are wet by this fluid.
There are but few exceptions to this rule, as some land crabs and
the sow bugs: these breathe air, either by means of the gills or by
special structures in the gill chamber to be mentioned later.

The branchiaw or gills are always placed where a rapid exchange
of water is possible. The appendages afford such a position, and
hence one finds the gills as thin-skinned vascular plumes or plates
(figs. 61, 437) either on the appendages or on the body near by,
or the whole appendage may take a leaf-like, thin-skinned shape
I. CRUSTACEA. 409

and thus serve as a gill (figs. 411, 451). Besides, the whole body
surface may be respiratory and in small forms may entirely replace
that of the gills, so that these organs become rudimentary or may
entirely disappear, there being a diffuse respiration with cor-
responding results in the circulatory system. With a localized
respiration heart, arteries, capillaries, and veins are well developed,
but with the diffuse respiration only the heart persists as a reduced
structure, or with its disappearance the last traces of a circulatory
system are lost.

Locomotion as well as respiration is related to the aquatic life,
and these animals usually possess a special form of appendage of
the biramous or schizopodal type, which at once differentiates these
forms from other arthropods. While in the latter, as every insect
shows, the joints of the limb follow in a single sequence, the crus-
tacean appendage has a two-jointed base (basiopodite), followed

  

en
Fig. 410.—Copepod appendages. I-IV, Diaptomus castor; I, a pair of schizopodal feet;
J/, second right antenna; JZ/, right mandible; ZV, right maxilla; V, right mandi-
ble of Cyclops coronatus, 1, 2, joints of basiopodite; 7, endopodite; a, exopodite.
by two many-jointed branches (fig. 410, /), an inner or endopodite
and an outer or exopodite.

The schizopodal appendage occurs only when the limb is used for
swimming; when it is used for walking upon the bottom, as in crayfish and
crabs, the exopodite is lost and only the endopodite persists as the func-
tional limb, which then closely resembles the appendages in the so-called
tracheates. This loss rarely occurs on all the appendages; usually the
abdominal feet and the mouth-parts retain the two-branched condition,
Embryology further shows that even in the case of the crabs all the feet are
at first schizopodal and that the walking legs lose the exopodite during
growth. There is some evidence to show that the schizepodal foot is not
the primitive type. This is furnished by the phyllopod foot (fig. 411, 77),
410 ARTHROPODA.

and consists of a medial axis, b, bearing on the inside six ‘ endites,’ 7, and
on the outer side two ‘exites,’ a flabellum or epipodite, a, and a gill, h.

 

Fia. 411.—Branchiopod appendages. 1 and JI first and sixth legs of Branchipus grubet
(after Gersticker); J1/, fourth leg of Daphnia simus (after Claus). a, flabellum;
h, basis; i, axon and its endites; k, gills.

This becomes modified into the schizopodal form by a loss of the four

basal endites (those nearest 6) and the development of the two terminal

endites into exopodite and endopodite. Still the schizopodal condition is

go nearly universal among Crustacea that it must be accorded great weight

in classification.

The appendages furnish a further diagnostic character in that
two pairs of antenne are present in the crustacea (see, however,
Trilobite). Antenne, it will be remembered, are preoral append-
ages innervated from the brain. In some cases, as many Ento-
mostraca, the second pair may lose their sensory functions and
become mere swimming organs.

A carapace, recalling the mantle of the molluscs, is widely dis-
tributed in the crustacea. It arises as a fold from the head, which
IL CRUSTACEA. 411

may extend backwards as a shield, completely covering some or all
of the thoracic segments (fig. 412), or it extends right and left on
the sides of the body (fig. 426) and produces two valves strikingly
like those of a lamellibranch, the resemblance being strengthened
in the cirripeds and ostracodes by the extensive calcification.
Concerning the internal organs but few general remarks can
be made. Salivary glands are wholly absent; on the other hand
the stomodeum is usually widened into a strong chewing ‘ stomach,’
and behind this empty the ducts of the so-called liver (better

 

Fie. 412. Fie. 413.

Fre. 412.—Apus cancriformis, (After Ludwig-Leunis.) The segments mostly covered
by the carapace.

Fig. 413.—Antennal gland of Mysis. (After Grobben.) Clr, blood lacune; ea, external
opening; h, bladder; rc, canal; s, internal vesicle.

hepato-pancreas). The liver itself differs widely from the two
simple blind sacs of the Daphnide (fig. 420) to the enormous
livers of the Decapoda (fig. 439, A). Excretory organs are repre-
sented by so-called green glands (antennal glands) and shell glands.
The latter, which received their name from the erroneous idea
that they produced the shell, open to the exterior on either side at
the bases of the fourth appendages, the maxille (fig. 420, s). The
green gland opens similarly on the basis of the second antenne.
Both have essentially the same structure (fig. 413); they begin with
a terminal vesicle (in the case of the antennal gland in close rela-
tions with the reduced celom), which passes into a slender, greatly
coiled tube. Their structure and development lead to the con-
clusion that they are modified segmental organs. Both occur
412 ARTHROPODA.

pressed. In some amphipods there are excretory diverticula
developed from the intestine (fig. 448), which resemble the Mal-
pighian tubes of insects, but differ from them in being of ento-
dermal origin. In some decapods cxca occur in the same region,
but nothing is known of their function.

Visual organs are either represented by the so-called nauplius
eye, consisting of a pigment spot with three lenses situated directly
on the brain, or by a pair of compound eyes. The nauplius eyes
are chiefly found in the lower, the compound in the higher, groups;
occasionally they coexist in the same species. Auditory (equili-
bration) organs (otocysts) occur only in the Malacostraca, either
in the base of the first antenne or in the endopodite of the last
abdominal feet (fig. 434, 0). These are rarely vesicular, but are

together only in the larva; in the adult one or the other is sup-

 

Fia. 414.—Otocyst of crayfish. as, auditory ridge; n, nerve.

usually grooves (fig. 414), bearing at the base a row of chitinous
sense hairs, the crista acustica, connected below with an auditory
nerve, while their free ends extend between a cluster of otoliths.

At ecdysis these otocysts with their sensory hairs and otoliths are
cast off. Ifa erayfish which has just molted be placed in perfectly clean
water, the otoeyst will remain without otoliths; but if some easily recog-
nizable substance, like urie acid crystals, be placed in the water, some of
these will soon be found in the sac, thus proving that the otoliths are
introduced from the outside.

Crustacea are only exceptionally hermaphroditic. The sperma-
tozoa are noticeable for their great size, in many ostracodes equal-
ling the body in length. Except in the Cirripedia the spermatozoa
lack a flagellum and are immobile. Their round or elongate body
is covered with rigid processes reminding one of the pseudopodia
of Actinospherium (fig. 36, y, 6). They are frequently enclosed
in spermatophores (fig. 422).

The typical development of a crustacean includes a metamor-
phosis, and where direct development occurs the metamorphosis is
I. CRUSTACEA. 413

either suppressed or, as is easily shown, the corresponding stages
are passed in the egg. Two of the larval stages are especially im-
portant, the nauplius and the zoea. The nauplius (figs. 7, 429)
consists of three segments covered by a dorsal shield and bearing
below three pairs of appendages. The first pair, developing later
to the first antenne, are simple; the others, corresponding to the

 

Fic. 415.—Zoea of Carcinus menas. (After Faxon.) h, heart; 7, intestine; I-VI,
cephalic appendages.

second antenne and mandibles, are schizopodal. Internally there
is a three-chambered alimentary tract, a supracesophageal ganglion
on which is an unpaired eye, and a ventral chain. The nauplius
is almost universal among the lower crustacea, and some writers
believe that it represents an ancestral form from which the crus-
tacea have descended, a view open to much objection.

The zoea is more complex. It consists (fig. 415) of cephalo-
thorax and abdomen, the latter without appendages, the former
414 ARTHROPODA.

with several pairs of schizopodal swimming feet. There are two
large compound eyes and, dorsal to the intestine, a heart. Fre-
quently the carapace is armed with very long spines projecting
from front, back, and sides, which are intended as protection from
enemies.

Nauplius and zoea are of systematic importance, since they
rarely both appear in the life cycle of one individual. The nau-
plius is characteristic of the lower crustacea—the ‘ Entomostraca.’
The zoea, on the other hand, has never been noticed in the Ento-
mostraca, but occurs in many Malacostraca. A nauplius appears
in only a few Malacostraca, like the schizopods and Peneus, and
there precedes the zoea stage. It must not be forgotten that
many forms among both Entomostraca and Malacostraca have no
zoeal or nauplius stage.

Frequently the lower crustacea are united under the name
Entomostraca, but, aside from the nauplius stage and the posses-
sion of a shell gland, the only characters of the group are negative.

Sub Class I. Trilobite.

The most important fossils of the class of crustacea are the
Trilobites which appeared in the Cambrian and died out in the
Permian, being extremely abundant in the Silurian. The body
(fig. 416) consists of head and trunk, the latter segmented. In
the young the segments are very few, but increase in number with
age (10-29, according to the species). The hinder segments fre-
quently differ from the rest and form an abdomen or pygidium.
Dorsally the animal is divided by two grooves into three lobes,
marking off in the head a glabella and two gene; in the trunk
rhachis and two pleure. On the head there are usually a pair of
compound eyes, which in the young were frequently ventral, but
are brought to the dorsal surface with growth. For many years
little was known of the under surface, but lately specimens of
Triarthrus becki (fig. 417) from the Utica slate have revealed the
appendages. On the head are a pair of simple antenne, and four
pairs of schizopodal feet, the bases of which acted as jaws. It is
a question whether the first pair of juw feet correspond to the sec-
ond antenne or whether these have been lost in the group. The
trunk segments bear biramous feet.

In some respects the trilobites resemble the Xiphosura (infra),
but the possession of antennxy and biramons feet place them among
the crustacea. Here their position is very uncertain. We have
I. CRUSTACEA: PHYLLOPODA. 415

little knowledge of but one species, and this with its single pair
of antenne differs from all recent crustacea.

    

y oS ee ay
Fia. 416. Fig. 417.

Fria. 416.— Paradoxides bohemicus. (From Zittel.)

Fia. 417.—Triarthrus becki, ventral surface, restored. (After Beecher.) The head
bears one pair of antenne and four pairs of biramous feet, the basal joints serv-
ing as maxille. Trunk with biramous feet.

Sub Class II. Phyllopoda.

The Phyllopoda are clearly the most primitive of crustacea.
The name is derived from the leaf-like feet (p. 410), which occur
upon the thoracic region. More anteriorly the appendages are
schizopodal, the second pair of antenne often being efficient swim-
ming organs. The number of body segments varies between very
wide limits, there being less than a dozen in the Cladocera, while,
if Savigny’s law (p. 401) holds true, there are over sixty in some
Apodide. In most forms (the Branchipodide excepted) a cara-
pace is developed by a backward growth from the head. This
forms a broad oval shell covering most of the body in the Apodide
(fig. 412); in the Estheriide and Cladocera it is divided into right
and left halves hinged together in the mid-dorsal line, thus giving
these animals the appearance of bivalve molluscs.

These forms have, besides the unpaired nauplius eye, a pair of
compound eyes which in the compressed forms are frequently
fused, although distinct in the young and retaining the double
416 ARTHROPODA.

optic nerve throughout life. The liver is present in the shape of
simple ceca; the heart, elongate, chambered, and with many ostia
in the Branchiopoda, a short sac with only a pair of ostia in the
Cladocera (fig. 420, 7), lies dorsal to the intestine. The shell
gland is well developed.

In development summer and winter eggs are distinguished. The sum-
mer eggs form a single polar globule and develop parthenogenetically.
The winter eggs form two polar globules and require fertilization. The
thin-shelled summer eggs are carried about by the mother in a brood
pouch and hatch in a relatively short time. The thick-shelled winter eggs
are cast off and fall to the bottom, where they require a long time for
development. They may be dried or frozen without injury, and at least in
some cases drying is necessary to their development. This feature explains
the appearance in early spring of large numbers of Branchipus and
istheria in snow pools which are dry throughout the summer. On our
western plains and in Europe Apus occurs in the same way. These pecu-
liarities of reproduction are readily understood when we recollect that the
phyllopods are largely inhabitants of fresh water. The winter eggs pre-
serve the species through times of drought and cold; the summer eggs are
for the rapid increase of the species during the wet season. The same
relations also explain the fact that males are rare and only appear at
intervals, indeed are not known in many species.

Order I. Branchiopoda.

The Branchiopoda are relatively large with numerous segments, leaf-
like appendages, long, chambered heart, and lack swimming antenne.
With few exceptions they are inhabitants of
fresh water. According to the development of the
carapace they are subdivided into three families.

1. APpopip%. Body depressed, with large oval
undivided carapace. Eggs carried in brood cap-
sules formed by a pair of appendages. Apus
(fig. 412) and Lepidurus occur in Europe, Green-
land, and the United States west of the Missouri.
Protocaris of the Cambrian rocks is apparently
an Apodid.

2. BRANCHIPIDA. Body without carapace, the
second autenne of the male large and modified
into an organ forclasping the female. The female
carries the summer eggs in a wide ‘uterus’ in
the abdomen. Branchipus lives in fresh water,
Artemia in brine, and in certain species one has
Fie. 41% —Apus equatise been transformed into the other by changing the

(After Packard.) water from fresh to salt or the reverse. Branchi-
pus vernalis (fig. 419) occurs in snow-water pools in our northern states,
Artemia in Great Salt Lake, around salt works or in tubs of brine near
the ocean,

 
I. CRUSTACEA: COPHPODA. 417

3. ESTHERUD. Body lateraily compressed and enclosed in a bivalve
shell, compound eyes fused; male very rare. Zstheria,* Limnadia,* in
fresh water,

Order II. Cladocera.

Like the estheriids the small Cladocera have the body enclosed in a
bivalve carapace, which, in some instances, is small and reaches back only
over the first trunk segments, in others is large, enclosing the body, with a
notch for the protrusion of the head, while behind it terminates in a sharp
spine. The head bears a pair of large swimming antenne and a much
smaller first pair bearing olfactory bristles and, in the male, hooks for

 

Fic. 419.—Branchipus vernalis,* fairy shrimp. (After Packard.)

clasping the female. The body consists of few segments, the heart is a
simple sac, and the fused faceted eyes, with paired optic nerves, are capa-
ble of motion in a special optic capsule.

The young eggs in the sexual organs always occur in groups of four
(fig. 420). Of these but one grows into an egg, the others breaking down
and serving this as nourishment. Larger eggs with more yolk occur when
several of these groups (2-12 fuse to form a single egg. The summer
eggs arise from a single group, the winter eggs from several groups of
primordial ova.

In all Cladocera the space between the back of the animal and the
shell serves as a brood pouch. In some cases this pouch contains an
albuminous fluid secreted by the mother, which nourishes the embryo.
The larger winter eggs—one or two in number—frequently remain for
awhile in the brood chamber and are there enveloped in a peculiar shell,
the ephippium, consisting of two chitinous plates, like watch crystals, their
edges closely appressed.

Darunip#. Shell well developed; Daphnia* (fig. 420), Bosmina.*
POLYPHEMID#. Shell small, only functioning as a brood case; head with
an enormous eye and large swimming antenna; no phyllopodous feet;
marine and lacustrine. Leptodora hyalina * appears at night, sometimes
in great numbers, in some of our lakes. Hvadne,* marine.

Sub Class III. Copepoda.

A general description of the copepods can only apply to the
non-parasitic forms, since many of the parasites have become so
418 ARTHROPODA.

 

Fre. 420.—Daphnia pulex. b, brood chambers with embryos; q, brain with nauphus
eye; go, optic ganglion; h, heart; 0, ovary; s, shell gland. The eggs arise at k, and
separate, forming in groups of four, as at ¢, of Which one becomes the egg, while
the others abort (0) and form food, ‘The egg then passes to the brood chamber.
1, 2, first and second antenne; 3, mandible (maxilla rudimentary and invisible);
5-9, legs.
I. CRUSTACEA: COPEPOD.\. 419

degenerate (figs. 6, 425) as to be recognized even as arthropods
only by a knowledge of the development. The sixteen somites of
the body are nearly equally divided among the three regions—

 

Fig. 421.—Evadue (orig.), showing the brood pouch filled with eggs and young. a?,
second antenna, av, adhesive organ; b, brain; f, furca; h, heart; 7, intestine; J,
liver; s, shell gland.

 

'

Fria. 422.—Diaptomus castor, hb, ventral nerve cord; g, brain with nauplius eye; Ju
heart, beneath it the ovary and digestive tract; sp, spermatophores; J, 2, first and
second antenne; 3, mandibles; 4, maxille; 5, maxilliped; 6-10, swimming feet.

head (6), thorax (5), and abdomen (5)—ofthe animal. (In Cyclops
the first thoracic segment is fused with the head, the first two
abdominal segments are fused—fig. 7.) The terminal abdominal
420 ARTHROPODA,

segment is two-forked, forming the ‘furca.? While the abdomen
lacks appendages, the thorax bears typical biramous appendages,
consisting of a two-jointed basiopodite, the basal joint being fre-
quently united with its fellow of the pair for common motion (fig.
410, Z). Exopodite and endopodite, usually three-jointed, are
fringed with bristles. Usually the fifth pair of thoracic append-
ages are not so well developed, and in some cases are represented
by two bunches of bristles.

The two pairs of antenne are frequently similar in size (whence the old
name Cyclops quadricornis). The first pair are always uniserial and in the
males may be hooked near the base for clasping; the second are some-
times biramous (fig. 410, 77). The mandible (fig. 410, 777, V) is instrue-
tive, since a study of several species shows that it is derived from a schiz-
opodal condition and that the first basal joint alone is used for chewing,
the rest being reduced to a palpus of varying development. Both basal
joints of the maxille (fig. 410, ZY) can be used in eating. Two maxilli-
peds (formerly regarded as the separated branches of an appendage) mark
the termination of the head (fig. 422, 5); both are hooked for holding the
food.

The internal anatomy is simple. There is no liver, and the
straight intestine (fig. 422) runs without marked changes in size
to the anus between the branches of the furca. The visual organ
is the unpaired nauplius eye (which has given the name to one
genus, Cyclops). It lies directly on the brain. The ventral chain
has its ganglia irregularly distributed. Gills are always absent, as
are usually the heart and blood-vessels. Only ina few parasitic
forms are there tubes which have been interpreted as parts of a vas-
cular system; in some free forms there is a short saccular slowly
pulsating heart. The gonads are unpaired in both sexes, but the
sexual ducts, which open at the base of the abdomen, are paired.
The females possess a receptaculum seminis distinct from the ovi-
ducts, to which the male attaches the spermatophores packed with
sperm (fig. 422, sp). As the eggs leave the oviduct they are fer-
tilized by the sperm issuing from the spermatophores, and num-
bers are enclosed ina gelatinous substance, thus producing bundles
of eggs, the so-called egg-sacs, attached to the abdomen, by which
one can easily recognize the females (fig. 7). A nauplius hatches
from the egg, and by budding segments and appendages at the
hinder end, and by a change of the nauplius appendages into
antenne and mandibles, passes through a ‘ cyclops-stage’ into the
adult.

The Copepoda have clearly descended from some phyllopod-
like form. The poorly developed ventral chain, the loss, partial
I. CRUSTACEA: COPEPODA. 421

or complete, of a circulatory system, and the absence of gills are
all evidences against the view which would consider them primitive.

Order I. Eucopepoda.

The forms to which the foregoing description will apply are the Euco-
pepoda, and include many species, which often occur in enormous numbers
in both fresh and salt water, forming the larger proportion of the plank-
ton. They thus furnish the most important food supply not only for fishes

 

Fia. 423. Fig. 424,

Fia. 423.—Female Lerneocera esocina, (From Lang, after Claus.) A, armlike proc-
esses of anterior end; d, digestive tract; es, egg-sacs; od, oviduct; ti-t,, rudi-
mentary thoracic appendages.

Fia. 424.—Argulus foliaceus. (From Ludwig-Leunis.) a, sting; a’, antenna: bh,
mouth; c, intestine with liver; d, abdomen; pm}, pm?, first and second maxilli-
peds; p!-p‘, biramous feet of thorax.

but for those giants among mammals, the baleen whales. Cetochilus sep-
tentrionalis occurs at times in such myriads that the sea for long distances
is colored red.

The CycLopip#, with no heart and paired egg sacs, are fresh-water
forms; Cyclops * (fig. 7). CALANID#, fresh water and marine; heart pres-
ent, single egg-sac. Diaptomus,* fresh water (fig. 422); Cetochilus,* Pon-
tilla,* marine. HARPACTIDA, creeping forms, mostly marine; Cantho-
camptus,* fresh water. The CorycaIpm, which are half parasitic aud
include the wonderfully iridescent Sapphirina * (upon pelagic animals, as
422 ARTHROPODA.

Salpa), and the NoTODELPHID®, parasitic in the gills of ascidians, form a
transition to the next order.

Order II. Siphonostomata (Parasita.)

There are also Copepoda to which the account in large type will not
apply, animals of such strange appearance that many of them were long
regarded as parasitic worms (figs. 6, 428, 425). Their mandibles are
altered to piercing bristles and enclosed in a piercing proboscis formed of
upper and lower lips. With this sucking organ they bore into the skin or
gills of fishes. They have cylindrical forms or bodies of the most bizarre
shapes, in which frequently no segmentation is visible, while the append-
ages are rudimentary or even entirely lost. Indeed one would not recog-
nize them as arthropods save for the following features :

(1) Most of them have the typical Copepod egg-sacs (sometimes elongate
and spirally coiled) attached to the hinder end. (2) In the course of years
a complete series of intermediate forms has been found, allowing one to
trace, step by step, the alterations of form from that of the free-living
species to that of the most modified parasites. (3) Ontogeny is convincing.
Most parasitic Copepoda leave the egg as a nauplius and pass through a
Cyclops-stage before attaching themselves to fishes and becoming the highly
degenerate parasites. These parasites are always females. The males
scarcely pass the Cyclops-stage, copulate with the females
and then die, or if they pass through the metamorpho-
sis, they remain small and different in appearance.
They occur attached to the female near the genital
openings. There is thus here a marked sexual dimor-
phism,

The ARGULIDZ (sometimes, but without warrant,
made a distinct sub order, Branchiura) are fresh-water
forms with compound eyes, liver lobes, and the second
maxillipeds metamorphosed into suckers. Argulus*
Fic. 425.—Lernea (fig. 424) causes considerable mortality to fish. The

branchialistorig.) marine and brackish-water CALIGIDE (Caligus *) have
similar habits. LERN#OPODIDaA. Fish parasites with maxille united into
an adhesive organ. Achtheres * (fig. 6), parasitic on perch. LERNEZIDE;
worm-like parasites. Lernea branchialis,* common on gills of cod;
Lerneocera* (fig. 423), on pike ; Penella.*

 

Sub Class 1V. Ostracoda.

Like the Cladocera and the Estheriidw the Ostracoda are en-
closed in a bivalve shell, which, when closed, includes not only
the body but the head and appendages as well, these being pro-
truded when the shell is opened. The valves are closed by an
adductor muscle, opened by a hinge ligament like that of lamel-
libranchs. This resemblance to the molluscs is heightened by
lines of growth upon the shell.
I. CRUSTACEA: CIRRIPEDIA. 423

The antenne, the first simple, the second frequently two-
branched, are used for swimming and creeping, and are bent back-
wards and provided with numerous joints and hairs. The
following appendages (mandible, maxille, and three pairs of legs)

 

 

Fia. 426.—Cypris fasciatus, adult female. (After Claus.) LIV, appendages; _c, furca;
e, eye; l, liver; m, adductor muscle of shell; 0, ovary; s, shell gland.

vary greatly from genus to genus. The internal structure is also
variable. The Ostracoda are largely bottom forms and live in fresh
and brackish water as well as in the sea.

CYPRIDINID. First two pairs of legs maxillary in character, the last
developed into a hook for cleansing the shell; heart present; marine.
Cypridina.* Cypripipz. First pair of legs maxillary in character ;
heart lacking; fresh water. Cypris,* Candona.*

Sub Class V. Cirrinedia.

The cirripeds, or barnacles, differ from all other crustacea in
that they have lost their locomotor powers and live attached to
rocks, floating timber, and the lke. In some cases they attach
themselves to other animals, as crabs and molluscs, or, as in the
case of Coronula, to whales. This leads
in Anelasma and the Rhizocephala to a
true parasitism, the barnacle not only
attaching itself to an animal but sucking
its juices as food.

The attachment is by the dorsal sur-
face in the neighborhood of the head,
and is initiated by the first antenne, in

 

which is a cement gland secreting a Fre. wQr—Batanushameri,t acorn
rupidly hardening cement. The region ek Hen tee
of fixation in the Balanide (fig. 427) ae ae Be ae aN
lies in the plane of the head; in the tts -

Lepadide it is drawn out into a long muscular stalk (fig. 114).

To this attached life are related all the peculiarities of structure,
494 ARTHROPODA.

It is clear that a fixed animal has greater need of protection than
one which can flee from its enemies, therefore we find not only a
right and left mantle and a shell capable of complete closure,
like that of an ostracode, but also in this calcified plates, the scuta
and terga (figs. 114, 427, s, ¢), the first cephalic, the other pos-
terior, in position.

Between the pairs of these is the gap through which the feet
are protruded. Besides there are other calcified portions, one of
which, the carina (fig. 114, ¢), eurresponds to the hinge-line of
the ostracode and in some Lepads is supplemented by a farther
unpaired piece, the rostrum. In the Balanide the rostrum and
carina are much stronger, while between them other paired pieces,
the lateralia, are intercalated. Lateralia, rostrum, and carina arise
from a base (usually calcareous) and form a capsule, closed above
by a double valve formed of the paired scuta and terga, between
which, when open, the animal can be seen (fig. 427).

The body in both lepads and balanids has essentially the same
structure. It is flexed ventrally, so that mouth and vent are near
each other, and bears six pairs of feathered feet, or cirri, which,
when extended, become widely separated and form a most efficient
means of straining small organisms from the water and conveying
them to the mouth. These feet are biramous, with their branches
ringed and thickly haired. Behind them is a rudimentary abdo-
men and an elongate penis; while the mouth is surrounded by a
pair of mandibles and two pairs of maxille.

In internal structure the most noticeable feature is that the
animals, in contrast to almost all other arthropods, are hermaphro-
ditic, a condition possibly correlated with their sedentary life and
the consequent need of self-impregnation. Yet it is to be remem-
bered that the common forms have a long
penis, so that these animals, crowded
closely together, can fertilize each other.
In cases of several species which live
solitary complementary males oceur.
These are very small, purely male forms,
with extremely simple structure (fig.
428), which live inside the mantle cavity
near the genital openings. The un-

 

Fig. 428.—Male of alcippe lam- gaomente © aan Soe ea? Sis hi
pas, an, antenna; 1, mantle segmente d hody is enclosed in a sac (a

lobes; m, muscles; oc, ocellus; gsoft-ski xd she ‘ ane . r
Pee eer eens ft DIO shell), and anchored by the
vesicle. antenne. ‘The long penis protrudes from

the mantle. In the genus Scalpellum there are purely hermaph-

 
I. CRUSTACEA: CIRRIPEDIA, LEPADID4, BALANIDA. 425

roditic species, hermaphroditic species with complemental males,
and purely dicecious species.
Since the hard shells of the barnacles resemble those of the molluscs, it

is not to be wondered that these forms were long regarded as belonging to
that group. It was not until the development (fig. 429) was studied that

 

Fia. 429.—Nauplius (A) and Cypris (B) stages of Sacculina carcini. (After Delage.)
1, 2, antenne ; 8, mandible; f, cirrhous foot ; m, muscles; oc, nauplius eye; ov,
anlage of ovary.

the error was corrected. A large nauplius comes from the egg and later

is metamorphosed into a second larval stage with bivalve shell which,

from its appearance, is called the cypris-stage. This becomes fixed and
develops into the adult, losing the compound eyes and retaining the nau-
plius eye.

Order I. Lepadide.

Stalked cirripeds, with shell largely formed of scuta, terga, and carina;
other parts may be added. Lepas anatifera*
(fig. 114) is the goose barnacle, which owes
its common name to a medieval myth
which claimed that the Ivish (or bernicle)
goose developed from these animals. J.
Jfascicularis,*(tig. 430), abundant floating on
the eastern coast. Anelasma squatlicola, a
thin-skinned barnacle, is parasitic on sharks
and forms a transition to the Rhizocephala.

Order II. Balanide.

Sessile cirripeds with calcareous shell
formed of carina, rostrum, and lateralia;
scuta and terga forming the valves (fig. 427).
Balanus balanoides,* common on our coast.

Coronula diademata, attached to the skin FIG. 430.—Lepas _ fascicularis,*
goose barnacle. (From Smith.)
of whales.

 
426 ARTHROPODA.

Order III. Rhizocephala.

These forms differ so much from the other cirripeds as to demand sepa-
rate mention. They are parasitic on the abdomens of various decapod
crabs and consist of a stalk which penetrates the body of the host and a
body which remains outside. The stalk, which branches in a root-like man-

 

Fic. 431.—Sacculina carcini attached to Carcinus mcenas, whose abdomen is extended.
m, shell opening; *, network of roots ramifying the crab; s, stalk; a, 0, d, anten-
nula, eye and anus of the crab.

ner, penetrates the cephalothorax and absorbs its juices. Since the stalk
furnishes the food, an alimentary canal is absent. The body lacks all ap-
pendages, is enclosed by a soft-skinned mantle, and is almost entirely
filled with the gonads. Since these forms lack, as adults, all arthropodan
features, their position is only settled by their development, which shows
(fig. 429) no great difference from that of cther cirripeds. These forms
are rare on the American coast. Sacculina, Peltogaster.*

Two more orders, ABDOMINALIA and APODA, parasitic in the mantle
and shells of molluses and other cirripeds, scarcely need mention.

Sub Class V. Malacostraca.

The Malacostraca are sharply marked off from the other Crus-
tacea by having a body which consists of twenty segments, of which
seven are abdominal (Nedalia has twenty-one, eight abdominal).
The excretory organs are represented by the antennal glands, and
shell glands are lacking except in some Isopoda. The male geni-
tal ducts open on the thirteenth, the female on the eleventh,
segment.
I. CRUSTACEA: LEPTOSTRACA. 427

Legion I. Leptostraca.

The Leptostraca connect the Phyllopoda with the higher
groups. They have twenty-one somites, eight abdominal, eight
thoracic, and five cephalic, and this and the openings of the genital
ducts ally them to the Malacostraca. On the other hand the
bivalve carapace covering the cephalothorax and part of the abdo-
men, and the leaf-like thoracic feet, are phyllopodan. They have
an antennal gland and a rudimentary shell gland; an elongate heart
which extends through cephalothorax and abdomen; and com-

 

Fie. 432.—Nebalia bipes.* (After Sars.) h, heart; 7, intestine; 0, ovary; a, adductor
of carapace ; b, brain ; 7, rostrum.

pound stalked eyes. The few species are all marine and belong to

the genus Nebalia. N. bipes* (fig. 452).

Legion If. Thoracostraca (Podophthalmia).

The names given this division have reference, first, to the fact
that the head and more or fewer of the thoracic segments are united
into an immovable part, covered by a firm carapace; second, that
the compound eyes (except in Cumacea) are placed at the ends of
movable eye stalks, a condition which has possibly arisen from
the inflexibility of the anterior part of the body. The first five
appendages are always two pairs of antenna, a pair of mandibles,
and two pairs of maxille. The remaining pairs vary greatly in
character and from one to three may be modified into maxillipeds,
while the abdominal somites except the last (¢e/son) usually bear
appendages, at least in the female. There is usually a metamor-
phosis in development in which a nauplius-stage may appear, most
frequently in the lower forms (schizopods), but even in the deca-
pods (Penevs).
498 ARTHROPODA.

Order I. Schizopoda.

These are small forms, mostly marine, in which the cephalo-
thorax is covered by a carapace with which some or all of the

 

Fie. 433.—Amphithoé. (From Gersticker.) a’, a?, first and second antenne; au, eye;
VII-XIII, the seven free thoracic segments ; 1-7, abdominal segments.

thoracic somites are firmly united. The eight thoracic feet retain
throughout life a biramous condition and are used in swimming.
The posterior pair of abdominal feet together with the telson form

 

 

Fie. 434.—Mysis elongata. (From Gerstii

 

stiicker.) a, 8, first and second antenne, a, ex-
podite; au, eye; 7, endopodite; 0, otocyst ; 1-7, abdominal somites.

a caudal ‘fin’ by means of which the animal can swim backwards.
The delicate skin permits of diffuse respiration, and gills are fre-
quently lacking. In some genera plates from the legs of the female
enclose a brood case beneath the cephalothorax, thus giving these
forms the common name of opossum shrimps.
I. CRUSTACEA: STOMATOPODA. 429

The Mysipip# are the most widely distributed, several species of Mysis
(fig. 484) occurring on our coasts and one inthe Great Lakes. In these the
endopodite of the sixth abdominal appendage contains an otocyst, with a
calcic fluoride otolith. Other families are the EupHaAvsiipas and LopHo-
GASTRID& of the deeper seas.

Order II. Stomatopoda.

In structure of the cephalothorax these forms, known as mantis
shrimps (from a resemblance to the insect, the praying mantis),
have not advanced as far as the schizopods, since the last three
thoracic somites remain free and are not covered by the carapace.

 

Fig, 435.—Squilla mantis, at, at’, first and second antenne; f, sixth abdominal feet:
k, gills; p, schizopodal thoracic feet; pr, pr’, raptorial feet; ps, pleopoda; sca,
telson.

The appendages, however, are more differentiated, since only the
three posterior thoracic feet are biramous and natatory. The
four in front of these are prehensile and bear a pincer formed of
the last two joints, the last being slender and usually toothed and
closing in a groove of the penult joint like a knife blade in the
handle. The first of these raptorial feet are the largest and are
used in capturing fishes, etc. Since the thoracic feet are of little
service for locomotion, the abdomen is long and stout, especially
the caudal fin. The five anterior abdominal feet bear the gills, and
correspondingly the elongate heart with many ostia extends into
the abdomen. The transparent pelagic larve were formerly re-
garded as adults and described as Alima and Erichthus.  Squilla
empusa lives on our east coast, Gonodactylus in Florida. They are
burrowing animals and deposit their eggs in their holes.

Order III. Decapoda.

The Decapoda is the most important group of Crustacea, since
it contains the shrimps, lobsters, crayfish, and crabs. It agrees
with the Schizopoda in having a cephalothorax composed of thirteen
fused somites, but differs in the structure and function of the
thoracic extremities. Only the last five pairs (whence the name
Decapoda) are locomotor. These lose the exopodite during de-
430 ARTHROPODA.

velopment and become strong walking legs, terminated either with
claws or pincers (chelw). Usually the first pair is distinguished
from the others by its size and by being chelate, and becomes not
locomotor but grasping in function. In the development of a
chela the penult joint sends out a strong process, the ‘thumb,’

 

Fie. 436.—Erichthus stage of Squilla (orig.),

which extends as far as the last joint (the ‘ finger’), which closes
against it.

The mouth parts—a pair of mandibles, two pairs of maxille, and three
pairs of maxillipeds (fig. 404)—lie in front of the first pair of legs. The
maxillipeds (7, 6, 5)show clearly a biramous condition, while the mayille
(4, 3) retain considerable of the original phyllopod character. In the man-
dibles (2) there is always a strong basal joint, the edge of which serves as a
jaw, while this may bear additional joints, the pazpus. Behind the mouth
are a pair of scales, the paragnaths or metastoma, formerly regarded as
appendages. The antenne are usually distinguished from their size as
antennae (second pair) and antennile (first pair, fig. 404). They have large
basal portions, which in the antennule bear two many-jointed flagella,
I. CRUSTACEA: DECAPODA. 431

while the antennez proper have but a single though usually much larger
flagellum. On the basal joint of the antennule is the auditory organ
(p. 412), while the green gland opens on the basal joint of the antenne
(fig. 439, gd).

When the abdomen is not rudimentary (as in the crabs) the appendages
of the sixth abdominal segment together with the telson form a strong
caudal fin (fig. 489); the cther appendages (fig. 404, Z) are small, bira-
mous organs to which, in the female, the eggs are attached. Inthe female
the first pair is reduced, but in the male except in Palinuride this pair is
well developed, curiously modified, and serves as a copulatory (intromit-
tent) organ. The condition of these appendages as well as the openings of
the genital ducts—on the base of the third walking foot in the female,
the fifth in the male—serve at once to distinguish the sexes. Frequently
also the males have the larger pincers.

The thickness of the integument prevents diffuse respiration
and accounts for the numerous gills (fig. 437) which are attached

 

Fie 487.—Gills of Astacus exposed by cutting away the branchiostegite. pdb, plb,
podo- and pleurobranchia of the corresponding segments; 7, rostrum, 1, stalked
es: 2, 3, antenne ; 4-6, mandibles and maxille; 7-7, maxillipeds; 10, 14, bases of

ic feet; 15, first pleopod.

  

to the bases of the appendages (maxillipeds and walking feet) or
to the sides of the body near them. (In the Thalassinida—forms
near the Astacide—the gills are on the abdominal appendages).
These gills are not visible externally, for the carapace extends
down on the sides of the body as a fold (branchiostegite) over
them, thus enclosing them in a branchial chamber. A process of
the second maxille—the scaphognathite—plays in this branchial
chamber and pumps the water over the gills, the water flowing out
near the mouth. All decapods can live some time out of water, a
fact readily explained when we remember that they retain some
water in the gill chamber, which keeps the gills in a moist con-
dition. In some of the tropical land crabs which live almost ex-
clusively on land there is a true aerial respiration, the lining of
the gill chamber becoming modified into a kind of lung traversed

 

 

 
432 ARTHROPODA.

by numerous blood-vessels. In Birgus latro the gill chamber is
divided into two portions (fig. 438), the upper part being pulmo-
nary, the lower containing the reduced gills.

 

Fia. 488.—Diayrammatic section through Birgus latro, showing lungs. (From Lang,
after Semper.) a, a4, afferent blood-vessels; ah, pulmonary chamber; ek, el, el’,
efferent blood-vessels; h, heart; k, gills; kd, branchiostegite; p, pericardium.

 

Fia. 439.—Anatomy of Crayfish (Astacus). 4, dorsal surface removed: B, scheme
of circulation ; C, viscera removed, showing green gland and nervous system. a,
anus; ad, hepatic artery ; ae, antenna; a, antennula, also sternal artery ; am,
muscles of stomach; «0, ophthalmic artery ; ap, abdominal artery ; av, ventral
artery; bl, urinary bladder ; br, gill arteries; c, @sophageal commiissures; gd,
green gland; gn’, brain; gn?-18, ganglia of ventral chain; hk, heart; hd, intes-
tine; k, mandibular muscles; 1, l’, liver and its duct; m, stomach ; o, otocyst ;
Cea ee eee i Pe, pericardium ; sgn, sympathetic nerve; t,t’,

€ ire bic oO es St 7 oa ra cj} S 5 4Uy rc .
DOE wenis Teste CHE e es; v, ventral blood sinus; vd, vas deferens ;
I. CRUSTACHA: DECAPODA. 433

Correlated to this localized respiration is the nearly closed cir-
culatory system (figs. 439, d, B). The heart (1), a compact
pentagonal organ, receives its blood from the pericardial sinus
(pe) through three pairs of ostia, and forces it out through five
arteries to the capillary regions of the body. The venous blood
collects in a large venous sinus at the base of the gills (v), passes
thence through gills, and is returned by several branchial veins
(vér) to the pericardium.

The alimentary canal is straight and has only one conspicuous
enlargement, the so-called stomach (fig. 439, A, m), divided into
two portions, an anterior sac (cardiac pouch), lined with chitinous
folds and teeth and serving to chew the food and bearing in its
walls the so-called ¢ crab-stones,’ which are masses of calcic carbon-
ate stored up to harden the armor rapidly after the molt. The
second or pyloric portion of the stomach is guarded by hairs and
serves as a strainer, allowing only food sufficiently comminuted
to pass. The two liver lobes—voluminous masses of branched

glandular tubes (7) open just behind the
stomach.
The two antennal glands (fig. 439, C,
gd), each provided with a large urinary
A bladder (07), are dirty green in color, whence
the name green glands often applied to
them. The gonads (figs. 440) lie close
beneath the heart, those of the two sides

  
     

z 7

 

Fia. 440. Fie. 441.

Fia. 440.—Reproductive organs of (4) female and (B) male crayfish. (From Hux-
ley.) od, oviduct; od’, its opening on llth appendage; ov, ovary; t, testes; vd,
vas deferens ; vd’, its opening on 13th appendage.

Fie. 441.—Nervous system of crab, Carcinus. (From Gegenbaur.) a,antennal nerves;
c, eesophagexl commissures; gi, fused ventral chain perforated for sternal
artery ; gs, brain ; 0, optic nerve.

 
434 ARTHROPODA.

being united behind, while their ducts remain separate. The
structure of the nervous system is in part dependent upon that
of the abdomen. In the Macrura (fig. 439, () the ventral chain
consists of six ganglia in the thorax, six in the abdomen, but in
the Brachyura (fig. 441) these all flow together in a common mass,
connected with the brain by two long wsophageal commissures.

The development of most decapods is interesting from the number of
larval forms. As a rule a zoea (fig. 415) is hatched from the egg ; this
passes next into a Mysis-stage (fig. 442) in which head, thorax, and abdo-
men are distinct, the thorax bearing biramous feet like those of schizo-
pods—a proof of the origin of the simple feet from the biramous type.
In the crabs (Brachyura) the Mysis-stage is replaced by a Megalops (fig.
443), in which the abdomen is well developed but the feet have lost their

 

Fria. 442. Fie. 443.

Fia. 442.—Phyllosoma larva (Mysis-stage) of Palinurus. (After Gerstiicker.) A, ab-
domen ; C, head; 7, thorax ; a and i, exopodites and endopodites of thoracic feet.
Fia. 443.—Megalops larva of Portunus. (From Lang, after Claus.) 2, antenne; IV-

Vili, thoracic appendages ; a?-a*, abdominal somites (a® is the seventh).
biramous character. In some prawns (Penews) the series is rendered more
complete by the appearance of a nauplius and a metanauplius with many
appendages, before the zoeal stage. In the crayfish and many land crabs
the metamorphosis has been lost, but the lobster leaves the egg in the
Mysis-stage. Differences may occur even in the same species; thus in the
European Palemonetes varians the embryo, in the sea, leaves the egg as a
zoea ; in fresh water in the Mysis-stage.

Sub Order I. MACRURA. Abdomen well developed ; antenna long ;
ventral nerve chain elongate; no megalops-stage in development.
CARIDEA. Body compressed; no sutures on carapace; feet weak, ex-
ternal maxillipeds pediform; a large scale on the second antenne. In the
Peneipe® there are weak exopodites. Peneus,* Sicyonia.* PAL#MO-
NIp&, mandibles bifid at tip. Palemon, Alpheus,* Hippolyte,* Panda-
dus.* In the CRANGONID& the mandible is simple. Crangon,* Sabinea.*
I. CRUSTACEA: DHECAPODA. 435

ASTACOIDEA. Carapace crossed by a transverse groove. The ASTACIDE
have well-developed chele. Cumbarus* includes the crayfish of the

 

Fic. 445.—ELupagurus bernhardus, hermit crab. (From Emerton.)

eastern states; those of the Pacific coast and Europe belong to Astacus.*
The lobsters belong to Homarus.* PALINURID (Loricata), no chele,
436 ARTHROPODA.

body with heavy armor; larva leaf-like and transparent ‘ glass crabs,’
called Phyllosome (fig. 442). Palinerus,* spiny lobster. PAGURIDEA,
hermit crabs; abdomen reduced, soft-skinned, and hidden for protection
in a snail shell which the animal carries about, which habit has resulted

 

Fra, 446.—A, Platyonichus ocellatus,* lady_crab; B, Libinia emarginata,* spider crab
(From Emerton.)

in a spiral twisting of the abdomen. Some hermits (Eupagurus) carry
sea anemones or hydroids on their shell, eases of symbiosis (p. 170).
Hupagurus,* Clibanarius.* Allied is Birgus, the palm crab of the East
I. CRUSTACEA: CUMACEA. 437

Indies, which is said to climb palm trees for the cocoanuts, which it eats.
Its respiratory organs have been referred to on p. 432.

Sub OrderII. BRACHYURA. Body depressed; abdomen rudimentary
and folded in a groove under the cephalothorax; antenne short; never
more than one pair of feet chelate; ventral nerve cord concentrated (fig.
441). Omitting some inconspicuons groups like the porcellain crabs (Por-
CELLANID#), the HIPPID#, and the LIrHopID&, which are united as a group
of Schizosomi from the fact that the last thoracic segment is free from the
carapace and its appendages are rudimentary, the sub order is usually
divided as follows: LEUCOSOIDEA (Oxystomata). Body oval or triangular,
area of mouth parts triangular, the apex anterior. Calappa, Matuta,*
Hepatus * of warmer seas. OXYRHYNCHA (Maioidea). Cephalothorax
triangular, narrowed in front; mouth area (as in the following tribes)
quadrilateral. Mostly tropical. Hyas,* Libinta,* Pugettia,* spider crabs.
CYCLOMETOPA. Body broader than long, regularly arcuate in front.
CANCRID, With last pair of feet pointed. Cancer,* shore crab; Pano-
peus,* mud crab. PoORTUNID#, with last pair of feet flattened paddles.
Platyonichus*,; Neptunus hastatus,* when thin-skinned after molting, is the
‘soft-shell crab’ of the markets. CATOMETOPA. Front of carapace
nearly straight; body from above nearly quadrilateral; Gelastmus,* the
fiddler crabs of our warm shores; Pinnotheres ostreum,* common in
oysters; GECARCINID® (Uca, etc.), land crabs of the tropics, which only go
to the sea at the reproductive season to lay their eggs.

Order IV. Cumacea.

Small marine forms with sessile eyes, three or four free thoracic somites;
appendages biramous; a brood sac beneath the cephalothorax. Of interest
because combining arthrostracan and thoracostracan features. Diastylis
(fig. 446).

 

Fic. 447.—Diastylis quadrispinosus.

Especial interest also centres in the little known Anaspides tasmanie
from lakes in Tasmania, which unites schizopod and amphipod characters.
It has the stalked eyes, caudal fin, and biramous feet of a schizopod;
otocysts in the antennule like a decapod; but agrees with the amphipods
in shape of body and in free thoracic segments. The epipodial plates
are paralleled elsewhere only in carboniferous species, with which these
forms apparently are closely allied.
435 ARTHROPODA.

Legion III. Arthrostraca (Edriophthalmata).

Although the head of the Arthrostracan consists of six seg-
ments, it is remarkably short. It bears six pairs of appendages,
one of the normal thoracic pair being added to it as maxillipeds.
Eyes, when present, are aggregates of ocelli situated on the sides
of the head. There are seven thoracic segments, the appendages
of which are walking feet which lack exopodites. The abdominal
appendages, when present, are always biramous, the telson never
bearing appendages, and in the Amphipods is greatly reduced,
sometimes being split nearly its whole length.

The nervous system (figs. 75, 448) is of the ladder type. The
alimentary canal is straight and has an anterior enlargement, the

 

Fie. 448.—Male Orchestia cavimena. (After Nebeski.) a’. a?, antenne :
anterior and posterior aorte ;

ao, aop,
1 a0 ¢, heart; d, digestive tract ; yg, brain and eye; h,
testes ; k, gills; kf, maxilliped; l, liver; m, excretory organ; nm, ventral nerve cord;
o, rudimentary ovary; vd, vas deferens ; I-VI, thoracie feet; I-). anterior, 4-4,
posterior abdominal feet.

 
 

chewing stomach, behind which empty one or more pairs of long
liver tubes, while in a few Amphipods a pair of excretory tubes,
the so-called Malpighian tubules, empty into the intestine near its
end. Respiratory and circulatory systems vary so that they are
best described in connexion with the two orders.

Order I. Amphipoda.

The Amphipods are almost exclusively aquatic, a few species
living on the shore near high-tide mark. A few live in fresh
water (Gammarus, Allorchestes), the majority being marine. On
land they move by a leaping motion, whence the common name,
I. CRUSTACEA: AMPHIPODA. 439

beach fleas. In swimming the abdomen is alternately bent against
the breast and then forcibly straightened.

The body is usually strongly compressed from side to side.
The thoracic feet generally bear large epineural plates (fig. 433),
which extend the sides of the body
downward, while on the inner side
delicate gills or gill sacs (fig. 449,
br) arise from their bases. In the
female brood lamelle (dr/) are
added—broad  chitinous _ plates
which enclose a brood chamber
beneath the body in which eggs or
young are carried. The three an-
terior pairs of abdominal feet are
two-branched, richly haired, and
serve to create currents of water Fia. 449.—Cross-section of Amphipod

: : (Corophium), (From Lang, after De-
which pass forward over the gills. lage.) bf, thoracic legs hm, ventral
Ee Pour a 2 nerve cord: hr, branchiz; brl, brood
T he remaining thoracic feet, elamella; d, intestine; h, heart; l, liver;

. 5 ov, eggs in brood chamber.
though biramous, are short and
stout and form springing organs. The position of the gills
explains why the abdominal part of the heart is degenerate and
only the anterior thoracic portion with three pairs of ostia persists.

 

   

Sub Order I. HYPERINA. Large head and eyes; strong prehensile feet.
Live attached to other pelagic animals on which they feed. Myperia
medusarum * lives on the jelly fish Cyanea; Phronitma,* warmer seas.

Sub Order II. GAMMARINA. Head much smaller; abdomen well devel-
oped; are mostly free swimmers. Numerous species in the sea. Gam-

 

Fig. 450.—Gammarus ornatus.* (From Smith.)

marus * occurs in shallow water, some being fluviatile; Orchestia * above
tide marks. Chelura terebrans * destroys piles and other submerged wood.

Sub Order III. LAAMODIPODA. Parasitic or semi-parasitic forms in
which the first (second) somite is fused to the head; appendages are lacking
44() ARTHROPODA.

from some of the thoracic segments and the abdomen is reduced. Species
of Caprella* are common on hydroids. Cyamus cet? is parasitic on whales.

Order II. Isopoda.

The Isopoda are readily distinguished from the Amphipoda by
their depressed (i.e. horizontally flattened) bodies. The feet are
adapted for creeping, and a brood pouch is formed as in the Am-
phipoda, but gills are lacking here since some of the abdominal
feet are modified for respiration (fig. 451, 4). In the abdomen,
the somites of which exhibit a great tendency to fusion, the telson,
as in all Malacostraca, is without appendages; the sixth somite

 

 

Fie. 451. Fig. 452.

Fic. 451.—Asellus aquaticus. (From Ludwig-Leunis.)  a!, a%, antenne; br, brood
pouch; k, pleopoda modified to gills; md, mandibles; p}-p’, thoracic feet;
pai-pa’, abdominal feet (pleopoda): I-VI, head; VII-XIIJ, thoracic segments;
XIV-XX, abdominal segments, partly fused.

Fig. 452.—Cymothoa emarginata. (After Gerstiicker.) p*, sixth pleopod.

bears, in the walking forms, long forked appendages (fig. 451); in
the swimming species (fig. 452) they are flattened and, with the
telson, make aswimming organ. ‘The five anterior pairs of pleo-
poda are modified for respiration, by the expansion of the endop-
odites into thin-walled plates, while the exopodites and the whole
first pair serve as opercula or gill covers. As a result of this posi-
tion of the gills the heart (usually with two pairs of ostia) is ab-
dominal in position.

In the terrestrial species the gills are adapted for breathing damp air.
In Porcellio and Armadillidum the first or first and second opercula are
permeated with a system of air tubes, which physiologically, though not
morphologically, are comparable to the trachez of insects.

In the Isopoda the tendency to parasitism is greater than in the
Amphipoda. Many swimming forms attach themselves to fishes and
feed by boring with their mouth parts, which are modified for the purpose,
I, CRUSTACEA: ISOPODA. 441

into the skin. The Bopyridz live in the branchial chamber of shrimps.
Cryptoniscus is a shapeless sac which attaches itself to the stalk of Saceu-
lina (p. 426), and, after causing the death of this parasite, uses its network
of ‘roots’ for its own nourishment. The Entoniscide (fig. 453) attack

 

Fic. 453.—Entoniscus porcellane. (From Gersticker, after Miiller.) 4A, male; B, female;
C, heart; he, liver; la, brood lamella; ov, ovary.

 

Fic. 454.—A, Idotea irrorata *; B, Limnoria lignorum *; C, <Ega psora * (‘ salve bug ’);
D, Leptochela algicola.* (After Harger.)

Decapoda and, pressing the skin before them, penetrate the interior. Their
strange shape is largely due to the lobe-like brood lamella. They are
449 ARTHROPODA.

usually hermaphroditic, but have besides complemental dwarf males (fig.
453, A).

Sub Order I. ANISOPODA. Six free thoracic segments; heart tho-
racic; first thoracic foot (ou head) chelate; abdomen with swimming feet.
A group intermediate between Amphipoda and other Isopoda. Tanais,*
Leptochela* (fig. 454).

Sub Order II. EUISOPODA. Seven free thoracic segments. ONISCID&;
terrestrial, familiarly known as sow bugs; Ligia, on seashore; Porcellio,*
Oniscus,* Armadillidum,* ‘pill bug.’ ASELLIDA (fig. 451), fresh water.
SPH#ZROMID#, head broad, body rounded and convex; Spheroma.*  Lim-
noria lignorum * (fig. 454), the gribble, attacks submerged wood and is
nearly as destructive as Teredo, IDOTEID#, free-living, mariue, with usually
elongate bodies; Idotea,* Cacidotea,* BOPYRID&, parasitic on Caridea;
body of female disc-like, asymmetrical, without eyes; Bopyrus.* CyMo-
THOIDH, parasitic on fishes or in their mouths. Cymothoa,* ya,*
Cirolana.*

Sub Order III]. ENTONISCIDA, parasites whose general features are
described above. Entoniscus, Cryptoniscus.

Class II. Acerata.

The animals comprising this group were formerly divided
among the tracheates (p. 408) and the Crustacea, but more recent
studies show that, although differing widely in respiration, the
forms included are closely allied in structure and development and
present many differences from both Crustacea and from other
tracheates (Insecta). The former views were based upon a con-
fusion between analogy and homology, it being thought that
trachew wherever found were homologous structures.

In the Acerata the body is usually divided into two regions.
cephalothorax and abdomen, though in some cases (mites) the
two regions become fused. The cephalothorax consists of six
somites which always bear appendages, and these appendages are
arranged ina circle around the mouth, the basal joints of one or
more pairs frequently serving as jaws. None of these appendages
are like antenne (whence the name of the group). The abdomen
consists of a varying number of somites, all of which may be free,
or, again, may be fused into a common mass. These abdominal
somites bear appendages in the embryo, but in the adults (except
the Xiphosura) these are usually lost or so modified that their
existence is only recognized by a study of development.

The alimentary canal is straight, without marked enlargements,
and lacks a chewing stomach. The liver is large and opens into
the intestine by two or more pairs of ducts. The nervous system
has some or all of its ventral ganglia arranged in a ring around the
Il, ACERATA: GIGANTOSTRACA. 443

cesovhagus, and in many forms is enclosed in the ventral artery.
Excretory organs, in the shape of neph-
ridia, are frequently present and open to
the exterior at the base of the second or
the fifth pair of appendages. Malpighian
tubes may occur, but these, unlike those
of other tracheates, are entodermal in
origin and hence not homologous with
them,

  

Fic, 455. Fic. 456,

Fia. 455.—Digestive tract of Ctenida ceementaria, (From Lang, after Dugés.) a, ab-
domen; au, anus; da, dt, diverticula (‘liver’) of midgut; y, brain; vb, rectal
bladder (stercoral pocket) ; vin, excretory tubules.

Fig. 456.—Lung book of Zilla cadophyla, (After Bertkau.) a,alung leaf separated

from the other leaves, b ; st, spiracle.

The respiratory organs are either gills, lungs, or trachee. The
gills are borne on some of the abdominal appendages. The lungs
are sacs onthe anterior abdominal somites opening hy narrow slits
(fig. 461) to the exterior. The anterior wall of each lung sac is
made up of thin plates arranged like the leaves of a book, and em-
bryology shows that these lung books are gill books drawn into the
ventral surface of the abdomen. The trachew in development
pass through a gill-stage and a lung-stage, the tracheal tubes being
outgrowths of the spaces between the lung leaves which penetrate
all parts of the body.

The reproductive openings are on the basal somite of the abdo-
men. The spermatozoa are motile. The development is direct,
there being no metamorphosis.

Sub Class I. Gigantostraca.

Marine forms with gills on the 2-6 abdominal appendages;
bases of five pairs of cephalothoracic feet masticatory; a pair of
median ocelli and a pair of compound eyes on the cephalothorax.
444 ARTHROPODA.

Order I. Xiphosura.

Cephalothorax large ; abdomen terminated by a long spiniform telson.
Limulus polyphemus of our east coast, commonly known as king crab

 

Fie. 457. Fie. 458.
Fie. 457.—Limulus polyphemus,* horseshoe crab (orig.).
Fia. 458.—Ventral surface of Limulus moluccanus. (From Ludwig-Leunis.) 1,
chelicere ; 2-5, walking feet ; ms pushing foot; 6%, flabellum; 7, genital operculum ;
9,

&, gills (there should be five); 9, base of telson.

or horseshoe crab. Other species on eastern shore of eastern continent.
They burrow beneath the sand and mud of the bottom and feed on worms.
In the spring they come to the shore to lay eggs.

Order II. Eurypterida.
Extinct Silurian and Devonian forms with small cephalothorax and
large twelve-jointed abdomen. The animals are intermediate between the

xiphosures and the scorpions. Hurypterus,; Pterygotus. some species
seven feet long.

Sub Class II. Arachnida.

Under this name are included a number of orders of greater or
less extent which can be arranged around the spiders, or Aranea,
asa centre. There is considerable modification of form, and the
following account applies only to the more typical groups. In
these the cephalothorax and abdomen are separated by a distinct
line, and since the abdominal appendages almost entirely disappear
in the adult, the number of somites can only be ascertained where
their boundaries are evident. The number varies between six in
the phalangids and thirteen in the scorpions.

The cephalothorax is, except in the Solpugide, a single piece
II, ACERATA: ARACHNIDA. 445

which bears six pairs of appendages; the four posterior pairs, con-
sisting typically of seven joints, are locomotor, so that the posses-
sion of eight legs is as characteristic for
an arachnid as ten for a decapod or six
for ahexapod. The first pair of append-
ages, the chelicere (fig. 459), are preoral,
the second, or pedipalpi, beside that open-
ing. The chelicere are short and con-
sist of two or three joints, the terminal
joint either folding back upon the other
or, pincer-like, meeting an opposable
thumb. In the spiders the last joint or

' i : : i . Fie. 459.—Mouth parts of Hpeira.
claw is forced into the prey, introducing 1, chelicera; 2% pedipalpi; p

6 ‘ ‘ 5 agar palpus; /, basal plate.
poison from a sac in the basal joint.
The pedipalpi are elongate, leg-like, their basal joints often form-
ing a lip, the other joints forming the palpus, which may end with
a claw or a pincer.

 

The question has often been discussed as to whether the chelicere are
the homologues of the antenne of other arthropods. The embryological
evidence, which cannot be detailed here, is in favor of their equivalence to
the second antenna of the crustacea, and to the mandibles of insects.

Since the Arachnida usually suck their food, the msophagus is
frequently widened to a sucking stomach, behind which comes the
true stomach, with which, as well as with the intestine, a number
of so-called liver tubes may arise (fig. 455, da, dt). These may
be restricted to the abdomen alone, as in the scorpions. The
hinder part of the intestine is often enlarged into a rectal vesicle
(stercoral pocket), just in front of which the excretory tubules (so-
culled Malpighian tubules) empty. These resemble the true Mal-
pighian tubes of insects in function, but differ in being entodermal
in origin. Besides there also occur coxal glands (modified ne-
phridia), of which only one pair comes to development, and this
may lose its external opening on the hase of the appendage.

The wsophagus is always closely surrounded by a nerve ring
composed of brain above and of part of the ventral chain on the
sides and below, the thoracic and more or fewer of the abdominal
ganglia entering into its composition (fig. 405, )). Of sense organs,
besides tactile hairs, only the eyes (fig. 406), 2-12 in number, are
well known. Hearing is well developed, but it is uncertain whether
certain hairs on the legs and palpi are the seats of the recognition
of sound. The function of the ‘lyriform organs,’ which occur in
the skin of body and legs in several groups, is unknown.
446 ARTHROPODA.

The respiratory organs already alluded to (p. 443) have their
spiracles, always few in number, on the anterior ventral part of
the abdomen and, it is stated, sometimes on the cephalothorax.
The internal organs are the lungs and the trachee. A lung isa
rounded sac just inside the spiracle and consists of numerous leaves
on the anterior wall of the lung sac. Each leaf is covered on each
side by a thin layer of chitin and contains a blood space in its in-
terior, while between the leaves are flattened spaces into which the
air enters (fig. 456). The trachez, on the
other hand, are branched tubes arising from
the abdominal spiracles and penetrating
the abdomen (fig. 460). These are lined
with chitin, and to strengthen them with-
out undue thickness this lining is thrown
into folds, usually arranged in a spiral.
In the scorpions and tetrapneumonous
Araneina only lungs occur. In other
spiders one pair of lungs is replaced by
trachee, while in most other arachnids only
trachew occur. (The smaller mites and
Fra. 460.—Beginning of paired parasites lack specialized respiratory or-

trachee of Anyphena accen- i:

tuata. (After Bertkau.) st, gans and circulatory organs as_ well.)

ay eter These facts, aside from embryological con-
ditions, show that lungs and trachee are morphologically equiva-
lent. The localization of respiration in the abdomen has resulted
in having the heart in the same region. It is noticeable that, as
the trachez are developed, the circulatory vessels are reduced. In
the scorpions, which have only lungs, the circulation is most
nearly complete.

 

In development the arachnidan trachee arise from the abdominal
appendages, as do the lungs. (In the Solpugide and some mites cepha-
lothoracic trachez occur, but nothing is known of their development.)
This fact shows that they are entirely different in origin from the tracheze
of insects, while numberless details show that these structures are only to
be compared with the gills of Limulus.

The gonads (only the Tardigrades are hermaphroditic) are
abdominal in position and open by paired ducts (sometimes with a
single mouth) on the first abdominal somite. In most cases the
animals are oviparous, but the scorpions and many mites bear liy-
ing young. Jn many instances the mothers care for their eggs and
young, the scorpions carrying their families on their bodies. Only
rarely is there a metamorphosis, and then in the aberrant forms
IT, ACERATA: SCORPIONIDA. 447

like the Linguatulida and Acarina, where the young have but two
or three pairs of appendages, acquiring the others later.

Legion I. Arthrogastrida.
Arachnida in which the abdominal somites are distinct.

Order I. Scorpionida.

The scorpions bear a superficial resemblance to crayfish and for
along time were associated with them, since (fig. 402) they have
four pairs of walking feet (3-6), while the pedipalpi (2) are large
and bear pincers. The chelicere are also chelate. ‘The pedipalpi
and the two anterior pairs of legs have the basal joint expanded
for chewing. The peculiarities of the abdomen mark the group
off from all other arachnids. It consists of seven broader somites
attached by their whole width to the cephalothorax and behind
six narrower somites, forming a tail or postabdomen. The last
somite is bent ventrally in a sharp spine and contains two large
poison glands. It is the ‘sting’ of the animal, which, in the case
of the small species, causes painful wounds in man; and in the
large tropical species is, perhaps, fatal. Usually scorpions feed
upon insects, which they seize with the pincers, and, arching the

 

Fia. 461.—Under surface of scorpion, showing the combs and the outlines of the lung
sacs with their spiracles (orig.).
tail over the back, kill with the sting. On the ventral surface of
the second abdominal somite (fig. 461) are a pair of appendages,
the combs or pectines; rods with teeth on one side of uncertain
function. They are clearly appendages with modified gill leaves,
and from their nearness to the sexual opening and their rich nerve
supply are supposed to be stimulating organs in copulation. The
448 ARTHROPODA.

next four segments bear spiracles which lead to four pairs of lung
sacs. The heart is abdominal and the ‘liver’ diverticula are con-
fined to the same region. The large number of abdominal ganglia
distinct from the esophageal ring is also characteristic. From
three to six pairs of eyes occur.

The scorpions are inhabitants of warm regions, ranging north with us
to the Carolinas and Nebraska. Buthus,* Centrurus.*

Order II. Phrynoidea (Pedipalpi, Thelyphonida).

The thoracic segments are fused, and of the appendages only
the last three are walking feet, the third pair having the last
joint (tarsus) developed into a long many-jointed tactile flagel-

   

~S

Fic. 462.—Phrynus (Phrynichus) reniformis, (From Schmarda.)

lum. The chelicere are strong and spined, but end in a claw, not
inapincer, The chelicere are also clawed and are possibly poison
organs, since the bite of these animals is feared. The abdomen
consists of eleven or twelve somites and contains two pairs of lungs.
There are eight eyes—two large ones in the middle of the cephalo-
thorax, and three small ones on either side.

The species are tropical. Phrynius (fig. 462) has a simple abdomen ;

Thelyphonus* (fig. 405, D) has a short postabdomen which bears a long,
many-jointed thread. One species in the southwestern United States.

Order III. Microthelyphonida.

Small animals as yet known only from Texas, Sicily, Paraguay,
and Siam. They have a general resemblance to a scorpion, the
chelicere are three-jointed and chelate, the pedipalpi simple, neither
these nor any of the legs having chewing lamelle. The head is
distinct from two ‘thoracic segments,’ the abdomen is eleven-
jointed and is terminated by a long many-jointed caudal flagellum,
Il, ACERATA: SOLPUGIDA. 449

Lung sacs, which are true appendages without lung leaves, occur
on abdominal segments four to six, and are eversible. The ovary

 

»
)
My
y
)
fh
)
)
»

SON

Fia. 463.—Kanenia mirabilis.* (From Wheeler.)

is unpaired, the testes paired. ‘There is a circumesophageal nerve
ring and a single abdominal ganglion. No Malpighian tubes
occur. Avwnenia.*

Order IV. Solpugida (Solifuge).

In these the cephalothorax is broken up into a head bearing the
chelicere, pedipalpi, and the first pair of legs; and three posterior
free somites, each bearing a pair of legs, thus giving these forms a
certain resemblance to the Hexapoda (infra). The chelicere are
strong and chelate, the pedipalpi are simple and are used in walk-
ing, while the first pair of legs are tactile. Respiration occurs by
four pairs of trachee, the first of which opens between the first and
450 ARTHROPODA.

second ‘thoracic’ somites, a condition which deserves embryologi-
cal investigation. The abdomen consists of nine or ten somites,
and the head bears two ocelli.

As the name implies, the Solpugidze are nocturnal, living by day in
holes in the sand and searching for their prey at night. In the Old World
they are reputed as poisonous, but no poison glands occur. Warmer parts
of U.S. Solpuga,* Galeodes,* Datames * (fig. 464).

 

Fie. 464. Fig. 465.

Fig. 464.—Datames formidibilis.* (After Putnam.)
Fie. 465.—Chelifer bravaisi. (From Schmarda.) 1, chelicerz; 2, pedipalpi.

Order V. Pseudoscorpii.

These small forms resemble the true scorpions in the chelate
cheliceree and pedipalpi (fig. 465), and in the abdomen joined by
its whole breadth to the thorax. They differ in the lack of post-
abdomen and sting. They breathe by trachew; have from two to
four ocelli, and spinning glands opening on the second abdominal
somite.

These animals, 2-3 mm. long, live in moss, ete., and among old and

dusty books, where they feed on mites and minute insects. Their bodies
are flattened and they run sidewise. Chelefer,* Obisium,* Chernes.*

Order VI. Phalangida.

The abdomen in the harvestman, or ‘daddy long legs,’ is less
evidently segmented than in the forms already mentioned, nor is
it sharply distinct from the cephalothorax. The small body bears
four pairs of exceedingly long legs; the chelicere are drawn out
II, ACERATA: ARANEINA. 451

in long horny processes; the pedipalpi are tactile organs as in the
true spiders. The males possess a long penis, and the females a

   

Fis
Fia. 466.—A phalangid laying eggs. (After Henking.)

long ovipositor (fig. 466). They have two or four ocelli and
breathe by trachee.
These largely nocturnal animals are predaceous, feeding upon small

mites. In structure they form in some ways an approach to the
Acarina. Phalangium,* Liobunum.*

Legion II. Spherogastrida.

Arachnida with the abdominal somites fused so that no traces
of segmentation remain.

Order I. Araneina.

In the spiders the soft-skinned
body is divided by a deep con-
striction into cephalothorax and
abdomen (fig. 467). The four pairs
of legs are adapted for springing
or for walking, the hinder pair
being also accessory to the spin-
ning. It bears a comb-like claw
with which several threads are
combined into a stronger cable.
The chelicera bears a sharp claw
(fig. 459), traversed by the duct
of the poison gland with which the
prey is killed, although but few
(species of Latrodectes, fig. 468, Fra. 467.—Epeira insularis,* round-web
the tarantula, and the bird spiders, ice ae
Mygalidw) can injure man. The pedipalpi are used as feeling

 
452 ARTHROPODA.

organs and with the basal maxillary process to comminute the
food. In the male the pedipalpi have the terminal joint swollen
to a pear-shaped structure (fig. 469) by which the sexes are easily

 

Fig, 468. Fic. 469. Fie. 470.
Fia. 468.—Latrodectes mactans,* poison spider. (After Marx.)

Fic. 469.—Pedipalp of Pardosa uncata. (After Emerton.)
Fic. 470.—Spinnerets of Epeira diadema., (After Warburton.) 1, 2, 3, first, second,
and third spinnerets; /, threads.

 

distinguished. This is used to convey the spermatozoa to the
female, a rather dangerous process, as the male is apt to be killed
by the much stronger mate.

At the hinder end of the abdomen, just in front of the anus,
are the spinnerets, which are reduced appendages, as is shown by
their paired arrangement and their jointing (fig. 470), as well as
by development. They are truncate and have at the tip a ‘spin-
ning field’ from which numerous minute, two-jointed spinning
tubes, resembling hairs, arise, each of which is the end of a duct
ofasilk gland. Different kinds of glands, producing silk for differ-
ent purposes, occur. The number of spinnerets varies between two
and three pairs, and in front of these may be an unpaired spinning
region, the cribrellum, so that hundreds or even thousands (Epei-
ride) of glands may be present.

The secretion of the glands hardens in contact with the air, and the
single threads are united by the combs of the hinder feet into a larger cord
which can be regulated in size according to the number of glands which
are active. Yet the largest cord is finer than the finest silkworm silk,
hence it is often used for the cross-hairs of telescopes. The spider silk has
many uses; it is used to line the nests, to form cocoons for the eggs, as a
means of descent from high places, and to form the well-known webs.

The nervous system consists of a brain and a circumm@sophageal ring,
and, in the Mygalidw, a single abdominal ganglion. The arrangement of
the six or eight occlli and the relative lengths of the legs are matters of
systematic importance. Two pairs of respiratory organs oecur. In the
Tetrapneumones there are two pair of lungs, but in the Dipneumones the
Il, ACHRATA: ACARINA. 453

hinder pair are replaced by trachez, which may open by separate spiracles
(Tetrasticta) or by a common opening (Tristicta, fig. 460).

Sub Order I. TETRAPNEUMONES. Four lungs, four spinnents and
cight eyes in two rows. The MyGauip&are the most important group, large
forms which spring upon their prey, capturing even small birds and mice.
To the genus Mygale* belong the spiders (commonly but erroneously called
tarantulas) which occur in banana bunches. Here also belong the trap-
door spiders, Cteniza,* of the southwest, which excavate burrows in the

Gs." 0d
Se

   
  

 

Fig. 471.—Cteniza cementaria in its tube, closing the lid. a, eyes; b, inside of lid
with places for the claws; c¢, egg cocoon.

soil, line them with silk, and close them with a hinged lid (fig. 471).
Atypus.*

Sub Order II. DIPNEUMONES, One pair of lungs, one of trachea;
six spinnerets. Here belong most of the native and numerous tropical
species. Some (VAGABUND-) use their webs only to line the nests and
enclose the eggs, which are either hidden away or carried about attached to
the body, while they spring upon or chase their prey. SEDENTARIA are
the web builders, their webs varying widely in structure. Of the first
group the SALTIGRADA include forms which jump upon their prey (Attus,*
Phidippus,* Habrocentrum*), and the Crrigrapa (Lycosa,* Dolomedes,*
Trochosa *), which run their prey down. Among these is the true Taran-
tula, T. apulie of Ttaly, whose bite was once believed to cause a frenzy ouly
to be cured by peculiar music (‘Tarantello’). The Sedentaria are divided
according to the web-building habits. The ORrBITELARLE or orb weavers
(Hpetra,* Argiope*) form vertical webs which in many instances are com-
plete circles. The RetirevarLe (Theridium,* Erigone*) build irregular
webs. The species of Latrodectes * are reputed poisonous to man (fig.
468). The TUBITELARL build horizontal webs with a tube to the mar-
gin in which they lay in wait for insects.

Order II. Acarina.

The mites, partly from parasitism, partly from other conditions
of life, have become, in some instances, considerably modified.
With the fusion of cephalothorax and abdomen the last traces of
segmentation in the body are lost. Yet they retain the six pairs
of appendages four pairs of legs which at once distinguish them
from the parasitic hexapods; and two pairs of mouth parts, modi-
fied into a sucking beak. This consists of a tube formed by the

 
454 ARTHROPODA.

basal joints of the pedipa) pi, in which the chelicere, either chelate,
clawed, or stylet-like, play.

Since the mites are small and half or wholly parasitic, they are much
simplified in structure. Frequently heart and trachew are lacking. The
larva as it escapes from the egg lacks the last pair of legs and then closely
resembles certain imperfectly segmented parasitic insects like the lice.

The red mites or TROMBIDUDZ and the water mites, HYDRACHNID& (Hy-
drachna,* Ataz*), are free-living in the adult condition, but parasitic as
young. The Ixopip# or ticks (Jvodes*), live in woods or on bushes, attack
man and other mammals, burrowing beneath the skin, sucking the blood un-
til they become enormously swollen and fall off. The much smaller males

  

Fia. 472. Fie. 473.

Fic. 472.—Sarcoptes scabei, female itch mite. (After Leuckart.)
Fie. 473.—Demodex folliculorum, follicle mite. (From Ludwig-Leunis.)

are attached to the females and take no food. Argas persicus, of eastern
lands, with habits like a bedbug, is poisonous. The GAMAsIDA are para-
sitic, species of Gamasus * occurring on beetles and Dermanyssus* on bats.
The AcaRID& include permanent parasites like Sarcoptes scabei * (fig. 472),
the cause of the ‘itch,’ and the closely allied cheese mite. The follicle
mite, Demodex folliculorum,* lives in the sebaceous glands of various
mammals, including man (fig. 456),

Order III. Linguatulida.

Elongate mites like Demodeyr lead to the Linguatulida, which
as adults live in the frontal sinuses of carnivorous mammals, as en-
cysted young in the liver of herbivorous forms, especially rodents.
The body is long, flattened and ringed, and hence somewhat tape-
worm-like (fig. 112). The adults have the mouth at the base of
a chitinous capsule, and on either side are two hooks regarded as
the claws of the first and second legs. Inside the body is a spa-
cious cavity traversed by the alimentary canal which is without
appendages. The nervous system is largely a circumesophageal
I. ACERATA: LINGUATULIDA, TARDIGRADA. 455

ring; the sexual organs are very complicated, the males having
the openings in front, the females at the hinder end.

The presence of these parasites in animals causes a profuse catarrh,
and the eggs pass out with the mucus. Falling on vegetation, these are

 

Fia. 474, Fie. 475.
Fie. 474.—Larva of Pentastomum proboscidenm. (After Stiles.) «, stomach; e, gland
cells; m, mouth ; st, stylet ; ¥, posterior larval hooks ; 1, 2, legs.
Fie. 475.—Macrobiotus hufelundi, water bear. (After drawings by Greef and Plate.)
LIV, legs; d, accessory glands; m, stomach; mk, mouth capsule ; ov, ovary 3 sp,
Salivary glands ; st, stylets; um, excretory tubules; blood cells in the body.

liable to be eaten by various animals. The larvee (fig. 474) have a boring
apparatus in front and two pairs of legs, the latter being lost in the
metamorphosis except for the hooks. It is by no means certain that
these are degenerate arachnids. The points in favor of such a position
are about equally balanced by those against. Pentastomum.

Usually associated with the Arachnida are two other groups of very
doubtful position, which until more definite knowledge is obtained, may
remain near them.

Tardigrada.

These are minute fresh-water forms, known to microsecopists as
‘water bears’ (fig. 475), which owe their name to their slow motions,
They have four pairs of short, hooked legs, their sole Arachnidan charac-
ter. The genital ducts empty into the rectum; the nervous system has
four ventral ganglia; heart and respiratory organs are lacking. In de-
velopment they are remarkable for the farge coelomic pouches. In the
456 ARTHROPODA.

feet are glands recalling nephridia in their history. It is possible that
these animals are to be placed among the Coelhelminthes. Macrobiotus.*

Pycnogonida (Pantopoda).

These marine animals have a cylindrical body, with a tubular probos-
cis in front and an abdominal appendage behind, and four pairs of very
long legs. In front of the legs is a pair of small chelate appendages and
usually a pair more like pedipalpi. In the male there is an additional
pair of ‘ovigerous’ legs to which the eggs are attached after being
deposited by the female, thus giving a total of seven appendages, a num-

ae

 

,
Fia. 476.—Nymphon stremii* (orig.). ¢, chelicere ; 0, ovigerous legs; p, pedipalpi ;
7r, rostrum.
ber not reached in any arachnid. Diverticula of the stomach extend into
the legs; a heart is present, but respiratory organs are lacking. The
Pyecnogonids, which creep slowly over seaweeds and hydroids, may be {1)
a distinet group of arthroproda, or (2) modified arachnids, or (3), and less

probable, Crustacea. Wymphon,* Phowichilidium,* Colossendeis,*

Class III. Malacopoda (Protracheata).

These forms, including only a single family PERIPATID#, show
a strange mixture of annelid and arthropodan (or ‘ tracheate’)

 

Fic. 477.—Peripatus capensis. (From Balfour, after Moseley.)

characters, so that they are usually regarded as representatives of
the stock, early separated from the annelids, from which the Insecta
have descended. They recall the annelids by the presence of
nephridia, so characteristic of that group, which begin by a closed
vesicle (reduced calom), pursue a short course, and expand into a
urinary bladder before opening at the bases of the legs (fig. 478,
so). On the other hand they possess trachee, long unbranched
III, MALACOPODA. 457

tubes which arise in numbers from the spiracles, which are irregu-
larly distributed in each somite (fig. 478, tr).

 

Fia. 478.—Anatomy of female Peripatus opened dorsally. (From figures of Moseley
and Balfour.) a, anus; at, antenne ; Vm, ventral nerve cords; d, digestive tract;
go, genital opening; 0, ovary; og, brain; p, pharynx; sd, slime gland; so, ne-
phridia ; sp, salivary gland; tr, trachee ; u, uterus.

The soft-skinned body, which shows no external ringing, bears
the legs, each terminated by claws. These legs somewhat resemble
the annelidan parapodia in that they are not jointed and are not
sharply separated from the trunk. Each segment bears legs, while
the head is provided with three pairs of appendages: a pair of
ringed antenne, a pair of mandibles, which lie in the oral cavity,
and a pair of mouth papillex, at the tips of which are the openings
of the slime glands, the sticky secretion of which is squirted out
and serves to capture insects (fig. 478, si).

The nervous system consists of a pair of cerebral ganglia (09),
supplying the antennew and a pair of very primitive eyes; and a
pair of ventral cords (dm), swollen slightly ia each segment, which
458 ARTHROPODA.

connect dorsal to the anus and are connected in the trunk by
numerous non-segmental commissures.

The description may be completed by saying that the straight aliment-
ary canal (p and d) bears only salivary glands (sp) ; that it is accompanied
throughout by a dorsal heart ; that the gonads (the sexes are separate)
open just in front of the anus (go), their ducts being modified nephridia.
The animals are viviparous, live in decaying wood, hide by day and hunt
their prey at night. The several species have a wide but discontinuous
distribution (South America, Cape of Good Hope, New Zealand, etc.), an
indication of great antiquity. Recently the forms have been divided into
several genera, Peripatus, Peripatopsis, Opisthopatus, etc.

Class IV. Insecta.

The Insecta is a distinct group marked off from all other
arthropods by several important characters.
The appendages show no signs of a schizo-
podal condition. The head is always a
distinct region, bearing a single pair of
antenney, a pair of mandibles, and two pairs
of maxille, the posterior pair often being
fused into a lower lip or labium.

The respiratory organs are trachee (figs.
479, 480), which resemble the trachea of

  

Fria. 479. Fia. 480.

Fia. 479.—Tracheal system of Machilis. (From Lang, after Oudemans.) Kk, head;
I-III, thoracic somites; s, spiracles; 1-10, abdominal somites.

Fig. 480.—Portion of trachea of caterpillar. (From Gegenbaur.) 4, main trunk;
B, C, D, branches; a, epithelium with nuclei, ); d, air in tracheal tube.
IV. INSECTA. . 459

man only in that they are tubes filled with air, and kept from
collapse by firm walls. They open to the exterior by openings
(spiracles, stigmata) on the sides of the body. They are inpushings
of the skin and consequently have the same structure, an epithe-
lium and an outer chitinous layer. The latter lines the lumen
of the tubes, and since it must be thin to permit the passage
of gases (oxygen, carbon dioxide), and at the same time firm,
to keep the tubes open, it is-thrown into folds which usually
pursue a spiral course. The turns of the spiral are so close
that it gives the tubes a ringed appearance. Inside the spiracles
the trachee branch repeatedly until they end in the tissues in
fine tracheal capillaries. In general it may be said that each
segment has a right and a left spiracle and corresponding tracheal
systems (fig. 59), but this scheme is complete in no known species,
for there are always some segments (especially in the head) which
lack these organs and are supphed from adjacent segments (fig.
479). Again, the trachex may be connected by longitudinal trunks
(fig. 494, ¢), so that spiracles occur in only a part of the segments,
these supplying the whole system. Although the trachee are for
aerial respiration, there are aquatic insects, but these also breathe
air, since they carry air about with them entangled among the
hairs which surround the spiracles. Then, too, aquatic Jarve often
have tracheal gills, thin-walled processes of the integument which
project into the water and are penetrated by numerous tracheal
twigs (fig. 495).

The alimentary tract always has excretory organs, the Mal-
pighian tubules, connected with it. These vary in number he-
tween wide limits, but are always placed at the junction of the
rectum with the rest of the tract. They differ from the physiolog-
ically similar tubes of the Arachnida in being of ectodermal origin,
so that no homology can be traced between them. The gonads
are always paired and placed dorsal to the intestine, while the
ducts (at least in some cases modified nephridia) open ventrally
at the hinder end of the body. The spermatozoa are motile.

In the subdivision of the ‘ tracheate’ arthropods a group of Myriapoda
is usually recognized, containing forms known as centipedes and ‘ galley
worms.’ These two types are in reality very different. The centipedes
(Chilopoda) show in all structural features close relationships to the Hex-
apoda, while the other group, Diplopoda, differ in almost every respect,
except the presence of numerous walking legs, from the Chilopoda.
Hence, since the object of classification is to show resemblances and dif-
ferences, the group of Myriapoda has been dismembered, the Chilopoda
460 ARTHROPODA.

being considered here, the Diplopoda as a distinct class at the end of the
group of Arthropoda.

Sub Class I. Chilopoda.

The most striking characteristic of the chilopods is their long,
flattened bodies, each of the numerous somites bearing a pair of

 

Fie. 481.—Diagram of transverse section of a centipede (orig.). d, digestive tract; g,
gonad; n, nerve cord; 8s, spiracle and trachee.

 

«

 

Fia@. 482. Fia. 483,
Fig, 482.—Mouth parts of Scolopendra morsitans. 1, antenne ; 2, mandibles; 3, maxe
illee; 4, second maxilla (labium); 4, poison feet. |
Fig. 483.—Ncolopendra morsitans, contipede, (After Schmarda.)

six- or seven-jointed limbs. The head bears a pair of long antenne
and usually numerous ocelli, which only in Scwtigera show a ten-
IV. INSECTA: HEXAPODA. 461

dency to become compound. The mouth parts (fig. 482) are a
pair of mandibles and two pairs of mavxille, both united in the
median line. Besides, the first pair of legs (fig. 482, 5), with their
fused bases, extend forward beneath the head and form the poison
claws, Their terminal joints are sharp and contain the ducts of
poison glands.

The spiracles (at least a pair to every other somite except those
of the head) are lateral in position in the soft integument between
the dorsal and ventral plates (fig. 481). The heart is elongate, with
chambers in each somite (fig. 66); there are two large Malpighian
tubes, and the nervous system is elongate, with ganglia in each
somite. ‘The gonads are dorsal to the intestine and are unpaired,
while the single duct opens ventrally in the preanal somite.

The Lrrnopipé, with 15 leg-bearing somites, have certain dorsal plates
enlarged and overlapping the succeeding somites ; Lithobiws,* common
under stones, etc. SCOLOPENDRID.E, centipedes; at least 17 legs and 5
ocelli ; Scolopendra,* in warmer regions (fig. 483). GEOPHILID®, not less
than 80 pairs of legs, spiracles 2 less than legs. Geophilus.* SCUTIGE-
RIDE, legs very long, 15 leg-bearing segments, but only 8 dorsal plates,
Scutigera,*

Sub Class Il. Hexapoda.

The Hexapoda is by far the largest division of the Arthropods,
since it contains at least ten times as many known species as all
the rest. The uumber is so large that it cannot be given with
accuracy; an estimate is 250,000. Since the tropics, which have
not been exhaustively studied, are very rich in insects, it is con-
ceivable that there are at least a million different species in the
world. On the other hand great uniformity of structure exists,
all adhering with great fidelity to plan of structure, regional divi-
sions, and number of appendages under the most diverse conditions,
so that the difference between the most extreme forms is far less
than that in Crustacea or Arachnida. But while hexapods thus
lose in morphological interest, they gain in their life relations, in
the way that they are injurious or beneficial to man, in their breed-
ing habits, and in their intellectual and social relations. From the
evolutionary standpoint they show marked adaptations to environ-
ment, and the large number of species is only possible by taking
advantage of every opportunity in nature.

Of systematic importance are the regional division of the body
and the number and character of the appendages. In the body
three regions are distinguished, often separated by marked con-
462 ARTHROPODA.

strictions: head, thorax, and abdomen. The number of abdomi-
nal somites, varies with the order and even with the family,

 

Fie. 484.—Schematic section of a hexapod through the Se cr, coxa; d,
digestive tract; f, femur; h, heart; 1, notum; pl, pleuron; st, sternum; t, tibia;
ta, tarsus; tr, trochanter.

ranging between eleven (in some larve and embryos twelve) in

the Orthoptera and five in many Diptera. Each cuticular abdo-

minal segment consists of two plates,
tergite (dorsal) and sternite (ventral),
united on the sides by a softer mem-
brane which contains the spiracles.

Head and thorax, on the other hand,

have a constant number of somites.

The thorax is plainly divided into three

segments, pro-, 2es0- and metathoraz,

each composed of three elements, an
unpaired dorsal portion, notum; a pair
f lateral plates, plewra; and an unpaired
ventral sternum (fig. 484). For sim-

Fic. 485.—Head of a grasshopper. Plicity one speaks of pronotum, meso-

ieee eens eae sternum, etc., to indicate the portions of

imaxillary pelpir it maxilie, the separate segments. The head is a

p OecipUbs Us vertex. continuous capsule in which the follow-
ing parts are recognized: in front and dorsal clypeus and frons;
dorsal and posterior a vertex and an occiput; laterally gene, ven-
trally a gula. The appendages show that the head is composed of
at least four somites.

 

The view that the head consists of six somites is based on the existence
of two more segments without appendages in the embryo, a preantennal
and a postantennal (intercalary, premandibular), as well as the knowledge
that the brain, in which formerly only antennal ganglia were recognized,
consists of three pairs of ganglia (proto-, deuto-, and trito-cerebrum).
IV. INSECTA: HEXAPODA. 463

The appendages (fig. 484), seven pairs, are confined to the head
and thorax (see, however, infra). The three thoracic segments
bear three pairs of legs, whence the name Hexapoda. The legs are
inserted between pleura and sterna and begin with a short coxa
(ce), followed by a trochanter (f7), also short. The two following
joints are long, the first, the femur (fe), being large and contain-
ing the muscles; the next, tibia (¢), being more slender; the foot,
or tarsus (/a), is composed of a series of joints, the last bearing a
pair of claws.

The first of the cephalic appendages, the antenne, are the
most leg-like, but normally are never clawed. They spring from
the frons above the mouth and are innervated from the brain.
The number and shape of the antennal joints varies with the
group, and according as the single joints are lengthened or short-
ened, narrowed or expanded, or provided with appendages, etc.,
different kinds of antenne—knobbed, club-shaped, toothed, feath-
ered, etc.—are recognized, distinctions of great value in classi-
fication.

The morphology of the three pairs of mouth parts, the mandi-
bles (md), maxille (mz), and second maxillew, or labium (ia, figs.
486-489), is more interesting. The labium, formed of united
tight and left appendages, lies behind the mouth and forms the
lower lip, and is in contrast to the upper lip, or labrum (ir),
which, however, is not appendicular in character. Both labium
and labrum may bear unpaired processes on their oral surfaces, an
epipharynx above, a hypopharynx below the mouth, neither of
them true appendages.

The different kinds of food necessitate differences in the char-
acter of the mouth parts,—chewing, licking, sucking, or piercing
—all referable back to the chewing kind, and these in turn are
modified legs. In the description of the chewing type it is well
to begin with the maziile (fig. 486), because of their easy com-
parison with the other mouth parts and with the legs as well.
These begin with a triangular joint, the cardo (c), which is fol-
lowed by a larger stipes (s/f). The stipes in turn supports two
chewing lobes, the inner, or Jacinta (//), and an outer, or galea (/e),
these being processes segmented off from the stipes. In the
Orthoptera and Coleoptera only the lacinia is sharp-pointed ; the
galea may either form a sheath for the lacinia, or, as in many beetles
(fig. 514), it may be tactile and jointed again. The stipes also
bears the maxillary palpus (pm), consisting of from three to six
similar joints, and is the mostly leg-like part of the appendage.
464 ARTHROPODA.

The /abiwm arises as a pair of processes which early approach each
other and fuse behind the mouth. All the parts of the maxilla
may be recognized, only it must be remembered that the basal
parts of the two sides are fused. The united cardines form an
under chin, the swbmentum, the stipites a chin or mentum, which
in the Orthoptera is cleft, a result of incomplete fusion. This
may bear inner and outer processes, the gloss (gl) and the para-
gloss@ (pg) respectively, and the labial palpus. The mandible con-

ES
|

 

Fig. 486. Fia. 487.

Fie. 486.--Chewing mouth parts of cockroach (Periplaneta orientalis). The letter-
ing is the same in figs. 486-489. c¢, cardo; gl, glossa: hy, hypopharynx; /, lobe;
le, li, external and internal lobes of maxilla; lr, labrum; m, memtum; md, man-
dible; ma, maxilla; p, pm, maxillary palpus ; py, paraglossa ; pl, labial palpus ;
sm, submentum; st, stipes.

Fig. 487.—Licking mouth parts of bumble bee (Bombus terrestris).

sists of merely the basal joint, altered for biting, while the rest of
the appendage, common in crustacea as the mandibular palpus, is
lacking.

The licking mouth parts, like those of the bees (fig. 487), stand
next to those already described, there being many transitional
stages. Labrum and mandibles retain their primitive condition,
while maxille and labium are greatly elongate, are connected at
the bases, and can be folded away beneath the head or extended at
will. The small submentum is followed by an elongate mentum
IV. INSECTA: HEXAPODA. 465

which bears the unpaired tongue or glossa (g/), which corresponds
to the fused glosse (or to the hypopharynx?) of the first type and
which is used for sucking honey and hence has the form of a
nearly closed tube. Beside it lie the rudimentary paraglosse (py)
and the well-developed palpi. Similarly the maxille have small
cardines and palpi, while the stipites and the undivided lobe (2)
are long and well developed.

The piercing mouth parts of the flies (Diptera) and bugs
(Rhynchota) can be compared with those of the bees in so far as
the labium forms the groundwork of the whole (fig. 488). The

Sli

Ty

 

 

 

 

 

 

 

Fig. 488. Fie. 489.
Fie. 488.—Sucking mouth parts of mosquito, Culex pipiens. (After Muhr.) The

groove of labium opened by removing labrum; the stylets separated.

Fia. 489.-- Sucking mouth parts of a butterfly. (After Savigny.) nec’, me’, shows how
right and left maxille unite into a tube; right labial palpus (pl) with hairs
removed.

 

beak (rostrum, haustellum) of these animals corresponds to the
labium; it isa grooved structure, either fleshy and flexible, or stiff
and jointed. The edges of the groove are inrolled so that there
remains a narrow dorsal slit, which can be closed by the slender
upper lip (ir). The tube formed of these parts contains four
stylets, toothed or with retrorse hooks at the tip. These are the
466 ARTHROPODA,

mandibles and maxille, and a fifth stylet, the hypopharynx (hy)
can be present. Palpi, which only occur in the Diptera, belong to
the maxille (7). Reduction in number of stylets to four or three,
or their complete absence (some flies), is brought about by fusion
or by degeneration. The haustellum serves as a case for the suck-
ing tube, which in the Rhynchota is formed by the united maxilla,
in the Diptera by labrum and hypopharynx.

The proboscis, or haustellum (the so-called tongue), of the
Lepidoptera (fig. 489) is a long tube coiled hke a watch spring
beneath the head. It consists of two long grooved maxillary galea
firmly united by their edges. The maxillary palpi are well de-
veloped in the moths; elsewhere they show ali stages of reduction
to complete disappearance. Labium and labrum are reduced to
small triangular plates at the base of the proboscis, the labium
bearing a pair of hairy palpi (p/). The mandibles are represented
by small plates or bunches of hair. These conditions gain in in-
terest when we remember that in the larva the mandibles are
strong biting organs, while the maxille are small hooks, and the
labium is better developed only in those parts connected with the
silk glands, a beautiful example of relations of structure to life
conditions.

In contrast tothe other regions, the abdomen lacks appendages in the
adults. Only in the lower group of Thysanura are small lobes present,
behind and in the same line with the thoracic feet, which may be regarded
as abdominal feet. Apparently, too, the appendages of the last segment,
the stylets and cerci, are modified limbs, but the parts (gonapophyses)
used in copulation and oviposition are different in character. False feet,
or pro-feet, occur on the abdomen of the larvee of the Lepidoptera and the
Tenthredinide, but since these are fleshy unjointed processes, it is
doubtful whether these are true abdominal limbs, like those of other
Arthropoda, or are structures independently acquired.

Besides ventral appendages the insects usually have two pairs
of dorsal outgrowths upon the meso- and metathorax, the wings.
They are lateral folds of the chitinous coat of the notum and con-
tain on their interior extensions of the blood sinuses and of the
trachexw, which are protected by thickenings of the chitin, causing
the network of ‘veins’ or ‘neryures’ in the wing. Both wings
may be elastic, flexible, and adapted for flight, or the hinder pair
may alone partake of this character (true wings or ale), while the
first pair may be thick and parchment-like wing covers, or elytra,
under which the true wings are concealed when at rest. When
only the base of the wing is thus thickened hemelytra result.
Between the origins of the anterior wings is frequently a chitinous
IV. INSECTA: HEXAPODA. 467

plate, the scutellum, while between the hinder wings is a similar
postscutellum. In many insects one pair of wings is lacking, the
anterior pair being retained in the Diptera, the posterior in the
Strepsiptera; these are clearly cases of degeneration. The entire
absence of wings may occur from two causes; wings have apparently
never been developed in some (primary lack of wings of the
Apterygota), while there are others in which we must believe that
wings once present have been lost, because nearly related forms—
bugs, lice, etc.—have wings, or because certain individuals (male
cockroaches, sexual ants and termites) are winged (figs. 506, 528,
529). The prothorax of all recent insects is wingless, but in some
of the Archiptera of the coal period wing rudiments occurred on
this somite.

As a result of differences in food the alimentary canal (fig.
490) varies greatly. The ectodermal
stomodeum begins with a pharynx,
which in the sucking insects is a
sucking apparatus with radial mus-
cles. The esophagus, which follows,
may be widened to a crop (ingluvies),
or it may have a cecal outgrowth
which in the butterflies may take the
shape of a stalked vesicle (falsely
‘sucking stomach’). Also ectoder-
mal is the gizzard (km, pv), or pro-
ventriculus, the chitinous lining of
which is toothed for grinding the
food. The true stomach, of ento-
dermal origin (m, cd), frequently
bears blind sacs or gastric ceeca (ap) ;
in general it is short and its junction
with the hinder ectodermal portion,
the proctodeum, is marked by the
entrance of the Malpighian tubules
(vasa Malpighii, vm). The latter,
excretory in function, arise from the
proctodeal region. The latter is Fro. 490— Alimentary tract of Cava-
usually differentiated into a small in- Dufour.) av, anal vesicle; ad, anal

foe : gland; cd, stomach with ceca ; ed,
testine and a two-regional (colon and — hind gut? i, ingluvies (crop); &,

: : head; oe, cesophagus; pv, proven-
rectum) large intestine. Therectum trieuius(gizzard): r,rectum; um,
may have enlargements called rectal #!pighan aes
glands. True glands, however, occur only at the beginning and

 
468 ARTHROPODA.

end of the alimentary tract; into the mouth empty from one to
four pairs of salivary glands (sp); at the anus are defensive anal
glands with their malodorous secretions of a protective character.
The alimentary tract with the other viscera is enveloped in the
fat body, a soft mass which contains, besides fat cells and connec-
tive tissue, concretions of uric acid.

The nervous system (fig. 405) has the ventral cord, especially
in primitive forms (Apterygota, Archiptera, Orthoptera, fig. 491),

   

Me Og. pa 2 ag a ee ie *

Fia. 491.—Viscera of male cockroach (Periplaneta orientalis). (Partly after Huxley.)
I-IIT, segments of thorax and corresponding legs; 1-10, abdominal segments: a,
anus; «9, ventral ganglia; ap, gastric ceca: at, antenna; bl, salivary bladder;
9,8 sexual opening ; , heart; kr, crop; kom, gizzard ; l, labial palpus; m, stomach
(the arrow shows i connexion between mand ki ), also maxillary palpus; mu,
male genitalia; oe, @sophagus;oy, brain; r, rectum; sp, salivary gland; ty,
thoracic ganglia: ug, infracesophageal ganglion ; um, Malpighian tubules.

 

and nearly all larve (fig. 59), long and composed of numerous
separate pairs of ganglia. In beetles, moths, bees (fig. 494), and
flies the cord is shortened and the ganglia are in part fused. The
brain arises by the fusion of three pairs of gangha (proto-, deuto-,
and tritocerebrum), and is, especially in the adult, very complex.
It is connected on either side with a large optic ganglion the size
of which is correlated to that of the eyes. In the adult condition
the Hexapoda have a single pair of highly developed compound eyes
(figs. 407, 408), which not infrequently occupy nearly the whole
of the top of the head. Between and in front of these small and
simple ocelli, usually three in number, frequently occur, especially
in insects which are strong fliers. These are either lacking or
poorly developed in the larvee, while the compound eyes are fre-
quently replaced by groups of from two to six closely crowded
ocelli. Of other sense organs only the tactile hairs of the skin are
known with certainty, while similar hairs on the antenne and
about the mouth are supposed to be organs of smell and taste, since
these senses are known to be well developed. The tympanal
organs of the Orthoptera are the only structures which can be with
IV. INSECTA: HEXAPODA. 469

much probability connected with hearing. These are thin drum-
like parts of the chitin, framed in thicker portions (figs. 492, 493),
beneath which is a tracheal vesicle, with a nerve ending in a < crista
acustica.? The power of producing sound is widely distributed and
often highly developed, the organs for this purpose varying widely
in character. Stridulating organs are formed by ridges on wings
and legs, which are rubbed against each other or against similar
ridges on the body. Humming is produced by the action of the

 
 

   

Fig. 492. Fie. 493.

 

Fia. 492.—Side view of grasshopper. s, spiracles; t, tympanic organ.
Fia. 493.—Anterior tibia of a Locustid with tympanum, ¢. (From Hatschek, after

Fischer.)
wings or by the passage of air through the spiracles, which are
often provided with vibrating membranes which also serve to close
these openings.

The trachee (figs. 477, 494) are usually united, just inside the
spiracles, by longitudinal trunks from which fine branches extend,
enveloping and penetrating all the organs with delicate silvery
threads. This connexion of trachew renders it possible for the
spiracles of some segments to disappear. The spiracles of the
abdomen are the most constant, usually occurring in the soft mem-
brane between the sternites and tergites; the thorax at most has
but two pairs, the head none. In insects with good powers of
flight many of the tracheal trunks are expanded to large air sacs,
which may be of value as reservoirs of air, so that the ordinary
respiratory motions are less necessary during flight.

An interesting adaptation of the tracheal system to aquatic life occurs
in the larvae of many Archiptera (Odonata and Mayflies) and Neuroptera,
and even among Lepidoptera (Paraponyx) and Coleoptera (Gyrinide).
The spiracles here are usually closed, and the taking of oxygen occurs
either through the skin or by means of so-called tracheal gills—bushy or
leaf-like appendages of the surface or the rectum, richly permeated by
tracheal branches (fig. 495). In such cases the tracheal system has two
portions, one which receives oxygen from and gives off carbon dioxide to
the water ; the other which supplies the tissues with oxygen and receives
carbon dioxide.
470 ARTHROPODA.

Since the trachex, with their fine branches, supply the tissues
directly with oxygen, the blood-vascular system is rudimentary.
Directly under the back lies the elongate tubular heart in a special

 

Fia. 494. Fie. 495.

Fie. 494.—Anatomy of honey bee. (From Lang, after Leuckart.) a, antenne: au,
eye ; b, legs; cm, chyle stomach; ed, rectum ; hm, honey stomach (proventriculus) ,
rd, rectal glands; st, spiracles; tb, tracheal chambers with trachez; vm, Mal-
pighian tubules.

Fie. 495.—Abdomen of Ephemera larva (from Gegenbaur) with tracheal gills, ¢: a.
tracheal trunks; 6b, intestine; d, caudal bristles (cerci).

pericardial sinus. This is a part of the hemocele cut off from
the gastric portion of this space by an incomplete partition in
which, right and left, are the wing muscles (ale cordis) of the
heart. The heart receives its blood through lateral ostia (eight or
fewer) from the pericardial sinus or (Orthoptera) through ventral
openings from the large hemocale. The blood passes forward
through an anterior aorta into the hemocele and thence back to
the pericardial sinus. The arrangement of the viscera, fat bodies,
and muscles gives a certain regularity to the circulation, especially
in the appendages. Accessory pulsating ampulle in the bases of
IV. INSECTA: HEXAPODA., 471

the antennez (Orthoptera) help in the flow of the blood. It is
noteworthy that many beetles (Meloide and Coccinellide) squirt
blood through the jointing membranes of the legs as a means of
protection.

The Hexapoda are diwcious. The gonads consist of a few or
many ovarial or testicular tubules (figs. 496, 497), the latter some-
times coiled into small oval bodies. Ovaries and testes are paired
and lie, right and left, in the abdomen. Their paired ducts (ovi-

 

Fic. 496, Fic. 497.

Fic. 496.—Male genitalia of Melolontha. (From Gegenbaur, after Fabre.) gl, accessory
glands; t, testes; vd, vas deferens; vs, seminal vesicles.

Fie. 497.—Genitalia of female Hydrobius. (From Gegenbauer, after Stein.) bc, bursa
copulatrix; gl, tubular glands; 0, ovarial tubes; ov, oviduct with glands; rs, re-
ceptaculum seminis; v, vagina.

ducts, vasa deferentia) open separately in the Ephemerida, but in
all other Hexapoda there is a single ventral unpaired sexual open-
ing just in front of the anus. This arises as a median invagination
of the ectoderm (hence lined with chitin), which extends inwards
and meets the genital ducts (modified nephridia), and forms the
ductus ejaculatorius of the male, the vagina of the female. Aside
from many accessory glands, the sexual apparatus shows the follow-
ing differentiations: in the male vesicule seminales, as widenings
or diverticula of the vasa deferentia; in the female the receptaculum
seminis and the bursa copulatrix. The latter may be either the
vagina or a blind sac arising from it, or a special invagination of
the ectoderm, emptying into the vagina by an internal canal. It
receives the penis. The receptaculum seminis, a stalked vesicle
connected with the vaginia or the bursa, has a special biological
interest. In insects which copulate but once during life it retains
the spermatozoa for a long time—four years in bees—in a living
condition. As the eggs are laid they are impregnated by sperma-

 
472 ARTHROPODA.

tozoa from it. Since a firm shell or chorion is developed around
the egg in the ovary, access of spermatozoa is only possible by the
existence of a micropylar apparatus, a system of tubes penetrating
the chorion at one end of the egg.

Oviposition occurs in many insects by means of an ovipositor
which may project free from the body (fig. 509) or may be re-
tracted into it. It consists of four or (Orthoptera) six parts or
gonapophyses developed from the eighth and ninth abdominal
segment, which form a tube. In many Hymenoptera this struc-
ture has become modified into a sting (aculews), and is provided
with poison glands, making if an efficient weapon of defence.
From its nature the sting is of necessity confined to the females.
In the males there is usually a protrusible penis which is frequently
composed of the same parts as the ovipositor; in others of metamor-
phosed somites. Further sexual differences lie in the form of the
antenne, shape and colov of the wings, modifications of the eyes,
etc.

In many insects the eggs may develop parthenogenetically.
Plant lice and scale insects reproduce for generations asexually, and
parthenogenesis is widely distributed among Hymenoptera, Lepi-
doptera, and Neuroptera. The conditions among the bees are
especially interesting, since here the determination of sex rests with
the existence or non-existence of fertilization (pp. 142. 487).
Much rarer than the ordinary parthenogenesis is that special form,
known as pedogenesis, which occurs only in certain Diptera like
Miastor. In the female J/tastor larva (fig. 498) the eggs develop

   

 

Fie. 498.—Larva of a Cecidomyid with pedogenetic daughter larvee. (From Hatschek,
after Pagenstecher.)

before the appearance of the ducts, so that the young can only
escape by rupture of the mother. After several pedogenetic
generations there appear at last larve which pupate and produce
adult male and female flies.

With the exception of these pedogenetic forms, the Pupipara,
many Aphide and a few other viviparous species, the Hexapoda
are oviparous. The development begins, after oviposition, by a
superficial segmentation of the egg. Later there appear two em-
bryonic structures, the yolk sac and the amnion; the first, m con-
trast to the vertebrate structure with the same name, is dorsal.
IV. INSECTA: HEXAPODA. 473

The amnion is a thin layer of cells which covers the ventral surface
and arises in a manner similar to the vertebrate amnion; folds aris-
ing from the blastoderm in front and behind, right and left of
the embryo, fuse with one another and produce a double envelope,
an inner amnion, an outer serosa.

With the rupture of the amnion and egg shell, the postembry-
onic development begins. This differs so in the different orders
that ametabolous, hemimetabolous, and holometabolous insects are
recognized, 7.e., insects with direct development without meta-
morphosis, those with partial and those with complete metamor-
phosis. The ametabolous young is closely like the adult, so that
it only has to grow, with periodic ecdyses, and to mature its re-
productive organs. Since no insect has wings when it leaves the
egg, this direct development is possible only in wingless forms like
the Apterygota and Aptera.

All winged insects, on the other hand have a more or less pro-
nounced metamorphosis, the final cause of which is the necessity
of developing wings. This view holds although there are wing-
less insects with a complete metamorphosis. These forms (fleas,
wingless moths, and ants) have undoubtedly sprung from winged
species and have inherited from them the metamorphosis which
has been retained after the wings were lost. In incomplete
metamorphosis the differences between the newly hatched young
and the adult, or imago, gradually
disappear (fig. 499). At the second
molt the wings often appear as small
folds in the chitinous wall of meso-
and metathorax; they grow with each
ecdysis, until at last, in size, form,
and movability, they are functional
wings. The chitinous coat of each
wing pad (fig. 499, B, 1, 2) encloses
the compressed and folded wing of
the next stage. Since the larve by

 
  

fi

\!

Fie. 499.—Hemimetabolous develop-

their lack of wings are placed in ment of Perlanigra. (From Hux-
‘ 2 ley.) A, wingless larva; B, ae
different circumstances from the with wing pads, 1, 2; C, adult;

i II, LI, thoracic ee
adult, the differences between the two

may be increased by the development of special larval organs.
Thus the aquatic larve of the May flies and dragon flies differ from
the adults not only in the absence of wings, but by the different
form and the tracheal gills, which are almost always lost at the last
molt (fig. 495).
474 ARTHROPODA.

Increase in the differences of environment and the correlated
increase in larval characters lead to complete metamorphosis. In
order to profit as muchas possible by its adaptation to its environ-
ment the larva retains its shape as long as possible; the gradual
change is suppressed and the alteration in form necessary to the
metamorphosis is postponed until the end of the larval life, to the
period between the last two molts. In this interval there is such
an energetic transformation of the organism that the performance
of ordinary vital functions, especially motion and feeding, is in-
terfered with or rendered impossible. This last stage therefore
becomes a period of rest, the pupal stage, upon the existence of
which great weight must be laid in the definition of complete met-
amorphosis. The more complete the condition of rest the more
pronounced is the holometabolous development. From this point
of view different types of pupe are distinguished: pup libere,
pup obtecte, and pupe coarctate. In a free pupa (pupa libera)
the appendages stand out from the body (fig. 500), so that not

 

Fie. 500.—Larva and pupa of May beetle. a’, a’, fore and hind wings: an, anus;
at, antenne, o, eyes; p’-p’”, legs ; st, spiracles.

only the segmentation of the body but the antenna, legs, wings,
and often the mouth parts of the imago are visible. Such pupe
have a certain power of motion, as, for instance, the pupx of many
Neuroptera and mosquitos, the latter rising and falling in the
water. The covered pupe (pupe obtecte) at the moment of
pupation have free appendages which with the hardening of the
chitin become closely appressed to the body, so that even by close
inspection only indistinct contours can be seen (fig. 501). Motion
is confined to bending of the whole body, as is familiar in the
pup of moths and butterflies. The pup coarctate are without
motion because here the pupa (in structure a pupa libera) is en-
closed in a larger coat, the last larval skin (Musearia).

The variations among larve are even greater than with pupe.
ILere structure and jointing of the body are so completely under
the influence of environment that with similar or different con-
IV. INSECTA: HEXAPODA. 475

ditions larve widely remote from the systematic standpoint may
closely resemble each other, while those of closely related species
may differ extremely. The leaf-feeding larvee of Lepidoptera (fig.

 

Fie. 501. Fie. 502. Fie. 503.

Fia. 501.—Pupa of Sphinx ligustri. (After Ludwig-Leunis.) 1, eye; 2, head ; 3, an-
tenne; 4-6, thoracic somites; 7, hind, s, fore wing; 9, legs; 10, proboscis; 11, ab-
dominal somites; 12, spiracles.

Fia. 602.—Larva of Sphing ligustri. (After Ludwig-Leunis.) 1, caudal disc ; p, tho-
racic feet ; ps, prolegs.

Fig. 503.—Larva (maggot) of blowfly, Musca vomitoria. (After Leuckart.)

 

502) and Tenthreds are brightly colored, the thoracic appendages
remaining small, and are reinforced by the fleshy ventral append-
ages, the prolegs or pedes spurii. The predacious larve of many
beetles and Neuroptera have long thoracic legs, strong mandibles,
and no prolegs. Other beetle larva, which burrow in wood or
live in the earth (fig. 500), have plump whitish bodies, with the
legs rudimentary or wholly lacking. These lead to the maggot-
like larve, in which the mouth parts are inconspicuous and the
distinction between head and thorax may vanish. Such soft-skinned
annulated sacs occur in the bees (fig. 59) and other Hymenoptera,
as well as in many flies (fig. 503); that is, in animals which live
in an abundance of food either because of parasitism or because
the mother has provided plenty.

From the outer appearance one would gain the impression that
these holometabolous larve not only lacked the wings, but that the
appendages of the imago were entirely absent or had an entirely
different form; farther, that wings, and frequently antenna, legs,
and mouth parts, come into existence at the moment of pupation,
and then in remarkable size and completeness. A more accurate
investigation shows that the anlagen of all these structures are
formed long before pupation, often at the first molt. The wings
476 ARTHROPODA.

of a butterfly are present in the caterpillar as small folds or proc-
esses of the surface which increase in size with each molt. That they
are not visible externally is due to the fact that they are pushed

 

Fie. 504.—Diagram of development of wings and legs from the imaginal discs of a
fly during metamorphosis. (After Lang.) kh, larval hypodermis; 7, imaginal
hypodermis ; J, , imagine] discs and legs and wings formed from them; s, con-
nexion of discs with hypodermis; x, chitinous larval skin.

  
 

into the body and enclosed in sacs opening to the exterior. Such
anlagen are called imaginal discs; with their recognition the dis-
tinctions between complete and incomplete metamorphosis in part
disappear, since in the first the structures of the imago, even if in
a modified shape, are outlined very early. Still there remains
much to be remodelled during the pupal rest. The muscles must
be adapted to the new locomotor organs, the digestive tract to the
altered food, the nervous system re-formed. Since a great part of
the previous structures must be broken down to afford material
for the reconstruction of the organs, the pulpy nature of the inside
of the pupa is easily understood. In a rapid degeneration of the
tissues the material, consisting of indistinctly separated cells, is
so homogeneous that it was formerly thought that the pupa re-
turned to the indifferent condition of the egg (Histolysis of flies).

In the classification four points are of special importance: (1) The
segmentation of the body, in which it is to be noted whether the segments
of thorax and abdomen follow without change of form, or whether the
thorax, by the closer union of its somites, is sharply marked off from both
head and abdomen, (2) The character of the wings, which are either
lacking in the lower forms or are delicate chitinous structures, with
numerous veins, the wings of the two thoracic segments similar. In the
higher forms a degeneration of the wing veins or a leathery consistence of
the membrane, together with a divergent development, partial reduction
of antennee and posterior wings may occur. (3) The structure of the mouth
parts, and (4) the type of development, both described above. From these
characters it is easy to differentiate six orders: Lepidoptera, Diptera,
Aphaniptera, Rhynchota, Hymenoptera, and Coleoptera. The remaining
forms were formerly divided among the Orthoptera and Neuroptera, but
IV, INSECTA: HEXAPODA, APTERYGOTA. ATT

these groups are not considered natural and the attempt has been made to
divide them into more or fewer groups. Here the Pseudoneuroptera or
Aphaniptera are separated from the Neuroptera, the wingless forms or
Apterygota from the Orthoptera.

Order I. Apterygota.

At the bottom of the Hexapoda come forms which lack wings
and which show no evidence of having descended from winged an-
cestors. They are regarded as slightly modified descendants of
the ancestral Hexapod. Besides the lack of wings they show many
primitive characters; compound eyes are poorly developed or lack-
ing; the tracheal system, when not degenerate, consists of isolated
tracheal bushes, rarely connected by longitudinal trunks (fig.
479); the mouth parts, resembling somewhat those of Orthoptera,
are for biting, though frequently rudimentary; the development
is always ametabolous.

Sub Order I. THYSANURA (Bristle-tails). Body  , '
elongate, with long bristles (cerci) at the hinder end. \,
Lepisma saccharina,* silver fish, common among old
books and papers, does considerable damage. It is
covered with shining scales. Campodea* (fig. 400),
with rudimentary abdominal appendages. Jachilis,*
Lapyx,* with caudal forceps.

Sub Order II. COLLEMBOLA (Spring-tails). Com-
pressed forms in which the bristles bent under the body
serve as a spring, throwing the animals (one to three
mm. long) forwards. Podura*; Anurida maritima,*
in tide pools ; Entomobrya*; Lipura*; Achoreutes ni-
valis,* the snow flea.

 

Order II. Archiptera (Pseudoneuroptera). }

Sater : Fie. 505.—Lepis
These represent the primitive forms of winged “'sicchasinat sil,

insects. The elongate body consists of numerous — [Ur ashe, (After
segments and usually bears the cerci of the Thysanura. The
wings are delicate and transparent, supported by a close net-
work of nervures, both pairs being very closely alike. The
mouth parts are of the typical biting kind; the maxille have
lacinia and galea; the labium, with glossa and paraglossa, is
frequently deeply cleft. These points of primitive structure
are correlated with a primitive, usually hemimetabolous de-
velopment. The distinction between larva and imago is largely
one of presence or absence of wings, although larval organs like
gills (Amphibiotica) may occur. Frequently the development is
direct when the imagines, as in some Termites and the Psocide,
are wingless.
478 ARTHROPODA.

The Archiptera were formerly united with the Neuroptera on account
of similarities of wings. The separation is due to characters of mouth
parts and development.

Sub Order I. CORRODENTIA. Larve distinguished from the im-
agines by difference in size and, in the winged forms, by lack of wings.
Best known are the TeERMITIDA (Isoptera), or white ants, which must not
be confused with the true ants (Hymenoptera), from which they are dis-
tinguished by the similar body segments, the mouth parts, and the simple
development. Like the true ants, they have a well-developed social state.
A colony of termites, consisting usually of thousands of individuals, forms
a nest with numerous chambers and passages. They are nocturnal, and
they burrow, without coming to the surface, through old wood (timbers
of houses, furniture, picture frames, dead wood in the forest, ete.). They
line these chambers with a cement-like substance composed of refuse which
has passed through the alimentary canal. Many species build dome-like
nests, ten or fifteen feet high, fifteen to twenty or twenty-five feet across,
of chewed earth. Ina colony are winged and wingless individuals, the
latter with ametabolous development (fig. 506). The wingless forms have
the sexual organs rudimentary, but, in contrast to ants and bees, may
belong to either sex. They are frequently blind, have strong mandibles,
and are of two kinds, the workers (c) and the large-headed soldiers (d).
The winged forms are sexually functional (0). Shortly after the metamor-
phosis they swarm, and then the wings are bitten off at the base and
‘king’ and ‘queen’ either form a new colony or enter one already in
existence. After copulation the abdomen of the queen, by the formation
of numerous eggs, swells to an enormous size (e}. Since the swarming

 

Fia. 506.—Termes flavipes,* white ant. (From Riley.) a, larva; b, winged male; ¢,
worker ; d, soldier; ¢, queen; f, pupa.

individuals form the prey of birds and other animals, it often happens that
a colony is left without a royal couple. In such cases the line is perpet-
uated by reserve males and females, sexual animals which have not com-
IV. INSECTA: HEXAPODA, ARCHIPTERA. 479

pleted the metamorphosis but are in the Wing-pad stage. The termites are
able, by quantity and quality of food, to modify the development of the
larvee and to determine which type of individual shall be produced. The
termites are farther noticeable for the bitter wars they conduct against
the true ants. Zermes flavipes* in our northern states. 7 Satalis,
Africa,

Allied to the Termites are the often wingless Psocip#, or book lice.
Troctes divinatorius* is the book louse. Other species are winged and
live in moss, ete. Near here also belong the MALLopHaGa, which, like
lice, live upon mammals and especially on birds. Like true lice they are
wingless, but they have biting mouth parts. 'richodectes,* on the dog,
ox, etc.; Gontodes,* Docophorus,* Nirmus,* ete., on birds.

Sub Order II. AMPHIBIOTICA, The three families united here
differ much in structure, but agree in having aquatic larvee with tracheal
gills (fig. 495). These are ventral bushes in the Perlide, wing-like or
bushy appendages of the abdomen in the Ephemeride, and
three-leaved appendages in those Odonata which do not
respire by tracheal branches in the rectum. All of these
larve are predaceous, especially the larvee as well as the
adults of the Odonata. The Odonate larve have a peculiar
apparatus for the capture of prey. The mentum and sub-
mentum of the labium are greatly elongate and when
folded bring the tip like a mask beneath the mouth. The
structure can be suddenly extended (fig. 507) and grasps

  

Fia. 507. Fa. 508,

Fic, 507.—Larva of -Eschna grandis. (After Risel von Rosenhof.) «, a?, wing pads;

m, mask ; st, spiracles. : j
Fic. 508.—Ephemera vulgata. (From Schmarda.) The caudal bristles incomplete.
the food. PERLID# (Plecoptera); hind wings the larger. Perla,* Ptero-
narcys,* stone flies. EPHEMERID#&, fore wings large, the hinder small or
absent ; May flies, life very short in the adult state. Hphemera,* (fig. 508)
Cleon,* Betisca.* OpboNATA (Libellulide), wings nearly equal, the hinder
slightly larger ; Dragon flies, veritable insect hawks destroying numberless
mosquitos. Libellula,* Aischna,* Agrion,* Gomphus.*

Sub Order IIT. PHYSOPODA (Thysanoptera). Wings slender, fringed
with hairs; tarsi bladder-like at tip; mouth parts bristle-like, probably
used for sucking. The position of this group is very uncertain. Thrips,*
Limothrips.*
480 ARTHROPODA.

Order III. Orthoptera.

Like the Archiptera these are hemimetabolous or in a few
cases ametabolous, and the mouth parts (fig. 486) are fitted for
biting, the mentum being cleft. On the other hand the wings
have lost the delicate membranous character and have become
more parchment-like, the fore wings being smaller and serving as
covers for the larger, softer, and folded hind wings, which are the
efficient organs of flight; the condition in these respects recalling
somewhat the Coleoptera. The abdomen bears cerci and _fre-
quently stylets. In internal anatomy the large number of Malpi-
ghian tubules is noticeable (fig. 491).

Sub Order I. CURSORIA. With rather long legs fitted for rapid run-
ning. Only the cockroaches (BLATTID®) belong bere. Wings may be
absent, according to the species, in either sex, but more frequently in
females. The more common cockroach, the ‘ Croton bug’ (Blatta ger-
manica *), is a well-known pest in houses. The larger Periplaneta orien-
talis * is common in ships and bakeries. Other species in our woods.

Sub Order I]. DERMATOPTERA (Euplexoptera). Front wings short
elytra ; the hinder wings being folded crosswise and packed beneath them,
or rudimentary ; circi developed to a forceps-like structure terminating
the body, whence the name Forficula* given one genus. Labidura.*
These forms are often called earwigs, from an erroneous belief that they
enter the human ear and injure the drum. The group on account of its
wing structure is often made a distinct order.

Sub Order III. GRESSORIA. Legs long, slender, adapted to a slow
walking motion. In the Mantip# the prothorax is very long and bears a
pair of long raptorial feet which when at rest are held in a position which
causes these insects to be known as ‘praying Mantes.’ Phasmomantis,*
warm countries. PHASMID&, with short prothorax, almost exclusively
tropical, represented throughout northeastern United States by Diaphero-
mera femorata,* the walking stick. The members of this family are
noted for their mimicry of twigs and leaves (fig. 12.)

Sub Order IV. SALTATORIA. Hinder legs long, strong, and for
jumping ; the other pairs much smaller. Hinder femora large and muscu-
lar, tibiee elongate and spined. Wings usually functional and in the
migrating species capable of sustained flight. Produce sound (stridulate)
by rubbing the anterior wings together (Locustide, Gryllide) or against
the legs (Acridiide). Tympanal apparatus (p. 468) on the anterior tibize
(Locustide, fig. 493, and many Gryllide) or on the first somite of the
abdomen (fig. 492). Stridulation occurs only in males, and in our com-
mon crickets the number of notes is directly dependent upon tempera-
ture, which, on the Fahrenheit scale, may be determined by the formula,

; n—d40. arate P
T= 50 4+ ae in which Z stands for temperature and 2 for number
IV. INSECTA: HEXAPODA, NEUROPTERA. 481

of chirps per minute. The females may readily be recognized by the ovi-
positor, AcRIDIID#; antenne and ovipositor short; tympani abdomi-

 

F1a. 509.—Locusta caudata. (After von Wattenwyll.) Jd, ovipositor.

nal. Acridium* ; Melanoplus* (Mf. spretus, the ‘grasshopper’ which
did such damage in the Missouri River States in 1878-75) ; Gdipoda* ;
Tettix.* LocusTip® ; antenne long; tympani on first tibiz ; ovipositor
long, flattened ; tarsi four-jointed. Hadenacus,* wingless, blind, in caves;
Conocephalus* ; Cyrtophilus* and Microcentrum, katydids ; Anabrus,*
wingless. GRYLLID#, Crickets: antennz long; ovipositor long, cylindri-
cal ; tarsi three-jointed ; tympani on first tibia. Gryllus*; Geanthus,*
tree crickets; Gryllotalpa,* mole crickets, burrowing.

Order IV. Neuroptera.

The Neuroptera closely parallel the Archiptera, and the twe
were formerly united, since they have the same wing structure and

 

Fig. 510.—Myrmeleo formicarius. (From Schmarda.) 1, imago; 2, larva; 3, pupa in its
cocoon.

show in general appearance great similarities. Thus the ant lions
(fig. 510) recall the dragon flies; the Chrysopine, the Perlide.
The Neuroptera, however, are holometabolous and have a resting
stage, although the pupx (pup libere) are capable of some mo-
tion. The mouth parts are for biting, and in some the labium has
no notch in the middle.
482 ARTHROPODA.

Sub Order I. PLANIPENNIA. Biting mouth parts. SIALIDA, wings
well developed, mouth not rostrate, larvae aquatic, commonly called dob-

 

Fia, 511.—Corydalis cornutus,* hellgrammite, male. (From Riley.)

 

Fig. 512.—Phrygqanea grandis, (From Schmarda.)
sons. Corydalis,* hellgrammite ; Sialis*  TleMEROBIID®, lace wings ;
wings well developed, mouth not rostrate; larve with sucking mouth
IV. INSHCTA : HEXAPODA, COLEOPTERA. 483

parts, predaceous. Chrysopa * feeds on plant lice ; Myrmeleo,* ant lions ;
larvee dig funnel-like pits in sand and capture the ants, ete., which fall
into them. PANORPID. (Mecoptera) ; mouth prolonged into a rostrum ;
Panorpa, * Bittacus.*

Sub Order Il. TRICHOPTERA (caddis flies). Wings usually large ;
mouth parts rudimentary, forming a short sucking tube which, with the
wings covered with hair-like scales, recalls the Lepidoptera ; larvee aquatic
with tracheal gills; build cases of foreign matter, stones, sticks, ete., in
which, like a hermit crab, they live; pupation occurs in these tubes.
Phryganea* (fig. 512), Hydropsyche.*

Order V. Strepsiptera.

These forms, comprised in a single family, STYLOPID#, are parasitic on
the Hymenoptera, The six-legged larvee (fig. 513, 3) press in between the
ventral abdominal plates of bees or wasps and pupate there. The quickly

 

Fic, 513.—Xenos rossi. (After Boas.) 1, female; 2, male; 3, larva; J-IIJ, thoracic
somites; «!, rudimentary fore wing ; a’, hind wing.

flying male (2) escapes from the pupal skin ; it recalls somewhat a beetle ;
has small fore wings and large hinder ones. The wingless, legless female
(1) remains in the pupal skin and is fertilized there; she is viviparous.
Insects infested with these parasites are ‘stylopized.’ The affinities of
the order are doubtful. The forms are frequently included with the

beetles. Stylops,* Xenos.*
Order VI. Coleoptera.

The beetles are the highest of the Ilexapoda with biting mouth
parts. They are closest to the Orthoptera, as is shown by the
structure of mouth parts and wings. The mandibles are strong;
the maxille (fig. 514) have Jacinia and galea; the labium consists
of asubmentum (often called mentum), behind which the rudi-
mentary mentum with its palpi, paraglosse, and gloss (the latter
frequently fused to a ligula) are retracted. (In the genus
Nemognatha the maxillary galea form a sucking organ.) The
group is distinguished from the Orthoptera by the holometabolous
development with pupx liber, while the larve (fig. 500) show
many modifications corresponding to the mode of lite. Another
character is afforded by the wings. The anterior pair, separated
at the base by a scutellum, are hard elytra not fitted for fight, and
484 ARTHROPODA.

from these the order receives its name Coleoptera, sheath wings.
Under the elytra are protected the delicate much folded hinder
wings, the organs of flight (lacking in insects with fused elytra).
Since the second and third thoracic wings and those of the abdo-

 

Fig. 515.
ec, cardo; le, galea ; li, lacinia ; pm, palpus;

Fie. 514.

Fig. 514.—Maxilla of Procrustes coriaceus.
st, stipes.
Fic. 515.—Calosoma sycophanta.

men are covered by the elytra, these are soft above. Externally
the relations of the elytra cause a regional division peculiar to the
beetles (fig. 515): head, prothorax, and a third division composed
of meso- and metathorax plus abdomen covered by the elytra.

The numerous species of beetles—over 100,000 are described—are sub-
divided into normal forms and Rhynchophora, the normal forms being
subdivided again upon characters derived from the tarsi as follows:

Sub Order I. PENTAMERA. Tarsus five-jointed, the last club-shaped
and bearing the claws, while the other four are short and somewhat heart-
shaped (fig. 516, a). This is the largest division and
contains the tiger beetles (CICINDELID#) and the pre-
daceous CARABIDS (fig. 515) ; the water beetles, HYDRO-
PHILIDEH and DyTiscip# ; the LAMELLICORNIA or SCARA-
BEID&, represented by the ‘ June bugs,’ Ifelolontha,* and
the large tropical Dynastes ; the fire flies, LAMPYRID£ ;
the rove beetles, STAPHYLINID.E, ete.

Sub Order II. HETEROMERA. First and second

(After Ludwig-Leunis.)

 

Fie. 516.—a, pen-
tamerous tarsus
of Dytiscus; b,
cryptopentamer-
ous tarsus of
Coccinella; t,
tibia ; *, reduced
tarsal joint.

TENEBRIONID® liv
Order ITT.

Sub

legs pentamerous, third apparently four-jointed ; few
forms belong here, among them the ‘oil bottles’ (ME-
LoID#) and the blister beetles, CANTHARID, both of
them containing a peculiar substance, cantharidin,
which renders the Spanish flies, Lytta vesicatoria, an
important ingedient of blistering plasters. Some of the

ein the larval stages in flour, ete.

TETRAMERA (Cryptopentamera). Tarsi with the
IV. INSECTA: HEXAPODA, HYMENOPTERA. 485

penult joint rudimentary, giving the impression of four joints. The
families included here, very numerous in species, are injurious to vegeta-
tion. The larvee of the long-horned CeRamBycip£ bore in wood. The
CHRYSOMELID#, of which the Colorado potato beetle (Dery-
phora decemlineata) is the most notorious, feed on
leaves.

Sub Order IV. TRIMERA ; tarsi with penult and anti-
penult joints rudimentary, so that they appear three-
jointed. Best known are the CoccINELLID&, or lady birds,
whose lurve, because of their destruction of plant lice,
etc., are of value to man.

Sub Order V. RHYNCHOPHORA, snout beetles ; head
produced into a long snout, at the apex of which are the
mouth parts. Here belong several families of weevils, pyo.517—xalani-
some of which do damage to grain, nuts, timber, ete. ene
Curculio,* Conotrachelus,* Calandra,* Hylesinus,* Bala- — vil.
ninus * (fig. 517).

 

Order VII. Hymenoptera.

The Hymenoptera, of which bees, wasps, and ants are well-
known representatives, have biting jaws, while the other mouth
parts are elongate and in a minority of the group converted into a
sucking organ. In the bees (fig. 487) the glossa unite, producing
a nearly closed tube, which lies in a sheath formed by the other
mouth parts, the mandibles alone retaining the primitive form.
Since mouth parts vary, the structure of the wings and body seg-
mentation have great value in defin-
ing the order. The wings are mem-
branous and are supported by few
nervures (fig. 518), and in flight they
act as one pair, since the two are
usually connected by hooked bristles
on the hind wing, which engage in
a groove on the hinder margin of
the front wing. The fore wings are
the larger and, correspondingly, the
mesothorax exceeds the other tho-
racic somites, so that these, especially
Fic, 518.—Sirer giyas, saw fly. (After the prothorax, seem but parts of the

Taschenberg.) strong mesothorax. Besides, the first
abdominal ring unites to the thorax so intimately in the En-
tophaga and Aculeata as to seem part of it. The constriction
which then separates thorax and abdomen comes between the first
and second abdominal somites, and when the second (petiole) is
elongate the stalked abdomen, familiar in the wasps, results.

 
486 ARTHROPODA.

The sexes are readily distinguished by the genital armature.
The female is provided with the ovipositor already described, which
when used for this purpose only, projects from the hinder end of
the body (fig. 518), but when used as a sting (aculeus), can be
drawn back in the body when at rest. The sting, naturally lacking
in the male, is connected with a poison gland, the secretion of
which owes its effect, not, as once believed, to formic acid, but to
a little known basic substance.

The distinction between ovipositor (terebra) and aculeus affords charac-
ters of systematicimportance; othersare furnished by the development, which
is always holometabolous. The pup, in all important points, are similar
(pupe liberee), but two kinds of larve are distinguished. Some have larve
with well-developed legs; many cf them are green in color and distin-
guished from the larve of Lepidoptera by the greater number of prolegs.
Others have footless larvee (fig. 59). The first occur where the larva must
shift for itself, the second where it is surrounded by an abundance of
food, either provided by the parents or by the host in which it is parasitic.

Sub Order J. TEREBRANTIA. Terebra present ; larvee with feet at
least on the thorax; eggs laid on leaves or in wood, usually without gall
formation; the larve therefore must move in order to feed. The TEN-
THREDINID®, saw flies, feed on leaves and have caterpillar-like larve.
Cimbex, Nematus. Srricipa (Uroceride), horn tails, the larve bore in
wood and are whitish.

Sub Order Il. ENTOPHAGA. Terebra present; larve legless, para-
sitic in galls or in animals, Some use the ovipositor to lay their eggs in
leaves, roots, or stems of plants. Galls are then produced, diseased struc-
tures by which the larve are nourished. Others use the ovipositor to lay
their eggs on or in other insects and larvee. The young feed on the tissues
of the host and at last cause its death, often before the completion of the
metamorphosis. The gall-producing forms are the CyNIPID.£, some of which
afford examples of heterogony (p. 145), in which the alternating generations
are distinguished by different structure, by sexual
and parthenogenetic reproduction, and by different
kinds of galls. So different are the two generations
that they have frequently been described as differ-
ent genera. The inquilines do not form galls, but lay
their eggs in the galls of other species. The insect
parasites are divided among several families, the more
prominent being the ICHNEUMONID.£, BRACONID.E, and
CHALCIDID.A, those of the first being large, the others
Fie. 519.—Chaleés flavi- small or minute. These forms are of immense value
pes.* (After Howard.) to agriculture, as they keep down, as no economic
entomologists or insecticides can, injurious forms.

Sub Order III, ACULEATA. Females with stings; larvae footless,
magyot-like. The digger wasps or Fossores excavate tubes in the earth in
which they lay their eggs and then bring into the holes as nourishment

 
IV. INSECTA: HEXAPODA, HYMENOPTERA. 487

other insects which they have paralyzed by a sting in the ventral cord.
Some true wasps have similar habits. Most wasps (VESPARL®) and bees
(APIARL#) have different habits. They build wonderful homes of chewed
wood (the first pulp paper) or skilfully trimmed leaves, earth, ete., or of
wax Which the animals (bees) secrete between the joints of the abdomen.
The nests, which are to contain the young, are either small tubes or
hexagonal cells which are united to horizontal or vertical combs; the food
is either honey, pollen, or chewed fruits. The fact that the offspring are
better protected when numerous individuals protect them has apparently
led in the wasps and bees to different grades of social states. The honey
bees (Apis mellifica*), which live in a colony, consist of three kinds of
individuals distinguished by structure of the head (fig. 520) and other

  

Fia: 520.—Heads of Apis mellifica. (After Boas.) a, queen; ), worker; c, drone with
the compound eyes meeting above.

features: a single queen, some hundred drones, and about ten thousand
workers. These last are females and hence have stings, but have rudi-
mentary functionless sexual organs; their work being to build the home,
to protect the young, and provide food for the winter. The business of
egg-laying belongs to the queen, who copulates but once, at the beginning
of her reign, when she and a drone take a wedding flight. For the four
years of her life the sperm retains its vitality in the receptaculum seminis.
In laying the eggs she can permit entrance or not of the spermatozoa at
will and thus produce males or females. A queen who has not been fer-
tilized or who has exhausted her supply can only lay drone eggs. The
further fate of the eggs depends upon the food of the larvie; with a small
amount of bee bread (pollen) workers are produced, but the same larva
placed in a larger cell and fed with the ‘royal jelly’ (much like blane
mange in appearance) will develop into a sexually mature queen. On
escape from the queen-cell the young queen with a part of the colony
swarm and start a new colony. This operation may be repeated once or
twice, but if there be danger of depleting the hive the remaining queen
larvee are killed. Wasps and bumble-bee coionies last but a year and are
reformed by a fertilized female which has lived through the winter.

The ants (FORMICARI&) have gone beyond the bees in the social organi-
zation. They have also departed most from the other Hymenoptera in
that the workers, sometimes the sexual individuals, are wingless and the
sting is rudimentary or entirely lacking. Only the Poneride sting like
bees and wasps; the others bite and squirt the secretion of the persistent
poison gland (formic acid) into the wound, The homes of the ants are
488 ARTHROPODA.

less wonderful than those of the
quently more complicated. In

bees, but their social organization is fre-
the colony occur the wingless workers

(rudimentary females with wing pads in larval life which are lost in pupa-

 

Fig. 521.

Fie. 522.

Fig. 521.—Myrmecocystus melliger,* honey-sac ant. (Orig.)
Fie. 522.—Plant of Hydnophyton. (After Forbes.) Showing the bulb occupied by

ants.

tion), and of these frequently there are different kinds, large-neaded
soldiers and small-headed workers ; ‘honey sacs’ in Myrmecocystus ; and
the sexual animals, queens and drones, which copulate in a marriage
flight. The queen after the flight returns to the colony. Frequently

 

Fig. 523.—Head of Cicada septendecim,
the mouth parts separated (orig.).
a, antenna; e, compound eye; /, la-
bium ; md, mandible ; ma, maxilla.

other insects, like the Aphides, are con-
nected with the colony, these being kept
for the honey dew they produce. Many
ants steal the pupe of others and keep
the adults when they emerge as slaves.
In Polyergus rufescens this has gone so
far that the masters cannot care for
themselves and must be fed by the slaves ;
otherwise they die. The ants possess
extreme interest on account of their care-
fully planned wars (Ecitons) ; on account
of their relations to plants, some species
making nests in the growing plant and
protecting it by their bites; the leaf-
cutting ants take leaves from trees and
carry them into their underground nests
for the cultivation of fungi on which
they feed; the agricultural ants from
their plantations and stores of grain, and
the honey ants from the fact that certain
workers (fig. 521) act as reservoirs of
honey, these ‘ honey sacs’ swelling up to
enormous size.
IV. INSHOTA: HWEXAPODA, RHYNCHOTA. 489

Order VIII. Rhynchota.

The Rhynchota, or bugs, in their external appearance are
nearest to the Archiptera and Orthoptera. The head, thorax, and
abdomen are joined in the same way; the development is hemi-
metabolous, and in the wingless species ametabolous. In some
cases, as the Cicadas with their membranous wings, the confusion
with the Orthoptera has led to these being called locusts; on the
other hand the delicate-winged Aphides resemble the Archiptera.
Yet all Rhynchota may be recognized by the sucking proboscis
(fig. 523), consisting of the grooved labium in which the needle-
like mandibles and maxille play. The wing structures afford the
basis of division into three sub orders.

Sub Order I. HEMIPTERA (Heteroptera). Anterior wings hemelytra,
z.e., leathery at the base, soft and elastic at the tip (fig. 524); between the

 

Fia. 524.—Pentatoma rujipes. (From Hajek.) s, scutellum.

hemelytra is a conspicuous triangular scutellum (s) which covers more or
less of the dorsal surface. Hemelytra and scutellum occasionally disap-
rear. A further characteristic is the presence of stink glands, producing
, most disgusting odor, which open in the adults ventrally on the meta-
thorax; in the larvee dorsally on the abdomen. According to habits the
many families may be grouped into the aquatic Hyprocores and the
terrestrial Grocores. Of the first the BELostomIDm are noticeable from
their size, Belostoma americana* being nearly 2} inches long and capable
of inflicting severe wounds. Other families are NEPID® (Ranatra, water
scorpion), NOTONECTID#, HyDROBATID#, ete, Of the Geocores the Repu-
vub#&, which feed on other insects; the ACANTHIIDA (Acanthia lectuaria,*
the bed bug); the LyGa#pa, containing the chinch bug, Blissus leecop-
terus,* so injurious to grain; and the PENTATOMID#, or stink bugs, may be
mentioned.

Sub Order II. HOMOPTERA. Wings, when not degenerate, similar in
texture throughout, although often differing in size. They are either parch-
ment-like or delicate membranes. The CICADID.Z, represented by Cicada
septendecim,* the seventeen-year ‘locust,’ and C. ¢ébicen,* or dog-day har-
vest fiy, are noticeable from their shrill notes, produced by a stridulating
drum on the abdomen. C. orn of the Old World fig. 526) punctures ash
trees, causing the flow of manna. The CERCOPID# contains: the spittle
bug (Aprophora *) which causes drops of foam on grass. The leaf hoppers,
490 ARTHROPODA.

 

Fia. 525.—Cicada septendecim,* seventeen-year locust. (From Riley.) a, pupa; b, pupa
case from which the imago, c, has escaped; d, twig bored for oviposition.

 

Fie, 526.—Cicada orni. (From Schmarda.)

or JASSID#, contain some injurious forms, Erythronura vitis damaging
the grape, while the true hoppers, MEMBRACID& (fig. 527), ave scarcely less
injurious. None of these, however, are such serious
pests as the plant lice and scale insects. In the
Coccip#, or scale insects, the wingless female dies
after laying the eggs and covers them with her dead
scale-like body. Here belong the cochineal insects,
Coceus cacti,* the dried bodies of which furnish the
pigment carmine, and the lac insects, Coccuws lacca,
as well as a host of injurious forms, like the orange
scale, Aspidotus aurantii,* and the worse San José
scale, A. perniciosus,* which has recently been spread
throughout the country. The APHID, or plant lice,
are soft-skinned and with their honey-containing
excrement form a substratum for the growth of
Fia. §27.—Cerese buba. Jurious fungi. They reproduce largely by parthe-
lus,* buffalo leaf hop- nogenesis, a reason for their rapid multiplication ;

per. (After Marlatt.) j : 2 ‘i nia
but their spread is not rapid, since the usually vivip-

 
IV. INSECTA: HEXAPODA, DIPTERA. 491

arous females are wingless. At times winged females appear and spread

 

Fie. 528.—Phyllorera _vastatrix. (From Ludwig-Leunis.) 1, winged generation:
2, grape root, with nodosities (a) caused by Phylloxera; 3, wingless root-generation.
the pests. Winged males appear in the autumn, and the fertilized eggs
endure the winter. Of all the species none
is more injurious than the Phyllowera vasta-
trix * of the grape, which with us does slight
damage, but in Europe has destroyed whole
vineyards. This is one of our returns for the
many pests the Old World has sent us.

Sub Order III. APTERA. Wingless bugs
with direct development, commonly known
as lice, of which three species attack man,
one living in the hair (Pediculus capitis*),
the others (P. vestimentorum * and Phthirius Fic. 529.—Phthirius inguinalis,
i : F eer crab louse. (After Leuckart.)
inguinalis*) upon the body. Other species
live cn other mammals.

 

Order IX. Diptera.

Like the Rhynchota, the Diptera, or flies, are sucking insects,
but the sucking tube or haustellum is different, here consisting
of a tube formed of both labium and labrum, and containing
stylets which include, besides mandibles and maxillw (often rudi-
mentary), the hypopharynx (fig. 488), the maxillary palpi being
present. Only the anterior wings (hence Diptera) are well de-
veloped, the hinder wings being replaced by the halteres or bal-
ancers, small drumstick-like structures richly supplied with nerves
and functioning as organs of equilibration. The thorax is, as in
492 ARTHROPODA.

 

Fra. 530.—Musca, house fly (orig.).

the Hymenoptera, sharply marked off from head and abdomen, its
somites being frequently fused. The development is holometab-
olous, two kinds of larve and pupe occurring
in its course. The larve are always apodal, but
have either a distinct head with biting mouth
parts or they are headless and have a rudimen-
tary sucking apparatus (fig. 531). The pup»
are correspondingly either free with powers of
motion, or are pup coarctate (p. 474). De-
velopment thus affords characters of systematic
importance, and these are supplemented by dif-
ferences in length of legs, antenna, haustellum,
Fie, 531.—Larva of and in body forms. In number of species the
rie (atige Geuck. Diptera stand next to the Coleoptera; in num-
oe ber of individuals they far exceed them.

Sub OrderI. NEMOCERA. Elongate with long, many-jointed antenne,
long proboscis, long legs. The larvie live in damp places or in water, where,
lacking legs, they swim by movements of the body. The pupz can also
swim well. Best known are the innocuous crane flies (TIPULID) and the
mosquitos (CULICIDA) with their numerous species affecting man, among
them the forms which carry yellow fever, and Anopheles,* which. distribute
malaria. The Cecrpomyipas include the injurious Hessian fly, Cectdomyia
destructor,* and the pwedogenctic Miastor (fig. 498).

Sub Order II. TANYSTOMA. Resemble the Muscarie (with which

 
IV. INSECTA: HEXAPODA, APHANIPTERA., 493

they were formerly united) in the short stout bodies, short antenne and
legs. They are distinguished from them and resemble the Nemocera in
their long proboscis and in development. The larve and pupe live in
damp places or in water and move rapidly, the larve having biting mouth
parts. Here belong the black flies, SImULmD#, which excel the mosquitos
in their viciousness, and the horse flies, TABANIDa&, the females of which
attack cattle and men, as well as horses, with their painful bites,

Sub Order III. MUSCARL# (Brachycera, after removal of Tanystoma).
Body short, stout; antenne three-jointed with a bristle (arista) (fig.
532) ; legs short, ending in an adhesive organ (pulvillus) ; larvee headless

   

Fic. 582. Fie. 533.
Fig. 532.—Left, Erax bastardi, robber fly ; right, antenna of Muscid showing arista

at a.
Fia. 533.—Gastrophilus equi,* bot fly. (From Hajek.) h, halteres.

living in decaying substances or parasitic in other animals. The Mus-
crip include the house flies (JZusca domestica * and other species), the blow
fly (Calliphora vomitoria*), and the flesh fly (Sarcophaga carnaria*), which
is viviparous. The ASILID.£, or robber flies, prey on other insects, as do
some of the SyRPHIDZ: Hristalis* of this family has an aquatic ‘rat-
tailed larva,’ one end being drawn out into a long breathing tube.
(EsTRID#H, bot flies ; the larvee always parasitic; those of the sheep bot
(Estrus ovis*) in the frontal sinuses of the sheep, causing the disease
called ‘staggers’; those of the ox warble (Hypoderma lineata*) just
beneath the skin of cattle; those of the horse (Gastrophilus equi,* fig.
533) in the stomach of the horse. In the tropics Dermatobia noxialis
lives as a larva in the human skin.

Sub Order IV. PUPIPARA. Very active, often wingless forms living
as parasites on mammals and insects; larval development inside the
mother; pupation occurring soon after birth.
Melophagus ovinus,* sheep tick; Braula ceca,*
bee louse.

Order X. Aphaniptera (Siphonaptera).

In spite of the lack of wings the fleas are
closely related to the Diptera, since they
have doubtless descended from winged forms,
as jig shown by the fact that they have a erent
holometavolous development. The larve,
long and footless, live in decaying wood or dust in cracks in the

 
494 ARTHROPODA.

floor, etc., and give rise to pupx, both without traces of wings.
Yet fleas and flies differ in that the fleas have similar body somites
but the haustellum is lacking, the sucking tube being formed
of labrum and mandibles, while the sharp maxille puncture the
skin. Besides Pulex irritans,* the flea that attacks man, many
other species occur on other animals. In warm countries the jigger
or chigoe, Sarcophsylla penetrans,* attacks man, the female boring
into the skin, usually under the nails, and there laying the eggs.

Order XI. Lepidoptera.

This group of butterflies and moths is the most sharply limited
of any order of Hexapods. The wings, both pairs of which are
well developed (rarely lacking, as in many female Psychide and
some Geometride), are covered with scales (flattened hairs), and
to these are due the frequently brilliant color patterns. Frequently
the fore and hind wings are united by hooks (frenulum) on the
latter, engaging in a redinaculwm in the fore wing. The mesothorax
is large and the smaller pro- and metathorax are closely united to
it, giving the region a distinctness from head or abdomen. The
mouth parts are peculiar (fig. 489), although foreshadowed in the
Phryganids, and not fully developed in the Microlepidoptera. The
mandibles are rudimentary or absent, while the fused maxilla,
greatly elongate, form the proboscis. Maxillary and labial palpi
are present, the former smaller and often degenerate. The de-
velopment is holometabolous; the larve, frequently called cater-
pillars (fig. 502), have biting mouth parts, the mandibles very
strong; and also silk glands (sericteria), a pair of internal organs
which open together on the labium and produce a secretion hard-
ening to silk; besides the thoracic legs, prolegs, two to five pairs,
are present. The pupe are usually pupe obtecte, and are rarely
free. In some the pupe are ornamented with golden spots, whence
the name chrysalides often applied to them.

Sub Order I. MICROLEPIDOPTERA. Small, inconspicuous; at rest
holding the wings horizontally over the back ; maxillary palpi very large ;
proboscis small. TINEID#; the larvee form a tube of the food material
which they carry around with them. Tinea pellionella,* the clothes
moth. TORTRICID ; the larvie roll leaves into a tube. Carpocapsa pomo-
nella,* the codlin moth, the larvae infesting apples.

Sub Order IT. GEOMETRINA. Moths slender, the wings in pattern
and shape recalling those of butterflies, but held horizontally when at
rest ; ‘tongue’ (proboscis) small; larvee with two, rarely three, prolegs,
known as span or measuring worms from their gait. Species numerous.
Canker worms (Paleacrita vernata,* <Alosophila pometaria,* females
wingless), Diastictis ribearia,* currant worm.
IV. INSECTA: HEXAPODA, LEPIDOPTERA. 495

Sub Order III. NOCTUINA. Owlet moths; with short bodies; fore
wings usually gray and ornamented by two spots and zigzag lines which

 

-
Fia. 535.—Leucania unipunctata, army-worm and moth. (From Riley.)

at rest cover the frequently (as in Catocala*) brightly colored hind wings,
1800 species in United States. AMypena humuli,* hop worm ; Aletia argil-
lacea,* cotton worm ; Leucania unipunctata,* army worm ; cut worms.

Sub Order IV. BOMBYCINA, silk worms. Body large, wooly, usually
broad dull-colored wings; occasionally lacking in the females ; proboscis
frequently rudimentary. Antenne long, pectinate; larve hairy, with
well-developed spinning powers. Most important are the true silk worms
(Bombyx mort*), natives of China, while others, like Telea polyphemus,*
furnish silk of value. Still others cause great damage to forest trees,
among them the tent caterpillars (Clistocampa*) and the imported gipsy
moth Ocneria dispar (fig. 72).

Sub Order V. SPHINGINA. Body long, stout ; fore wings long, slender,
hind wings shorter; proboscis very long 2: autennes short ; larva with a

 

Fic. 536.—Everyx myron. (From Riley.)

caudal spine. Phlegethontius celeus,* tomato worm ; P. carolina,* tobacco
worm. Considerably different are the Sesip, or ‘clear wings,’ which
resemble bees and wasps.

Sub Order VI. RHOPALOCERA, butterflies. Body slender; wings
held vertically when at rest, proboscis long ; antenne clubbed at the tip ;
496 ARTHROPODA.

larve usually spiny ; pupe hung by a thread, never acocoon. Specics
numerous. Vanessa antiopa* lives over winter ; the species of Pieris *
attack cabbages, etc.; Papilio,* swallow tails.

Class V. Diplopoda (Chilognatha).

The Diplopoda are usually united with the Chilopoda in a
group of Myriapoda; but while they agree in having a head fol-
lowed by numerous foot-bearing segments, they differ so greatly
that no union is possible. The body is nearly cylindrical, although
in the Polydesmids by lateral growth it may be flattened above;
the legs are close together on the ventral surface, with the tracheal
openings near them, while on the sides of the body are other
openings of defensive glands, the foramina repugnatoria.

 

Fig. 537. Fie. 538.

Fia. 537.—Schematic section of Diplopod (compare with fig. 481). d, digestive tract;

g, gonad; h, heart; 7, repugnatorial gland ; s, spiracle and trachee.

Fic. 5388.—Mouth parts of Julus. (After Latzel.) 2, mandibles of J. molybdinus ; 3,
gnathochilarium (fused maxilla) of I. luridus.

A more marked feature is that each segment of the body except
the first four or five bears two pairs of appendages, which, with a
similar duplicity in chambers of the heart, trachex, ganglia, etc.,
shows that a fusion has occurred. The anterior somites bear at
most but a single pair of legs; both legs and antenne are short.
The head bears, besides the antenne, but two pairs of appendages,
a pair of several-jointed mandibles (fig. 538), and a pair of rudi-
mentary maxille fused to a single plate, the gnathochilarium.

The gonads lie ventral to the intestine far back in the body, those of
the right and left sides enclosed in a single sac ; the ducts open separately
on the second somite of the trunk. The spermatozoa are not motile. The
legs of the seventh segment of the male are used in copulation. The
V. DIPLOPODA, SUMMARY OF IMPORTANT FACTS. 497

young escape from the egg with three pairs of legs, a point once thought to
show resemblances to the Hexapoda, but which does not, for these legs are

 

Fig. 539.—Hexapod young of Strongylosoma. (After Metschnikoff.)

on the fourth, sixth, and seventh somites of the body. The IuLID& have
elongate cylindrical bodies ; Spirobolus.* GLoMERID& short, capable of

 

Fic. 540.—Iulus maximus. (After Schmarda.)

rolling into a ball; PoLypesmip#, with wing-like processes to the
segments, giving thema flattened appearance. PAUROPIDA: minute ; body
with twelve segments, tending to fuse to six. Pauropus,* Eurypauropus.*
More uncertain in position are the SYMPHYLA (Scolopendrella *), but from
the position of the genital opening they are placed here,

Summary of Important Facts.

1. The ARTHROPODA are animals with evident internal and
external segmentation (metamerism).

2. The metamerism is expressed internally in the ladder-like
nervous system, in the structure of the heart, and in the arrange-
ment of segmental organs and trachez so far as these are present.

3. The outer segmentation is expressed in the rings of the chi-
tinous coat of the body as well as in the metameric arrangement
of the appendages.

4, From the similarly metameric Annelida the Arthropoda are
distinguished by the presence of jointed appendages, at most a
pair toa somite. The appendages may be divided according to
function into antenne, jaws, accessory jaws, feet, and swimmerets.

5. A further distinction is the grouping of the somites into
regions of which usually head, thorax, and abdomen are recognized.

6. The head bears the tactile and eating appendages; the
498 ARTHROPODA.

thorax those used in locomotion (pereiopoda), the abdomen the
swimmerets (ple-noda), or it lacks appendages.

7. By fusion of head and thorax a cephalothorax is produced;
a postabdomen may be separated from the abdomen.

8. The eyes are either ocelli or compound eyes.

9, Tlermaphroditism is rare; reproduction is by eggs; fre-
quently there is parthenogenesis, rarely pedogenesis. The eggs
usually have a superficial segmentation.

10. The Arthropoda are divided into Crustacea, Acerata,
Malacopoda, Insecta, and Diplopoda.

11. The Crustacea respire by gills; they usually have two
pairs of antenne, and usually biramous feet; the reproductive ducts
open near the middle of the body.

12. The Crustacea are divided into Trilobite, Phyllopoda,
Copepoda, Ostracoda, Cirripedia, and Malacostraca.

13. Phyllopoda, Copepoda, Ostracoda, and Cirripedia are fre-
quently called Entomostraca; they have a shell gland and the
nauplius as a larval stage.

14. The 7rilobite are extinct forms with one pair of antenne,
and the body divided by longitudinal grooves into three regions.

15. The Phyllopoda have variable segments and primitive leaf-
like feet recalling the parapodia of the annelids.

16. The Copepoda are without shells and have biramous feet.

17. The Ostracoda have reduced bodies enclosed in a bivalve
shell.

18. The Cirripedia are usually hermaphroditic and are sessile.

19. The Malacostraca have 20 (21) segments, of which 7 (8)
are abdominal; the male sexual openings are on the 13th, the
female on the 11th, segment; the excretory organ is the antennal
gland; the larva is a zoea, rarely a nauplius.

20. The Malacostraca are divided into Leptostraca, Thoracos-
traca, and Arthrostraca.

21. The Leplostraca have twenty-one somites; they are closely
related to the Phyllopoda.

22. The Thoracostraca or Podophthalmia (Schizopoda, Stoma-
topoda, Decapoda) have stalked eyes and some or all of the tho-
racic somites fused with the head to a cephalothorax.

23. The Ardhrostraca or Edriophthalmia have sessile eyes and
have seyen free thoracic segments. They are divided into Am-
phipoda and Isopoda.

24. The Acrrata lack antenne; the body is divided into ceph-
alothorax and abdomen; the cephalothorax bears six pairs of
V. ARTHROPODA, SUMMARY OF IMPORTANT FACTS. 499

appendages; the genital ducts open on the seventh somite; the
respiratory organs—gills, lungs, or trachea—develop from the
abdominal appendages.

25. The Acerata are divided into Gigantostraca and Arachnida.

26. The Gigantostraca are large, and breathe by gills. The
only living forms are Xiphosures.

27. The Arachnida breathe by lungs or by traches derived
from lungs, the openings to which are on the abdomen; they have
a pair of chelicere, a pair of pedipalpi, and four pairs of legs; they
have in addition several pairs of highly developed ocelli.

28. The Arachnida are divided into nine orders: Scorpionida,
Phrynoidea, Microthelyphonida, Solpugida, Pseudoscorpii: Pha-
langida, Araneina, Acarina, and Linguatulida.

29, The Scorpionida have chelate pedipalpi and a postabdomen
terminated by a sting.

30. The Phrynoidea have the first pair of legs tactile and not
used in walking, and a continuous cephalothorax.

31. The Microthelyphonida and the Solpugida have three
‘thoracic’ segments free. The Microthelyphonida have a long,
jointed postabdomen, lacking in the Solpugida.

32. The Pseudoscorpit resemble the Scorpionida, but lack the
postabdomen and sting.

33. The Phalangida have very long legs and spider-like bodies.

34. The Araneina have an unsegmented abdomen, bearing four
or six spinnerets and numerous spinning glands. They are divided
into Tetrapneumones, with four lungs, and Dipneumones, with
two lungs and two tracher.

35. The Acarina have cephalothorax and abdomen fused and
the mouth parts for sucking. Several species are parasitic on man.

36. The Linguatulida are complete parasites, ribbon-like and
without legs; the young live in the lungs and liver.

37. The Tardigrada and Pycnogonida agree with the Arachnida
in the number of walking legs. Their position is very uncertain.

38. The Manacopopa are intermediate between Annelida and
Insecta. They have indistinctly segmented bodies with parapodia-
like feet, segmental organs, and tracheex.

39. The Insecta breathe by trachew; the head bears four pairs
of appendages: antenne, mandibles, mavxillw, labium; since
trachew are present the circulatory system is reduced; the repro-
ductive organs open at the hind end of the body.

40. The Insecta are divided into Chilopoda and Hexapoda.

41. The Chilopoda have numerous body segments with a pair
500 ARTHROPODA.

of long legs on each; close behind the head are a pair of poison
feet.

42. The Hexapoda have the body divided into head, thorax,
and abdomen.

43. The abdomen consists of a varying number of somites and
lacks appendages.

44. The thorax consists of three segments, pro-, meso-, and
metathorax, each bearing a pair of legs, and meso- and metathorax
usually a pair of wings each.

45. The head bears, besides three pairs of mouth parts, an un-
paired upper lip (labrum) and two compound eyes, besides usually
one to three ocelli.

46. The structure of the mouth parts varies with the food;
they are either biting, licking and sucking, or piercing in function.

47. Wingless insects usually have a direct (ametabolous) de-
velopment with numerous ecdyses.

48. Winged insects (and many without wings which have de-
scended from winged forms) have a metamorphosis in which the
larva differs more or less from the imago (metabolous insects); the
larva never has wings.

49. An incomplete metamorphosis (hemimetabolous develop-
ment) occurs when the larva with each molt becomes more like
the adult, developing wing pads which with each ecdysis become
larger.

50. In complete metamorphosis (holometabolous development)
the changes occur in the last molting stage, which is a stage of
rest, the pupa.

51. Classification of Hexapoda is based upon structure of mouth
parts and wings as well as upon regional relations and development.

52. The Apterygotu are wingless, ametabolous Hexapoda with
biting mouth parts.

53. The Archiptera have biting mouth parts with incompletely
fused labium, net-veined wings, and incomplete metamorphosis.

54. The Orthoptera resemble the Archiptera in mouth parts
and development, but have parchment-like wings.

55. The Newroptera have net-veined wings and a holometabolous
development; the mouth parts are modified.

56. The Coleoptera are biting insects with the fore wings
changed to elytra; they differ from the somewhat similar Orthop-
tera by the complete metamorphosis.

5%. The Strepsiptera are parasitic forms allied to the Coleoptera.

58. The Hymenoptera have partly biting, partly licking mouth
CHORDATA. 501

parts; membranous wings with few nervures and holometabolous
development.

59. The Rhynchota are hemimetabolous or ametabolous, with
piercing mouth parts; the bed bugs and the Pediculina are parasitic.

60. The Diptera are holometabolous, with piercing mouth parts
and not more than one pair of wings. The larve of the Céstride
are parasitic.

61. The Aphaniptera are holometabolous, wingless, parasitic,
with sucking mouth parts.

62. The Lepidoptera have the wings covered with scales; labium
and labrum rudimentary, the mavxille altered to a sucking tube;
the development holometabolous.

63. The Diptopopa have a head with three pairs of appendages;
the trunk with double segments, each bearing two pairs of legs,
the genital openings anterior.

64. The term Myriapoda is frequently used to include Chilop-
oda and Diplopoda.

PHYLUM VII. CHORDATA.

Within recent years it has been realized that a number of ani-
mals, formerly distributed among various groups, possess structural
features of great importance which ally them to the vertebrates.
On the other hand they lack the vertebre and many other features
characteristic of that group, so that the name cannot be extended
to include them. Yet since all these forms possess, as a temporary
or a permanent feature, a structure known as the chorda dorsalis
or notochord, the term Chordata has been adopted to include them.

The notchord is a smooth elastic rod arising, in development,
from the entoderm and coming to le between the digestive tract
and the nervous system (fig. 9). In all Chordates the anterior
(pharyngeal) portion of the alimentary canal develops one or
more pairs of pockets which grow outwards and fuse with the
ectoderm. The fused portion then breaks through, and the pock-
ets become converted into gill shts (branchial clefts), which, in
the lower forms, allow the passage of water over the gills which
line the slits.

The central nervous system lies on one side of the alimentary
canal, there being no ring of nervous matter (Enteropneusta ex
cepted) around the w@sophagus, such as is so common in the in
vertebrata. It arises as a medullary plate on the dorsal side of the
body around the blastopore. The edges of this plate are rolled
502 CHORDATA.

inwards, converting the plate into a tube with nervous walls and
a central canal. From this, as will readily be seen, it happens,
when the blastopore remains open behind (fig. 547, ne), that a
temporary communication, the neurenteric canal, exists between
the neural and alimentary canals.

On the other hand the chordates share with the annelids and
arthropods a segmentation of the body which, however, is internal
and only exceptionally is visible from the surface.

The Chordates include the Leptocardii, the Tunicata, doubt-
fully a group of Enteropneusta, and the Vertebrata.

Sus Puyritcm J. Leprocarpir (CEPHALOCHORDIA, ACRANTIA).

Until recently but a single genus (Amphioxus) was recognized
as belonging to this group, and this form, known for over a hun-
dred years, was at first described as a molluse (Limazx lanceolatus).
Its chordate nature was first recognized by Johannes Miiller,
while the embryological researches of Kowalewsky showed its close
relations to the Tunicata.

In structure it is comparatively simple. The fish-like body,
pointed at either end (whence Amphioxus), lacks paired appendages,
but has a median fold or fin best developed at the caudal end. The
epithelium covering the body is but a single cell in thickness and

  
  

 

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Fia. 541.—Amphioaus lanceolatus. (Diagram after Boveri.) a, anus; 1, eye; >, peri-
branchial space; c, notochord; g, gonads; /, liver; m, muscles; 1, nephridia ; 0,

mouth; p, atrial opening ; 7, spinal cord; sp, gill slits.

allows the underlying muscle segments to show through. It differs
from the fishes in lack of skull (Acrania), vertebre, brain, heart,
and kidneys, although the rudiments of brain and excretory organs
are present. Connective tissue is almost entirely absent, the body
consisting of much-folded epithelia separated by thin gelatinous
layers.

An axial skeleton is present in the notochord, which extends the
whole length of the body (fig. 541, ¢). Above it lies the spinal
cord, with a central canal, which expands in front into a rudi-
mentary cerebral vesicle. A pigment spot in this brain is the
I. LEPTOCARDII. 503

primitive eye, but other places are sensitive to ight. The olfac-
tory organ is an unpaired pit on the anterior end of the body;
and at its bottom, in the young, is an opening, the anterior neu-
ropore, which leads into the anterior end of the neural canal. It
is a point of delayed closure of the embryonic medullary folds.

Of the alimentary tract more than a third is occupied by the
pharynx with the gill slits. It begins with an oval mouth, sur-
rounded by cirri, and is perforated by numerous gill slits, be-
tween which elastic gill arches form a support for the walls (fig.

5

542, hb). In the young the gill slits open directly to the anterior,
but later, somewhat as in Tunicata, into a peribranchial chamber

 

Fig. 542.—Section of the gill region of Amphiorus. (After Lankester and Boveri.)
a, aorta descendens; b, peribranchial space ; ¢, notochord; cd, c@lom (branchial
body cavity) ; e, hypobranchial groove, beneath it the aorta asc endens; 4, gonad;
kb, gill arches; kd, pharynx: l, liver; m, muscles; 7, nephridia, on the left with
an arrow; 7, spinal cord; sx, spinal nerve; sp, gill slit.

    
 

(2) which allows the escape of the water through a porus branchialis
(fig. 541, p), behind the middle of the body. On the ventral floor
of the pharynx is a ciliated hypobranchial groove (fig. 542, ¢), the
homologue of the tunicate endostyle and of part of the vertebrate
thyroid. It leads back to the straight digestive tract which opens
on the left side near the end of the body, and bears in front a
blind liver sac which extends forward into the gill region (figs.
504 CHORDATA.

541, 542, 7). The vascular system, with colorless blood, consists
of a dorsal arterial (@) and a ventral venous trunk connected by
lateral loops or arches. The ventral trunk begins as a subintestinal
vein under the intestine, branches as a portal vein over the liver,
and, reuniting again in a ventral vessel, continues forward, as the
aorta ascendens, below the gills. From this the vascular arches—
gill arteries—pass up between the gill slits and form the dorsal
vessel, the aorta descendens. A true heart is lacking, but various
parts of the vessels—a part of the ventral trunk and the bases of
the gill arteries—are contractile, whence the name Leptocardii.

As the pharynx liesin the peribranchial chamber, the digestive
portion of the tract lies in a true body cavity or colom, which ex-
tends forward (fig. 542, cé) into the branchial region as well as
into the gill-walls (branchial celom) and into the outer walls of
the peribranchial chamber (peribranchial celom). In the peri-
branchial ceelom are the gonads (g), a series of pouch-like cell fol-
licles which, by dehiscence, allow their products to escape into the
peribranchial chamber. Into this chamber also empty the excre-
tory organs which were long sought for in vain. These are (7) a
series, on right and left sides, of ciliated canals apparently cor-
responding to the pronephros of the vertebrates. Each canal
begins with at least one ciliated nephrostome in the celom and
opens separately like an annelid nephridium.

Like the structure, the development is comparatively simple. The
following points deserve special mention: (1) The eggs have a nearly
equal segmentation (fig. 96). (2) A typical invaginate gastrula (fig. 105)
occurs. (3) The mesoderm arises as a series of pouches, right and left,
from the mesenteron, which later separate and represent the primitive
segments. Hence these are clearly mesothelial in nature. From the cavi-
ties of these arises the body cavity, which is consequently an enteroceele.
(4) The dorsal surface of the entoderm between these ccelomic pouches
becomes folded off from the rest and forms the notochord, which lies
between the digestive tract and the nervous system. (5) The nervous
system arises from a longitudinal groove which becomes folded into a
tube and is connected for a while with the digestive tract by a neuren-
teric canal.

Amphioxus* contains a few closely related species which oceur on our
southeastern coast, in Europe, Indian Ocean. Recently other genera have
been described—Asymmetvon * in America, Heteropleuron in the South
Seas. The animals live in quiet bays and bury themselves in the sand,
with only the mouth above the surface. Like all animals with rudimen-
tary eyes, they shun the light and are greatly excited by strong illumi-
nation.
IT, TUNICATA. 505

Sus Puyius II. Tunicata (URocHorpa).

In their adult condition the Tunicata, or sea-squirts, bear some
resemblance to the siphonate mollusca, especially in the posses-
sion of incurrent and excurrent orifices, usually close together,
anda mantle. Hence these forms were long associated with the

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Fic. 543.—Diagram of a tunicate (orig.). a. atrium; b, nervous ganglion; e, endo-
style; i, intestine; m, mouth; 2, subneural gland; s, stomach; ?, tunic. In the
centre the branchial basket with the gill slits communicating with the peri-
branchial space, and this in turn with the atrium.

molluscs; later they were associated with the worms, but their de-

velopment shows them to be more nearly related to the vertebrates.

The group owes its name to the tunic or mantle—lacking in the
Copelatee—an envelope (fig. 543, ¢) which is formed like a cuticle
by the epithelium of the skin, but which is distinguished from
ordinary cuticula by its structure. It much resembles connective
tissue in that cells from the mesoderm wander into the ground
substance, which is sometimes fibrous, sometimes homogeneous,
and has an interesting chemical nature. It consists of the same
proportions of carbon, oxygen, and hydrogen (C,H,,0,) as cellulose
and agrees with this substance, so characteristic of plants, in its
reactions (blue color with iodine-iodide of potassium and sulphuric

 
506 CHORDATA.

acid, violet with chloriodide of zinc). Nowhere else among ani-
mals is there such a rich formation of cellulose.

The anterior part of the digestive tract is modified into a
pharynx or branchial chamber, the walls of which become per-
forated with a varying number of gill slits, these leading either
directly to the exterior or, more usually, into a peribranchial
chamber, and from this to a cloaca or atrium (a), before reaching
the outside world. While the respiratory water passes through
the gill slits the food particles which it contains are received by a
ring-shaped ciliated band (peripharyngeal band) and, enveloped
by mucus, are led to the esophagus. This mucus is formed by a
ciliated glandular groove, the endostyle (¢), on the ventral surface
of the pharynx.

Between the gill region (end of the endostyle) and the stomach
lies the ventral heart enclosed in a pericardium. It has the
peculiarity, met nowhere else, of changing the direction of its
contractions at frequent intervals; after the heart has driven the
blood for a time to the gills it rests a while and then begins to
force the blood in the opposite direction, pumping it from the
gills and sending it towards the stomach.

If we add to the foregoing that a dorsal ganglion and a her-
maphroditic gonad are present, the striking features of the group
are enumerated. The extreme forms, the Copelate and the
Thaliacea, are rather remote, but they are connected by interme-
diate forms, the Ascidize and Pyrosomas.

Order I. Copelate.

These small forms—one or a few centimeters in length—are
pelagic; they have the anterior end inserted ina gelatinous envelope
or ‘house’ which replaces the lacking tunic. They swim like a
tadpole by means of a tail which arises from the hinder end of
the trunk. The alimentary canal (fig. 544) is bent on itself, and
both it and the two large gill slits, in contrast to all other tuni-
cates, open directly to the exterior. The heart (lacking only in
the Kowalewskidz) is ventral and the hermaphroditic gonads and
the nervous system dorsal. The latter corsists of a cerebral
ganglion, with beside it an extremely simple auditory organ and a
ciliated groove, and farther a chain of ganglia extending into the
tail. The notochord, a gelatinous structure enclosed by a sheath
of cells, forms the skeletal axis of the tail ventral to the nerve cord
and gives attachment to muscles. Otkopleura,* Appendicularia,*
Fritillaria; Kowalewskia.
Il, TUNICATA: COPELAT_A. 507

 

Fic. i44.—Ockopleura cophocerca. (After Fol.). .4,the whole animal, remov i
5 : faees j : se ssh eens , , removed from its
‘house,’ dorsal view; B, body, side view with base of tail. «, anus: c, notochord:
ae Lonel geist te stomach ; en, endostyle; f, ciliated peripharyngeal
«4 € C : org r aj]: jar . a .
pan ae , brain an rst ganglion of tail; h, testis; m, mouth; o, ov, Ovary;

 

Fia. 545.—Ciona intestinalis. .A, from the left side, the cellulose tunic and dermal
muscular sac removed; &, from the right side, the tunic entirely removed, pharynx
opened from the mouth. a, anus; c, cellulose tunic below with adhesive processes;
cl, cloaca; d, rectum; ¢, atrial opening; en, endostyle ending above in the peri-
pharyngeal band; g, ganglion; kh, mouth of the ‘hypophysis’; hr, heart, with peri-
cardium ; ho, branched testes; ¢, oral opening; k, gill sac ; #7, muscular sac ; oe,
cesophagus ; od, oviduct, the black line beside it the vas deferens; ov, ovary; s,
partition between atrium and body cavity ; st, stomach ; ¢, crown of tentacles.

   

 
508 CHORDATA.

Order II. Tethyoidea (Ascidiaformes).

With the exception of the pelagic Pyrosomide all of the ascidi-
ans are attached to rocks, etc., in the sea. The greater necessity
for protection caused by this sedentary life has resulted in a great
development of the cellulose tunic, which, enveloping the internal
organs, gives these animals a swollen, somewhat shapeless appear-
ance. Two openings, mouth and atrial opening, lead into the
interior, and the water which issues from these, when the animals
are taken from the ocean, has given them the common name 6f
* sea-squirts.’

On removing the tunic, which is but slightly attached to the
other parts except at mouth and atrial opening, a muscular sac is
seen (fig. 545), the fibres running circularly and longitudinally. In-
side this sac are the viscera, the pharyngeal region by far the most
conspicuous. The mouth leads to a short tube with tentacles (t),
and then to the pharynx, a wide sac which
lies in a large cavity, the peribranchial
chamber, the walls of the pharynx and
the enclosing space uniting on the ventral
side (fig. 543). The pharyngeal walls are
perforated like a net by small ciliated
gill slits, arranged in longitudinal and
transverse rows (fig. 546), through which
the water received from the mouth passes
Bu eh a ek Neen into the peribranchial chamber and thence

bit of the wallof the gillsac ¢o the atrium, and so out to the external
enlarged to show the gill
slits. world.

While the respiratory water thus passes out in a nearly direct
course, the food particles which it contains pass into the digestive
tract. By means of a ciliated tract (peripharyngeal band) just
inside of the tentacles and surrounded by mucus secreted by the
endostyle (or hypobranchial groove), the food is carried back to
the wsophagus (9e) at the base of the gill chamber, and thence to
the stomach (usually provided with liver glands), and on to the
intestine. The anus is at the base of the special portion of the
peribranchial chamber, which also receives the genital ducts and
hence is known as the cloaca or atrium.

In the body cavity, which is greatly reduced in the species
with concentrated bodies, occur the digestive tract, the sexual
organs, and the heart; the latter, frequently S-shaped, extends be-
tween the stomach and the endostyle. Opposite to the endostyle

 
II, TUNICATA: TETHYOIDEA. 509

is the ganglion in the dorsal wall between oral and atrial open-
ings. Below it (rarely above it) is a branched subneural gland
which, from its relations and its opening into the prebranchial
part of the alimentary tract, has been compared to the vertebrate
hypophysis. In many there exist special excretory organs, numer-
ous blind vesicles filled with excreta.

From the eggs are hatched small swimming tadpole-like larve
(fig. 547), resembling Appendicularia and, like it, consisting of

Kee ab

 

a

Fia. 547.—Development of an Ascidian. (After Kupffer and Kowalevsky.) 1, larva,
just hatched ; 2, cross-section through the tailof a slightly younger larva; 3, much
younger stage, formation of notochord and nervous system; 4, anterior end of a
larva just before attachment. (2, Phallusia mentula ; 5,4, Fh. mammillata.) aw,
eye; ¢, notochord ; cl, tunic; d, digestive tract; d’, its nutritive, d’’, its respira-
tory division; e, atrial vesicle; ek, ectoderm ; en, entoderm; /, brain; 7, oral in-
vagination ; m, muscles of tail; n, neural tube; xe, neurenteric canal; 0, otocyst.

 

trunk and tail, in which the chordate features are strongly marked.
The digestive tract is confined to the trunk; dorsal to it lies the
tubular nervous system in which three parts are recognizable: in
front a vesicular brain with a simple eye and an otocyst imbedded
in its walls; farther back a narrower portion (‘ medulla oblongata ’) ;
lastly, a spinal cord extending into the tail. In the axis of the
tail is a notochord which extends forward a short distance into the
trunk between digestive tract and nervous system.

In the metamorphosis of the free larvie into the sessile ascid-
ians four processes are important: (1) The larve attach themselves
by means of three ventral anterior papille; (2) The tail is retracted
and, after preliminary fatty degeneration, is absorbed; (3) The
body becomes more or less spherical by development of the tunic;
510 CHORDATA.

(4) Two dorsal invaginations are formed, these grow together, en-
velop the pharyngeal region, and form the atrium and peribranchial
chamber. It is to be noted that this arises from the dorsal sur-
face and extends ventrally, while the peribranchial chamber of
Amphiocus arises by folds which grow ventrally over the pharynx.

Besides sexual reproduction many ascidians reproduce by bud-
ding. Where this occurs it results in the formation of colonies, a
matter of systematic importance.

Sub Order I. MONASCIDLA. Simple ascidians of considerable size ;

sometimes with transparent, sometimes with thick opaque tunic. The
CLAVELLINIDE produce small colonies by basal budding, each individual

 

Fia. 548, Fig. 549.
Fic. 548.—A. Molgula manhattensis*; B, Eugyra pillularis.** (From Verrill.)
Fig. 549.—Botryllus violaceus. (After Carpenter.) 4A, small colony of eighteen indi-
vidual groups; B, two individual groups somewhat enlarged.

with its own test; Perophora.* CyNTHIIDs, test leathery, oral and atrial
openings four-lobed; Cynthia.** MoLeuLipm, oral opening, six-lobed,
atrial four-lobed. Molgula,* Eugyra.*

Sub Order I]. SYNASCIDLAH. Compound ascidians consisting of
numerous small individuals imbedded in a common cellulose tunic and
forming considerable crusts on stones, plants, ete. Usually (fig. 549) the
individuals of a colony are divided into small groups, the oral openings
(6-20 in number) forming a rosette around a common central atrium.
Distaplia,* Leptoclinum,* Polyclinwm,* Amaroucium,* Botryllus.*

Sub Order III]. LUCIA, Free-swimming pelagic synascidians, having
the form of a hollow cylinder closed at one end. The animals imbedded in
the tunic vertically to the axis of the cylinder, the oral apertures on the
outside, the atrial in the central cavity. Pyrosoma, very phosphorescent,
tropical, some species four feet long.

Order III. Thaliacea (Salpe#formes).

These, like the Lucie and Copelate, are pelagic, and play an
important part in the plankton, either by the vast numbers of
small individuals or by the formation of colonies of considerable
size. In form a Salpa may be compared to a barrel formed out-
side of a cellulose tunic, ined internally with a muscular wall.
The muscles run circularly (fig. 550), are six or eight, not always
II, TUNICATA: THALIACEA. 511

closed rings, like hoops. By their contraction the water is expelled
through the posterior or atrial end of the body, while fresh water
on their relaxation enters the other or oral aperture. By the
reaction the animals swim through the water with the oral end in
front. The cavity of the barrel corresponds to pharyngeal and
peribranchial chambers of the ascidian. In the Dolioliide the two

 

Fic. 550.—A, B, Salpa democratica with stolon, ventral and lateral views; C, Salpa
mucronata, part of a young chain not yet separated. a, anus; ¢, tunic; d, diges-
tive tract; e, atrial opening: en, endostyle; f, peripharyngeal groove, g, gan-
glion with horseshoe-shaped eye, and near it the tentacle and hypophysial
groove; i, testis; 7, mouth; i, gill; m, muscle hoops; st, stolo prolifer.

chambers are separated by a partition perforated by gill slits (fig.

551); in the common Salpe the partition is reduced to a bar with

transverse rows of cilia go that branchial and peribranchial cham-

bers are not distinct; yet the endostyle and the peripharyngeal
band are retained.

The viscera lie in the muscular sac, where the branchial bar and
the endostyle meet and are usually compacted into a mass, the
‘nucleus’ (intestine, liver, gonads, heart). The ganglion is dis-
tinct and lies dorsally opposite the endostyle, just in front of the
branchial bar. Associated with it is a horseshoe-shaped eye.

For a long time two kinds of Salpe have been known, one
solitary, the other consisting of numerous individuals connected
together like a chain or a rosette (fig. 550, C). At the beginning
of the last century the poet Chamisso discovered that the chain
512 CHORDATA.

salps were produced by the solitary individuals, and that these in
turn came from the chain form, a peculiar type of reproduction
to which Steenstrup later gave the name alternation of generations.
The solitary salp is asexual; gonads are lacking, but near the
hinder end is abudding cone or stolo prolifer from which ene
after another bud colonies of salps. When
the first is separated a second matures and
a third begins. These colonial forms, the
chain salps, are sexual, and each produces a
single egg from which a solitary individual is
formed.

Since both the solitary and the chain forms have
received names, the species of Salpa* now have
double names like Salpa democratica-mucronata,
democratica being the asexual, meeronata the sex-
ual, individual, ete. From the true Salpw Dolio-
eer eee Ean dum* is distinguished by the better developed gills,

tion of letters sec fig. the complete muscular bands, and a more compli-
eat) cated alternation of generations.

 

Susp Purim III. ENrEropNeusta (TMEMICHORDIA).

The few marine forms here included are decidedly worm-like,
and, like many worms, they burrow in the mud. The body con-
sists of three parts—proboscis, collar, and body (fig. 552). The
proboscis contains a cavity opening to the exterior by a dorsal
pore, while two similar cavities in the collar open separately.
These can be filled with water, and by alternately enlarging and
contracting these parts the animal is able to burrow like a razor
clam (Hnsis). The mouth lies ventral and in front of the collar
and leads into a digestive tract, which in its anterior part is per-
forated by numerous paired gill slits, while the part behind it is
covered with hepatic ceca. The intestine is supported in the
celom by dorsal and ventral mesenteries, and is accompanied by a
dorsal and a ventral blood-vessel, to which are added lateral canals
and numerous anastomoses. <A vesicle on the dorsal vessel in the
proboscis is called the heart. The nervous system is very peculiar.
There is a dorsal portion lying in the collar region, which is pro-
duced by inrolling, as is the central nervous system in the Chor-
dates, and a ventral part, as yet lying in the ectoderm, the two
being connected by nerves in the collar. The gonads are numerous
follicles lying between gill and liver region and opening to the
exterior.
Ill, ENTEROPNEUSTA. 513

 

Fig. 552.—Balanoglossus kowalewskii.* (From Korschelt-Heider, after A. Agassiz.)
db, dorsal blood-vessel; e, proboscis; g, sexual region; k, gill region; kr, collar;
vb, ventral blood-vessel.

The systematic position of the Enteropneusta is not settled beyond a
doubt. In the possession of gill slits and in the formation of the dorsal
nervous system it closely resembles the
other chordates, and the resemblance
is strengthened by similarities in de-
tails of structure of the gills. The
advocates of this view recognize the
notochord in a blind tube, sur-
rounded by tough membrane and
thickened beneath, which extends
from the pharynx into the proboscis. ’

. . . Fie. 553.—A, Tornaria larva of Balano-
Embryology throws but little light on ~ Gyosens. (After Morgan.) «, apical
the problem. Some species have a Plates ae, preoral part of ciliated

: band; be!, be?, he’, ecelomic pouches ;
direct development (fig. 553, B, C), m, mouth ; p, postoral part of ciliated
g e : band 5b, ©, two stages of Balano-
while others lave alarva (Tornaria, — giossus with direct development. (at
552 hie @ 4g 4 ter Bateson.) @, anus; le, branchial
fig. 558, A) which so resembles the clefts; ¢ collar? de, digestive part of
larvee of certain echinoderms that alimentary canal; in, intestine; nc,
it was long held to belong to that OI CAEEE TS Hy DUCE OS ET:
an 5 ‘ 5
phylum. The chief resemblances are in the relations of the ciliated

bands to the alimentary tract and in the presence of the proboscis cavity,

Og en,

A

   
514 CHORDATA,

which, like the ambulacral system, opens to the exterior. Two deep-sea
forms, Cephalodiscus and Rhabdopleura, have the same type of ‘noto-
chord,’ and the first has a pair of gill slits, In other respects these are
strikingly Polyzoan in appearance.

Sup Puytum IJV. VERTEBRATA.

In the vertebrates only the internal segmentation occurs. This
is shown, and most clearly, in the lower Vertebrata, in the muscles
(myotomes, myomeres), the myocommata or myosepta which sep-
arate them, and the protovertebre from which they arise; in the
nerves (neurotomes), the skeleton (sclerotomes), the blood-vessels,
and in the excretory organs (nephrotomes). In the higher verte-
brates this metamerism is visible only in the embryonic stages.
In part this absence of external segmentation has its cause in the
heteronomy (p. 399) of the body and the obliteration of segmental
boundaries, consequent upon the union of somites into body re-
gions, of which at least three—head, trunk, and tail—at most six
—head, neck, (cervical) thorax, lumbar, pelvic (sacral), and tail
(caudal)—occur. Not less important in this respect is the charac-
ter of the skeleton. The cuticular skeleton, which is the cause of
the annulation of the arthropod, is entirely lacking. The skin
remains soft, or contributes to a subordinate degree, more for pro-
tection than for support, to the formation of a skeleton (dermal
skeleton of fishes, alligators, turtles). On this account firmer
tissue is formed in the axis of the body, which, in the lowest ver-
tebrates and the embryos of the higher, appears as the notochord
already mentioned, but in the higher is supplemented by the verte-
bral column and skull.

The skin of the vertebrates is distinguished from that of all
invertebrates by two characters (figs. 26, 27): the many-layered
condition of the epidermis, and the considerable thickness of the
derma, The epidermis is but rarely covered by a delicate cuticle;
usually such a protection is unnecessary since—and especially in
the land forms—the superficial layers become cornified and hence
furnish the necessary resistance without a cuticle. There are two
layers to be distinguished, the deeper stratum Malpighii and the
superficial stratum corneum (s.J/ and se; see p. 76).

The second constituent of the integument, the derma (cutis,
corium), arises from the mesoderm (mesenchyme). It consists of
many layers, often stratified, of close connective tissne, and is
usually separated from the underlying structures, especially the
muscles, by a loose tissue rich in l7mph spaces, the subeutaneous

 
IV. VERTEBRATA. 515

tissue. Both of these constituents of the skin, aside from their
own firmness, can give rise to protective structures. The horny
layer of the epidermis in places becomes greatly developed and
thus forms the tortoise shell of the turtles, the scales, shields, and
scutes of the snakes and lizards, the feathers of the birds, the hair
and horns of the mammals. Other epidermal products are the
claws, nails, and hoofs of the terrestrial vertebrates. The derma
is often the seat of ossifications which, in contrast to the deeper
bones, are called the dermal skeleton.

First of the dermal skeletal parts are the scales of the fishes,
which, in spite of similarity of name,
are different from the epidermal scales
of the reptiles. They may be traced
back to the primitive form, the pla-
coid scales of the Elasmobranchs.
These are rhombic plates, bearing in
the middle pointed spines, which are
called dermal teeth from similarity in
structure and development to the teeth
of the mouth cavity (fig. 554). They
consist of dentine (d) and have a Jarge
pulp cavity (p), with numerous blood-
vessels in the interior. Whether the
thin layer (sch) covering the tip can
be called enamel is disputed. Der-
mal teeth and true teeth are identical

. ° Fic. 554.— Sagittal section of ascale
structures which, in consequence of © of seyilium stellare. (After Hofer.)
different position and consequent dif-  Pypasnieiets: Gamers
ference of function, have developed differently.

The scales of fishes have a wider anatomical interest, since from
them have arisen, besides the bony plates which form the resistant
armor of the turtles, alligators, and many mammals (Armadillos),
important parts of the axial skeleton, the secondary or membrane
bones. A membrane bone isa bony plate which has arisen from
a fusion of dermal ossifications, becomes transferred to a deeper
position, and contributes to the completion of the axial skeleton.
After what was said above about the relations of dermal and true
tecth it is readily seen that a further source of formation of mem-
brane bones lies in the lining of the mouth cavity.

In describing the axial skeleton, the notochord comes first.
This has already been mentioned in connexion with the lower
Chordates. It persists in the cyclostomes, but from them upwards

  
516 CHORDATA.

it is gradually replaced by the vertebra arising around it. It is of
entodermal origin (fig. 9), arising as a longitudinal band of the
epithelium of the archenteron (J, ch), and, becoming cut off,
comes to he in the long axis of the body between digestive tract
and nervous system (//, J//); here it forms a cylindrical rod con-
sisting of a connective tissue which, as already said, resembles
plant tissues because of the vesicular nature of its cells (fig. 38).
In transverse section (fig. 555) the chorda is surrounded by
three layers, internally by a fibrous noto-
chordal sheath, then an elastic layer (not
always present), the elastica externa, so
called because an elastica interna is some-
times present inside the notochordal
sheath; and lastly a skeletogenous layer
(SS), also called the outer notochordal
sheath. This last is a mesodermal con-
nective-tissue layer and is therefore con-
nected with the other connective-tissue
sheaths which surround muscles, nerves,
etc., and deserves special mention because
in it the cartilages and bones arise from
which the vertebre and skull are formed.
Ceils from it can penetrate the notochor-
Fig. 555.—Transverse section dal sheath, converting it into fibrous car-
of axial skeleton of Pe- |. ; _ dee :
tromyzon. (From Wieders- tilage, thus enabling it to participate in
heim.) C, notochord; Cs, ‘
notochordal sheath; #&, the formation of the vertebre.

elastica externa; F, fatty Ae -
tissue ; M, spinal cord; P. Since the notochord and its sheaths

its meninges; Ob, upper : 4 ‘ * ‘
process of Skeletczenous are elastic and give under the strain of the

fssue: Tp jowen seacess, muscles, they are unsegmented. The seg-

of skeletogenous tissue. mentation of the axial skeleton begins with
the appearance of firmer tissue in cartilage and bone. Then there
is a separation of successive parts, and with this the gradual forma-
tion of vertebral column and skull. For both there is a con-
nected series of developments, if studied with reference to the
ontogenetic processes or in the comparative manner from the
lower to the higher forms.

The first parts of the vertebral column to appear are the upper
and lower (figs. 555, 556), or neawral and hemal arches. These
consist of paired parts in the skeletogenous layer which abut
against the notochord, and which are usually a pair to the somite,
although occasionally two or more pairs, the arches proper and
the intercalaria, may occur. The neural arches (arcus vertebra

 
IV. VERTEBRATA. 517

of human anatomy) enclose a spinal canal surrounding the spinal
cord, the parts of the arch, newrapophyses, uniting above the cord
to form the spinous process (frequently an independent part of the
skeletal axis). In the caudal region, in the same way, hemal
arches may be formed of hemapophyses and hemal spine, the arches
surrounding the blood-vessels of the tail (fig. 557). In the trunk
region the ventral arch behaves differently. Since the large body

 

Fie. ee ere of cet owe se notochord; f, exit of nerve ; 7, dorsal and ventral
intercalaria; n, neural canal; ob, neural arch; s, chordal sheath; r, rib;
hemalarch. Bone white, cartilage dotted. a Be aes

 

Fie. 557. Fic. 558,

Fic. 557.—Caudal vertebre of a carp, section (4) and nearly side view (B). ch, space
filled by notochord; h, hemal arch: w, neural arch; ob, neural spine; wb, hemal
spine.

Fic. 558.—Thoracic vertebra, ribs, and sternum of a mammal. (From Wiedersheim.)
Cu, capitular head of rib; Co, neck of rib; Cp, bony rib; An, cartilaginous rib; Ps,
spinous process; Pt, transverse process (diapophysis); St, sternum; 1b, tuber-
cular head of rib; IVA, vertebral centre.

cavity with its viscera, varying in size (digestive and reproductive

organs), is here, the hemapophyses extend outwards and down-

wards and are divided into two parts, a basal apophysis and a

lower movable portion, the rid (fig. 556). Also the lower union

of hemapophyses with hemal spine does not occur; the ribs are

either free (fishes) or are (at least in part) connected ventrally by
518 CHORDATA.

a breast bone or sternwm (Amniotes, fig. 558). The sternum is
a derivative of the ribs. In development the ventral ends of the
ribs of a side fuse and then these fused tracts of the two sides
unite to form the sternum.

The heemal arches lie internal to the longitudinal muscles of the body,
and in the trunk region they lie in the same position just beneath the
peritoneum. These are hemal ribs and are found only in teleosts and
ganoids. The ribs of all other vertebrates (elasmobranchs, amphibia,
amniotes) are morphologically different and are called lateral or pleural
ribs, They develop independently of the vertebral column in a horizontal
connective-tissue septum which extends out through the longitudinal mus-
cles from the axial skeleton to the skin, dividing the musculature into
dorsal (epaxial) and ventral (hypaxial) portions (fig. 89). In the elasmo-
branchs these pleural ribs are attached to the hemapophyses, in the
others to the transverse processes (diapophyses), which arise from the
neurapophyses, and parapophyses, which arise from the vertebral centres.
In the caudal region, often also in the cervical, lumbar, and sacral regions,
the pleural ribs and dia- and parapophyses fuse to form lateral processes.
These occur concurrently with hemal arches in the tails of many Am-
phibia and reptiles and some mammals, forming the chevron bones which,
as in fishes, enclose the caudal blood-vessels. The presence of intercalaria
in cyclostomes, sharks, and ganoids indicates that primitively a double
vertebra arose in each somite. Paleontological and embryological re-
searches on reptiles support this view.

In most vertebrates either the basal ends of the arches broaden
out around the notochord and fuse with one another, or perichordal
cartilages arise independently, furnishing in either case firm sup-
ports, the vertebral bodies, or centra, for the system of arches.
These increase in size at the expense of the notochord on the in-
side, sometimes leading to its almost complete obliteration, as in
the mammals; in others, as the fishes, the reduction is less com-
plete. The fishes have amphicele vertebre (fig. 557), that is, the
centra are hollow at either end. In these cups the notochord
exists even in the adult, and when small connecting portions ex-
tend through the centra the notochord takes the form of a rosary
with alternating enlargements and contractions.

Histologically the vertebral column may be either cartilage or
bone; usually it is first formed in cartilage, which is later replaced
by bone. If the ossification does not occur, the column remains
cartilaginous; if incomplete, cartilage and bone appear side by side.
Since these histological differences are combined with varying de-
grees of persistence of the notochord and with modifications in the
form of the vertebre and their processes, there results an extraor-
dinary variety in the appearance of the vertebral column.
IV. VERTEBRATA. 519

In order to allow for bending where complete centra are present vari-
ous conditions oceur. (a) Opisthocale vertebree have a socket on the
hinder surface which receives the convex anterior end of the succeeding
centrum, forming a ball-and-socket joint. (6) Procalous vertebrae have
these relations reversed, the socket being in front. (c) The vertebrze may
articulate with a ‘saddle joint’ (birds). (d@) Between two successive
vertebra an elastic intervertebral ligament may occur (mammals), The
neurapophyses may bear, in addition to the transverse processes, anterior
and posterior articulating processes (zygapophyses) connecting the sepa-
rate vertebra,

The skull, the anterior continuation of the axial skeleton,
occurs in all vertebrates; it appears before the vertebra, for it is
found in the cyclostomes, which lack these. It surrounds the brain
as the vertebre do the spinal cord; and, like them, its first stages
are formed in the skeletogenous layer surrounding the anterior end
of the notochord. It is so related to the surrounding parts that
it may in general be said to be equivalent or homodynamous with
the vertebre, although we cannot agree with Oken and Goethe, the
founders of the vertebrate theory of the skull, that it has arisen
by the fusion of vertebra. On the other hand skull and vertebrae
are parts arising in the common basis of the skeletogenous layer,
but which have developed in different directions.

Three stages are recognized in the development of the skull:
the membranous, the cartilaginous cranium, and the bony skull.
The first, which consists of connective tissue, occurs only in the
early embryonic stages, scarcely a trace of it persisting in the
adults. It is early replaced by the cartilaginous skull, which may
persist unaltered throughout life in the lower fishes (elasmo-
branchs, sturgeon). In most vertebrates, however, ossification sets
in, embracing a part (fishes, amphibians) or the whole of the carti-
lage (birds, mammals), converting it in the latter case into a bony
capsule. In the bony skull two kinds of bone, primary and sec-
ondary, are recognized, these varying in their origin. The pri-
mary or cartilage bones develop from the cartilage itself, either in
its interior (evtochondrostoses) or in its enveloping perichondium
(ectochondrostoses). The secondary or membrane bones are, in
their origin, foreign to the axial skeleton and arise from the ossifi-
cations in the skin (scales) or in the mouth (teeth), already re-
ferred to (p. 515). They sink into the deeper portions and apply
themselves to the axial skeleton, especially to those parts where,
from lack of cartilage, no primary bones can be formed (parostoses).
Still it is not settled how far these distinctions may be carried.
According to Gegenbaur all ossifications arose primarily in the skin
520 CHORDATA.

or mucous membranes, and primary bones are merely membrane
bones which have entered the cartilages and replaced them. Ac-
cording to this view it is conceivable that the same bone in one
animal may arise as a membrane bone and in another as a primary
bone, « view which is of importance in the homologies and no-
menclature of many bones. It is but just to say that this view is
not universally accepted.

The cartilaginous cranium (chondrocranium) is most complete
beneath the brain. This basal portion is a direct continuation of

 

 

 

Be a ee re

Fie. 559.—Chondrocranium of Amphiuma. anp, antorbital process; ap, ascending
process of quadrate; c, cornu trabecule; ¢, ethmoid plate; ef, endolymph fora-
men; j, jugular foramen; /, lamina cribrosa; m, Meckel’s cartilage ;7, notochord:
oc, oculomotor foramen; ocp, occipital process; of, optic foramen ; Pp, parachor-
dal; pal, palatine foramen ; pf, pevilymphatic foramen; gq quadrate; s, stapes;
sp, stapedial process ; t, trabecula; tre, crest of trabecula; V, VI, TI, foramina
for V, Vi, Vill nerves.

 

 

  

the vertebral column, and a part of it (the parachordals) embraces
the anterior end of the notochord, while part (the frabecule) ex-
tends in front of the end of the notochord. The side walls of the
skull are increased by the cartilaginous envelopes of the two sense
organs, the nasal and otic capsules, around the nose and ear. Be-
tween these is a hollow for the eye which contributes nothing to
the skull. In only a few forms is the chondrocranium completely
IV. VERTEBRATA. 521

closed; usually gaps ( fontunelles) occur in its roof, and frequently
in its floor. The higher the animal intellectually and the larger
its brain the more the connective tissue (primordial cranium) is
called upon to roof in the chondrocranium. Hence it is that in
the reptiles, birds, and mammals, where it is also confined to
embryonic life, the chondrocranium is relatively the smallest.
Since it only closes above in the occipital (hinder) region, while
it gaps widely in front, it follows that the secondary bones play an
important part in the completion of the skull.

The bony skull presents great difficulties from the standpoint
of comparative anatomy, in part from its varving appearance in
the different groups, in part on account of the number and com-
plicated arrangement of the constituent bones. It may be said in
beginning that as a rule the same bone reappears in the separate
classes, and that the difficulties are connected with the fact that
certain bones may fail to develop (Amphibia), or they may fuse
to larger elements (mammals). <A further complication results
from the intimate union with the cranium of bones of the visceral
arches, which, strictly speaking, do not belong to it.

Jilo

    

ae,
[is fro as
: , a mie

—s a the visceral skeleton removed. (A) Cartilage bones: oc
ee ee ee a supraoccipitals ; epo, epiotic ; pto, pterotic 5 spho, sphe-
notic; pro prootic sas, alisphenoid ; 0s, orbitosphenoid; me, mesethmoid 3 ee, oo
ethmoid. (B) Ventral membrane bones : ps, parasphenoid ; vo, vomer. (C) Dorsa

membrane bones: p, parietal; fr, frontal; 1-4, exits of nerves.

The primary bones (preformed in cartilage) can be divided ac-
cording to the cranial regions into four groups: (1) bones of the
hinder part of the head—occipitalia; (2) bones of the ear region
—otica; (3) bones near the eye sphenoidalia; and (4) of the
nasal capsule—ethmoidalia. The occipitalia—four in eve
(figs. 560-562)—united in the higher mammals to a single occipita

 

 
522 CHORDATA.

bone, surround the foramen magnum, the opening through
which the spinal cord passes to connect with the brain. These are
a pair of exoccipitals, right and left, a supraoccipital above and
a basioecipital below. The otica depend in their development
upon the extent of the otic region. In the fishes, where this part
is large, several bones may be present: epiotic, pterotic, sphenotic,
prootic, and often opisthotic. In the mammals, on the other hand,
these are fused to a single petrosal bone (figs. 466, 467) of small
size.

Since the otic bones usually do not reach the middle line below,
the sphenoidalia rest direct upon the basioccipital behind and in
front upon a presphenoid bone, both unpaired but arising from

 

Fig. 561.—Skull of goat. (From Claus.) Als, alisphenoid; Bs, basisphenoid: C, occip-
ital condyle; Eth, mesethmoid, covering the ectethmoid; Fo, optic foramen in
orbitosphenoid; Fr, frontal; Zn.w, premaxillary; Jp,interparietal; Ju, jugal
(malar); La, lachrymal ; Mx, maxillary; Ne, nasal; Ob, basioccipital; Ol, exoc-
cipital; Ors, orbitosphenoid ; Pa, parietal; Pul, palatine; Pe, petrosal; Pm,
paramastoid process; Ps, presphenoid ; Pt, pterygoid ; Sf, frontal sinus in frontal
bone ; Spb, basisphenoid ; Sg, squamosal; J'y, tympanic; Vo, vomer.

paired centres. With each is connected, right and left, a pair of

bones; with the basisphenoid the alisphenoids, with the presphenoid

the orbitosphenoids, just as the exoccipitals flank the basioccipital.

In the region of the nasal capsule there is an unpaired mesethmoid

with a pair of ectethmoids. Ience the cranium of primary bones

may be described as a chain of four median basal bones, basioccipi-
tal, basisphenoid, presphenoid, and mesethmoid; right and left of
this a row of exoccipital, alisphenoid, orbitosphenoid, and ecteth-
moid. The position of the otic capsule results in the sum of the
otic bones, the petrosal, being wedged in between the exoccipitals
IV. VERTEBRATA. 523

and the alisphenoid. Only behind is there a dorsal element, the
supraoccipital.

The skull must be roofed in by membrane bones, and of these
three pairs are almost constantly present. These are, from behind
forwards, a pair of parietals, a pair of frontals, and a pair of nasals,

 

Fia. 562.—Sagittal section of hinder part of goat skull. (From Gegenbaur.)
For lettering see fig. 561.
the latter covering the nasal capsules. Confined to the lower
vertebrates is a large membrane bone on the floor of the skull, the
parasphenoid, which reaches from the basioccipital to the meseth-
moid.

The scheme of the cranium thus outlined undergoes the most modiii-
cations in the sphenoidal region. Parasphenoid, on the one hand, and
basi-and presphenoid, on the other, may be substituted for one another,
so that when the parasphenoid is present (fishes, Amphibia) the others
are small or absent and vice versa (mammals). In the mammals, besides,
the alisphenoids fuse with the basisphenoid (greater wings), the orbito-
sphenoids with the presphenoid (lesser wings), so there arise here an an-
terior and a posterior sphenoid, fused in man to a single sphenoid bone.
Mesethmoid and ectethmoids likewise fuse in the mammals to an eth-
moid bone.

The brain case, or cranium, is developed into the complete skull
by the addition of the visceral skeleton, a series of arches which,
like ribs, embrace the beginning of the alimentary tract and are
related to the cranium, much as are ribs to the vertebre. These
must be considered as parts of the skull, although in part they are
shoved backwards and lie under the anterior end of the vertebral
column. As the ribs arise in alternation with the musculature
(myomeric), so the visceral arches are similarly related to the gill
formation (branchiomeric). Analogous to the cranium the visceral
skeleton has a cartilaginous anda bony stage. The visceral skeleton
is entirely cartilaginous only in Elasmobranchs, and here it is so
524 CHORDATA.

loosely connected with the cranium as to be easily separated from
it. It consists in these forms usually of eight (rarely eleven)
arches (fig. 588); these are, from in front backwards, the rudi-
mentary labial cartilages, then the large mandibular arch, the
hyoid arch, and five (r mel seven) gill or branchial arches. The
mandibular arch consists, on either aie of two pieces which bear
teeth and oppose each other in biting; the upper half, attached to
the skull in front and behind, is the ptlerygoquadrate (is not the
upper jaw of higher forms). The lower part, which is hinged to
the Shee is the mandibulur or Meckel’s cartilage. In the same
way the hyoid arch is divided into an upper, or hyomandibular,
and a lower hyoid proper on either side, the hyomandibular being
fastened to the otic capsule. The hyoids are united below by an
unpaired piece, the copula. A copula also exists between the
halves of the branchial arches, each of which consists of four parts
on either side. Hyoid and gill arches bear gills. Certain features
(existence of rudimentary gills and a rudimentary gill cleft, the
spiracle) indicate also that the mandibular arch was once gill-
bearing and that it lost its original function upon being converted
into an organ of mastication. Recently the labial cartilages have
been regi arded as remnants of a support for tentacles around the
mouth like those of Amphiorus and Myzine, and which reappear
anew in the barbels of bony fishes. Hence they are not compar-
able to the other arches.

3y ossification the visceral arches of the higher fishes and all
higher vertebrates produce a great modification of the skull, this
being increased by a progressive change of function of the arches,
which depart more and more from their relations to the respiratory
apparatus. From this standpoint they may be divided into two
groups, an anterior, consisting of labial cartilages, mandibular arch,
and the hyomandibular; and a posterior, of the hyoid and the gill
arches. The hinder arches are well developed as long as branchial
respiration persists. With the loss of gills they largely disappear,
but what remains forms the hyoid or tongue bone (not to be con-
fused with the hyoid proper), its body being composed of the
copula, its anterior horns of the hyoid, and its posterior horns of
the remnants of a gill arch. Other gill arches contribute to
laryngeal cartilages, the epiglottis and the cartilages of the andi-
tory meatus.

The anterior members of the visceral skeleton (labials, pterygo-
quadrate, Meckelian, and hyomandibular) become developed
further, but lose more and more their individuality and unite with
IV. VERTEBRATA. 525

the cranium; in the mammals forming the ‘bones of the face.’ It
is therefore a source of additional bones which are difficult to fol-
low from class to class, since they change in their functions and
consequently in shape and relative size.

All vertebrates with bony visceral skeleton (figs. 561, 591)
have two pairs of membrane bones, right and left, in front of the
pterygoquadrates, the premaxillaries (intermaxillaries) and max-
illaries. They bear, in toothed vertebrates, the marginal row of
teeth, which are distinguished from the palatopterygoid teeth in
that they are opposed by the teeth of the lower jaw. The pterygo-
quadrates are thus forced backwards and form a second series of
bones, parallel to the maxillary series, which likewise may bear
teeth. This row of bones consists of an anterior palatine portion
and a posterior quadrate part. The cartilages of the palatine part
largely disappear and are replaced, in front, by a pair of vomers
followed by a pair of palatines, while farther back are a pair of
pterygoids. The quadrate portion ossifies into the quadrate bone,
which affords the articulation for the lower jaw. The ossifications
for the lower jaw occur in a similar way; in front a series of mem-
brane bones, of which the dentary is most important, surrounding
Meckel’s cartilage, while the hinder part of the Meckelian ossifies
into the articulare, so called because it articulates with the quad-
rate. The hyomandibular forms only one constantly present bone
known. by the same name.

If all vertebrates with bony skeletons be compared, it is found
that those with terrestrial habits have a sound-conducting apparatus
in connexion with the ear. This is composed of elements which,
in the fishes, He in the neighborhood of the otic capsule, the
hyomandibular, the quadrate, and the articulare, to which is
added another element, the stapes, which occupies the fenestra
ovalis (p. 544) and is derived from the otic capsule itself. In
Anura, reptiles, and birds the hyomandibular apparently gives
rise to an element, the columella, which abuts against the stapes.
In the mammals stapes and columella are possibly fused, while
quadrate and articulare undergo a change of function, losing
their position in connexion with the articulation of the jaws and
being converted into part of the sound-conducting apparatus, the
quadrate furnishing the incus, the articulare the malleus (figs.
576, 577). Since the lower jaw in this way loses its articulation,
a new one is formed by a process from the membrane bones.

According to this view the lower jaw of a mammal is not equivalent
to the lower jaw of a bird, since in the latter the hinge is furnished by the
526 CHORDATA.

quadrate-articulare joint. It should be said that another view obtains,
though not so well supported, which considers the ear bones as exactly
homologous throughout terrestrial vertebrates, and which recognizes incus
and malleus in the columella and maintains that quadrate and articulare
form the hinge of the mammalian jaw.

In conclusion three other bones, widely distributed, must be
mentioned—the squamosal, the tympanic, and the jugal. The
squamosal is a membrane bone arising at the boundary of quadrate
and otic capsule (petrosal), and hence with relations to both these
bones. It increases in size as the quadrate diminishes in changing
to the incus, and in the mammals fuses with the petrosal to form
the temporal bone. In common with the tympanic, which in
mammals also fuses with the petrosal, it forms a frame for the
attachment of the tympanic membrane of the ear. The jugal
(malar, zygomatic) belongs to the maxillary series. In many ver-
tebrates this series is articulated only in front, its posterior end
terminating freely in the soft parts, but when the jugal occurs it
forms a jugal or zygomatic arch which bridges the gap between
the maxillary and the quadrate region of the skull. When the
quadrate becomes modified to the incus, the jugal articulates with
its companion, the squamosal, which extends a zygomatic process
forward for this purpose.

Difficulties in ascertaining the morphological relations of bones arise
where the visceral and cranial parts join and where primary and second-
ary bones touch, especially since in the latter no general criteria of
distinction can be drawn. Thus the pterotic, sphenotic, and ectethmoid of
the fishes are often replaced by other secondary bones in the Amniotes ;
the primary pterotic by the secondary squamosal ; the primary sphenotic
and ectethmoid by two membrane bones in front of and behind the frontals,
the prefrontals and postfrontals of reptiles and other forms.

Just as skull and vertebral column form a firm axis for the body,
the appendages are supported by axial skeletal structures. Two
kinds of appendages are recognized, paired and unpaired, which
generally occur together only in fishes (figs. 598-608). The un-
paired consist of a fold of the skin beginning in the sagittal plane
behind the head, running back around the tail and forward on
the ventral surface to the anal region. This continuous fold is
nearly always divided into three parts, a dorsal fin (often sub-
divided into smaller fins), a caudal fin, and an anal fin. In a
similar way, apparently, the paired appendages—an anterior or
thoracic and a posterior or pelvic pair—have arisen from a pair of
continuous folds, by development of the appendages themselves

 

 
IV. VERTEBRATA. 527

and suppression of the intermediate regions. Of these the un-
paired are possibly the oldest, since they occur not only in the
cyclostomes, but in Amphioxus and the tunicates as well, where
paired appendages are lacking; on the other hand they disappear
in the higher forms. Since they are of service only in an aquatic
life, they are lost in the Amphibia, in which a continuous fin, un-
supported by skeletal elements, occurs only in larval life. On the
other hand the paired appendages gain in importance with terres-
trial habits.

In the fins of fishes two kinds of skeletal elements occur which,
in the Elasmobranchs, are distinguished by their histological
structure, since the one, the fin supports (basalia and radialia),
consist of cartilage, the others (actinotrichia, dermal skeleton)

 

 

¥Fia. 563.—Pectoral girdle and left fin of Heptanchus. (After Wiedersheim.) a, prin-
cipal row of the cartilaginous fin supports; h, horny threads or fin rays cut
across at ih’; nl, foramen for nerve; 7, accessory cartilaginous fin supports ;

s,s’, scapula; u, ventral portion of girdle.
are of horny consistency (fig. 563). Since in the teleosts both
kinds of supports may ossify, the distinction is here less striking,
yet the basalia and radialia arise from cartilage and lie in the basal
part of the fin, while the others are never cartilagimous and-occur
in the distal portion. These distinctions are of importance, since
the actinotrichial portions play no part in the development of the
extremities of the higher groups. These arise from the basal sup-
ports of pectoral and pelvic fins, which therefore alone need further
mention.

The skeleton of the paired appendages, preformed in cartilage,
consists of two parts, the girdles lying in the lateral walls of the
trunk, and the skeleton of the limbs themselves. A girdle—a
shoulder or pectoral girdle in front, a pelvic girdle for the hind
528 CHORDATA.

limbs—is in its simplest form an arch with right and left halves,
each half with an articular surface for
the limb, dividing it into dorsal and
ventral portions (fig. 563). The dorsal
portion is the scapula (shoulder blade) in
the pectoral, ilium in the pelvic girdle.
The lower portion is usually split into
anterior and posterior parts (fig. 564).
The anterior of these is the clavicle in
the pectoral girdle, pubic bone in the
pelvis; the hinder part is the coracoid
or the ischium in the two girdles re-
spectively. These parts are most con-
stant in the pelvic girdle. In the pec-
tora] girdle either coracoid or clavicle
may be lacking, at times both are absent;
but no vertebrate with fore limbs lacks
a scapula. In the clavicle there is fre-
quently an element, preformed in carti-
lage, the procoracoid,to be distinguished
Fie. 564,—Rigit half of shoulder {TOM & membrane bone, the clavicle in
girdles of (4) frog, (B) turtle, the strict sense.

(C) lizard. (After Gegenbaur,

slightly modified.) cl, clavicle; 3 i é f
cht ao AaGde Genco ute In the fishes the girdles are largely

eo “Guprascapula; st, or entirely held in position by muscles;
ribs. in most terrestrial vertebrates there is «
more intimate connexion with the axial skeleton and especially with
the vertebral column. In the case of the pelvic girdle the connexion
is direct, since the ilium is articulated with one or more sacral
vertebra (in reality not with the vertebra themselves, but by the in-
tervention of sacral ribs). The connexion of the pectoral girdle is
less direct and is looser. This is effected by clavicle and coracoid.
The latter connects with the sternum, which in turn is connected
to the vertebral column by the ribs; the clavicle articulates with
a bone, the episternum, which rests upon the breast bone, the
morphological relation of which is doubtful, since under this term
have been included different structures (the membrane bone of
Reptiles, episternum in the strict sense, the cartilage bone, the
prosternum of the monotremes and the preclavia of the mammals).
Since only the free portions of the appendages are concerned
directly in locomotion, aud since the various modes of motion—
swimming, flight, running, leaping, climbing—demand special
modifications, the skeleton of the limbs shows great variety. It

 
IV. VERTEBRATA. 29

or

is usually believed that all these forms are to be traced back to
an ancestral type, the archipterygium. In this (fig. 563) are
numerous skeletal parts which vary little in size and form and are
arranged in mauy closely appressed rows. One of the rows has
acquired prominence and is called the prmeipal row; it begins
with a larger piece, the metapterygium, which articulates with the
girdle and bears either on both sides (archipterygium biseriale) or
caly on one (archipterygium uniseriale) the lateral rows of skeletal
elements. Usually most of the lateral rows are not attached to
the principal row, but arise independently from the girdle, and
may begin with larger parts, the propterygium and mesopterygium.

From this archipterygium can be derived a primary form which
serves for all terrestrial vertebrates from the
Amphibia onwards; it is the pentadactyle ap-
pendage (fig. 565). In tracing this from the
archipterygium (of either uniserial or biserial
type) the following modifications must be sup-
posed. First a reduction in the number of
rows to five, a principal row and four acces-
sory rows. The terminal portions of the prin-
cipal row produce the bones of the fifth, the
accessory rows of the other fingers. Then
there is an unequal growth of parts; the meta-
pterygium, already in Elasmobranchs a con-
siderable element, increases in size and forms 4
in the fore limb the humerus, in the hind Fie. 565.—Schema of a

3 & pentadactyle appen-
limb the femur. In like manner the second dae rater Gexen:
* Q r) The dottec

element of the principal row and the first of lines indicate the lat-
5 ; eral rays; the names

the first accessory row increase and form re- for the ‘hinder ex-
ji i A . tremitiesin parenthe-
spectively ulna and radius in front, fibula and ses. H, humerus (fe-
oy : 1 . ay mur), U, ulna (fibula);
tibia behind. Then follow parts which remain __ &, radius (tibia). Car
f ‘ pus (tarsus) consist-

small and somewhat cubical, carpal bones in ing of two rows and
5 2 : MEA ee a two central portions:

the fore limb, tarsals in the hinder extremity; — Row I: r,radiale tibi-
: ale); ¢, intermedium;

they bear in turn slender bones, the meta- — w uinare (fibularei: ¢,
centralia. Row II:

carpals or metatarsals, and these at last the 7 °s Carpalia (tarsalia):
the metacarpals (met-

phalanges. (For the nomenclature of carpals — atarsals) and pha-
and tarsals see the explanation of fig. 565.) daneesinoupetiered:
The third and most important modification is brought about
by the development of joints. So long as the appendage served
ag an oar it must act asa single plate with its parts firmly held.
On the other hand, when it must act as a system of levers to sup-
port and move the body, as is necessary in a terrestrial animal, it

 
530 CHORDATA.

must be divided into sections, jointed to each other. By this
there are developed two joints of importance in both fore and hind
limbs; the elbow (knee) joint between humerus (femur) on the
one hand and radius and ulna (tibia and fibula) on the other; and
the wrist joint (ankle) between the bones of the fore arm (shank)
and the carpals (tarsals). Less important are the joints of the
fingers and toes.

If the limbs of terrestrial vertebrates be compared with this
primary form, variations are seen in two directions. Rarely are
there more bones than in the schema; then there occur remnants
of a sixth or even a seventh row or finger. More frequently there
is a reduction in the number of parts, either by fusion or by abso-
lute loss. Fusion accounts for the fact that with complete pen-
tadactyly the number of carpalia is usually less than ten, as would
be expected from the schema. Degeneration and loss explain the
existence of animals with four, three, two, or even one digit, and
one can say with certainty that the missing parts are in most cases
lost, though a fusion of digits is not unknown. Paleontology, for
example, teaches that the one-toed horse has descended, by gradual
reduction, from five-toed ancestors.

The completeness and character of the skeleton thus sketched

 

Fie. 566.—Horizontal section through the anterior trunk region of a young Rhodeus
amarus at the level of the ventral arches. c, notochord; h,skin; /¢, intermuscu-
lar ligament; m, longitudinal muscles; r, rib end of the cartilaginous ventral
arch ; v, osseous centrum.

in outline has a great influence on the rest of the organism. It
has already been pointed out that the external appearance of ver-
tebrates has been influenced by it, since the skin is no longer, as in
arthropods, a supporting structure and has consequently lost its
segmentation. More immediate is its influence upon the arrange-
ment of the musculature. The development of an internal skele-
ton renders it necessary that the point of resistance of the muscles
IV. VERTEBRATA. 581

must be transferred from the skin, where it is found in annelids,
molluscs, and artbropods, to the interior. A dermal musculature
occurs only as an inconspicuous remnant in vertebrates; it is re-
placed by a body musculature. This latter consists primarily of a
longitudinal system of muscle fibres on either side of the vertebral
column (fig. 566), which are divided by connective-tissue partitions,
the myosepta or myocommata, into successive segments, the myo-
tomes. Thus when the connective tissue of a fish is dissolved by
cooking the muscles fall into disk-like parts. The myosepta ex-
tend from skin to axial skeleton. Since they run obliquely back-
wards from the skeleton to the skin, they serve to render the
skeleton a point of resistance for the action of the muscles.

A segmented trunk musculature occurs in the Myxinoids (and
in Amphioxus), in which the axial skeleton consists only of noto-
chord and is consequently unjointed. The segmentation of the
muscles is therefore older than that of the skeleton and, as we can
further say, is the cause of it. The action of the muscles prevents
the formation of a cartilaginous or bony vertebral continuum such
as the notochord and skeletaginous layerare. It produces at in-
tervals joints or flexible parts separating the cartilaginous or bony
column into vertebra. Naturally these flexible portions cannot
coincide with the boundaries of the muscles, but must le between
them; in other words, muscle segments and skeletal segments—
myotomes and sclerotomes—must alternate. Segmentation is lack-
ing in the cranium, since the myotomes here have no locomotor
significance, are reduced, and only small remnants of them persist.

In the mammals only a little of this segmental arrangement of
muscles is recognizable, a result of the
development of the appendages; and the
more these gain in importance as the ws
locomotor structures, the more the mus-
cles are modified and grouped for the ser- wh
vice of the limbs, so that only the inter-
costals anda part of the muscular system
to the sides of the vertebral column show
clearly the primitive metamerism. Yet

ch

 

Fic. 567.—Horizontal sec-

in all vertebrate embryos the muscles ap- Ggnok jai embryo of
: A * Triton. (From O. Hert-

pear at first strictly segmental, m the wig.) ch, notochord; uh,
BR aes SG > rey myocele; ws, primary

form of the primitive somites (fig. 967), Toi aele: Gcetene Gh woe

formerly called protovertebre. vome.:

Another important point in the musculature lies in the fact
that it is dorsal in origin and therefore in fishes is largely dorsal
552 CHORDATA.

in position throughout life. The muscles which are ventral have
largely been transferred from the back, and the cause of the migra-
tion is to be recognized to a large extent in the progressive devel-
opment of the appendages. The dorsal position of the muscles is
only a part of a general fact, that the skeletal axis divides the body
into a dorsal zone, containing only animal organs, and a ventral
zone, chiefly vegetal in character. Besides, the muscles, the cen
tral nervous system, and the most important sense organs—eyes,
nose, ears—belong to the dorsal zone.

The central nervous system of vertebrates consists of brain and
spinal cord. Like that of all chordates it is distinguished from
that of other segmented animals—annelids, arthropods, in which
there is a dorsal brain and a ventral nerve chain—in its purely
dorsal position. It is further distinguished from that of all non-
chordates by its tubular character, that is, by the presence of a
central canal in the axis of the elongate central system (fig. 76),
lined by a special epithelium, the ependyma, and containing a
fluid, the liquor cerebrospinalis. This central canal is the result
of the mode of development, the nervous system arising by an in-
rolling of the ectoderm and not by a splitting from it as in the
invertebrates (fig. 9). Besides the neurenteric canal already
referred to (p. 502), there long persists at the anterior end an
opening to the exterior, the neuropore. In all vertebrates, in con-
tradistinction to the lower chordates, the brain is large and sharply
marked off from the spinal cord.

The spinal cord is a cylindrical structure (flattened in Cyclo-
stomes, fig. 555) which, in the middle line above and below, is
marked by two longitudinal grooves, the dorsal and ventral fissures
of the cord (sp, sa, fig. 76). The central canal (Cc) has its lumen
greatly narrowed by the growth of the nervous tissue, in which,
as in the ganglia of the invertebrates, two layers are distinguished,
one containing almost solely nerve fibres, the other both fibres and
nerve or ganglion cells. The arrangement of these layers is con-
trasted with that of the invertebrates in that the ganglion-cell
layer—the gray matter—lies in the centre, the fibrous layer—white
matter (JI’)—on the periphery, a reversed position consequent
upon the development by infolding. The distinction in color
indicated in the names depends upon the fact that white medullated
fibres run in the cortex, while in the gray matter gray non-medullated
fibres are present between the nerve cells. The color distinctions
fail in the cyclostomes (and inphiorus), which have no medullated
fibres, although the same general structure occurs.

 

 
IV. VERTEBRATA. 598

The gray matter surrounds the central canal, but extends on
either side dorsally and ventrally into the white matter, so that
in section it resembles somewhat the letter H, with its dorsal
(fig. 76, HH) and ventralhorns (VH). By means of these horns
and the dorsal and ventral nerve roots arising from them, the white
matter on either side is divided into three tracts, the dorsal (/7),
ventral (s), and lateral (SY) columns of the cord.

Corresponding to each muscle segment two nerve roots arise
from the cord, a dorsal root, with a ganglion (spinal ganglion) at
some distance from the cord, and a ventral root, without a ganglion.
The dorsal root contains only sensory fibres—i.e., those carrying
nervous impulses to the cord—and is afferent, while the ventral
roots are efferent and contain only motor elements (Bell’s Law).
These roots unite into a mixed root, which then divides.into dorsal
and ventral branches.

The brain of vertebrates in general corresponds in its funda-
mental plan (fig. 568), best seen in development, with the brain of
man. At an early stage it consists of three
vesicles, one after the other, a fore brain
(prosencephalon), a mid brain (mesencepha-
lon), and a hind brain (metencephalon).
Usually this stage is reached before the
closure of the medullary folds. Formerly it
was stated that a condition with five vesicles

  

Fie, 568. Fic. 560.

Fig. 568.—Diagram of a vertebrate brain. (From Wiedersheim.) Aq, aqueduet ; Ce,
central canal; FM, foramen of Monro (connexion of lateral ventricles with each
other and with the third); HH, cerebellum ; MH, corpora bigemina, (optic lobes) ;
NH, medulla oblongata; A, spinal cord; SV, lateral ventricles * VH, cerebrum;
ZH, optic thalami (twixt prain); JI, IV, third and fourth ventricles.

Fic. 569, Scheme of brain in sagittal section. c, cerebrum; ch, cerebellum; cc, canal
of spinal cord; ch, notochord ; ¢s, corpus striatum; , hypophysis; ¢, infundibu-
lum; m, medullary region ; 0, optic chiasma ; of, olfactory lobe; ol, optic lobes ;
Pp, pinealis.

followed upon this with three, the mid brain remaining undivided,
while the hind brain divides into cerebellum (cd) and medulla
oblongata (7); the fore brain into cerebrum and ’twixt brain.
This is unnatural so far as the hind brain is concerned, for cere-
584 CHORDATA.

bellum and medulla are related to one another as roof and floor of
one and the same cavity (fig. 569). The distinction between the
first and second vesicles is problematical. The fore brain becomes
divided into three parts by an inpushing at its anterior end: an
unpaired middle portion, and in front a right and a left diver-
ticulum. These paired portions, increasing in size, form the cere-
bral hemispheres, and together with a small connecting part
represent the first cerebral vesicle, while the unpaired portion
forms a second vesicle, the ’twixt brain.

Introducing the terms of human anatomy for the separate parts
of the brain, the first vesicle consists of the two cerebral hemi-
spheres whose dorsal and lateral walls are usually thick and are
called the pallium, while in the floor of each hemisphere is an
enlargement, the corpus striatum (cs). The spaces in the hemi-
spheres are the first and second ventricles (sv). From the front
portion of each hemisphere arises a distinct region, the olfactory
lobe (of), which gives origin to the olfactory nerve. Since the
organ of smell is frequently at some distance from the brain, the
olfactory nerve must be elongate, as in the Amphibia (fig. 614), or
the olfactory lobe must lengthen, as in many Elasmobranchs (fig
592). In the latter case the swollen end of the lobe is close to
the olfactory epithelium and is connected with the brain by a long
stalk, the tractus, while the swelling is called the bulbus olfacto-
rius. Both, as parts of the brain, must be distinguished from the
olfactory nerve.

In the region of the second vesicle only the lateral walls become
thickened, producing the optic thalami, directly adjoining the
corpora striata; the roof of this vesicle develops no nervous sub-
stance, but remains a thin layer of epithelium closing in the third
ventricle above (f//). The floor is also thin-walled between the
thalami and is pushed downwards, forming a funnel-like pocket,
the infundibulum (7). The third vesicle, as a rule, is divided by
a deep longitudinal dorsal groove, dividing the cavity into a right
and left ventricle, while the two halves of the roof are known as
the optic lobes or corpora bigemini. In the mammals alone (in
which there is also a transverse groove dividing the optic lobes
into the corpora quadrigemini) the cavity of this mid brain is re-
duced, by thickening of the walls, to a narrow canal, the iter or
aqueduct of Sylvius, with the result that the term fourth ventricle
is transferred to the cavity of the hind brain.

This last region is called the medulla oblongata; it isa prolonga-
tion of the spinal cord, and in many respects shows a similar struc-
IV. VERTEBRATA. 000

ture. It is distinguished from the cord externally in that it
gradually increases in size in front, while its roof is reduced to a
thin epithelium, often torn away in dissection, leaving an opening,
the fossa rhomboidalis, into the ventricle. In front of this fossa is
the cerebellum, often a thin transverse nervous lamella, but usually
is a considerable part of the brain, composed of a median ‘ vermis’
and two lateral cerebellar hemispheres.

Although these five parts are present in all vertebrates, the
appearance of the brain in the various classes is very different,
because the relative size and form of the parts undergo great
yariations. In the lower vertebrates optic lobes and medulla
oblongata are disproportionately large, while the cerebrum, and
often the cerebellum, are insignificant in size; in the cerebrum,
again, the hemispheres may be smaller than the corpora striata
and the olfactory lobes. In the higher vertebrates, on the other
hand, the cerebrum and cerebellum far surpass the other parts,
the increase in size of the cerebrum being proportional to the in-
crease in intelligence. The cerebral hemispheres grow backwards,
in man and the apes covering the other parts, while in front the
olfactory lobes are carried by a similar overgrowth to the lower
surface. Since the capacity of the skull is limited, the cortex of
the cerebrum, the seat of intelligence, is increased in amount by
the development of folds, gyri, separated by sulci. Somewhat
similar conditions exist in the cerebellum, which in mammals and
birds is, next to the cerebrum, the largest part of the brain.

Connected with the ’twixt brain are two problematical organs, one, the
epiphysis (pinealis), being dorsal; the other, the hypophysis (pituitary
body), ventral. The hypophysis arises like a gland by an outgrowth from
the embryonic mouth, This hypophysial pocket cuts off from its source,
inereases by budding, and fuses with parts derived from the end of the
infundibulum to a single two-lobed body. It has been compared with the
subneural gland of the Tunicata (p. 509), The epiphysis is an outgrowth
from the roof of the brain, from which develops in many vertebrates the
parietal organ. In many reptiles this has the structure of an eye (pineal
eye), and in these, separated from the brain, but connected with it by a
nerve, it lies in a special cavity in the parietal bone, which occurs not only
in recent but in fossil forms. Above the eye the skin may be transparent.

The nerves which come from the brain mostly arise from the
region between the mid brain and the spinal cord, especially from
the medulla oblongata. The olfactory and optic nerves are an
exception, the one arising from the cerebrum, the other from the
*twixt brain, but both, and especially the optic, differ so much from
the peripheral nerves that they can hardly be classed with them.
536 CHORDATA.

Development shows that the optic nerve is a part of the brain.
Following custom, however, and including these two, the pairs of
cranial nerves may be enumerated in the terms of human anatomy
as follows: I, N. olfactorius; H, N. opticus; III, N. oculomotorius;
IV, N. trochlearis (patheticus); V, N. trigeminus; VI, N. abducens;
VII, N. facialis; VIII, N. acusticus; IX, N. glossopharyngeus;

po

ive)

<<
—

 

Fia. 570.—Diagram of cranial nerves (shark). a, alveolaris; h, buccalis; c, cere-
brum ; cb, cerebellum; ct, chorda tympani; e, ear; er, external rectus muscle;
f, inferior rectus muscle; g, Gasserian ganglion; h, hyoid cartilage; hm, hyoman-
dibular; 7, internal rectus muscle; io, inferior oblique muscle; j, Jacobson's
commissure ; J, lateralis of vagus ; m, mouth; mc, Meckel's cartilage: md, mandi-
bularis ; mx, maxillaris superior; n, nose ; 0, optic lobes; op, ophthalmicus profun-
dus; os, ophthalmicus superficialis; p, pinealis; pl, palatine; po, posttrematic
branches; pr, pretrematic branches; pn, pneumogastric (intestinal) of vagus:
ptg, pterygoquadrate; s, spiracle; so, superior oblique muscle; sv, superior rectus
muscle; ft, ’twixt brain; J-X, cranial nerves: 1-5, gill clefts.
X, N. vagus (pneumogastricus), XI, N. accessorius; XII, N.
hypoglossus. The accessorius in fishes and amphibia is a part of
the vagus; the hypoglossus, strictly speaking, belongs to the spinal
nerves and only secondarily is associated with the cranial nerves,
which explains its course, outside the skull, in eyclostomes and
amphibia.

Since the head undoubtedly consists of several coalesced body seg-
ments (at least as many as there are visceral arches, and apparently
more), the question arises whether the cranial nerves are as evidently seg-
mental as are those of the trunk. To this is allied the further question
whether Bell’s Law that a mixed nerve consists of dorsal sensory, and
ventral motor components is appleable here. Both problems have been
much discussed in recent years, but as yet the final answers have not been
given, It is probable that the present cranial nerves, the optie and olfac-
tory excepted, have arisen by manifold rearrangements of segmental
nerves. On the other hand it seems impossible to accept Bell's Law here
without considerable modification, since many cranial nerves (facialis,
trigemenus, ete.) contain motor fibres, although they are formed like
dorsal roots.
IV. VERTEBRATA. 537

Besides the nervous system of the body already outlined, the vertebrates
have a special nervous system supplying the viscera,—the sympathetic
system,—and in this a special central organ consisting of right and left
cords beneath the vertebral column, in which ganglia are incorporated.
The last of these ganglia lies at the base of the caudal vertebree, the most
anterior at the beginning of the neck. From the latter nerve cords
extend into the head and are connected with ganglia (otic, sphenopalatine).
This system sends out nerves in the form of delicate networks (plexus
sympathetici) which usually accompany the blood-vessels to the vegeta-
tive organs (intestine, sexual apparatus, etc.). It is also connected with
the spinal nerves.

Regarding the sense organs of the vertebrates we stand on
firmer ground than with the invertebrates, since their great simi-
larity to those of man supports the ideas of their functions derived
from studies of their structure. The tactile organs make an ex-
ception, since only in land animals, and not in fishes, do they
resemble those of man. These organs, in all forms above fishes,
have the peculiarity that the nerves do not end in epithelial cells,
but in special tactile cells of the derma, which either lie isolated in
the connective tissue (Amphibia, reptiles), or, grouped together,
produce tactile corpuscles (birds, mammals,
fig. 571). These are oval bodies and are im-
bedded in special papille of the derma. In
form and position they are much like the
Vater-Pacinian corpuscles, which are distin- 4m
guished by their histological structure (fig. 78)
and, since they also occur in internal organs
(mesentery of cat), are of problematic function.
Besides these mesodermal nerve endings there Fie. 571.—Tactile cor-

m a Sen 1 puscle from hbird’s
are present in all vertebrates intraepithelial — fongue. H, outer en-
nerve branchings which are best seen in the fe ee
cornea of the eye and in animals, like pigs and Partitions.
moles, with sensitive snouts. Even here the finest nerve twigs do
not end in epithelial cells, but in small knobs between them.

Fishes lack tactile cells, tactile corpuscles, and end bulbs;
hence the skin is provided with sense organs in which a sensory
epithelium occurs. The dermal nerves pass into the epidermis
and end in oval corpuscles, which, while imbedded in a stratified
epithelium, consist of a single layer of sense cells. According to
structure, nerve hillocks and nerve-end buds are distinguished,
The first are the specific organs of the lateral line, to be men-
tioned later, of fishes and branchiate amphibians and amphibian
larve, and therefore appear to subserve special and important sensa-

 
5388 CHORDATA.

tions connected with aquatic life; hence the idea of a ‘sixth
sense,’ lacking to man (c/. p. 125). The end buds are especially
collected in the neighborhood of the mouth, on the lips and bar-
bels. Since they also occur in the mucous membrane of the mouth,
especially in the palatal regions, they connect with the taste organs.
The taste buds have the same structure as the end buds of fishes.
They occur in all classes of vertebrates, and are most abundant in
man in the walls of the circumvallate papille at the base of the
tongue; in rodents on the large foliate papille, etc.

The end buds also lead to the olfactory organs. The olfactory
epithelium of many fishes and amphibia is a stratified epithelium
with closely arranged end buds (fig. 572). By disappearance of

 

Fia. 572.—Section of olfactory epithelium of a fish (Belone). (From O. Hertwig, after
Blaue.) e, epithelium: k, olfactory buds; n, nerves.

the isolating parts of the ordinary epithelium the end buds form

a continuous sensory epithelium, which is the rule in most ver-

tebrates.

The olfactory organ, the nose, lined with its sensory epithelium,
acquires a special interest both from its grade of development and
from the important systematic distinctions it affords. Except the
cyclostomes, which have an unpaired nasal sac, all vertebrates have
paired olfactory organs. In adult fishes and in the embryos of
higher forms are two pits which he in front of or dorsal to the
mouth; they are either distinct from it or only connected with
it by an oronasal groove in the skin (fig. 599). If the animal
be terrestrial and replace branchial by pulmonary respiration, a
respiratory canal is developed in connexion with the nose. The
eronasal groove closes to a tube which begins with an opening
(nostril) on the surface and ends with a second opening (choana)
in the mouth cavity. The olfactory sac proper is included in the
wall of this tube, usually on its dorsal surface (fig. 573). In Am-
IV. VERTEBRATA. 539

phibia, lizards, snakes, and birds the choana is far forward, behind
the upper jaw; in alligators, turtles, and mammals it is carried far
back, in crocodiles and some mammals
(edentates) nearly to the vertebral col-
umn. ‘This position is brought about by
the development of the hard palate, a
parting wall which divides the primitive Ly
mouth cavity into two portions, a lower, ‘ a \

the persistent or secondary mouth cavity, od Ch
and an upper, which, as secondary nasal Fig. 573.— Diagram of nose of
: i . izard. (After Wiedersheim.)
cavity, contributes to the air passages, 4X, outer nasal cavity; €,
olfactory sac; Ca, canal from

The bones of the maxillary and palatine Jacobson’s organ to mouth:

3 3 - Ch, choana; IN, inner nasal
series contribute to the hard palate, since cavity; 4/8, roof of mouth;

premaxillaries, maxillaries, and rarely the ian Weoreen anteroe
pterygoids send out horizontal processes "**

which meet in the middle line. In the mammals this partition
is continued back by the muscular soft palate. In crocodiles there

is a fibrous palate.

       

In the olfactory organ of the chordates two constituents must be
recognized, an unpaired and two paired portions. The unpaired portion
alone occurs in Amphioxus, this being supplied by the lobus olfactorius
impar ; in all vertebrates there are paired sacs with paired olfactory lobes.
The unpaired sac of the cyclostomes has apparently arisen from a union of
paired and unpaired parts, hence the double olfactorius.

A further increase in the nasal cavity is brought about by complicated
folds in the walls supported by special skeletal parts, the turbinal bones,
and also by the outgrowth of chambers, lined with mucous membrane
which extends into the neighboring bones. Thus are formed the sinus
frontalis in the frontal bone ; behind, the sphenoid sinus in the sphenoid,
and the antrum of Highmore in the maxillary. Again, a part of the primi-
tive chamber lined with olfactory epithelium can be cut off from the rest
and form an accessory nose, Jacobson’s organ, which opens into the
mouth behind the premaxillaries by ‘Stenson’s duct’ (fig. 578, P). This
organ is best developed in lizards, monotremes and ungulates, but often
oceurs in a reduced condition in other terrestrial vertebrates.

Tn all vertebrates with the exception of Myzine and a few forms
living in the dark the eyes are composed of all the principal con-
stituents which occur in the human eye and which have already
been briefly described (p. 131, fig. 83). In most vertebrates it is ¢
nearly spherical body with the optic nerve entering it from behind,
with its interior occupied by transparent, refractive substances
(lens, vitreous body), and its walls of three concentric layers.
The outer of these is the tough protecting sclera (sclerotic), a
540 CHORDATA,

usually fibrous, but in many fishes a cartilaginous, layer, which in
front becomes transparent and strongly curved, forming the cornea.
The second layer, the choroid coat, is richly vascular and pig-
mented; at the boundary between sclerotic and cornea it is changed
to the iris. The inner layer is the retina, the structure and
arrangement of which are characteristic of the vertebrates.

From the developmental standpoint the retina (fig. 82) con-
sists of two parts, the retina proper and the tapetum nigrum
(pigmented epithelium), formerly regarded as part of the choroid.
In the retina the following layers are distinguished: (1) the limi-
tans interna; (2) nerve-fibre layer; (3) ganglionic layer; (4) inner
molecular layer; (5) inner granular layer; (6) outer molecular
layer; (7) outer granular layer; (8) limitans externa; and (9) layer
of rods and cones. The limitans externa is the bounding mem-
brane of the embryonic retina, which is later penetrated by the
rods and cones. Between the two limiting membranes Miiller’s
fibres (m) extend, large supporting cells occurring in other sensory
epithelia, the nuclei of which lie in the inner granular layer, and
which are aided in their supporting function by the fine horny
framework of both molecular layers. The nervous elements
which are imbedded in this support are best understood by begin-
ning with the optic nerve. This spreads out in the nerve-fibre
layer, and on its way to the end apparatus comes twice into relation
with ganglion cells; first in the ganglonic layer, second in the
inner granular layer. Thus a great part of the retina (layers 1 to
6) are to be considered as an optic ganglion, such as occurs in
molluscs and arthropods, but which there lies outside the sensory
apparatus. The sensory epithelium (the retina in the sense this
term is used in invertebrates) consists of but two layers, the
outer granular layer and the rods and cones. The outer granules
are the nuclei of the extremely slender epithelial cells which bear
the rhabdomes (rods and cones) on their peripheral ends. Pigment
cells are lacking between these visual cells, but the pigment so
necessary for the visual function is supplied by the tapetum
nigrum already mentioned. This isa layer of hexagonal epithelial
cells which lies on the tips of the rhabdomes and sends pseudopodia-
like processes between them, and since the tapetum is rich in
black pigment granules, the rods and cones are enveloped ina
close pigment mantle.

Although in this relation of pigment and in the union of the
optic ganghon with the sensory cells important differences are to
be noted from the eyes of the invertebrates, even of the closely
IV. VERTEBRATA. 541

similar cephalopod eye (p. 385), the most striking difference re-
mains to be mentioned. The retina, with its limitans interna and
nerve-fibre layer, abuts against the vitreous body; with its rhab-
domes and tapetum against the choroid. Hence the incoming
light must traverse the optic ganglion and pass through the layer of
sense cells before reaching the end organs, the rhabdomes. In
nearly all invertebrates, for example the Cephalopoda (fig. 383),
the light falls directly on the peripheral end of the rhabdome.
The rhabdomes in cephalopods, as in most invertebrates, are
turned towards the light, in the vertebrates away from it.

This peculiar and functionally purposeless inversion of the vertebrate
retina is explained by the development of the eye. This can be divided,
according to origin, into two parts, a cerebral (optic nerve, retina, tape-
tum) and a peripheral (all other parts). As the eye in tunicates and Am-
phioxus is permanently a part of the brain, so is the retina of vertebrates
genetically, and of the first cerebral vesicle. An outgrowth occurs on
either side (fig. 574, B) of the ’twixt brain and becomes expanded distally

 

Fic, 574.—Diagram showing the inversion of layers in the formation of the retina

(orig.). The nuclei are placed in the (morphologically) deeper ends of the cells.

In 4 the brain ()) has been closed in; in B the optic vesicle (v) has reached the

liens (1) and on the right is being converted into the double-walled optic cup

with, as shown in C, an outer tapetal (¢) and an inner retinal layer (").

to an optic vesicle which is connected with the brain by an optic stalk.
The vesicle extends out to the periphery and, coincidently with the de-
velopment of the lens, is folded into a double-walled optic cup with outer
or tapetal, inner or retinal layers. If the position of the epithelial cells
be followed, it will be seen that the peripheral ends rest upon the tapetum,
and when these ends develop the rhabdomes, these must grow into the
tapetal layer.

In contrast to the retina, the lens develops as an invagination from
the epithelium of the body (fig. 574) ; sclera, cornea and vitreous body from
connective tissue. Thus the important part of the eye arises from the
brain and is later provided with accessory apparatus which arise from
peripheral parts. The invertebrate eye, on the other hand, with all its
parts arises from the skin.

The vertebrate eye is furnished with secondary structures: with mus-
cles which move it, with lids which protect the cornea from injury and
drying. The lids are dermal folds which extend over the eyeball from
above and below. To these a third lid, the nictitating membrane, may
542 CHORDATA,

be added. It arises from the inner angle of the eye, and can extend over
the cornea beneath the upper and lower lids. A special lachrymal gland,
which occurs at the outer angle of the eye, provides the fluid to moisten
the cornea, while a second or Harder’s gland occurs at the inner angle
when a nictitating membrane is present. Both are lacking in the An-
amnia,

The ear, at the level of the medulla oblongata, rivals the eye in
its complication of structure. In development it has one point in
common with the invertebrate
otocyst—it arises as an ecto-
dermal pit which is usually
completely cut off from its par-
ent layer, and only in elasmo-
branchs remains connected with
the exterior by a tube, the
elsewhere closed endolymphatic
duct. In the cyclostomes it con-
sists of a single vesicle with a
single macula acustica; from
the fishes upwards the vesicle
becomes divided by a constric-
tion into an upper utriculus
and a lower sacculus (fig. 575),
the connecting utriculosaccular
Fic. 575.—Diagram of membranous laby- duct being narrow in the mam-

rinth of a fish. (From Wiedersheim.) ed ‘

au, ae, ap, anterior, external, and poste- mals. Both utriculus and sac-
riz mpulle amy superiet MinewAT onlns receive a part of the
ee ee canalss usutri: macula acustica.  Diverticula
colemphaticus: & lagena.s zee Teesseus from the vesicle occur, giving
a Pec aiGg Eo cr the whole the name of labyrinth.
dolymph-duet: From the utriculus arise three
semicircular canals, connected at either end with this cavity, each
swollen at one end to an ampulla, containing a special nerve
termination, the crista acustica, These canals stand at right
angles to each other in the three dimensions of space and with-
out doubt subserve the sensation of equilibration (p. 128). They
are an outer horizontal, an anterior vertical (nearly sagittal), and
a posterior vertical (nearly transverse). The non-ampullar ends
of the two vertical canals unite, a condition which is understood
when it is recalled that in cyclostomes these canals alone are
present, and in Myzxine form a single canal with two ampulle.
A later formation is a diverticulum from the saeculus, which

 
IV. VERTEBRATA. 543

appears even in the fishes as a small pocket, the lagena, containing
a part of the macula acustica; in the reptiles and birds the lagena
becomes much larger, and in the mammals is spirally coiled and is
known asthe cochlea. A part of the macula acustica of the lagena
develops into a special nerve-end apparatus, the organ of Corti.
The membranous labyrinth described above is partially or en-
tirely enclosed in the side wall of the skull in the otic capsule,
which may ossify to the otic or petrosal bones. In the birds and
mammals the enclosure is such that the structure is duplicated in
bone, so that the membranous labyrinth lies in a bony labyrinth,

We

ILO {De

I

 

 

 

 

 

    

rot

Fig. 576.—Diagram of human ear. (From Wiedersheim.) a, h, vertical semicircular
canals ; ¢, their upper connexion; Co, the connexion in bony labyrinth; Con,
ductus cochlearis; Con’, cochlea; Cr, canalis reuniens; Ct, tympanic cavity
(left), cupula terminales (right); d, perilymph; De ductus endolymphaticus; Dp,
Dp’, ductus perilymphaticus; Kl, AU, bony labyrinth surrounding the mem-
branous labyrinth, the perilymph space black; MW, conch of ear (left), membrane
closing fenestra rotunda (right); Mae, external auditory meatus; Mt, tympanic
membrane; NS, sacculus; SAp, ear bones (represented as a rod); Ne, sacculus en-
dolymphaticus; St, Sv, scale tympaniand vestibuli; Th, Th’, Eustachian tube and
its entrance into pharynx; *, connexion between scale tympani and vestibuli;
+, insertion of ear bones in fenestra ovalis; 2, utriculus.

the two being separated by lymph spaces (fig. 576). These spaces
are developed in the cochlea into two tubes, the scala tynipani and
scala vestibuli, the two connecting only at the tip, being separated
elsewhere in part by the membranous cochlea (the ductus cochlearis
or scala media). The spaces of the bony labyrinth are filled by
two different fluids: inside the membranous labvrinth an en-
dolymph, and between this and the walls of the bony labyrinth a

 

perilymph.
o44 CHORDATA.

Accessory structures may be added to this auditory apparatus
proper, their purpose being to bring sound waves to it. Such
structures are but occasionally present in fishes (it is not certain
that they hear), since the sound waves are easily carried by the
water to the tissues and thence directly to the ears. On the other
hand, with the change to terrestrial hfe such a sound-conducting
apparatus is necessary on account of the differing densities of the
air and the tissues. So we find from Amphibia onwards a vibrat-
ing membrane—the tympanic membrane—which receives the sound
vibrations from the air and carries them to a chain of ear bones
(ossicula auditus), which in turn transmits them to the inner ear
orlabyrinth. These structures are not always functional (cetacea),
and they may be wholly or in part rudimentary (urodeles, snakes,
Amphisbenids).

To understand this apparatus it must be recalled that the ear
lies between the hyoid and mandibular arches in the neighborhood
of a canal which leads from the surface to the pharynx. In
the fishes this canal is the spiracle, a reduced gill cleft. In the
Anura and amniotes it consists of an air chamber closed exter-
nally by the tympanic membrane, stretched on a tympanic an-
nulus, while the opening to the pharynx is retained. The
part next the membrane becomes expanded into the tympanic
cavity, this with the membrane forming the tympanum or drum.
The part connecting with the pharynx is usually narrowed and is
called the Eustachian tube. The membranous labyrinth lies in
the wall of the tympanic cavity and touches it at one or two points
where the bony auditory capsule is interrupted, the always present
fenestra ovals, and the fenestra rotunda, lacking in Amphibia.

When it is recalled that the mandibular arch lies just in front
of the spiracle, and the hyoid close behind it, it is readily under-
stood how parts of these arches can
enter the tympanum and produce the
ear bones. In Anura, reptiles, and birds
a columella has one end attached to
the stapedial plate, which lies in the
fenestra ovalis, while the other is in-
serted in the drum membrane, the
whole conveying the waves across the
tympanum to the labyrinth. In the
F1a. 517.—Ear bones ofman. mammals the structure is different, since

ae ele ene 4 the columella is replaced by two bones,
nS the malleus, which is attached to the
drum membrane, and the ineus, which articulates with the

 

 
IV. VERTEBRATA. 545:

Most students believe incus and malleus to be parts

stapes.
a view which

(quadrate and articulare) of the mandibular arch
has its opponents, who believe these to be a divided columella
(fig. 577).

The tympanic membrane is usually flush with the surrounding
skin or only slightly below its level. In the mammals it is pro-
tected by being placed at the bottom of a deep tube, the external
auditory meatus. The ear conch, a fold of skin supported by
cartilage, is also confined to the mammals.

The more important vegetative organs of the body are enclosed
in a large body cavity or celom beneath the vertebral column.
This is, as development shows, an outgrowth from the primi-
tive digestive tract, an enterocwle (pp. 109 and 158), lined
with epithelium. Since it arises, as in other colomate animals,
by paired outgrowths from the archenteron, it follows that
at first the two cavities must be separated by a_ partition

 

My

CEL

ZZ

LLLCOS

LAE:

Lb

   

Rox SX
AS
F1a. 578.—Section of vertebrate in abdominal region. (From Kingsley.) a, dorsal
aorta; ¢, ccelom; g, gonad; gl, glomerulus; i, digestive tract; l, iver; m, mesen-
tery; mu, muscular part of myotomes; my, its ca@lom (myoccrle); 0, omentum ;
s, Spinal cord; so, sp, somatic and splanchnic epithelia; t, nephridial tubule; vm,
s,s alc atic
ventral mesentery ; w, Wolffian duct.

which also encloses the intestinal tract (fig. 578).
walls furnish the mesentery which supports the intestine in its

 

These
546 CHORDATA.

whole length from the vertebral column, but ventral of the diges-
tive tract (as the mediastinum, omentum minus, and suspensory
ligament of the liver of human anatomy) only reaches as far back
as the liver, so that right and left coloms unite behind. Some
other organs are also suspended in the body cavity by membranes:
the testes by the mesorchium, the ovary by the mesovarium.

The body cavity is frequently called the pleuroperitoneal cavity,
since in mammals it is divided by a partition, the diaphragm, into
an anterior or pleural and a posterior or peritoneal (abdominal)
cavity. The lining membranes of these cavities are called pleura
and peritoneum respectively. The pericardial cavity is also a de-
rivative of the celom, and the lining, the pericardium, but a part
of the pleuroperitoneal membrane. Hence it is that in many
fishes (sharks, sturgeon) a communication persists between the
pericardial and the other celom. In most fishes and in many rep-
tiles there is a direct connexion of the celom with the exterior by
one or two pori abdominales, beside or behind the anus.

The alimentary tract possesses the greatest systematic interest
of the vegetative organs, for it not only is concerned with diges-
tion, but furnishes, as in all chordates, the respiratory organs (gills
and lungs) as well, these arising in the non-chordates from the
ectoderm. It begins with the anterior ventral mouth and ends
ventrally with the anus, some distance in front of the tip of the
tail; it is almost wholly entodermal in origin, there being but
slight ectodermal portions at either end.

The first division is spacious and consists of the ectodermal
mouth cavity and the entodermal pharynx, two spaces which, in
most vertebrates, are not sharply marked off, but in alligators and
mammals are separated by the soft palate. Then begins the
narrow cesophagus, wideuing behind to the stomach. From the
hinder or pyloric end of the stomach begins the small intestine,
which enlarges into the large intestine, separated from the small
intestine in the higher vertebrates by a valve and one or two ecxca.
The terminal portion in most vertebrates is called the cloaca be-
cause it receives the urogenital ducts. The liver is the only gland
constantly present; it is a large compact brown organ, generally
provided with a gall bladder. Usually a smaller gland, the
pancreas, occurs. The ducts of the liver (bile duct, ductus
choledochus) and pancreas empty into the small intestine near the
pylorus. The mouth cavity may have salivary glands connected
with it, while the rectal region occasionally has blind sacs and

glands.
47

or

IV. VERTEBRATA. :

A striking vertebrate characteristic occurs in the dentition.
In the cyclostomes there are horny teeth—strongly cornified epi-
thelial products seated on connective-tissue papille; in the higher
groups occur true teeth of dentine and enamel, enclosing a richly
vascular pulp. They occur in places where the underlying skele-
ton affords them a firm support, especially on the upper or lower
jaws, but they may occur on other bones of the mouth and
pharyngeal cavities (roof of the mouth, gill arches). They have
apparently arisen from a diffuse dentition, recalling the scales of
the skin, since many elasmobranchs possess, besides the ordinary
teeth, rudimentary teeth in mouth and pharynx. Where teeth are
lacking (birds, turtles, baleen whales) they have been lost.

The respiratory organs arise from the pharynx. In the fishes
and some Amphibia its walls, right and left, are perforated by
gill clefts, each of which lies between two successive visceral arches
(fig. 570). These are canals which open internally into the
pharynx, while the outer gill openings are on the outer surface.
The anterior and posterior walls of the clefts bear delicate vascular
folds of mucous membrane, the gill filaments. These are the in-
ternal gills, in contrast to the external gills of Amphibian larve,
which are dendritic external ectodermal growths arising above and
between the gill slits (figs. 4,5). Itis important for the phylogeny
of the vertebrates to note that reptiles, birds, and mammals, which
never breathe by gills, have gill clefts outlined and later lost with
the exception of the Eustachian cleft.

Two problematical organs, the thymus and the lateral lobes of the thy-
roid gland, develop from the epithelium of the gill clefts. The middle
unpaired part of the thyroid has been regarded as a modification of the
endostyle of the Tunicata (p. 506). The thyroid, which produces iodine
compounds, is doubtless very important; disease or extirpation of it
causes serious nervous disturbances.

The lungs also arise from the pharynx as two sacs (one oc-
casionally remaining rudimentary), which grow downwards and
backwards. They retain their opening into it either directly or
by means of a trachea or windpipe, which just before its entrance
into the lungs usually divides into two bronchi (figs. 579, 620).
At the opening into the pharynx (glottis) the supporting cartilages
(remnants of the visceral skeleton, p. 524) are strong and form
the larynx, which in mammals may be closed from the pharynx
by a valve, the epiglottis. The lungs and trachea have their
counterparts in the fishes in the swim bladder, a hydrostatic
apparatus, and its duct.
548 CHORDATA.

The swim bladder of fishes and the lungs of most amphibia are smooth-
walled sacs, but in some have greater respiratory surface since folds ex-
tend into the central space. This peripheral folding increases in the rep-
tiles at the expense of the central chamber, this in some being completely
divided by the partitions, which extend inwards from the walls to the
bronchus. In the mammals a central chamber is lacking; the bronchi
extend into the lungs, branching again and again to the fine bronchioles
which give off alveolar ducts lined with minute air cells or alveoli.

The circulatory apparatus is easily derived from that of annelids,
and, like it, is completely closed. In the annelids (p. 307, figs.
272, 275, 276) above and below the
digestive tract is a longitudinal blood-
vessel, these being connected in each
somite by loops which pass around
the intestine. The vertebrate scheme
varies in the development of a heart
in the ventral trunk (the dorsal of
the annelid). In the lower verte-
brates, the fishes (figs. 65, 597), the
heart lies close behind the gills and
sends to them the blood which it
receives from the body. Hence, like
the whole ventral trunk, it carries
Fic. 579.—Lungs of man, ventral view. Venous blood. Since the anterior

(From Wiedersheim, S, sulcus for loops, the gill arteries, pass through

subclavian artery; Tr, trachea di-
viding below into the two bronchi; the gills, the dorsal trunk, which

Z, position of diaphragm; 1, 2, 3, 2a,

3a, lobes of right and left lungs. — ggjlects from these, must contain
oxygenated blood, which is sent by the carotids to the head, and
by the dorsal aorta and the vascular loops to the body. It thus
becomes venous and flows back into the ventral trunk.

This scheme of circulation in fishes needs further description.
The heart, a strong muscular organ enclosed in a pericardium, con-
sists of two parts, auricle and ventricle, separated by valves. The
trunk (ventral aorta) arising from the auricle is arterial and cor-
responds to the ascending aorta and pulmonary artery of man.
The arterial arches of the gill region which arise from it pass di-
rectly into the dorsal vessel only in young fishes (fig. 597); later
they furnish the branchial circulation of gill arteries, gill capillaries,
and gill veins (fig. 65). The dorsal trunk is the dorsal aorta
(aorta descendens); the ventral trunk, which only occurs in the
embryo, is the subintestinal vein, from which the portal vein arises.
To this are added a system of paired veins, consisting of Cuvierian

 
IV. VERTEBRATA, 40

ducts and jugular and cardinal veins, the latter with growth en-
croaching more and more into the territory of the subintestinal
vein.

The circulation of the fish type undergoes a great modification
with the loss of gillsand the appearance of pulmonary respiration.
Gills and gill capillaries disappear, and the branchial circulation is
reduced to arterial arches leading direct from the ventral to the
dorsal aorta. The swim bladder received its blood from the body
(systemic) circulation, but with the functioning of the lungs pul-
monary arteries and veins come into existence, while the arterial
arches in part disappear, in part are divided between the pulmonary
Lil LIV

 
     

 

gi

uc

fic

  

yr
SSL

  

(nt

     

Fig. 580.—Diagram of modification of arterial arches in various vertebrate classes. White,
vessels which degenerate; cross-lined, vessels containing arterial blood; black, vessels
containing venous blood. J, Dipnoi; J/, Urodeles witb pulmonary respiration; JJIJ,
Reptiles; JV, Birds (in mammals the left instead of the right aortic arch persists). ao},
venous aorta of reptiles; co?, arterial aorta: ast, arterial trunk; a, bh, arches which
usually disappear; ad, dorsal aorta; d¢.8. ductus Botalli; k, gill capillaries; pu, pul-
monary artery; 1-4, persistent arterial arches.

and systemic circulations (fig. 580). Of the six arches which

usually appear in the embryo, the first and second, and the fifth

in animals with lungs, usually disappear. The last arch (4), which
even in the Dipnoi supplies the swim bladder, becomes a pulmonary
artery, the other arches (7 and 2) furnish the systemic portions,

the dorsal aorta (2) and the carotids supplying the head (2).

Since special pulmonary veins, distinct from the systemic circula-

tion, carry the blood from the lungs to the heart, the heart be-

comes divided by a septum which separates it into right and left
halves. The right half retains the venous character of the fish
heart; since the right auricle receives the systemic veins, the right
ventricle gives off the pulmonary artery. The left half is purely
arterial, receiving arterial blood by the left auricle from the lungs

and sending it out through the aorta ascendens to the body. A

complete separation of pulmonary and systemic circulation, and a

corresponding division of the heart, occurs only in birds and mam-
550 CHORDATA.

mals. Reptiles and amphibia show how the modification has been
accomplished. In these the separation begins in the venous sys-
tem and extends to the auricle, in the reptiles the septum arises in
the ventricle. In the arterial system remnants may persist, such
as a connexion (ductus Botalli) of the puJmonalis with the aorta
(//, d.B), or an aortic arch may arise with the pulmonalis from
the right side of the heart (JZJ, ao).

Besides blood-vessels, lymph vessels occur in the vertebrates as com-
plements of the venous system. The fluids which collect in the spaces of
the connective tissue are taken by them and carried into the large venous
trunks. Usually the action of the heart and the movements of the body
are sufficient to cause the flow of this lymph, but special lymph hearts
may occur. The lymph vessels distributed to the digestive tract play an
important role, since they serve in the resorbtion of digested food. They
are called chyle ducts because their contents, the chyle, rendered white
by oil globules at the time of digestion, distinguishes them from other
lymphatics. The most important features of lymph and blood have
already been noticed (p. 88). In special places small bodies, the lymph
glands, are inserted in the course of the lymph vessels, in which lymph
corpuscles arise. Among these from its structure is to be enumerated the
spleen, colored bright red by its rich blood supply.

The sexual and excretory organs are so closely associated that
they are generally united as the urogenital system. The sexual
products are formed in the embryo from a special region of the
peritoneal epithelium on either side of the vertebral column.
These primordial cells early leave their primitive position, and sink
into the underlying connective tissue (fig. 33), forming in the
male glandular tubes, in the female cords which break up into
numbers of round follicles, each containing a single larger cell, the
ovum. In the male the gonads thus formed are compact and fre-
quently oval, the testes; in the female they are looser and follic-
ular ovaries.

The deposition of the sexual cells occurs in many fishes by way
of the body cavity and the abdominal pores, and in this case a part
of the celom may be cut off as a special vas deferens or oviduct.
In most vertebrates the ducts are formed from a part of the
nephridial system. Embryology shows that there are three kinds
of nephridia in vertebrates: (1) the pronephros, or head kidney;
(2) mesonephros, or Wolflian body; (3) metanephros, or kidney
proper, with the corresponding pronephric, mesonephric (Wolf-
fian), and metanephric (ureter) ducts. The first two of these
ducts are genetically connected, since the development of the
elasmobranchs shows that the pronephric duct, by splitting, gives
IV. VERTEBRATA. 551

rise to two canals, the Wolffian (mesonephric), and the Millerian
ducts, the latter retaining its relation to the pronephros.

The pronephros is usually functional only in embryonic life
and then only in early stages, possibly in some cases not at all.
Its relations to the other parts are yet in question. In most

 

Fie. 581.—Scheme of urodele urogenital system based on Triton. (From Wieders-
heim, after Spengel.) A, male; B, female. a, excretory ducts; yn, sexual part
of mesonephros; Ho, testis; lg, Leydig’s duct (ureter); mg, Millerian duct
(oviduct); mg’, its vestigial end in male: NW, functional part of mesonephros;
Ov, ovary; Ot, ostium tube; Ve, vasa efferentia; *, collecting duct of vasa effer-
entia (rudimentary in B).

teleosts the mesonephros is equally developed in nearly the whole
length of the body cavity, but in the Amphibia (fig. 581) and
many elasmobranchs its anterior part is smaller than the rest, a
condition which has its explanation in its relations to the sexual
apparatus.
552 CHORDATA.

In the males (excepting many fishes) the testes become con-
nected with the anterior end of the Wolffian body (fig. 581, A),
so that the urinary tubules of the latter come to be seminal ducts,
while the hinder portion remains excretory, this condition being
permanent in the Amphibia. In the amniotes the anterior meso-
nephros retains its connexion with the testes, forming the vasa
efferentia, while the Wolffian duct forms the vas deferens, a por-
tion of it greatly coiled being the epididymis. The remainder
of the Wolffian body degenerates, a portion only persisting as the
paradidymis,

In the females (fig. 581, 2) the mesonephros is smaller in front,
as in the males, but the connexion of this with the ovary does not
exist, so here the Wolffian duct is solely excretory, and not, as in
the males, excretory and seminal duct. In the female amniotes
the Wolffian body almost entirely disappears, for in both sexes of
the reptiles, birds, and mammals the metanephros or kidney proper
is a new formation, growing forwards from the posterior end of
the Wolffian duct. In the females of elasmobranchs, Amphibia,
and Amniotes the Millerian duct serves as an oviduct, its anterior
end opening by the ostium tube into the abdominal cavity and
receiving the eggs as they escape from the ovary. In the male the
Millerian duct disappears early.

The union of sexual and excretory organs to a urogenital system arises
from the same relations as in the annelids; both organs arise from the
celomic epithelium and have temporary or permanent connexion with the
body cavity. This has already been described for the gonads. The
urinary tubules of both pro- and mesonephros are derivatives of the celomic
epithelium and possess an arrangement recalling that of the annelids in
a striking manner. As is shown (fig. 70) in the scheme of the embryo
selachian, the nephridial system consists of numerous canals, segmentally
arranged, connected by funnels (nephrostomes) with the body cavity;
and differs from the segmental organs of the annelids in that they do not
open singly to the exterior, but by a common duct. They also differ in
their further development by increasing greatly in number and forming
a compact organ, and, finally, by the formation in a certain part of a
network of blood-vessels, the glomerulus, which pushes into the lumen of
the tube.

The ducts of the urogenital system open behind the anus in
most fishes on a urogenital papilla; in the elasmobranchs, amphib-
ians, birds, and most reptiles dorsally into the hinder part of
the digestive tract, which thus becomes a cloaca. In turtles and
mammals the urogenital canal opens into the urinary bladder, a
ventral diverticulum of the rectum which first appears in the
IV. VERTEBRATA, 553

Amphibia. Urinary and sexual ducts then either open into the
urogenital sinus, the lowest part of the bladder leading to the
cloaca (turtles, monotremes), or this part receives only the geni-
tal ducts, while the ureters enter the base of the bladder. The
urogenital sinus remains in connexion with the cloaca in the
turtles and monotremes; in the other mammals a cloaca occurs
only in embryonic life. Later, by formation of the perineum, the
cloaca is divided into a hinder digestive and an anterior urogenital
canal. Step by step the stages may be followed from urogenital
ducts opening behind to those opening in front of the anus.

Asexual and parthenogenetic reproduction are unknown in the
vertebrates. The impregnation of the eggs in the lower groups
is usually external and occurs during oviposition; in the higher
internal copulation is effected by opposition of the genital ori-
fices or by the development of an intromittent organ, the penis.
The fertilized egg can undergo a part or the whole of its devel-
opment in specialized parts of the oviduct (uterus). Accordingly
viviparous and oviparous forms are distinguished, and between
these extremes those that are ovoviviparous (cf. p. 161). Most
elasmobranchs are viviparous, but many are oviparous. In the
teleosts oviparous forms predominate, but there are viviparous
exceptions. So, too, among the reptiles and Amphibia there are
some viviparous species among the egg-laying majority. The
birds and mammals are most constant, the first being exclusively
ovoviviparous, while all the mammals bring forth living young
with the exception of the ovoviviparous monotremes,

Three embryonal appendages may occur in the development,
the yolk sac, the amnion, and the allantois. The yolk sac is small
in those vertebrates which have some yolk, but not enough to
cause meroblastic segmentation (Amphibia), yet it is everywhere
present and is best developed in those groups (fishes, fig. 582,
reptiles and birds) with discoidal segmentation, and is the result
of the accumulation of food material in the digestive tract, which
forces out its ventral wall like a hernia. Its presence in the mam-
mals, which have small eggs lacking in yolk, is an indication that
these have descended from large-yolked forms, such as the mono-
tremes yet are. The embryo either lies directly on the yolk or is
connected with it by a yolk stalk.

While the yolk sac is widely distributed, the amnion and allan-
tois are restricted to reptiles, birds, and mammals, which are con-
sequently spoken of as Amniota or Allantoidea, in contrast to the
fishes and Amphibia, which are frequently called Anamnia or Anal-
d+ CHORDATA.

lantoidea, from the absence of these structures. The amnion isa
sac which envelops the whole embryo and is connected with the
rest only at the umbilicus, that is, the point where the yolk sac
projects from the ventral wall. In this sac is an albuminous

 

Fie. 582. Fig. 583.

Fie. 582.Shark embryo. (From Boas.) y, part of yolk sac; yg, external gills in front
of pectoral fins. ;

Fig. 583.—Embryonic envelopes of amammal. (Diagram after KGlliker.) ah, amni-
otic cavity ; al, allantois; am, amnion; dg, yolk stalk ; ds, yolk sac; e, embryo:
hh, ventral wall of embryo; r, extra-embryonic coelom; sh, serosa; sz, serosal
villi.

amniotic fluid. The amnion is genetically a part of the ventral
surface; it develops ventrally as folds—lateral, anterior, and pos-
terior—which grow up over the back on all sides and unite above
the embryo.

The allantois is an enlargement of the urinary bladder. This
grows out from the body cavity at the umbilicus and extends be-
tween yolk sac and amnion and then grows in all directions until
its folds meet above the back. The part of the allantois which re-
ceives the urine may be enlarged or not. The rest of the out-
growth consists of blood-vessels and connective tissue. The blood-
vessels are the most important, for the allantois forms the respira-
tory apparatus of the embryo, and in the mammals it develops the
placenta, by which nourishment as well is conveyed to the young.
Yolk sac, amnion, and allantois are enveloped in a common coat,
the serosa.

Aristotle and his followers recognized four divisions of vertebrates, and
these were retained by Linné and Cuvier under the names Pisces, Reptilia
or Amphibia, Aves, and Mammalia. Blainville (1818) divided the second
of these into two classes, retaining the name Reptilia for the one, Amphibia
for the other. Milne Edwards showed that this division corresponded
IV. VERTEBRATA: CYCLOSTOMATA. 555

with one between the higher and lower groups, the amniote and the anam-
niote divisions. Later Haeckel divided the fishes, separating the Cyclo-
stomes from the others as a distinct class, while Huxley pointed out the
close resemblances between the reptiles and birds, grouping them as
Sauropsida, Another division of convenience but not of much systematic
importance contrasts the fishes with all other forms, the Tetrapoda, so
called from the possession of legs rather than fins.

Series J. IcuTHyopsipa (ANAMNIA, ANALLANTOIDA).

Vertebrates respiring for a time or throughout life by means
of gills ; neither amnion nor allantois present in the embryo.

Class I. Cyclostomata (Marsipobranchii, Agnatha).

The class of Cyclostomes contains but few species, among
which the lamprey eels and the slime or hag fishes are best known.
In shape they are eel-like. They are distinctly vertebrate in the
possession of large liver and nephridia; of a muscular heart with
auricle and ventricle, lying in a pericardium; olfactory lobes,
epiphysis and hypophysis, and the higher sense organs. In the
brain, cerebrum and cerebellum are not so prominent as are the
optic lobes and medulla. The inner ear is not divided into utric-
ulus and sacculus, and it has but one or two semicircular canals,
but always two ampulle. The skin (fig. 26) consists of derma
and a stratified epidermis.

The cyclostomes are distinguished from the true fishes by the
lack of a vertebral column. The axial skeleton of the trunk consists
either of the notochord alone or of it and small neural arches. A
cranium and a basket-like gill skeleton are present, but so different
are these from those of other vertebrates that homologies are dif-
ficult. The absence of paired fins isimportant. Since the median
fins are supported by horny threads alone, the cartilaginous appen-
dicular skeleton—alone of importance—is entirely wanting. Then
the skin lacks scales, and the mouth true dentine teeth, for the
pointed brown teeth arranged in circles in the mouth of the lam-
prey (fig. 584), and the fewer teeth of the myxinoids, are purely
epidermal products and cannot be compared with the teeth of
other vertebrates. Other important differences have given rise to
names applied to the group.

The name Cyclostomata refers to the circular mouth, an ex-
ternal feature, which, however, rests on the important fact that
the jaws are absent or extremely rudimentary, and do not close on
each other as do the jaws of other vertebrates. This cyclostome
condition is of value to the animals, as it aids them in sucking on
556 CHORDATA.

to other animals. At the base of the dome-like mouth cavity is
the so-called tongue, which is the sucking apparatus, since it can be
drawn backwards like a piston (fig. 584).

The name Marsipobranchs has been given on account of the
form of the gills, which are usually six or seven in number, but in
Bdellostoma may be twelve or fourteen on either side. Each gill
cleft consists of three parts, the gill sac (marsupium), which alone
contains gills, and the afferent and efferent ducts (fig. 585). These
canals arise separately, and may continue so (Bdellostoma), but in
Petromyzon the afferent ducts unite to a single tube which opens
ventrally in the pharynx. In Myzine (fig. 585) the conditions are
reversed, the efferent canals uniting to empty through a single
external opening.

A third name, Monorhina, has been given, since these forms,
in contrast to all other vertebrates, have an unpaired olfactory
organ. The single nostril, lying in the mid line of the head,

 

Fic. 584. Fig. 585.
Fig. 584.—Mouth of Petromyzon marinus with horny teeth and tongue. (From Gegenbaur.)

Fie. 585.—Gill apparatus of Myxine glutinosa. (After J. Miller.) a, atrium; «bd, gill artery
and gill arch: br, gill sac (the lines show the gills): hr’, efferent canal; c, cesophaged-
cutaneus duct; ¢, skin turned away; ‘, afferent gill canal; 0, esophagus; s, mouth
of atrium; v, ventricle of heart. 5

 

opens into a nasal sac, from the bottom of which a canal descends
towards the roof of the mouth, ending blindly in Petromyzontes
(Hyperoartia), or penetrating it in the Myzontes (Hyperotretia),
so that an inner nasal opening (choana) oceurs. A paired olfac-
tory nerve supplies the organ.

Sub Class I. Myzontes (Hyperotretia).

Semiparasitic eyclostomes with cirri around the mouth, very primitive
nephridia, right and left rows of slime saes, eyes rudimentary (lens, sclera,
IV. VERTEBRATA: PISCES. 557

and choroid lacking). From the large amount of mucus they are known
as slime eels. They bore into fishes and eat the flesh. Myaine* on the
east coast, Bdellostoma* (Polistotrema) on the west.

Sub Class If, Petromyzontes (Hyperoartia).

Several American species of lampreys, all belonging to Petromyzon*
(with sub genera), have well-developed dorsal fins, and seven branchial
openings. They occur in salt and fresh water, some marine species

 

Fig. 586.—Petromyzon marinus,* sea lamprey. (After Goode.)

ascending streams to lay their eggs. The young pass through a larval
(Ammoceetes) stage with rudimentary eyes and slit-like mouth. Many of
the species live on the mucus and blood which they rasp from fishes.

Here may be mentioned a group of fossils, the OSTRACODERMI, of
uncertain position. They have fish-like bodies, but no skeleton or jaws are
known. They flourished in paleozoic seas. Preraspis, Cephalaspis,
Pterichthys.

Class II. Pisces (Fishes).

The term fish is used in a wider and a narrower sense. In the
first it includes any aquatic vertebrate swimming by means of fins
and breathing by gills; in the more strict sense, as used here, it
means aquatic branchiate forms with vertebral column, cranium,
and well-developed visceral skeleton; with paired as well as
unpaired fins, these supported by a cartilaginous or bony skeleton
in addition to horny rays; with double nasal pits; with a skin and
oral mucous membrane which can produce ossifications, the scales
and teeth. The cyclostomes are thus excluded. The fishes are
the best adapted of all vertebrates for an aquatic life, and their
whole organization must therefore be considered from this stand-
point.

The epidermis consists of numerous layers of protoplasmic cells
with an extremely thin external cuticle. Cornifications of this
epidermis are lacking under ordinary conditions, with the excep-
tion of a thin portion of the external subcuticular layer. At the
time of sexual maturity cornifications increase greatly in most
Cyprinoids and many Salmonids, producing hard bodies in the skin,
dos CHORDATA.

the ‘pearl organs.’ Enormous numbers of large slime cells give
the fishes their well-known slippery skins. Since the epidermis
contributes nothing to the firmness of the body walls, all protective
structures arise from the derma, which is composed of many layers
of dense connective tissue and furnishes the characteristic dermal
skeleton, the scales. These lie at the boundary of epidermis and
derma, commonly imbedded in pockets of the latter, and are, on
account of their different structure, of systematic value, although
the classification based entirely upon them is no longer retained.

The placoid scales (fig. 554, 587, 4) have already been men-
tioned, because they form the
starting point for dermal ossifica-
tious and teeth (p. 515). They
are rhombic bony plates, usually
close together like a mosaic, but
not overlapping. In the centre
of each is a spine, directed back-

yards, In which is a pulp cavity,
while the tip of the spine is coy-
ered witha cap of hard substance,
variously called enamel or vitro-
dentine.

The ganoid scales (fig. 587,
3) are usually rhomboid and
Fig. 587.—Scales of fishes. 1, cycloid; 2, arranged like parquetry. In the

SETTLE RS See en ye early stages they may bear teeth,
but these are lost in the adult. The outer surface is always covered
with a thick layer of ‘ganoin,’ which gives, even in fossils, an
iridescent effect, a most characteristic feature. The ganoin is no
longer regarded as enamel, but the most superficial layer of dentine
(vitrodentine).

Cycloid and ctenoid scales are closely related. They are always
more loosely placed in the pockets, from which they are easily with-
drawn as in ‘scaling’ a fish. They are arranged in oblique, trans-
verse, and Jongitudinal rows, and overlap like shingles, one scale
covering the parts of two scales behind. The eycloid scales (fig.
587, 1) are approximately circular with a middle point, surrounded
by concentric lines, from which go radiating lines. The ctenoid
seale (2) has the radial and concentric lines of the eycloid, but has
the hinder edge truncate and the free portion bearing small spines
or teeth, processes of the concentric ridges.

Besides these types of scales many fishes bear considerable

 
IV. VERTEBRATA: PISCES. 559

spines (strongly developed single scales) and larger bony plates,
these last usually resulting from the fusion of numerous scales.

The coloration of fishes is threefold in origin. The silvery lustre is
due to crystals of guanin which occur not only in the skin but in the peri-
toneum and pericardial walls. In some fishes from their iridescence
(Alburnus lucidus) these crystals become of commercial value. They are
freed from the skin by boiling with ammonia and, suspended in the fluid,
form the important part of essence of pearl (essence d’orient) which is
used in making artificial pearls, being either applied to the outside of ala-
baster balls (Roman pearls) or as a coating to the inside of glass beads
(Paris pearls). The other colors of fishes are due in part to the numerous
strongly pigmented fat cells, in part to ‘chromatophores’ in the derma,
which, under control of the nervous system, can alter their form and
extent and thus produce color changes in the fish. It is by means of
these chromatophores that fishes adapt themselves to their surroundings.
It is of interest to note that destruction of the eyes results in loss of power
to change color.

The axial skeleton shows many conditions which are unknown
outside the class, and varies in character from group to group, the
most important differences consisting in its cartilaginous or bony
character. The vertebre are nearly always amphicelous, the
notochord persisting in the cavities between the successive centra
(fig. 557). Neural and hemal arches occur, these having as key-
stones the unpaired spinous processes. The neural arches extend
throughout the columns; the hemal are complete only in the tail;
in the trunk the hemal spines are absent and the hemal processes,
divided into basal processes and ribs, surround the viscera. A
sternum is everywhere lacking. When ossification is lacking or
is incomplete, two pairs of arches may occur in each segment, the
anterior being the stronger and alone persisting in fishes with ossi-
fied vertebre; the second is much smaller, so that its elements are
not called arches, but intercalaria (figs. 556, 588).

The great number of visceral arches, and their independence
from the cranium, are characteristic of fishes. After removal of
these the cranium in all cartilaginous fishes is very simple (fig.
588), but in the teleosts, with the appearance of ossification, he-
comes very complicated, since the bones are very numerous and
are not, as in mammals, in purt fused to larger bones. There are
also great differences between the different families of fishes, some
having bones which are lacking in others (figs. 560, 589), The
large membrane bones of the cranial roof (parietals, p, frontals, /,
and nasals, na) and the large ventral parasphenoid (ps) are
especially constant. The vomer in front of the parasphenoid is
dO0 CHORDATA,

unpaired, while in all other vertebrates it is paired. Most con-
stant of the cartilage bones are the ethmoids (the paired ecteth-
moids, ee, and the sometimes paired mesethmoid), and the four
occipitals. On the other hand the otic and optic regions vary
considerably; the otic region, from its great size, has several bones,
usually (fig. 589) five in number: pterotic, pto, often called

 

Fig. 588.—Cranium, visceral arches, and part of vertebral column of Mustelus vulgaris. aor
antorbital process; co, copula; gp, foramen for glossopbaryngeal; H, otic capsule and
hyoid; Hm, hyomandibular; ic, intercalare; Md, mandible (Meckel’s cartilage) ;
N, nasal capsule; o, optic foramen; ob, neural arcb; po, postorbital process; Py.
pterygoquadrate ; ps, spinous process; R, rostrum; 7, ribs; tr, trigeminus foramen ;
v, vagus foramen; J-‘, visceral arches: 1, labial; 2, mandibular; 3, byoid; 4-8. gill
arches.

squamosal; sphenotic, spo, frequently called postfrontal; epiotic,
epo; prootic, pro; and opisthotic, oo, the last sometimes lacking.
In the region of the eye the cartilaginous sphenoids are rarely
well developed, the large parasphenoid taking their place. The
same is true of the ali- and orbitosphenoids, these sometimes form-
ing an interorbital septum (fig. 560) or a more or less wide in-
terorbital fenestra (fig. 589).

The character of the visceral skeleton is related to the aquatic
life. All fishes have numerons gill arches (five to seven, mostly
five), which, since their function—gill supporting—is similar, are
similar in structure. So far as they are not degenerate they con-
sist each of four parts and are connected by unpaired copule, these
often being fused. The upper ends are frequently toothed and,
in chewing, are opposed by the rudimentary last arch, on which
account these are spoken of as the superior and inferior pharyngeal
bones. The anterior visceral arches are greatly different in car-
tilaginous and bony fishes. In the former (fig. 588) the pterygo-
quadrate (pg) and the Meckelian cartilage bear teeth and oppose
ach other in biting. In the bony fishes (fig. 589) the teeth of

c

 
IV. VERTEBRATA: PISCES. dL

the lower jaw oppose the tooth-bearing elements, premaxillary
and maxillary, of the maxillary series, while the pterygoquadrate
clements—the palatine and the series of pterygoids—are the an-
tagonists of the hyoid.

A second characteristic of the bony fishes is already outlined in

the ae an the nea ingeion of the o pyomandibulyy, to

 

FG. 589. —Skull of haddock. Infraorbital ring and operculum outlined in red. a,

; ar, articulare ; as, alisphenoid; de, dentary; ee, ectethmoid; ekt,

ory goid; eng, O8 entoglossum ; ent, entopterygoid; epo, epiotic; fr, frontal;

l'-h3, hyoid elements: hm, hy mandibular; ih, interhyal; ma, maxilla: me,

mesethmoid; mt, metapterygoi na, nasal; och, ocl, ocs, basi-, ex-, and supra-

occipital 3 00, opisthotic ; p, parietal; pa, palatine; prm, premaxillary ; pro, pro-

otic; ps, parasphenoid ; pto, pterotic; qu, quadrate; rbr, branchiostegals ;_ spho,

sphenotic; sy, symplectic; vo, vomer; w, vertebra. Bones outlined in red: inf,

infraorbital ; io, interoperculum ; 0, operculum ; pro, preoperculum ; so, suboper-
culum ; 1,2, 3, axes of ,abial, mandibular, and hyoid arches.

  

   

 

a suspensor of the jaws. In the elasmobranchs (especially the
skates) the parallelism of hyoid and mandibular arches is lost, the
hyomandibular separating from the hyoid and attaching itself to
the hinge of the jaws. In the teleosts the hyomandibular is thus
brought in connexion with the quadrate, and lies between it and
the cranium, the joint being thus indirectly supported from the
cranium, a bone, the symplectic (known only in fishes) helping
out the suspensor, while another bone, the interhyal, connects this
with the hyoid, which itself divides into two, so that the hyoid
arch, like a gill arch, consists of four clements.
562 CHORDATA.

The opercular apparatus does not occur in all fishes. It isa
number of bony plates and processes which arise from the hyoid
arch and extend backwards over the gills, protecting them. It
arises in part (opercular bones—O, Pro, So, Jo, fig. 589) from
the hyomandibular, in part (branchostegal rays) from the hyoid
bone. The significance of this apparatus will be spoken of in con-
nexion with the gills; it gives the fish head a definite character,
but covers its structure, on which account it, like the infraorbital
ring, is shown in red in the figure 589.

The appendages are also influenced by the aquatic life. In
contrast to the cyclostomes, they have two pairs of paired fins, the
thoracic or pectoral, and the pelvic, ventral, or abdominal fins; in
contrast with Amphibia, reptiles, and mammals, which occasionally
have fin-like structures, the fishes have three unpaired fins, dorsal,
caudal, and anal fins. Only rarely, as in the eels, the ventral fins
are lacking; more rarely (Murenidz) the pectorals are lost. The
function of the fins in swimming and in balancing makes it neces-
sary that they be broad and well-supported plates. Hence it is
that numerous skeletal parts are present; besides those preformed
in cartilage, numerous horny or bony rays; further, that all parts
should be similar and closely, even if flexibly, bound to each other.
Joints are unnecessary except at the base where the fins join the
supports and move upon the body. The supports of the paired
fins are the girdles, arched skeletal parts, which in the sharks are
held only by muscles, a statement which is true for the pelvic
girdle of all fishes. This is why the ventral fins so readily change
their place. Their primitive position is at the hinder end of the
body cavity (Pisces abdominales, figs. 598, 601). From this point
they can move forward to beneath the pectorals (Pisces thoracici,
fig. 602), or may even come to lie in front of them (Pisces jugu-
lares) in the throat region (fig. 606). The pectoral arch is united
to the vertebral column in the skates; to the skull by a series of
bones in the teleosts. =

The dorsal and anal fins are supported by clements preformed
in cartilages which rest upon the neural or hemal spines and in
turn support the fin rays. In the caudal fin the rays rest directly
upon the spinous processes. Three types of caudal fin are rec-
ognized—diphycereal, heterocercal, and homocereal (fig. 10),
distinctions of great importance. The primitive type is the diphy-
cercal, in which the vertebral column extends directly into the
middle of the fin, dividing it into symmetrical halves. In the
heterocercal type the vertebral axis binds slightly upwards at the
IV. VERTEBRATA: PISCES, 563
base of the fin, so that the dorsal part is reduced, the ventral greatly
enlarged, the result being extremely asymmetrical, as seen from
the exterior. The homocercal fin is symmetrical externally, but
in reality is extremely asymmetrical. The end of the vertebral
column, the unossified notochord, is bent abruptly upwards, and
hence the fin is almost entirely formed of the ventral portion,
which is usually divided by a terminal notch into upper and lower
halves. The homocercal fin begins with a diphycercal and passes
through a heterocercal stage in development.

In correspondence with the simple motions the musculature is simple
and consists largely of longitudinal muscles divided into myotomes, which

       

  
    

are conical with the apex in front, and are y ;
so inserted in each other that a cross-sec- i RN tbl a
tion gives concentric circles. In a section ! h (MAALEZ. =o Wy

there are at least two such systems, the
muscles being divided by a lateral in-
cision into dorsal and ventral halves.
There are also smaller groups of muscles
related to fins, gill arches, jaws, ete., but
of much smaller size, derivatives from the
larger mass. Electric and pseudelectric
organs, which occur in different fishes,
sometimes in the trunk, at others in the
tail, are formed by the modification of mus- qr ji
cles. Each organ consists of numerous if

lwo |

/
: : Fig. 5$0.—Driag’ atic secti

closely packed vertical or horizontal col- ena an oe

of electrical apparatus. (From
umns, each column, like a Voltaic pile, Wiedersheim.) The arrow points
consisting of layers of gelatinous plates

(equivalents of muscle bundles) in which
the nerves, with special end plates, termi-
nate. The discharge is electronegative.

dorsally or anteriorly. BG, con-
nectiv issue framework; FP,
electrical plates; G, gelatin-
ous tissue; , nerves entering
through the septa; NN, nerve
terminations.

  

The brain shows the low position of the class in the slight development
of the cerebrum (fig. 591). This is especially true of the teleosts, in which,
in place of a cortex, there is only a thin epithelial layer (Pall), what was
formerly called cerebrum being only the corpora striata. The independent
olfactory lobes lie either close to the cerebrum (most teleosts, ol) or are
separated from it by an olfactory tract (fig. 592, 2). The optic thalami
are small (@), but below them are enlargements characteristic of fishes, the
lobi inferiores, and between them the sacculus vasculosus. Both optic
lobes and cerebellum are greatly developed.

The nose consists of two preoral pits, the opening being divided by
a bridge of skin into anterior and posterior nostrils. In many selachians
the nostrils are connected with the mouth by a groove covered by a fold of
skin, and in the Dipnoi there is a choana. The eye has several peculiari-
ties. The lens is very convex, almost conical, due to the slight refraction
caused by the passage of light from the water into the cornea. Further,
564 CHORDATA.

the eye is very short-sighted because light is so absorbed by water that
objects forty feet away are invisible. With this is connected the cam-
panula Halleri. The processus falciformis, a sickle-shaped outgrowth of
the choroid, extends from the entrance of the optic nerve into the vitreous
body as far as the lens, swelling out into the campanula; this contains a
muscle which draws back the lens and so is an apparatus of accommoda-
tion. Near the entrance of the optic nerve is a problematic organ, the

 

Tie. 591. Fie, 592.

Fia. 591.—Brain of trout. (After Wiedersheim.) BG, corpus striatum; GP, pine-
alis; HH, cerebellum; Lol, olfactory lobes; MH, optic lobes; NH, Medulla
oblongata; Pali, pallium, in part cut away; VH, cerebrum; XJ, nerves.
(See p. 536.)

Fig. 592.—Brain and nasal capsules of Scyllium catulus. (From Gegenbaur.) a, me-
dulla; 6, cerebellum ; c, optic lobes; d, *twixt brain; g, cerebruin; /i, bulbus and
tractus olfactorius ; 0, nasal capsules.

 

choroid gland, consisting largely of blood-vessels (rete mirabile). Chon-
drifications and ossifications of the sclera are common. Lids are weakly
developed or absent, and only some elasmobranchs have a nictitating mem-
brane.

The ear has a relative size found in no other vertebrates, the labyrinth
corresponding well with fig. 575. The labyrinth contains in many teleosts
two otoliths, the asteriscus and sagitta, the first being especially large.
Experiments show that the ears are primarily for balance, and hearing is
doubtful. Strychninized fish do not respond to sound, if in its production
mechanical vibrations are avoided.

Of all sense organs the most noticeable are those of the skin, especially
those of the lateral line, which are nowhere else so well developed and
which occur elsewhere only in cyclostomes and aquatic amphibia. In
fishes a line on either side usually begins at the tail and extends to the
head, where it divides into several curved lines (fig. 602, SZ). Its position
is marked by a groove or a canal in the seales which opens to the exterior
by numerous canals through the scales. Branches of trigeminus, facialis,
glossopharyngcus, and especially the lateral branch of the vagus (fig. 570) go
to these organs, the latter extending back to the tail. These supply special
IV. VERTEBRATA: PISCES. 565

sense organs, which may be grouped in several lines or occur in pits (am-
pullz) in the skin in other places. Their function is obscure, since noth-
ing of the sort occurs in man or mammals. They are specific organs of
aquatic vertebrates and possibly have to do with the perception of water
pressure.

The alimentary tract is spacious only in the oropharyngeal
region. Then it narrows to a tube in which the various regions
are not sharply marked off from each other. Mouth and pharynx
frequently bear teeth. In the teleosts the bones of the floor of
the cranium and those of the visceral arches may be covered with
coalesced heckel-like teeth. In the elasmobranchs the teeth are
mostly confined to the lower jaw and the pterygoquadrate, but are
in rows, the anterior row alone being functional; but as these are
loosely held they are easily torn out, when they are replaced by
the row behind. Liver and spleen are always present; pancreas
and gall bladder usually occur. In many fishes blind sacs, the
pyloric ceca, occur at the junction of stomach and intestine (fig.
593, B); others have a spiral valve (4), a fold of mucous mem-
brane, which extends like a spiral stairway into the lumen of the
intestine, increasing the digestive surface. Czeca and spiral valve
rarely occur in the same fish.

 

Fia. 593.—Digestive tracts of (A) Squatina vulyaris (partly opened) and (B) Tra-
chinus radiatus. (From Gegenbaur.) ap, pyloric ceca; c,rectuin; d, bile duct;
dp, duct of air bladder; i, intestine; 0e, esophagus; », pylorus; 1, stomach; vs,
spiral gland ; x, rectal gland.

Gills of two types occur (fig. 594, 4 and BL). In both the gill
clefts, which le between successive branchial arches, begin by
openings in the pharynx, but differ in their external openings.
In the elasmobranch type (.1) the external openings are a series of
slits separated by broad dermal bridges which cover the gills and
gill clefts (fig. 598). The gills are vascular folds of mucous mem-

 
566 CHORDATA,
brane with secondary folds which extend on anterior and posterior
sides of the cleft. Hach arch except the last, as the sections (fig.

594, A, and 595) show, bears two rows of gill folds (demi-

A. B

   

 

 

y /
We aad ‘ WAN

Fic. 594.—Pharynges of (4A) Elasmobranch (Zygena) and (B) Teleost (Gadus), the
skull removed and on the left the gill slits cut across. a,attachment of upper
jaw to cranium, cs, outer gill slit; b, gill arch; bU, DI2, anterior and posterior
gills (demibranchs); h, dermal projection ; lon, hyomandibular ; is, inner gill
cleft; m, mouth ; ma, maxillare ; 0, esophagus; op, operculum ; ops, opercular
opening; pea, palatine; phi, inferior pharyngeal bones; pq, pterygoquadrate;
prim, premaxilla; s, shoulder girdle ; wk, lower jaw; 2, tongue.

branchs) which belong to different clefts and are separated from

each other by tissue containing the cartilaginous gill rays.

In the second type (2), which occurs in all other fishes, the
dermal bridges are lacking, and the tissue between the demi-
branchs has more or less completely disappeared, so that the
demibranchs of one arch become connected, their free ends pro-
jecting into the water like the teeth of a double comb. Here,
on account of their very delicate structure, they would be ex-
posed to serious injury were they not protected by the opereulum
or gill cover, This isa fold of skin arising from the hyoid arch
and extending back over the gillregion. It is supported by two
groups of bones, the opereular bones proper (fig. 589, O, Se, Jo,
Pro), attached to the hyomandibular, and the branchiostegals
1V. VERTEBRATA: PISCES. 567

(ror) from the hyoid, these latter supporting the branchiostegal
membrane. Between the free edge of the
operculum and the branchiostegal miem-
brane and the skin of the body behind is
the opercular cleft (fig. 594, ops), which is
obviously not identical with a gill cleft. but
leads into an atrium into which the gill
clefts empty. In many elasmobranchs and
ganoids there is a rudimentary cleft, the
spiracle, between the pterygoquadrate and
hyomandibular, in which arudimentary gill,
or pseudobranch, may occur, this often per-
sisting when the spiracle is closed.

Besides gills, fishes, with the exception
of elasmobranchs and some teleosts, have a

  

Fig. 595.—Sections of gill
arches of Cradus (left)

swim bladder which is usually regarded as oe a

the homologue of the lungs. It is often — jnteyy.s aE
shaped like an hour glass, filled with air, and My er ee naar
may open into the esophagus bya pneumatic — veins 2, tooth.
duct (Physostomi), or this, appearing in development, may be lost
in the adult (Physoclisti). The air Dladder serves for respiration
in the Dipnoi and possibly in some ganoids (Lepidosteus aud
Amia), but is usually a hydrostatic apparatus, its enlargement or
compression altering the specific gravity of the fish. In fishes
brought up from great depths the expansion of air in the swim
bladder frequently forces the viscera out through the mouth.

The heart, enclosed in the pericardium, lies immediately
behind the gill region, and is protected by the shoulder girdle.
It always consists of auricle and yentricle (fig. 506), separated by

 

a pair of valves to prevent back-flow of the blood; it sends the
blood to the gills by the arterial trunk (veritral aorta), and receives
it from the body through a thin-walled sac, the venous sinus, in
which the hepatic veins and the Cuvierian ducts (formed by union
of jugular and cardinal veins) empty (figs. 65, 597).

The most important differences lie in the development of conus
and bulbus arteriosus. These are muscular accessory organs, the
first arising from the heart, the other from the arterial trunk; and
correspondingly the conus has striped, the bulbus smooth musele
fibres. The anterior end of the heart contains ‘semilunar’ valves,
which, like the auriculo-ventricular valves, prevent the back-flow
of the blood. When, by increase in the number of valves, this part
becomes clongute, a conus arteriosus (fig. 596, vf) is formed. The
568 CHORDATA.

bulbus ((’) is a muscular swelling in front of the conus, in the
arterial trunk.

The connexion of ventral and dorsal aorte is effected in young
fishes (fig. 597) by the gill wrteries directly; later by means of the
complicated loops of the gill circulation. When these are de-

 

Fic. 596.—Forms of hearts of fishes in schematic long section. (After Boas.) 4. sela-
chian and most ganoids: B, Amid; C, Teleost. a, auricle; b, bulbus arteriosus: ©. conus
arteriosus; /, valves; s, sinus venosus; f, truncus aorte; v, ventricle.

   

clr vy Zz
Fia. 597.—Head of embryo teleost. (Diagram from Gegenbaur.) «a, auricle; abr, ventral
aorta with arterial arches; ad, dorsal aorta: c, carotid ; dc. Cuvierian duct, formed by

union of jugular and posterior cardival veins

a ; 2, Dostril; s, gill clefts; sv. sinus veno-
sus; ¢, ventricle.

veloped, afferent branchial arteries, gill capillaries, and efferent
arteries cun be recognized, the latter uniting to form the dorsal
aorta and also giving off the arteries (carotids), which go to the
head.

The nephridia are a pair of large reddish-brown organs lying
outside the body cavity to the right and left of the vertebral
column, usually extending from heart to anus. Their ducts empty
behind the anus or in the dorsal wall of the intestine and are often
provided with enlargements called, from their functions, urinary
bladders, although totally different morphologically from the
urinary bladder of the higher vertebrates. The gonads, suspended
IV. VERTEBRATA: PISCES. 569

by mesorchia or mesovaria, are large and project into the body
cavity. They are rarely unpaired. In the elasmobranchs and
most ganoids these products pass out by the urogenital system (p.
552), in other forms by the pori abdominales or by special ducts.

Cuvier divided the fishes into cartilaginous and bony groups, an im-
portant step so far as the extremes (elasmobranchs and teleosts) were
concerned, Agassiz recognized a middle group which he named Ganoidei,
from the character of the scales, but his account was modified and made
more accurate by Johannes Miiller, who also included the Dipnoi among
the fishes. At present the group of ganoids is retained largely as a matter
of convenience. Its members are more closely related with the teleosts
than with the elasmobranchs, and in America Ganoids and Teleosts are
united under the head Teleostomi, the name alluding to the presence of
a true upper jaw comparable to that found in higher vertebrates,

Sub Class L. EHlasmobranchii (Plagiostomt, Chondropterygit).

The elasmobranchs, the shark-like fishes, are almost exclu-
sively marine, varying in length from a foot and a half to sixty
feet, living almost exclusively on other vertebrates, and noted for
their voracity. Sometimes slender and cylindrical, as in the sharks
(fig. 598), sometimes flattened dorsoventrally, as in the skates (fig.

 

Fia. 598.—Acanthias vulgaris,* dogfish. (From Claus.)7 2B, ventral fin; Br, pectoral fin;

Ks, gill clefts ; 1, nostril ; R’, R?, dorsal fins ; S, heterocercal caudal fin ; Spl, spiracle.
599), they agree in form in that the head is prolonged into a snout,
which is usually supported by a cartilaginous prolongation of the
cranium, the rostrum (fig. 588, #). The mouth lies ventrally, at
more or less distance from the anterior end, and is transverse,
whence the name Plagiostomi—transverse mouth. This position
makes it necessary that a shark approaching its prey from below
must turn on its back before biting. The tail is heterocercal or is
drawn out in a long filament. The skin is covered with placoid
scales, usually close together, these being so small in some cases
that the skin—shagreen—is used instead of sandpaper for polish-
ing. More rarely the scales are larger, and the spines, which
project from the skin, justify in size and form the term dermal
teeth. Such strong spines occur especially at the front of the
570 CHORDATA.

dorsal fins (ichthyodolurites of paleontologists). The skeleton is
cartilaginous, frequently calcified on the outside. The calcifica-
tion can also extend into the vertebra, producing star-like figures.
Since bone is lacking, the sharks have no upper jaws, but bite with
the pterygoquadrate. The amphicwlous vertebre (lacking in the
Holocephali and the extinct Cladoselachii, Ichthyotomi, and
Acanthodide), have neural arches, small ribs, and intercalaria.
The number of gill arches and clefts varies between five and seven,
the first cleft lying between the hyoid and the first branchial arch.
Besides, most elasmobranchs have a spiracle and pseudobranch
(fig. 598, Spl). Except in the Nolocephali the gill clefts open sep-
arately, the hyoid arch being without an operculum.

In the visceral anatomy these points are of importance as dis-
tinguishing elasmobranchs from Teleostomes. (1) The heart hasa
large conus, with several rows of valves (fig. 596, A), but lacks a
bulbus. (2) The alimentary tract (fig. 593, 4) has a spiral valve,
but lacks swim bladder and pyloric ceca. (3) The sexual products
are carried to the exterior by the urogenital ducts. The eggs
escape from the follicles of the ovary (occasionally unpaired) by
dehiscence into the body cavity, and from thence by the unpaired
ostium tube and the paired Miilerian ducts to the exterior. The
spermatozoa traverse the anterior part of the Wolffian body (‘kid-
ney’). Sexual and reproductive ducts open dorsally into the
cloaca.

Male elasmobranchs are distinguished by the presence of a copu-
latory structure (mixipterygium) developed by enlargement of some radii
of the ventral fin (fig. 599, c). The large eggs, rich in yolk, are fertilized
in the oviducts and usually develop in uterine enlargements of the ducts.
The embryos (fig. 582), with long gill filaments protruding from the gill
slits, are nourished by the yolk in the yolk sac. In Mustelus aud Car-
charias, as Aristotle knew, there is the formation of a placenta, which
differs from that of the mammals in that the embryonic blood supply
arises from the blood-vessels of the yolk sac and are not allantoic. There
are oviparous elasmobranchs, and in these the egg is surrounded by
albumen and a shell, but these eggs differ from those of birds in that the

skull is horny and is usually drawn out at the four corners, sometimes
with threads for attaching the egg to plants, ete.

Order I. Selachii.
With the notochord more or less completely replaced by verte-
bral centra; no dermal bones.
Sub Order I, DIPLOSPONDYLI. Gill slits lateral, six or seven in
number, a single dorsal fin. Ch/amydoselachus with terminal mouth.
Hexanchus,* mouth normal, six gill slits ; Heptanchus, seven gill slits.
IV. VERTEBRATA: PISCES, SELACHIL. 571

Sub Order II. SQUALI (Euselachii). Normal sharks, with cylindrical
bodies, free thoracic fins, heterocercal tail, lateral gill slits. Most of them
are fast swimmers and are rapacious, the teeth being usually pointed, with
sharp or toothed edges, but in some the teeth are pavement-like and are
used for crushing shell fish. The numerous families are distinguished by
vertebral characters, number of dorsal fins, presence of nictitating mem-
brane, ete. In the GaALEID&, in which the nictitating membrane is
present, belong, besides the dog-sharks (Jfustelus * and Galeus*), the largest
of all sharks, Carcharinus,* some of which have man-eating reputations.
The hammer heads (Zygeena*) are closely allied. The mackerel sharks
Lamna*) and the great white‘ man-eater,’ Carcharodon,* lack nictitating
membranes. All of the foregoing have star-shaped figures in the verte-
bree (p. 570). In the dog-fishes, represented by Acanthias vulgaris * (or
Squalus acanthias, fig. 598), there is a spine in front of each dorsal fin.

Sub Order III. RAIA. In the skates the body is flattened horizontally
fig. 599), and the pectoral fins, also flattened, are united to the sides of

 

Fic. 599.—Raia batis, male, ventral view. (After Mébius and Heincke.) B. ventral,
Br, pectoral fin; R, rostrum, a, anus: ¢ copulatory part of ventral; ks, gill clefts:
m, mouth; 7, nostril; between them the oronasal groove. :

the body, the union usually extending clear to the tip of the snout, and

frequently back to the pelvis, giving the body a rhombic appearance from

above. The animals swim by undulating motions of these fins. They

mostly lie quiet on the bottom, and hence the lower surface is white,

the upper colored. The union of the fins to the side has resulted in trans-
572 CHORDATA.

fer of the gill slits to the lower surface, the spiracles to the upper. The
teeth are usually pavement-like. ‘The PRisTiDa, or sawfishes, are the most
shark-like, but are readily recognized as belonging here by the position of
the gill slits. The common name is due to the fact that the snout is pro-
longed into a paddle-shaped blade, the edges armed with teeth. Pristis.*
Raup#; the typical members of the group; Raia.* Closely allied are the
TRYGONIDA, or sting rays, with whip-like tail with one or two spines, the
‘stings’ at the base; Dasyatis.* The torpedos (TORPEDINIDZ) have
smooth skins, and have electrical organs, kidney-shaped bodies, on either
side between gill arches and pectoral skeleton, Torpedo.*

Order II. Holocephali.
These forms, which have no common English names, differ from
the selachii in having the pterygoquadrate arch, which bears a few
large chisel teeth, fused with the cranium without a suspensor; in

 

Fia. 600.—Chimera monstrosa. (From Kingsley.)

having a dermal fold constituting an operculum, which covers the
gill slits; and corresponding with this, the gills more on the teleost
type (p. 566). Lastly, the vertebral centra are not developed.
Chimera.* Fossils appear in the Devonian.

The CLADOSELACHI (Cladoselache), ICHTHYOTOMI (Pleuracanthus), and
ACANTHODID# are paleozoic forms in which vertebral centra were lacking.
In Cladoselache the skeleton of the paired fin consisted of numerous simi-
lar radii and was more primitive than the archipterygium; Plewracanthus
was diphycercal, and the head, as in Acanthodes, bore dermal bones.

Sub Class II, Ganoidei.

The ganoids form a transition group in which elasmobranch
and teleost characters are mingled ina notable manner. They
have the spiral valve of the sharks, the swim bladder of the telosts;
the heart with the conus is selachian, the respiratory structures—
the comb-like gills and the operculum—are as distinctly teleostean.
The hyoid arch, with the development of the operculum, has not
entirely lost its respiratory function, since in garpike and sturgeon
it bears an opercular gill, and often there is a psendobranch in
the spiracle. The skeleton is always ossified in certain parts; large

 
IV. VERTEBRATA: PISCES, GANOIDET. 5738

membrane bones lie on the shoulder girdle, on the roof and floor
of the skull (parasphenoid); the horny threads of the fins are bony
rays. In general the skeleton ranges between two extremes—an
extremely primitive cartilaginous condition with persistent noto-
chord, and one with a more than ordinary degree of ossification.
It is important for the systematist to find characters in all ganoids
which occur only in the group. The ganoid scales, used by Agassiz,
are not sufficient, since the sturgeon has bony plates free from
ganoin, while the paddle bill (Polyodon*) has almost no dermal
skeleton, and Amia has cycloid scales. Most recent and fossil
forms possess fulcra, bony plates with forked ends lying shingle-
like in front of the fins (fig. 10, B), but these are not universal,
and are absent, ¢.g., in Amia and Polypterus (fig. 10, 4 and C).
The group is largely American. The few recent ganoids fall into
three distinct groups.
Order I. Crossopterygii.

These are largely extinct, but two genera persisting to-day. The tails
are diphycercal or heterocercal; the pectoral fins have the basal portion
scaled; broad gular plates beneath the jaws in place of branchiostegals; the
skeleton well ossified. Polypterus and Calamoichthys from Africa. The
order was probably ancestral to the Amphibia.

Order II. Chondrostei.

These forms resemble the sharks externally in the heterocercal tail,
spiracle, ventral position of the mouth; internally in the cartilaginous
skull and (except Polyodon) in the pterygoquadrate serving as upper jaw.
In the vertebral column they are more primitive than most selachians,
since centra are lacking, the neural and hemal arches and the intercalaria
resting direct on the notochordal sheath (fig. 556). ACIPENSERID&, with

 

Kia, 601.— Acipenser sturio,* common sturgeon. (After Goode.)

large bony dermal plates.  Acipenser,* sturgeon. The swim bladder
furnishes isinglass, the ovaries make ecaviare. POLYODONTIDE, with
naked skin and long paddle-like snout, toothed maxillaries present.
Polyodon,* paddle fish.

Order III. Holostei.

In these the skull is ossified as in teleosts; maxillary and premaxillary
bones are present, the pterygoquadrates reduced and not meeting in front,
and the mouth terminal. The body may be covered either with ganoid or
574 CHORDATA.

eycloid scales. The living forms (the group appears in the trias) have ossi-
fied opisthocelous vertebree and diphy- or homocercal tails.

Lepiposreip#&. Scales rhomboid, branchiostegal rays present, a pseudo-
branch, but no spiracle. Lepidosteus,* garpike. AMIDA, distinctly teleos-
tean in appearance with cycloid scales, amphiccelous vertebre, and heart
with reduced conus (fig. 596, B). Améa,* bow fin.

Sub Class ILI. Teleoster.

The teleosts owe their name to the extensive ossification of
the skeleton, which consists, in the trunk, of amphicclous vertebra,
and in front a skull with numerous primary and secondary bones,
already enumerated (p. 560, fig. 589). Maxillaries and premaxil-
laries are present, but these are frequently without teeth, since
other bones of the mouth (vomers, palatines, hyoid, gill arches,
superior pharyngeals—the latter alone in Cyprinoids) may bear
teeth. Frequently there are present small bones, usually forked,
lying in the intermuscular septa above the ribs, which are not pre-
formed in cartilage. These are the epipleurals, and are distinct
from the ribs. In the fins both cartilage and dermal rays are ossi-
fied, the former remaining small, the rays forming most of the
support. These rays may either be soft and flexible (Malacopteri)
or hard and spine-like (Acanthopteri), a matter of classificatory
value. In the first case they consist of numerous small threads

 

Fic. 602.—Perea fluviatilis. (From Ludwig-Leunis.) 4, anal fin; B, ventral fin; Br,
pectoral fin, K, operculum; NV, nostrils; R,, Ry, spinous and soft dorsal fins; S,
caudal fin ; Si, lateral line.

(fig. 602, Br, A, B, F,), in the other the parts of a ray are fused

to a spine which, sometimes provided with poison glands (Scorpena,

Amphacanthe, etc.), become good defensive weapons. The tail is

usually homocercal; the diphycercy of eels and other fishes is sec-

ondary. The dermal skeleton consists of ctenoid or cycloid scales,
sometimes of spines or body plates. In rare instances the skin is
naked.
IV. VERTEBRATA: PISCES, TELEOSTEI. 575

The hyoid arch always bears an operculum and _ branchiostegal
membrane, but there is no opercular gill. The gills of the
comb-like type, are confined to the four anterior gill, arches, but
they may be reduced to even two and one-half pairs of demi-
branchs. Instead of a conns (present in Butrinws), the bulbus
arteriosus is well developed; a spiral valve is lacking, but pyloric
appendages are common. A swim bladder is usually present, but
its duct is frequently closed.

The teleosts are distinguished from all vertebrates except the cyclo-
stomes and perhaps some ganoids in that the nephridial system does not
form part of the sexual ducts. The eggs and milt are deposited through
the abdominal pores or by special canals developed from the body cavity.
Copulation occurs in only a few viviparous forms (Embiotocide, Gambu-
sta, etc.). The rule isthat males and females deposit their reproductive
products in the water at the same time, and this leads to the enormous
schools of herring and other fishes which occur yearly at certain times.
This also explains the ease with which artificial impregnation in fish
culture is performed,

In rare instances the males care for the young, as in the case of the
sticklebacks; more noticeable are the conditions in the lophobranchs (sea
horses and pipe fish), where the males receive the eggs in a brood pouch on
the ventral surface. A metamorphosis is known only in the eel-like fishes,
the larvee of which—originally described as distinct under the name Lepto-
cephalus—are flat, transparent forms with colorless blood, enormous tails,
and extremely small trunk. These larvee normally occur in the sea at the
depth of some hundred fathoms. The fresh-water eels go to the ocean
for propagation. On the other hand many salt-water fish go to fresh
water for reproduction.

The classification of the fishes is yet in an unsettled state, partly owing
to the large number of forms, partly to the fact that the groups intergrade.
Most European writers recognize six divisions, Physostomi, Anacanthini,
Pharyngognathi, Acanthopteri, Cheetognathi, and Lophobranchii. Our
authorities separate the Ostariophysi from the Physostomi, the Pediculati
and Hemibranchii from the Acanthopteri, and unite the Anacanthini and
some of the Pharyngognathi with the Acanthopteri and make a distinct
group, Synentognathi, of the others. The characters on which these divi-
sions are based are less convenient for the tyro than those adopted here.

Order I. Physostomi.

The character to which this name refers is not readily seen
without dissection, the persistence of the duct of the swim bladder.
This is, however, correlated with the soft character of the fin
rays (few exceptions) and the abdominal position of the ventral
fins. The Ostariophysi are remarkable in having a chain of bones
connecting the swim bladder with the ear. More than a third of
the food fishes and nearly all of the fresh-water fishes belong here.
576 CHORDATA,

The Ostariophysial families are the Sirurip# (1000 species),
or cat-fish, with barbles about the mouth, of which Malapterurus,

 

Fra, 603.—Salmo salar,* Atlantic salmon. (After Goode.)
the electric cat of Africa, is most noteworthy. The CyPRINIDZ,
or carp (1000 species), and the suckers, Carostomip®, have little
food value. The electric eel of South America belongs to the
Gymnonoti. The other families are true Physostomes. The Sa.-
MONID# are easily recognized by the ‘adipose dorsal,’ a fin formed
of a fold of skin without fin rays. The trout and salmon (Sa/mo *)
belong here and are among the most important food fishes.
Osinerus,* smelt; Coregonus,* white fish; CLUPEIp£, herring,
shad; ANGUILLID#, eels, the breeding habits referred to above.
Esocipm, pike and pickerel. AxmBLyopsip., blind fish of Mam-
moth Cave.
Order II. Paryngognathi.

In many fishes the inferior pharyngeal bones (7.e., the last
rudimentary gill arch) fuse to form a single bone, and these forms
are called Pharyngognathi. Some have spiny fins, among the
Lasrip.x, including Clenolabrus,* the cunners, and Tautoga,* the

 

Fria. 604.—Clenolabrus caruleus,* cunner. (After Goode.)
tauutog. These are placed among the Acanthopteri by American
authors, Others have only soft fin rays. These are the Synento-
gnathi and include the Exocarrrp.x, or some of the flying fishes,
in which the pectoral fins are very large, acting as parachutes
when the fish leap from the water.  Lrocetas.*
LV. VERTEBRATA: PISCES, TELEOSTEI. BUT

Order III. Acanthopteri (Acanthopterygii).

This is the largest group of fishes, its members usually having
the ventral fins thoracic in position and more than three rays spiny
in dorsal, anal, and ventral fins. The sticklebacks (GAsTERo-
STEID.£) and some other forms have the pharyngeal bones reduced,
the ventral fins farther back, and form the group Hemibranchii.
Gasterosteus.* The perch of fresh water (PERCID®), Perca*
and Micropterus* (black bass), and the marine SERRANIDAZ, some
of which are hermaphroditic, have ctenoid scales. The Scomprip,
with Scomber,* the mackerel, and Thynnas,* the horse mackerel, and

 

Fic. 605.—Scomber scombrus, mackerel.

the XIPHIID#, or sword fishes, in which the snout is prolonged into
along sword, are the most important edible fishes of the group. The
Lonrivatr, including the sculpins (Coltus,* Hemitripterus,* fre-
quently have the body armored with bony plates. The Emprotocirps,
or surf perches of the Pacific, are viviparous. The suck fishes,
Remora,* Echeneis,* have the first dorsal modified into a sucker on
the top of the head.

Order IV. Anacanthini.

These are soft-finned fishes in which the ventral fins lie in

 

Fic. 606.—Gadus morrhua,* cod. (After Storer.)

front of the pectorals. Structure goes to show that these have
descended from <Acanthopteran forms. With few exceptions
578 CHORDATA.

(Lota,* burbot), all are marine. The Gavipm, with Gadus,* in-
cluding the cod and haddock, and the
PLEURONECTIDA, with Hippoglossus,*
the halibut and other gevera, the floun-
ders, turbot, and sole, make this the
most important group of marine fishes.
The Pleuronectide, from their asym-
metry, need a word. The young are
perfectly symmetrical, but the animals
turn on one side, the lower becoming
white. The eye of this side gradually
works over to the upper side, twisting
the bones of the skull in its progress.
Order V. Lophobranchii.

A small group of marine species,
having in common gills composed of
small rounded tufts, the body covered
with a segmented armor of bony plates
and peculiar breeding habits, the male
carrying the eggs and young in a brood
pouch. The sea horses, Hippocampus,”
with their horse-like heads, and the
Fra. 607.— Hippocampus hepta- slender pipe fishes, Syngnathus,* belong

gonus, sea horse.
eee ) here.

 

Order VI. Plectognathi.

A small group of peculiar compact fishes, in which the bones
in each jaw are coossified, the ventral fins reduced or absent. In
the trunk fishes, Ostracodermi, the body is enclosed in a firm angu-

 

Fie. 608.—Chilomycterus geometricus,* swell fish. (After Goode.)

lar box of bony plates. The Gymnodonta, or swell fishes (fig. 608),
have the power of inflating the body to spherical sacs. The flesh
is poisonous.
IV. VERTEBRATA: DIPNOI.

Or
al
©

Sub Class IV. Dipnoi (Dipneuste).

The lung fishes have the form of true fishes, with scales and
paired fins, supported by a single or a doubly pinnate archiptery-
gium. The median fin is not separated into dorsals, caudal and
ventral, and the caudal part is diphycercal. The skeleton is very
primitive, consisting largely of cartilage, the notochord being re-
tained to a great extent. The animals live in fresh water and,
under ordinary conditions, breathe by gills which are covered by
an operculum. In the gills there are some peculiarities recalling
amphibian structures, Protfopterus, and the young of Lepidosiren
having external ag well as internal gills. The resemblances are
strengthened by the periodic appearance of pulmonary respira-
tion. The lung fishes live in the tropics in pools and swamps
which, during the hot season, may be more or less completely dried
up. When the water becomes too foul for branchial respiration,
the swim bladder is used. This is a paired or unpaired sac with
a duct leading to the esophagus, and the interior has its respira-
tory surface increased by the development of air cells. Protopterus
indeed can live out of water; it burrows in the mud at the dry
season, and builds a cocoon lined with mucus in which it remains

 

F 1a. 609.—Protopterus annectens, lung fish. (From Boas.)

é
choana opening into the mouth cavity. The last gill vessels give
off pulmonary arteries, and there are veins carrying the blood back
to the heart. The heart itself shows the beginning of division
into arterial and venous halves, especially in the regions of the
conus and auricle.

quiescent until the wet season. The nose is respiratory, with a

The few species now living have a wide and discontinuous distribution,
and are the remnants of a much richer group which appeared in the
paleozoic. MONOPNEUMONIA, with one swim bladder: Ceratodus of Aus-
tralia. DIPNEUMONIA, with two bladders: Protopterus, Africa; Lepido-
siren, South America. Possibly the paleozoic ARTHRODIRA, some of
gigantic size (Dinichthys), belong here.
580 CHORDATA.

Class III. Amphibia.

There are two views as to the origin of the Amphibia. Accord-
ing to the one they have descended from
Crossopterygian ganoids (and this seems the
better supported); the other is that they
have come from the Dipnoi. The group is
distinguished at once from the fishes by the
absence of fins. There is, it is true, a median
fin in larval life, and this may persist (Peren-
nibranchs, Triton), but it is never divided
into dorsal, caudal, and anal, and it lacks
any skeletal support (figs. 4, 5). The paired
fins are replaced by pentadactyle feet (p. 524).
These are often webbed and are used for
swimming; they are also used for creeping
and leaping, and are consequently jointed
between the separate skeletal elements (fig.
610). Besides the shoulder and hip joints,
which alone occur in fishes, there occur also
elbow (knee), wrist (ankle), and finger joints.
The number of digits is not always five, for
a reduction to four, three, or even two
occurs.

The connexion of the girdles with parts

of the axial skeleton (lacking in most fishes)
ae oe Se iy eae ad OL importance. The pelvic girdle is con-
losa, larva. (From Ge- é : 3
wenbaur.) ¢,centrali; F, nected with the vertebral column by means
ae ae ua of the ilium, which articulates either directly
tibia; t, tibiale; 1-5fcar- eee cae pet
palia‘and corresponding or by a sacral rib with the single sacral ver-
metacarpals and digits. tebra, Ventrally the two halves of the girdle
fuse, and usually the limits of ischium and pubis cannot be traced.

The attachment of the pectoral girdle is less firm (fig. 564, 4).
The dorsal portion, the scapula, ends free in the muscles; the
ventral, differentiated into coracoid and clavicle, is often connected
with the sternum, but this is not connected with the vertebral
column, since the ribs are too short to reach it. The sternum is
frequently connected with an episternum.

The vertebral column often (Perennibranchs, Derotremes,
Cecilians, and many Stegocephali) resembles that of fishes in
amphiceslous centra and persistence of notochord. The notochord
may disappear, there then occurring opisthoccelous (Salamandrina)

 
IV. VERTEBRATA: AMPHIBIA. 581

or procelous centra (most Anura). There is also an articulation
of skull with vertebral column, rare in fishes but characteristic of
land animals, by which the first vertebra (atlas) becomes distinct
from the rest.

The skull is remarkable for the extent to which the chondrocra-
nium is retained and the consequent small number of primary
bones (figs. 611, 612). The bones of the orbital region are repre-

 

Fic. 611.—Frog skull from below. (From Wiedersheim.) For letters see fig. 612.

sented by a pair each of ali- and orbitosphenoids in the urodeles, by
a ring of bone, the sphenethmoid (os en ceinture), in the anura.
The auditory region usually contains only prootics, the occipital
only exoccipitals. The absence of other occipitals is often of value
in distinguishing between amphibian and reptilian skulls, since in
the former the articulation with the atlas is consequently by double
occipital condyles. Of secondary cranial bones are to be men-
tioned the nasals, frontals (in many prefrontals also), and parietals,
the latter two fused in anura to frontoparietals; ventrally the large
parasphenoids.

The cranium is increased by the addition of the large quadrate
cartilage, which becomes applied to the otic capsule and (Anura)
fuses with it, while the rest of its arch (pterygoquadrate) extends
forward in a more or less complete condition, reaching the nasal
capsule in the Anura. The quadrate cartilage is covered externally
by the squamosal (paraquadrate), and supports the lower jaw, com-
posed of Meckel’s cartilage surrounded by membrane bones
582 CHORDATA.

(dentary, splenial, angulare, etc.); its articular portion, like the
quadrate, being rarely incompletely ossified. Vomers, palatines,
and pterygoids appear in the base of the skull, all three forming a
continuous arch in the Anura; in front of them lie the premaxil-

pro. eee PE Le. pet. pe.
G ae a :

    
  

“x mk. dn. cm

Fi. 612.—Lateral and hinder views of frog skull, (After Parker.) Letters for this
and 611: an, angulare; As, alisphenoid cartilage; co (Cocc), occipital condyles;
col, columella; d, dentary; KE (e), sphenethmoid; fo, foramen magnum, FP,

frontoparietal ; Gk, otic capsule; h’, h’’, hyoid and copula; jg, jugal; M (m),

maxillary (in lower jaw mento-Meckelian); mk, Meckel’s cartilage; N,N},

nasal capsule; 2a, nasal; ob, os, cartilages from which basi- and supraoccipitals

arise elsewhere ; ol (Olat), exoccipital; pf, frontoparietal; Pal, palatine; p (PP),
palatine arch; Pmx, premaxillary ; Pro, prootic; Ps, parasphenoid: Pt, pterygoid;

Qu, quadrate; Qjy, jugal; sy, squamosal; Vo, vomer. Cartilages dotted.
laries, and in most cases maxillaries. Between the hinder end of
the maxillaries and the quadrate there may be a gap or it may be
bridged by a jugal. By the modification of the quadrate into a
suspensor the hyomandibular loses
its function, and if represented at
all, it is as part of the columella.
The character of the remaining Vis-
ceral skeleton depends upon the
respiration (fig. 613). Where gills
occur, the copula and hyoids—repre-
Fra. 613.—Hinder visceral skeleton of senting body and cornua—as well as

(A) larva of a salamander; (B) of : = 3

toad. (From Gegenbaur.) «, body four gill arches are present, but with

of hyoid; }, anterior horn (hyoid); Sy F ; ‘ :

¢, rest of branchial skeleton. pulmonary respiration the hyoid ap-
paratus is reduced to a hyoid with anterior and posterior cornua,
the gill arches being contained in the posterior horns.

With the assumption of a terrestrial hfe changes occur in the
sense organs. The organs of the lateral line, which occur in all
larve and are persistent in the aquatic perennibranchs, and the
nerves which supply them, disappear; the eyes in the Salamandrina
have upper and lower lids; in the frogs an under lid (really nicti-
tating membrane). The nose becomes respiratory and is provided
with choanw opening into the mouth. Especially noteworthy is
the auditory apparatus. This, in the urodeles and eecilians, is

 

 
IV. VERTEBRATA: AMPHIBIA. 53838

very primitive, even the tympanum being absent, but in the Anura
a sound-conducting apparatus appears.
The spiracular cleft persists asa canal,
opening into the pharynx by the Eu-
stachian tube, its outer end expanded into
the tympanic cavity and closed externally
by the tympanic membrane, supported by
the cartilaginous tympanic annulus (dot-
ted circle in fig. 611, B). The connexion
of the labyrinth with the tympanum is
by an opening in the otic capsule, the
fenestra ovalis, in which is the stapes
(? part of capsule), the columella extend-
ing from this to the tympanic membrane
and carrying the sound waves across to
the inner ear. The brain (fig. 614) has
advanced above that of the fishes in the
stronger development of the cerebrum,
but remains behind in the small size of the
cerebellum, which is but a thin lamella.
The respiratory organs afford impor- Fie. 614.—Brain of frog. f, line
tant characters, since both gills and lungs Ce ee ee en and

: ; boidalis;'’ HI’ cerebellum ;
occur. Of gills there are two kinds, inter-  1/Sltactory nerve; Lol, olfac.

nal (found only in Anura), of entodermal [0fF (ones: MU optic lobes:
origin, and ectodermal gills, external in 74 "tw!xt brain.

position (figs. 4, 5), occurring in all. These ectodermal gills, three
in number, are richly vascular and arise from the skin at the
upper part of the gill clefts. The paired lungs open into the
hinder part of the pharynx, either directly through the glottis or
more rarely by a short trachea. Cartilages, the remnants of gill
arches, may support trachea and glottis, and on the latter support
vocal cords (larynx). Breathing is accomplished by a kind of
swallowing, the air being forced into the lungs by the muscles of
the floor of the mouth and the pharynx. Persistent gills and
lungs are found only in the Perennibranchs. Usually the young
breathe by gills, the adults by lungs, the origin of the metamor-
phosis to be described below.

Besides gills and lungs the skin is an important respiratory
organ, as are pharynx and mouth cavity, in which the air must re-
main for some time on account of the respiratory mechanism. This
renders intelligible the fact that many Salamandrina (Spelerpes,
Desmognathus, Plethodon, Gyrinophilus, etc.) have neither gills

 
584 CHORDATA.

nor lungs, but have only pharyngeal and cutaneous respiration.
The capillary network in these parts is greatly developed and may
extend into the epithelium. Thus, also, it happens that in the
Anura the skin receives as large an artery as the lungs (fig. 616, cz).

The skin is thin and slimy from the numerous mucous glands,
which not infrequently produce a poisonous secretion (so called
parotid gland in the ear region). The epithelium bears a thin
horny layer which at intervals is molted as a continuous sheet.
The derma in the Anura is undermined by large lymph spaces,
the presence of which makes the skinning of a frog such an easy
matter. Ossifications in the skin—enormously developed in the
fossil Stegocephali — occur but rarely (Gymnophiona) in recent
Amphibia. On the other hand the abundance of chromatophores is
noticeable, these, under the influence of the nerves, changing their
shape and thus producing color changes in many Amphibia.

The heart (figs. 615, 616) has two auricles, distinctly sep-
arated in Anura, a right with venous blood, a left which with

 

Fia. 615, Fic, 616.
Fie. Gis, Haar and arterial arches of salamander larva. (After Boas.) a a2, right

   

and left auricles; ad, arterial trunk; ad, dorsal aorta; as, left aortie arch; b,
dir connexion between afferent and efferent arteries; c, carotid; l, afferent
art ip, pulmonary artery; v, ventricle; 1-4, afferent arteries: 1’-3’, gills.

3—Heart and arches of frog (diagram). a, a,,, right and left auricles: aa,
ventral aorta; ad, as, right and left aortic arches (radices aorte): c, carotid; cu,
cutaneus; 1, lingualis; p, pulmonary artery; ss, subclavian; v, ventricle; ve,
vertebralis; 1, 2,4, persisting arches.

   

pulmonary respiration receives arterial blood. There is, however,
but a single ventricle, and the arterial trunk is, at least externally,
single. The arterial arches show different relations and have
different fates. With branchial respiration the first three afferent
and efferent arteries are connected in two ways, the one by the
IV. VERTEBRATA: AMPHIBIA. 585

capillaries of the gills, the other direct (fig. 615, b). In the fourth
arch there is no gill system, but on the other hand this arch gives
off the pulmonary arteries (jy) to the lungs.

With the loss of gills (fig. 616) the third arch frequently dis-
appears entirely (Anura), as well as the gill circulation of the
others, while the direct circulation persists, at least in part. The
first arch gives rise to the carotids, supplying the head (c); the
second unites with its fellow of the opposite side to form the dor-
sal aorta; the fourth forms the pulmonary artery and, in the
Anura, gives off a cutaneus artery (vw) to the skin. A longitu-
dinal fold inside the arterial trunk is so arranged that the venous
blood from the body coming to the heart through the right auricle
is mostly sent out through the fourth arch to the lungs and the
skin, while the blood returned from the lungs by the pulmonary
vein passes through the left auricle and then through the first
and second arches (carotid and aortic arches). So there is here a
separation of pulmonary and systemic circulations, although the
blood all passes through a common ventricle.

The sexual organs (fig. 581) are similar to those of selachians.
The eggs pass from the ovary to the oviducts (Miiller’s duct), and
in this are enveloped with a gelatinous layer. The spermatozoa,
on the other hand, pass through the anterior part of the Wolffian
body (‘kidney’) and thence out through the ureter. The distinc-
tion from the selachians lies in the fact that a urinary bladder,
lying ventrally to the rectum, is present, at some distance from the
urinary ducts, which open dorsally into the cloaca. Besides sexual
organs fat bodies frequently occur, lobed and often brightly col-
ored structures, best developed between the reproductive seasons.

A sort of copulation occurs, and internal impregnation is effected in
many urodeles and in the Gymnophiona, but not in the Anura. The Anura
and most other forms are oviparous, but occasionally, as Salamandra
maculosa and S. atra of Europe, viviparous species occur. Many inter-
esting brooding habits are known. The male of Alytes obstetricans wraps
the cords of eggs about his legs and crawls into a hole until the young are
hatched, while the females of Amphiuma and Ichthyophis watch over the
eggs. The male of Rhinoderma darwinii has a large sac arising from the
pharynx in which the eggs and young are cared for until the completion
of the metamorphosis. In Pipa americana the male places the eggs on its
back, the skin thickening around them so that each les in a separate
pocket, from which the young escape at length in nearly the adult form,
In Nototrema and Notodelphys there are dermal sacs upon the back for the
reception of the eggs.
586 CHORDATA.
The development of the Amphibia possesses special interest,
since it affords the only easily observable instances of a metamor-
phosis among the vertebrates. This metamorphosis is the more
marked the wider the adults are from the fishes. In the Anura a
larva, the tadpole (fig. 4) escapes from the egg. It lacks lungs,
but has three pairs of external gills, no legs, but a swimming tail
with afin-like fold. In the metamorphosis the gills and tail—larval
organs—are lost, while Iungs and legs are formed. A complica-
tion is introduced into the metamorphosis in that, for a time after
the loss of the external] gills, internal branchix, lying in gill slits,
occur. These, however, are not visible from the exterior, since a
fold of skin grows back over them, thus forming a cavity, the
atrium, into which the gill slits open, and which in turn opens to
the exterior by an opening (rarely paired), usually on the left side
(fig. 617). In the tailed forms the metamorphosis is simplified,

    
 

ee

 

NRA RNR SAS
-

VISIT TTT TTT TTI TI

Fis. 617.—Side view of tadpole. e, eye; g, opening of atrium; 1, hind leg; m, mouth;
v, vent.

usually consisting in the loss of the external gills and sometimes
in the change of form of the tail, which may lose its fin fold and
become cylindrical. The last traces of a metamorphosis disappear
in the perennibranchs, where lungs occur and the gills persist
(Siren is said to lose the external gills and then re-form them). In
the Anura the metamorphosis is lost when, as in Hylodes mar-
tinivensis, the whole development occurs in the egg, the young
hatching in the adult form.

Order I. Stegocephali.

Extinct forms with well-developed tail, numerous membrane
bones in the skull, and frequently a bony armor, at least on the
ventral surface. Some were of gigantic size, and some from the
folded condition of the enamel of the teeth are known as Laby-
rinthodonta. The group appears in the carboniferous (footprints
in the Devonian), and died out in the trias.
IV. VERTEBRATA: AMPHIBIA. 587

Order II. Gymnophiona (Cecilie, Apoda).

These are the nearest of living forms to the Stegocephali,
but fossils are entirely unknown. The
group is exclusively tropical, occurring
in Ceylon, African islands, and America,
a discontinuous distribution indicative
of great age. They are burrowing ani-
mals and feed on small invertebrates.
As a result of this subterranean life the
eyes are small and concealed under the
skin, the legs are entirely lost, so that
the animals are snake-like in appearance.
In the skin there are usually small bony
scales; the drum of the ear is lacking;
the vertebre are amphicelous. Inside
the egg many species have three pairs
of beautifully feathered gills (fig. 618), a e
proof of their pertinence to the Am- ee ee
phibia. Later, for a time, there isan arasins.)
external gill opening which finally closes. Jchthyophis, Ceylon;
Hypogeophis, Seychelles; Cecilia, America.

 

Order III. Urodela (Gradientia).

Of recent forms of Amphibia the urodeles are the most fish-
like. The vertebral column consists of numerous vertebre, and
of these a large part are behind the sacrum and consequently
belong to the tail. Ribs are present, but so short that they do not
reach the sternum, which is weakly developed or is entirely absent.
Tympanum, and Eustachian tube are entirely lacking, as are the
vocal chords and the production of sound.

Sub Order I. PERENNIBRANCHIATA. Two or three gill slits, three
bushy gills, and a swimming tail persist throughout life. Mecturws,* mud
puppy, with legs and two gill slits. Stren,* three gill slits, hind legs lack-
ing. Proteus, of Austrian caves, much like Vecturws, but nearly blind,

Sub Order IT. DEROTREMA. External gills lost, but an opening in
the neck leading to the gill slits. Menopoma* (Cryptobranchus), hell-
bender, legs strong ; Amphiuma,* legs rudimentary.

Sub Order III. SALAMANDRINA (Myctodera). After the loss of gills
the gill slits close. Amblystoma,* remarkable for the length of time the
larvee retain their gills, A. tigrinum (fig. 5) and the Mexican axolotl even
breeding in the larval stage. The adult of the true axolotl is unknown.
Plethodon,* Spelerpes.* The European Salamandra atra and 8. macu-
588 CHORDATA.

lata are viviparous, the former undergoing its metamorphosis inside the
mother.
Order IV. Anura.

The anura have the compact bodies familiar in frogs and toads,
with a small number (7-1) of trunk vertebree and complete absence
of tail; the caudal vertebre being represented by a long bone, the
urostyle. Ribs are sometimes distinct, sometimes fused to the
transverse processes; the limbs are larger than in other Amphibia,
and are frequently used for leaping and climbing. Ear drum and
tympanic membrane are lacking only in the Pelobatide; their
presence is correlated with the existence of vocal cords and the pro-
duction of sound. The metamorphosis includes a tadpole stage.

Sub Order I. AGLOSSA. Toad-like anura with degenerate tongue and
unpaired opening of the Eustachian tube. Pipa (p. 585), South America ;
Dactylethra, Africa.

Sub Order II]. ARCIFERA. Tongue present, Eustachian tubes widely
separate, coracoids of the two sides overlapping. BuFoNID&, toads, tooth-
less; Bufo,* the dermal glands poisonous. PELOBATID£, with teeth,
usually no tympanum, Scaphiopus,* burrowing toad, with tympanum.
HYLID#, tree toads, toothed ; tips of toes with sucking discs ; Hy/la,*Acris.*

Sub Order III. FIRMISTERNIA. Tongue present, Eustachian tubes
distinct, coracoids firmly united in the middle line. Ranip#, frogs.
Rana catesbiana,* bull frog, the largest frog known ; numerous other
American species.

Series I], AMNIOTA.

Vertebrates with amnion and allantois (p. 554) in embryonic
life; with the pro- and mesonephros functional only in the em-
bryos, and replaced in the later stages by the true kidney (meta-
nephros); ducts of the embryonic excretory system retained only
so far as they have genital functions; gill shts appearing as trans-
itory structures, but without gills and never functional. There
are two great divisions of the Amniotes, the Sauropsida and the
Mammalia. The Sauropsida include the Reptilia and the Aves,
which agree with each other and differ from the mammals in
having a single occipital condyle, the quadrate acting as suspensor
of the jaws; ankle jomt between the first and second rows of
tarsals; the presence of epidermal scales, nucleated red blood
corpuscles, and a cloaca.

Class I. Reptilia.

On account of similarity of form, the reptiles and Amphibia
were long united. They form parallel groups: urodeles and liz-
ards, frogs and turtles, cecilians and snakes. Hence the points
IV. VERTEBRATA: REPTILIA. 589
of distinction must be emphasized. The most important are two:
the reptiles belong to the Amniota and, as such, have the em-
bryonal features of the group; second, although often aquatic,
they are, in the entire absence of branchial respiration, in character
of skin and skeleton, in their entire structure, like the true land
auimals.

The skin, the better to withstand desiccation by the air, is
strongly cornified, so that in the epidermis a many-layered stratum
corneum and a many-luyered stratum Malpighii can be distin-
guished. At the tips of the toes the stratum corneum develops
strong claws. Further protection is afforded by the thick derma,
often capable of being tanned into leather, in which not infre-
quently bony plates occur. Dermal glands are very rare, the
femoral pores of the lizards (fig. 625, 0), which appear like the
ducts of glands, being produced by the ends of cornified epithelial
cones. The axial skeleton, both skull
and vertebral column, is nearly always
ossified; only exceptionally (Spheno-
don and the amphicwle Ascalabote)
are considerable parts of the noto-
chord retained. The vertebre are
usually procelous.

In the skull of reptiles (as in the
allied birds) are many characters
which they share with Amphibia and
which distinguish them from mam-
mals. This is especially the case
with the visceral skeleton. As in
the Amphibia, the hinder end of the
pterygoquadrate is attached to the
otic capsule; the quadrate is ossified
and affords the articulation for the
lower jaw, which is composed of many

 

bones. The squamosal lies at the base
of the quadrate and, in the Squamata,
is intercalated between it and the
cranium. Behind it isthe columella,
its inner end inserted in the fenestra
ovalis. From the quadrate the palatine
series of bones—pterygoid, palatine,
vomer—extends forward, these being

Fia. 619.—Ventral view of skull of
Tropidonotus. (From Wieders-
heim.) Bp, basioccipital; Bs, basi-
sphenoid (in front also parasphe-
noid); Ch, choana; Coce, occipital
condyles; Eth,ethmoid cartilage;
Fy, frontal; Fo, fenestra ovalis;
M, maxillary; ol, exoccipital; P,
parietal; Pf, prefrontal; Pl, pal-
atine ; Pmax, premaxillary ; Pt,
pterygoid; Qu, quadrate; Sa,
squamosal; 7s, transversum; Vo,
vomer; IJ, optic foramen.

frequently toothed; and in front of and parallel to it the pre-
590 CHORDATA.

maxillaries and maxillaries. Extremely characteristic of the
reptiles, the turtles excepted, is an os transversum, which appears
in no other vertebrates. It extends from the hinder end of the
maxillary to the pterygoid (figs. 619, 626, 627, 630, Tectia
jugal is also frequently present. Of the other visceral arches,
since gills are lacking, only the hyoid bone and laryngeal carti-
lages persist.

In the cranium the complete ossification of the occipital region
is noticeable, the four occipital bones being present. The basi-
occipital forms the larger part of the single occipital condyle, in
which parts of the exoccipitals participate, the single condyle
being the sharpest distinction between the reptilian and amphibian
skull. The basisphenoid, which lies in front of the basioccipital,
has an anterior process or rostrum, representing the rudimentary
parasphenoid (possibly presphenoid). Above, the skull is roofed
in with membrane bones: parietals (frequently fused and perforated
by the parietal foramen for the pineal eye), frontals, nasals, as
well as pre- and postfrontals and postorbitals, and usually lachry-
mals as well.

The ethmoidal region is largely cartilaginous ; ali- and orbitosphenoids
are small and variable. Only the prootic is constant in the otic region;
epiotic and opisthotic usually fusing with the occipitals, the opisthotic
being large and distinct only in the turtles. The zygomatic arch (lost in
snakes) is formed of jugal and quadratojugal, while above it may be a
second arch formed of postorbital and squamosal.

The convex occipital condyle forms, with the concave surface
of the first vertebra (atlas), an articulation for motion in the ver-
tical plane and lateral motions, while a twisting around the long
axis of the body is perniitted by the joint between the atlas and
the second vertebra, the axis or epistropheus. The atlas is a bony
ring, its centrum having separated and united with the body of
the axis, forming a pivot around which the atlas turns. There are
two sacral vertebra, and the vertebrae of the trunk are divided into
thoracic and Iumbar, the former bearing long ribs which reach to
the sternum, while the shorter ribs of the neck end freely.

Limbs are lacking in snakes and some lizards. When present
the number of digits varies between three and five (usually four or
five). Inthe pelvis ischium and pubis are separated by an obturator
foramen and are united with the corresponding bone of the oppo-
site side by a double symphysis. In the shoulder girdle scapula
and coracoid alone are constant, a clavicle occurring in turtles and
lizards, in the latter an episternum (fig. 564) as well. Of con-
siderable systematic importance is the position of the ankle joint.
IV. VERTEBRATA: REPTILIA. 591

This is intertarsal in character, in that it occurs between the first
and second rows of tarsal bones (fig. 636, C).

 

Fic. 620.—Viscera of Alligator. (From Wiedersheim.) ED, rectum; H, heart; L,
liver; Lg, lung; M, stomach; MD, intestine; Oc, esophagus; P, pylorus; Tr,
trachea ; ZB, body of hyoid; ZH, its cornua ; *, perforations of hyoid.

Since reptiles lack even transitory gills, the gill slits are com-

pletely degenerate before the young escapes from the egg. Dermal
592 CHORDATA.

respiration is far less important than with the Amphibia, lungs,
as in birds and mammals, being the respiratory organs, and in
these a progressive development may be followed. The larynx is
followed by a trachea with cartilage supports in its wall, and this
either opens directly into the two lungs or divides into two bronchi,
which, in Varanus, may divide again inside the lungs. 'The lungs
in the more primitive forms are subdivided only peripherally, but
in the higher groups the whole is chambered, partitions extending
inwards to the intrapulmonary bronchus.

Since the respiration is entirely pulmonary, the heart is divided
into a left arterial and a right
venous half, and a corresponding
separation of systemic and pulmonary
blood-vessels occurs (fig. 621). The
two auricles are completely separated,
while a septum extends into the ven-
tricle, complete in the crocodiles,
but not in turtles, lizards, and snakes.
Yet even in the crocodiles a mixing
of arterial and venous blood occurs
siuce in the large aortic trunks which
arise from both ventricles a commu-
nication, the foramen Panizze, per-
sists. The arterial trunk is divided
by internal partitions into three ves-
sels, which are but rarely visible from
the exterior. One of these arises
from the right ventricle, carries
venous blood, and takes over the
JE fourth arterial arch, which gives off
Fic. 621.—Heart of crocodile with ar- the pulmonary arteries (4, p). A

teries (diagram). a, a?, right and be ie
loft auricles: ci, os, right and iert Second vessel arises from the right

aortic arches; ¢, ce id's; ‘0%, 07, iets oo SA lea teas
Hohtand. lett atiogleesiptiag Ventricle, is purely arterial and con-
ostia; p, pulmonary artery; s, sub- S ry P > q7n]
clavinns'v}, 0% right and left ven- nects with most of the remaining
tricles (the arrows show the direc- yptapja] arceheag 5 fire hie yes
tion of the blood flow): 1, 2, 4, arterial arches, the first, which gives
arches comparable with those of Pp AN YPOt) ‘ a O olf
amphibia. Notice the origin of the off the carotid, and the right halt
vessels from the heart, and the (aortic arch ad) of the second
connexion (foramen Panizza) be- ” ae :

tween the arterial trunk and the greh. The third vessel connects on
left aortic arch, just in front of A ate
the heart, the one hand with the remaining

(left, second) arch and on the other with the right or venous half
of the heart. The foramen Panizze occurs between this and the
right aortic arch.

 
IV. VERTEBRATA: REPTILIA. 593

The venous character of the left aortic arch and the incomplete
ventricular septum (or presence of foramen Panizze) prevent a
complete separation of systemic and pulmonary circulations. In
the turtles a third element enters, the persistence of a ductus
Botalli (as in Urodeles, fig. 580, //, dB).

To the foregoing adaptations to a terrestrial life may be added
indications of higher development. The brain shows two advances.
The cerebellum, especially in turtles and alligators, has be-
come large, and the cerebrum grows dorsally and backwards over
the *twixt brain and forms the temporal lobes of the hemispheres.
The parietal organ is developed as nowhere clse. In many lizards
it forms an unpaired dorsal eye lying beneath the skin in the
parietal foramen. The paired eyes possess hds (usually upper and
lower as well as a nictitating membrane), and frequently (turtles,
lizards, and many fossils) a ring of bony plates (sclerotic bones)
in the sclera. A new opening in the petrosal, the fenestra rotunda,
places the tympanic cavity and the labyrinth in close relations.

In the excretory system amniote characters prevail. The
Wolffian body with its ductis functional in the embryo. Later
there arises behind it the permanent kidney (metanephros) with
the ureter, while the embryonic structures disappear with the ex-
ception of those retained as accessory to the genital apparatus.

Thus in the male the vas deferens and epididymis are formed
from the Wolffian duct; in the female the Millerian duct (early
lost in the male) becomes the oviduct. Usually the urogenital
canals open dorsally in the cloaca, rarely in an elongation of the
urinary bladder (Chelonia). This latter is lacking in snakes and
crocodiles.

Almost all reptiles lay eggs; only in the Squamata (some snakes
and lizards) are viviparous or ovoviviparous forms present. The
eggs much resemble those of birds, in that the large yolk is sur-
rounded with a layer of albumen and enclosed in a fibrous, often
calcified shell. To open the egg the embryo has an egg tooth on
the tip of the snout; this consists, of dentine in the Squamata, but
elsewhere, as in birds, is horny. From these relations it follows
that internal impregnation must occur; the eggs undergo a discoidal
(meroblastic) segmentation. Copulatory organs to accomplish
this internal fertilization occur, and these are of classificatory im-
portance, since they differ in character in the Squamata on the one
hand, the turtles and crocodiles on the other. These differences
are correlated with differences in the form of cloacal opening and
in structure of skull and skin, so that all living species may be
594 CHORDATA.

divided into two groups, the Lepidosauria, containing the lizards,
snakes and Sphenodon, and the Hydrosauria with turtles and croc-
odiles. This, however, ignores the fossil forms. When these are
taken into consideration another grouping must be adopted.

Order I. Theromorpha.

Extinct reptiles from the Permian and triassic which are closely re-
lated to the stegocephalous amphibia; with amphicclous vertebre, im-
movable quadrate, and from two to six sacral vertebre, The ANOMODON-
TIA, with partial or complete loss of teeth, stand near the turtles, while
the THERIODONTA, in which a heterodont dentition is developed, resemble
in this and some other respects the mammals, which, by many, are sup-
posed to have descended from them.

Order II. Plesiosauria.

Extinet aquatic forms from the triassic to the cretaceous, some forty
feet in length. They had long necks, and the limbs were modified into
swimming paddles recalling the flippers of the whales. The quadrate was
immovable, and the jaws, with numerous teeth in sockets, were long.

Order III. Ichthyosauria.

These forms resembled the Plesiosaurs in skin, swimming feet, elongate
jaws, and quadrate, but had the teeth (sometimes absent) in grooves rather

 

Fia. 622.—Restoration of Plesiosaur. (After Dames.)

than in sockets, and short necks. Some species at least were viviparous.
Their range in time was like that of the preceding order.

Order IV. Chelonia (Testudinata).

The turtles form in external appearance a sharply circumscribed
group, with the short and compact body enclosed in a bony case,
from which only head, tail, and legs protrude (fig. 623). The
case consists of a convex dorsal portion, the carapace and a flat-
tened ventral plastron, the two being united in most forms at the
margins. Hach consists of bony plates, the positions and names
of which may be learned from the adjacent eut. It only needs
mention that the neural plates are united with the spinous pro-
cesses, the costals with the ribs, and that the entoplastron is re-
IV. VERTEBRATA: REPTILIA, CHELONIA. 595

garded as an episternum. It is not connected with the internal
skeleton, since the sternum is lacking. The pelvis is only rarely
fused with the plastron. This bony case is usually covered with
horny shields, their number and arrangement usually agreeing
with the plates of the case, although without their contours exactly
coinciding.

More important are the great firmness of the skull and the
immovable condition of the quadrate, the lack of an os transver-
sum and of any but basisphenoid of the sphenoidal bones, and by

 

Fie. 623.—Carapace (A) and Plastron (B) of Testudo greeca. (From Wiedersheim.) C,
costal plates; E, entoplastron ; Kp. epiplastron: H, posterior; Hp, hypoplastron;
Hy, hyoplastron ; M, marginal plates ; N, neural plates; Np, nuchal plate ; Py,
pygal plate; R, ribs; V, anterior ; Xi, xiphisternum.

growth forward, and backwards by which the girdles are brought
inside the ribs. The teeth are entirely lost, and, as in birds, the
jaws are enclosed in sharp horny beaks, in many cases efficient
weapons against larger vertebrates. The cloacal opening is oval,
its major axis corresponding to that of the body, and in its anterior
end is an unpaired erectile penis used in copulation. Turtles
appeared in the Permian, and the group has persisted until now.

Characters of armor and legs serve to contrast sharply the land and
sea turtles; the first with well-developed legs, five-toed in front, four-
toed behind, the toes with claws; the carapace arched, into which legs,
head, and tail may be retracted. In the sea turtles the feet are flipper-
like (fig. 624), claws mostly absent, and the carapace weakly united to or
free from the plastron, flat and incapable of covering head or appendages.
The fresh-water species are intermediate in position.

Sub Order I. ATHECA. Carapace of numerous mosaic scales and not
connected with ribs and vertebra; skin leathery. Dermochelys (Sphargis)
coriacea,* the leather-back tortoise of warmer seas, reaches a weight of
1500 pounds.
596 CHORDATA.

Sub Order II. TRIONYCHIA. Fresh-water forms with poorly ossified
carapace, but ribs and vertebrae connected with it. Our leather turtles
(Amyda*) and soft-shelled turties (Aspidonectes*) of savage habits belong
here,

Sub Order III. CRYPTODIRA. Carapace well developed and united
with ribs and vertebree, but the pelvic arch free. The species are numer-
ous, including terrestrial, fresh-water, and marine forms. CHELYDRIDA,
fresh water, taillong. Chelydra serpentina,* snapping turtle; Machrochelys

 

Fa. 624.—Chelone imbricata, tortoise-shell turtle. (From Hajek.)

lacertina,* alligator turtle. CHELONID®, marine, paddle-like feet. Tha-
lassochelys caretta,* loggerhead; Chelone mydas,* green turtle, the favorite
of epicures; Hretmochelys imbricata, whose horny shields furnish tortoise
shell. TESTUDINIDA, terrestrial, including Xerobates,* the ‘gopher turtle’
of the South, the giant Testudo of the Galapagos Islands, and the enormous
fossil Colossochelys atlas of India, 18-20 feet long, 8 feet high. Other
families contain our mud turtles (Kinosternon *), box turtles (Cistudo*),
and terrapins (Malaclemmys *).

Sub Order IV. PLEURODIRA. Pelvis united to carapace and plastron.
All belong to the southern hemisphere.

Order V. Rhynchocephalia.

These resemble the lizards not only in body form (four five-
toed feet) and in scaly skin, but in certain anatomical matters as
well: lack of hard palate, presence of epipterygoid, transverse
cloacal opening, and heart, lungs, and brain. On the other hand
they recall the crocodiles in having two postorbital arches and
immovable quadrate. The large abdominal sternum and abdominal
ribs are noticeable as well as the uncinate processes of the true
ribs. The notochord is but incompletely replaced. The group
appears in the Permian and is thus one of the oldest of reptilian
types, and is usually regarded as ancestral to all the orders yet to
be mentioned. The only living species, Sphenodon (Hatteria)
punctata, belongs to the New Zealand region.

Order VI. Dinosauria.

This order included some of the largest land animals which have ever
existed. Some of them were from forty to one hundred feet long and
twelve to twenty feet high (Amphiculias, Camarasaurus). In some there
IV. VERTEBRATA: REPTILIA, SQUAMATA. 597

was an exoskeleton, some of the plates of which in the stegosaurs measured
a yard across. Among the characters of the group are the fixed quadrate,
jugal and postorbital arches, three to ten sacral vertebrae, and ilium
elongate in front of and behind the acetabulum. Some of these forms
(Orthopoda) in pneumaticity of bones, in having the pubic bones directed
backwards, and in the formation of an intratarsal joint, resembled the
birds, and have been regarded as the ancestors of that group. The Dino-
saurs were confined to mesozoic time.

Order VII. Squamata (Lepidosauria, Plagiotremata).

One of the characters which unite lizards and snakes and which
has given the name Plagiotremata is the transverse form of the
cloacal opening (fig. 625), behind which, in the male, are the

ay jer Sr pi

 

Fig. 625, Fia. 626.

Fie. 625.—Hinder trunk and hind limbs of a lizard. (From Ludwig-Leunis.) a,
cloacal slit ; b, femoral pores ; sca, anal shield.

Fia. 626.—Skull of Ameiva vulgaris. an, angulare ; ar, articulare ; co, epipterygoid ;
cr, coronoid ; d, dentary ; fr, frontal; j, jugal ; la, lachrymal; m, maxillary ; na,
nasal; p, postorbital, above and behind it the parietal; pf, prefrontal; pr, pre-
maxilla; pt, pterygoid ; y, quadrate ; gj, quadratojugal; sy, squamosal ; tr, trans-
versum.

paired copulatory organs, each lying in a sac from which they can

be everted like the finger of a glove. The names Squamata and

Lepidosauria refer to the scaly condition of the skin. These

scales are horny structures and somewhat distinct from the bony

scales of fishes. The derma forms flattened papille which resemble
the scales of fishes in that in many species they contain bony
plates. These papille determine the character of the epidermis.

Since the stratum corneum is especially thick on the top of the

papille and thinner between them, rhomboid and oval plates occur,

which either lie flush with each other (shields) or overlap like
shingles (scales). The rule is that the head is covered with reeu-
larly arranged shields, each with its name, the trunk with scales

in longitudinal, transverse, and oblique lines. Outside these is a

layer of cornified cells, the pseudocuticula, and outside of all an

inconspicuous true cuticle. Since all cornified cells are dead and
598 CHORDATA.

require periodic removal, the horny layers are cast yearly and re-
placed by new. During this periodic molting, which recalls that
of arthropods, the animals are sickly and apt to die in captivity.

All Squamata are characterized by the slenderness of the
cranial bones (fig. 619, 626, 627), which, especially in the Lacertilia,
incompletely close in the cranium. The quadrate is movable,
and the squamosal is intercalated between it and the cranium. A
hard palate is lacking, and the choane, as in the amphibia, lie far
forward (fig. 619, Ch). There is a wide gap in the partition
between the two ventricles of the heart.

Sub Order I. LACERTILIA (Saurii). The lizards are usually distin-
guished from the snakes by the possession of limbs, but a few forms,
undoubted lizards, like the glass snakes and Amphisbene, lack limbs.
These are distinguished by the existence of the scapula and the iliac bone
united to the vertebra, and especially by the presence of a sternum, which
never occurs in snakes. In the skull is a peculiar bone (lacking only
in Chameleons and Amphishzene), found nowhere else, the epipterygoid
(fig. 626, co); it reaches from the pterygoid to the parietal, and from its

WLS hr Tf Pa Sq Os

  

Fia. 627.—Skull of rattlesnake. (From Boas.) Fr, frontal; h, hyomandibular (colu-
mella); Ma, maxillary; N, nasal; Os, supraoccipital; Pa, parietal; Pul, palatine;
Pf, postfrontal; Prf, prefrontal; Pt, pterygoid ; Px, premaxilla; Q, quadrate; Sq,
squamosal; 7, transversum; 1, dentary; 3, articulare.

slender shape is sometimes called columella, but is not to be confounded

with the true columella of the ear. The bones of the jaws are firmly united,

so that the mouth has no special capacity for opening widely. The jugal-
quadratojugal arch is present.

In external appearance the presence of eyelids, nictitating membrane,
tympanic membrane, and Eustachian tube are noticeable, these being
absent only in the Amphisbeenee. In the Ascalabote, as in snakes, the lids
grow together, forming a transparent covering over the eyes. Fossil
lizards are rare, but the group dates back to the eretaceous.

Section I. AScALABOT.E (geckos). Skeleton incompletely ossified, noto-
chord persistent, amphiccele vertebrie; skin granular rather than scaly,
usually adhesive dises on the toes by which they climb vertical surfaces or
ean walk upon ceilings. Two hundred species. Phyllodactylus.*
IV. VERTEBRATA: REPTILIA, SQUAMATA. 599

Section II. Crassmuinauta. Tongue thick, fleshy, not protrusible from
the mouth, or only slightly so. IGuaNrpa; American, often a comb of
spines on the back, teeth pleurodont, ¢.e., firmly united to the inner side
of the jaw. Three hundred species. Anolis,* Sceleporus,* Phrynosoma,*
‘horned toads.’ AGAMID&; Old World, teeth acrodont, @.e., seated on the
angle of the jaw bones. One hundred and fifty species. Chlamydosaurus,
Draco volans, with ribs greatly elongate and supporting a dermal fold
which acts as a parachute.

Section III. Fissruincura. Tongue long and thin, divided at the tip,
and capable of wide protrusion from the mouth, and in Varanus retractile
into a sheath. Trsipa#; American, teeth acrodont; Cnemidophorus,*
Tejus. NHELODERMATID#, pleurodont; Heloderma,* the ‘ Gila monsters,’ are
the only poisonous lizards. Lacertinipas (Lacerta) and VARANID& (Vara-
nus, the monitors) are Old World forms, Lacerta vivipara bringing forth
living young.

Section IV. Brevitineura. Tongue short, slightly notched at the tip,
slightly protrusible. Four hundred species. Scrncip#®, with tendency to
reduction of the limbs. Humeces,* Oligosoma.* In Anguts and Typhline
the legs are absent. ZONURIDa&, with a finely scaled groove along the side;
all Old World except our Ophisaurus ventralis,* the glass snake, a limb-
less form with brittle tail.

Section V. ANNULATA. In many respects snake-like; legs and epi-
pterygoid, tympanum, and movable eyelids lacking and usually girdles ;
tropical or subtropical. In Chirotes sternum and reduced fore legs
retained. Amphisbena.

Section VI. VERMILINGUIA; includes the Old World chameleons (our

    
 

 

_
Te
BE Manze

 

Fia. 628.—Head of chameleon with tongue extended.

‘chameleon’ is Anolis,—supra) with long fleshy tongue, lying rolled up in
the mouth, but protrusible and used for catching insects, its end being
covered with a sticky mucus. Other characteristics are the ring-like eye-
lids functioning as an iris, the climbing feet in which the toes are united
into two opposable groups; epipterygoids, clavicle, sternum, and tympanic
membrane lacking. The chameleons are best known from their changes
of color, produced by rapid alterations in the size and shapes of the
chromatophores. Color changes occur in other lizards, but not to such an
extent as here.
600 CHORDATA.

Sub Order II. PYTHONOMORPHA. Large, extinct, extremely elon-
gate reptiles with four flipper-like limbs and strong swimming tail.
Flourished in the cretaceous. Josasaurus, Clidastes.

Sub Order III. OPHIDIA. The snakes are distinguished from most
lizards by the absence of limbs, and connected with this the similar verte-
bree in which only trunk and caudals can be distinguished. The caudals
lack ribs, but these are present and long in the trunk region, serving for
locomotion and supporting the body on their distal ends. Since there are
legless lizards, it is further necessary to say that in the Ophidia the girdles
and sternum are lost, only the Peropoda having remnants of the hinder
appendages and pelvis, but these not connected with the vertebral column.

Further distinctions exist in sense organs and jaws. The columella is
indeed present, but tympanum and Eustachian tube are lacking. The eye-
lids also seem to be wanting, but examination shows, in front of the cornea
and separated from it by a lachrymal sac, a transparent membrane, com-
posed of the fused eyelids (outer cornea). The apparatus of the jaws (figs.
619, 627) is remarkable for its great extensibility, which enables snakes to
swallow animals larger than themselves, after coiling around them and
crushing them. This extensibility is in part due to the fact that the bones
of the lower jaw are bound together at the symphysis by elastic ligaments,
in part to the freedom of motion of the bones of the upper jaw (excepting
the small premaxillaries) and the palate. Further, the squamosal (Sq),
quadrate (Q), and transversum (77) are elongate and slender, the quadrate
being widely separated by the squamosal from the skull, while the zygo-
matic arch is entirely absent. The food is forced down the throat by
hook-shaped bones on palatines and pterygoids. A wide distension of
the stomach is rendered possible by the elasticity of its walls and the great
mobility of the ribs, which are not united ventrally by a sternum.

In the non-poisonous snakes the dentition is similar on jaws and
palate bones (fig. 619). The vomer and, usually, the premaxilla are tooth-
less. In the poisonous serpents poison
fangs appear on the maxilla (fig. 627)
and are distinguished from the other
teeth by their greater size and connex-
ion with a large poison gland. The
duct of the gland opens at the base of
the tooth; the peison which is pressed
out by the pressure of the jaw muscles
is led to the tip of the tooth either by a
groove (proteroglyphie tooth, fig. 629, A)
or, when the groove is closed to a canal
(solenoglyphie tooth, B), through this

Fie. 629.—Poison fangs. 4, -4,, pro- canal which opens at base and tip of

teroglyphic (grooved) tooth of co-
bra, and section of same; B, 2,, so- the tooth.

 

lenoglyphic tooth (tubular) of rattle- The asymmetrical character of the
snake; g, poison canal; p, pulp eee 5
cavity. lungs is interesting. In the Peropoda

one lung (apparently the left) is much
smaller than the other; in the poison snakes and some others it is rudi-
IV. VERTEBRATA: REPTILIA, CROUVODILIA. 6OL

mentary orevenabsent. In the Typhlophide, on the other hand, the right
appears to be degenerate. The urinary bladder is always absent. The
excreta, chiefly urie acid, accumulate as a solid mass in the cloaca and
form the chief part of the excrement; the faces, on account of the
extraordinary digestive powers, being small in amount.

Section I. OporeroponTa (Angiostoma). Burrowing blind tropical
snakes with the mouth incapable of distension, the animals living on
small insects. Typhlops.

Section II. Peropopa. These large snakes have paired lungs and rudi-
ments of hind extremities ; lack poison fangs, and kill their prey by mus-
cular power. Python, Africa; Boa and Hunectes (anaconda), South
America,

Section III. CoLuBRIFORMIA. Ordinary snakes (over 500 species) with
numerous teeth in the upper jaw, but with appendages entirely absent.
Some are poisonous, some not, but no structural lines can be drawn be-
tween them. The AGLYPHA have no grooved teeth. Tropidonotus,* water
snakes ; Bascanion,* black snakes; Hutainia,* garter snakes. The Pro-
TEROGLYPHA, with grooved teeth, perma-
nently erect, are poisonous. Most are
brightly colored. Elaps,* the coral snake;
Naja tripudians, the cobra of India; NV.
haje, Cleopatra’s asp. Here belong the
pelagic sea snakes of the Indo-Pacific,
which are viviparous.

Section TV. SOLENOGLYPHA. With the
maxilla reduced and serving as a socket
for the single large tubular tooth with
one or more reserve teeth (fig. 627).
VIPERID, Old World, no pit between
nostril andeye. CroTaLip#, New World
and Asia, with a pit between nose and
eye. Crotalus,* with the tail ending in
arattle formed by remnants of cast skins,
is common throughout the United States.
Agkistrodon contortrix,* copperhead,
and A. piscivorus, moccasin, lack the
rattle, Bothrops lanceolatus of the An-
tilles, possibly the most poisonous snake.

71,

eli

Order VIII. Crocodilia (Loricata).

 

The crocodiles, alligators, ete.,

eee ro : 0 Fie. 630.—Ventral surface of skull
agree with some of the forms already ~ of crocodile. (From Wiedersheim.)

: : " Coce, occipital condyle: Ch, cho-
mentioned in the oval cloacal open- San aaeal: ML maxillary; Oh,
] a ae oe ahi soe ah basioccipital ; Orb, orbit ; Qi, quad-
ing with single copulatory organ, — patojusal: Qu, quadrate § Pl, pala-
] : . Albay) ea tine ; Pmz, premaxilla; Pt, ptery-
immovable quadrate, and the bony oid! 7s. transversum.
plates in theskin. Inshape they are

lizard-like, but in structure they differ from all other living reptiles
GOL CHORDATA.

and approach most nearly to the Theromorphs. The maxillaries,
palatines, and pterygoids have united in the hving species in the
middle line, forming a hard palate and forcing the vomers upwards
into the nasal region. This same process has carried the choana
(fig. 630, Ch) to the back of the skull. Some of the ribs have
two heads; the ears and nostrils are provided with valves. A
sternum is present and, farther back, abdominal ribs and an ab-
dominal sternum. ‘The jaws are extended into a long snout, and
the teeth, which occur only on the margins, are placed in sockets
(alveoli). The four-chambered heart has already been described
(p. 592). The animals move slowly on land, but in the water,
thanks to their strong, keeled tail, they are very active. They have
a strong smell, owing to musk glands in the cloaca and on the
under jaw. The group appeared in the trias, and of the three sub
orders two, the Pseudosuchia and Parasuchia, are extinct.

Sub Order EUSUCHIA. External nostrils united, choana posterior;
five toes in front, four behind. Gavialis, India, snout long and slender.
Alligator luctus,* alligator ; Crocodilus,* most species Old World, one, C.
americanus,* occurring in our southern waters.

Order IX. Pterodactylia (Pterosauria).
Extinet reptiles of the Jurassic and cretaceous, adapted for flight.

The bones were hollow and the wings were broad membranes, supported,
like those of a bat, by the body and the greatly elongated fifth digit of the

  

_

Fra. 631.—Dimorphodon, a pterodactyle. (After Woodward.)
fore limbs. Some were sparrow-like in size and some, Pferanodon, had a
wing expanse of twenty feet. Yet one of these large forms from Kansas
had its pelvic opening so small that its eggs could not have been more
than half an inch in diameter.
IV. VERTEBRATA: AVES. 603

Class II. Aves.

While structurally the birds stand very near the reptiles, yet by
the development of wings and the feathering of the body the group
is one strictly circumscribed. The skin is in some places, as the
lower part of the legs, covered with horny scales and shields, on
the toes are claws, but as a rule the fingers are feathered. On
most places the skin is soft and thin, since the derma and stratum
corneum are poorly developed. Periodic molts of the integument
do not oceur, since the horny layer, as in mammals, undergoes a
constant renewal. These peculiarities of the skin are correlated
with the appearance of the protecting plumage.

The feather, like the hair of mammals, is exclusively epithelial
in character, but of a much more complicated structure. The cor-
nified epithelium forms a firm axis, the scape, from which, right
and left, arise branches, or barbs. The scape is solid as far as the
barbs extend (rachis, or shaft), while below it is hollow (quill, or
calamus). The quill is inserted deep in the derma, in a follicle,
and is provided with muscles for its movement. Its hollow in
most fully developed feathers is empty save for the ‘ pith,’ a small
amount of dried tissue. In young growing feathers it is occupied
by a richly vascular connective tissue, the feather papilla, which,
for purposes of nourishment, extends inwards from the derma.
The feather may therefore be regarded as a cornified outgrowth
from the skin which has arisen on a papilla of the derma, a view
which corresponds well with its development and shows its
homology with the scales. In many birds (cassowaries) two well-
developed feathers arise from the same follicle—a fact which
explains the existence of a rudimentary feather, the hyporachis,
or after-shaft, attached to the scape below.

In contour feathers the barbs are, to a great extent, united into a
vane. Right and left of the shaft they lie close together and parallel,
each repeating in miniature the entire feather, the barb having branches
or barbules, which, overlapping the barbules of adjacent barbs, give the
vane its close texture. The vane is held together by minute hooks on the
barbules of one barb interlocking with those of the next. Down feathers
(plumes) differ from contour feathers in the absence of hooks and the
loose arrangement of the barbs. Since feathers consist of cornified epithe-
lium and these cells are held firmly (only in powder down is there a
gradual loss), they, like the scaly coat of the snakes and lizards, must be
molted yearly and replaced by new.

Young birds or embryos have only down feathers. Later the contour
feathers arise in regular order in the feather tracts, or pteryle, between
604 CHORDATA.

which are apteria in which no contour feathers appear (fig. 682). Since
the contour feathers overlap like shingles, they form a firm coat of
plumage beneath which the down and semiplumes form a warm coat.

 

Fia. 632. Fic. 633.
Fia. 632.—Feather tracts and apteria of pigeon, dorsal view. (From Ludwig-Leunis.)
Fia. 633.— Regions and feathers of Falco lanarius. (From Schmarda.) As, secondaries ;
Ba, belly ; Br, breast; Bz, rump; D’-D’’, wing coverts; Di, gonys of bill; EF,
alula ; F,culmen of bill; H, occiput; HS, primaries; K, throat ; L, legs; N, neck;
Sch, crown; SF, parapterium; St, forehead, lower tail coverts; Sz, rectrices; W,
cheek; WH, cere with nostril; Zh, toes.

Besides these covering feathers (coverts, or tectrices, fig. 633, D) there are
the longer feathers of the wing, the remiges, and the tail feathers, or
rectrices (Sz). The larger remiges form the chief part of the wing; they
spring from the part of the limb corresponding to the hand (carpus,
metacarpus, phalanges) and are known as primaries (HS), while the
secondaries (As), arising from the forearm, are shorter. These are over-
lapped at the base by the coverts (D, D’, D’) and by the parapterium (SF)

ye

Sere

  

Fic. 634.—Wing skeleton of stork. (From Gegenbaur.) ¢, c’, carpalia of first row:
h, humerus; m, fused metacarpals and carpals of second row; p-p’’, phalanges of

first three fingers ; 7, radius; u, ulna.
springing from the shoulder. A few feathers arising from the first finger
remain distinct from the remiges and form the alula (ZF). In the water
birds especially the feathers are oiled by the secretion of a pair of glands
at the base of the tail above the coecyx.

Since the feathers are not only for protection, but give to most
birds the power of prolonged flight, they predicate a special mode
IV. VERTEBRATA: AVES. 605

of life, under the influence of which all of the other organs exist.
The character of the skeleton, the respiratory organs, and in part
the sense organs and brain, are connected with the powers of flight.

As the feathers of the wings, like the fins, form what may
be called a paddle working as a whole, the skeleton of these limbs is
simplified (fig. 634), first, by the reduction of the fingers, of which
only three with a small number of phalanges persist (p, p’, p’’);
second, by fusion of the corresponding metacarpals (m) with each
other and with the adjacent carpal
bones. On the other hand, in order
that there may be the necessary en-
ergy and the most complete transfer
of the same to the body, the con-
nexion with the skeletal axis is
strengthened by special development
of the parts. In the shoulder girdle
(fig. 635) all three elements are firm,
asword-shaped scapula (s), a colum-
nar coracoid (c), and clavicles which
are usually united to a ‘wish-bone,’
or furcula (f). Clavicles and furcula
are united directly or by ligaments
to the broad sternum, the anterior
face of which is developed into a
strong keel, the carina, or crista
sterni, in order to give the largest

 

7 2 . Fia. 635.—Trunk skeleton of stork.
surface for attachment of the large “(From Gegenbaur.) as, sternal

muscles of flight. Usually the greater PR fehuP iced clinical mp.
the powers of flight the more devel- [wsed,spinons processes of thoracie
oped the carina, yet in somé cases Sorte irel part oe ee ee eee ae
(albatross) the weak carina is com- ee ae
pensated for by the enormous width !™-.
of the sternal plate. In running birds (ostriches, etc.) the
carina is entirely gone. The thoracic framework is rendered
more firm by the development of uncinate processes from the ver-
tebral parts of the ribs (w) which overlap the succeeding ribs.
Since the fore limbs are no longer used for walking, the sup-
port of the body depends upon the hinder extremities, and this
has brought about two striking characteristics—the broad union of
the pelvis with the vertebral column, and the simplification of the
leg skeleton. In the embryo the ilium (fig. 635, 7) is connected
only with the two sacral vertebre present in most reptiles, but
606 CHORDATA.

later it extends forward and back, uniting with at least nine ver-
tebre and sometimes with as many as twenty-three; while the iliac
bones of the two sides meet dorsal to the vertebral column. This
extensive union of pelvis and axial skeleton is understood when
we recall that in walking or at rest the vertebral column is not
vertical as in man, but isinclined. Ischium and pubis are peculiar
in that they extend backwards, parallel to each other, from the
acetabulum, and that only exceptionally (ostrich) are the bones
of the two sides united by a symphysis.

In the hind limbs occur conditions similar to those which will

B Cc

 

Fic. 636.—A, leg of Buteo vulgaris. a, femur; b, tibio-tarsus; b’, remains of fibula; c,
tarso-metatarsus; c’, same, front view; d!-/3, toes. B, lower leg of bird embryo;
C, of lizard. f, femur; t, tibia; p, fibula ; ts, tarsales of first row (talus); ti, tar-
sales of second row; between these intertarsal joint; I-V, digits. (From
Gegenbaur.)
be repeated in the ungulates. The weight of the body makes it
necessary that the simplification found in the wing should be re-
peated in the lower leg and foot, and that the numerous bones
usually occurring in these regions be replaced by one which shall
support the pressure (fig. 636). Therefore the fibula, well de-
veloped in the embryo (2), becomes reduced to an inconspicuous
rudiment; the metatarsals, distinct in the embryo (B), fuse toa
IV. VERTEBRATA: AVES, 60T

single tarso-metatarsus (4, ¢), which has below as many articular
surfaces as there are toes (since the fifth toe only appears in the
embryo, at most four, in some three or even two, d-d’’’). At the
same time the tarsals disappear by fusion with adjacent parts.
Even in reptiles (C’) a part of the tarsals unite with the bones of
the shank, and the remainder with the metatarsals; in the birds
the union is completed, the proximal series fusing with the lower
end of the tibia to form a tibio-tarsus, the distal with the metacar-
pus to form the tarso-metatarsus, in this way producing the inter-
tarsal joint so characteristic of birds.

In respect to the vertebral column, it only needs mention that
the vertebra articulate with each other by a so-called saddle-joint,
that (in living birds) only a few caudal vertebre persist behind the
pelvis, that these are partially fused to a single bone, the pygo-
style, which supports the tail feathers, and that, corresponding to
the well-developed neck, there are many cervical vertebre, among
them an atlas and an axis, all except the last two fused with the
corresponding cervical ribs.

The skull (fig. 637) resembles closely that of the lizards in the
presence of a single occipital condyle, in the movable condition
of the quadrate upon the cranium, and in the presence of a slender
columella. On the other hand an os transversum is lacking. The
cranium, as a result of the increase in size of the brain, is more
spacious; the bones of its walls fusing early so that the sutures

Fr

   

a SZ)
=
Pal GF Pe Ang
Fia. 637.—Skull of young bustard. (From Claus.) Als, alisphenoid ; Ang, angulare;
Art, articulare; D, dentary ; Et, mesethmoid; Fr, frontal; Jmxr, premaxillary ;
J, jugal; L, lachrymal; Mx, maxillary; NV, nasal; Ol, exoccipital; Os, supra-
occipital; Pa, parietal ; Pal, palatine; Pt, pterygoid; Q, quadrate; Qj, quadrato-
jugal; Sm, interorbital septum ; Spb, basi- and presphenoid.
between them are obliterated. The occipital condyle is on the
under surface, so that the skull is carried at nearly right angles
to the axis of the vertebral column. Teeth are lacking in living
birds, although they occurred in some fossil forms. In their place
608 CHORDATA.

are hard horny sheaths covering the jaws which are frequently car-
ried back on the outside into a softer cere (fig. 634, WAH).

The cranium consists of four occipitals, a basi- and a presphenoid; above,
the parietals and frontals; and on the sides prootics, alisphenoids and
orbitosphenoids, while the broad squamosals also enter its wall. The large
mesethmoid lies in the interorbital septum ; the nasal cavity is roofed by
the nasals, and beside them are the lachrymals. The quadrate articulates
with the squamosal, and from it extend forward internally the pterygoid,
palatine, and vomer; externally a zygomatic arch of quadratojugal and
jugal to the maxillaries and premaxillaries. The maxillaries are hinged
in the ethmoidal region, so that in opening the mouth there is besides the
depression of the lower jaw an upward motion of the upper jaw.

The pneumaticity of the bones is an important feature of the
skeleton. In place of marrow and bony tissue, the inside of the
bones in strong flying birds is more or less completely occupied by
wir spaces, around which, as a sheath, is the compact bone. This
gives the greatest possible lightness and strength to the skeleton.
In Buceros and Palamedea all of the bones are pneumatic; in
others (Pelecanus, Sula, Tachypetes, etc.) only the phalanges of
the toes contain marrow, while in the penguin and Apteryz, as in
mammals, air spaces occur only in some of the cranial bones.

The air spaces of the bones are in part (skull) connected with
the nose and tympanum, but most of them, by means of the air
sacs, communicate with the lungs. The long trachea forks at its
lower end into two bronchi. At its upper end is a larynx, as in
other vertebrates, but this is not vocal; the notes of birds are pro-
duced by the syrinx, which lies at the division of trachea into
bronchi. It is usually formed of both trachea and bronchi, but
more rarely of either trachea or bronchi alone. Its vocal cords
are regulated by special muscles, which in the singing birds have

a complicated arrangement. The relatively

‘small lungs send out from their surface air
sacs, especially well seen in embryos (fig. 638,

I-56). These later become large, thin-walled

spaces, easily torn away in dissection, leaving

large openings on the surface of the lungs

(fig. 639, 1-3). Usually five pairs of these

ai ote air sacs are present, largely in the cclom,
ee Sea ree ca but extending in between the muscles (breast

trachea ; 1-%, lung sacs. and axillary region), and also into the bones.

 

The spongy lungs lie on either side of the vertebral column and are
IV. VERTEBRATA: AVES. 609

united to the ribs. On entrance to the lung the bronchus (fig. 639, 7) loses
its cartilage supports and enlarges into a
vestibule (v) and extends thence asa
mesobronchus (0m) backwards, termi-
nating in the abdominal air sac (4). A
side branch supplies the hinder sub-
costal sac (4). Secondary bronchi arise
from the vestibule and mesobronchus;
of these there are three to five ento-
bronchi (J-JV ) supplying the remaining
air-sacs and six or more ectobronchi.
Arising from the mesobronchi and
secondary bronchi are tertiary bronchi,
or air pipes, running parallel to each
other and anastomosing frequently.
Each air pipe has a thick spongy wall un eee aa eb e
(tig. 640) composed of numerous thin- shows a mesobronchus with its
: branches. a, artery; bm, meso-
walled sacs, the lung vesicles, closely bronchus, arising from the vesti-
enveloped by capillaries, and connected Dule: br, bronchus swelling to
with the central air-conducting tube, lung pipes; I-IV, mesobronchi;
% 1-5, ducts of lung sacs.
the lumen of the pipe.

Inspiration is effected by raising the framework of the chest, this
causing a straightening of the hinged ribs and an increase of the sterno-
vertebral diameter ; expiration by the reverse motion. By this the lungs,
attached to the ribs, are alternately enlarged and contracted in spite of
their slight elasticity. This is also true of the lung sacs, which, on account
of their poor blood supply, are not respiratory but serve as accessory air

 

 

Fia. 640.—Section of lung pipe. (After Schulze.)
pumps. It is probable that in flight this air-pump action occurs espe-
cially with the subpectoral and axillary air sacs, drawing air through the
lungs and rendering other respiratory motions superfluous, thus enabling
the thorax to remain quiet, an important matter. If the trachea be
closed and the air canal in the humerus opened. the bird can breathe
through the latter.
610 CHORDATA.

The circulation in the birds has arisen from that of the reptiles
by complete separation of systemic and pulmonary systems. Of
the three great arterial tranks present there (fig. 621), the pul-
monary artery and the right aortic arch, arising from the left ven-
tricle, are retained, the left venous arch being lost. The septum
between the ventricles is complete. The striking features of the
alimentary canal (fig. 60) are the crop (not always present), a
glandular stomach or proventriculus (¢), and a muscular chewing
stomach or gizzard (d), as well as two long, rarely rudimentary,
ceca (k) at the junction of small and large intestine. Liver and
gall bladder (¢, f), pancreas (g), and spleen are present. A blind
sac (the bursa Fabricii), the paired ureters (mm), and the sexual
ducts (2) open into the cloaca. The latter show the peculiarity
that the right oviduct and ovary are degenerate, while those of the
left side are correspondingly larger. Since copulation occurs the
large eggs (the ‘ yolk’) are fertilized in the oviduct (fig. 99). As
they pass slowly through the duct, they become enveloped first
with a thick layer of albumen, ‘white’ (w), then with a double
egg membrane (ism, sm,) the two parts being separate and enclos-
ing an air chamber at the larger end of the egg. Lastly comes the
shell. All of these accessory structures are secreted by the gland-
war walls of the enlarged oviducts. During the passage down the
oviduct the first phenomena of development (segmentation, gastru-
lation) occur, and after oviposition the development stops and again
starts when the necessary warmth is supplied.

The care for the young, the sexual life connected with copula-
tion, and the complicated conditions of ex-
istence connected with flight have resulted in
an intelligence far superior to that of the
—vx reptiles, which finds its expression in the bet-
Z ter development of sense organs and brain.
--t In the brain (fig. 641) the cerebellum, which
en ua isthe central organ for the coordination of the
action of parts, is strikingly developed. Cor-
a4 respondingly large are the cerebral hemi-
Fie. 641.—Brain_ of pig- Spheres, the frontal lobes of which begin to

as eee: cover the olfactory lobes, the temporal lobes
ee bell en aot in like manner extending back over the *twixt
tory ees Te Ontis brain and optic lobes. Corresponding to the
ee iT cave: Vocal apparatus, the ear is highly organized,
brum ; Z, pinealis. the lagena of the labyrinth being greatly en-
larged and the sound-conducting apparatus (columella, tympanum,

IT

 
IV. VERTEBRATA: AVES. 611

etc.) well developed. The beginnings of an external ear are seen in
the deeper position of the drum membrane. Since the power of
flight necessitates vision at great distances, most birds have exceed-
ingly sharp sight, and the eye itself (fig. 642) is in general con-

Co

 

Fig. 642.—Eye of owl. (From Wiedersheim.) Ch, choroid; CJ, ciliary muscle; Co,
cornea; Cv, vitreous body; J7, iris; L, lens; Op, optic nerve; OS, sheath of nerve;
P, pecten; Rt, retina; Sc, sclera; VK, anterior chamber; +, sclerotic bones.
structed for distance. Peculiarities of the bird’s eye, already
weakly developed in the reptiles, are the pecten (P), a comb-
shaped growth of the choroid into the vitreous body, and the
scleral ring, a circle of bones developed in the sclera and support-

ing the outer part of the eye.

Among birds there is spirited rivalry for the females, especially
among polygamous species. At the time of mating the males seek to win
the favor of the females either through striking motions (dances), by
singing, or by beauty of plumage. All of these peculiarities are confined
to the male and frequently lead to a marked sexual dimorphism. The dis-
tinction in plumage is commonly strengthened at this time, the male
receiving the brilliant wedding dress. Thus we speak of the spring molt,
although there is only a color change and only exceptionally a renewal of
the feathers. The return to every-day clothes only occurs with a molt, and
this comes at the close of the reproductive season.

The reason for the dull plumage of the female is due to the fact that
she usually sets on the nest, at which time inconspicuous colors protect her
from destruction by enemies. In only a few instances is the heat neces-
sary for incubation produced by other causes, such as the heat of the sun
upon the sand in which the eggs are buried, or the increase of temperature
caused by fermentation in decaying vegetation (Megapodes). The rule is
612 CHORDATA.

that both sexes build the nest, which with the weaver birds is most skil-
fully constructed; occasionally among social species the nests are placed
under a common roof, When the clutch of eggs is complete the female
(rarely the male) begins the incubation, at this time in some instances
losing the feathers from certain regions the better to warm the eggs.
Many birds, like hens and ducks, are so far advanced when they leave the
nest that they can follow the mother and feed themselves. Such birds
are called Praecoces—in contrast to the Altrices, which hatch with incomplete
coat of feathers and therefore need the warmth of the nest and the pro-
tection and care of the parents.

The migrations of birds possess great interest. We distinguish among
birds permanent residents and others which, in order to obtain food, take
long journeys, the migratory species. At the approach of cold weather
these seek the south, following regular paths in their travels. They can-
not, like reptiles and amphibians, hibernate at the period when insects
and fruit are scarce, because their greater intelligence and their more ener-
getic vital processes demand a more rapid metabolism and a continuous
food supply. Hence the birds, like the mammals, in contrast to the
‘cold-blooded’ reptiles, amphibia, and fishes, maintain, under all extremes
of external temperature, a body heat of 38-40° (44° ?) C. (100-104° F.).

The classification of birds is in a state of change. The older system
based upon adaptive characters is not in harmony with the results of care-
ful anatomical study, which would divide the whole class into many small
groups. For this reason it has been thought best to retain the older sys-
tem of larger, easily recognized divisions, and to call attention, where
necessary, to the contradictions with later results.

Order I. Saurure.

The view that birds are closely related to reptiles has received
considerable support by the discovery of fossil birds with teeth.
The most reptilian of these occur in the Jurassic of Bavaria, and
only two specimens have been found. In these (Archeopteryx
lithographica) the carpals and metacarpals have not fused, the
three fingers are well developed and clawed, and the caudal verte-
bre, although bearing feathers, form a long slender tail like that
of a lizard (fig. 2).

Order II. Odontornithes.

These forms, from the cretaceous of Kansas and Colorado, also
had teeth. In the Opontorm.© (/chihyornis) there was a keeled
sternum and normal pygostyle. In the OponToHoLc.® (Hesper-
ornis) the wings were reduced (only the humerus persisting), the
sternum was without a keel, and the caudal vertebrae formed a
broad paddle.

Order III. Ratite.

Here are included several families, very different in structure,

which agree in having the feathers not arranged in feather tracts;
IV. VERTEBRATA: AVES, CARINATA. 613

and in that, together with the lack of flight, many structures
normally connected with it are absent. The bones are but slightly
pneumatic, the sternum has no keel, and a furcula is not formed,
the clavicles being rudimentary (Vromeus) or not present as dis-
tinct bones. The wings are small and lack primaries and seconda-
ries adapted for flight, for typical contour feathers with close
vanes, as well as typical down feathers, are absent.

Since several structures apparently adapted for flight occur here
(fusion of hand bones and often of caudal vertebre; arrangement
of wing muscles), it is probable that the Ratites have descended
from carinate forms by loss of power of flight. The anatomical
distinctions between the various families lead one to believe that
they have arisen from different groups of carinates and hence do
not form a natural assemblage.

Section I. STRUTHIONES, with long humerus, long legs and neck.
STRUTHIONID#, two-toed ostriches of Africa, Struthio camelus. RHEIDA,
South American three-toed ostriches, Rhea americana, nandu. Section
Il. CASUARINA ; three toes, humerus short. Dromeus, emus; Casu-
arius, cassowaries. Section III. APTERYGES, bill long, nostrils near the
tip, rudimentary wing skeleton; four toes. Apteryz, kiwi, of New Zealand.
The DINORNITHID#, three toes, wing skeleton absent ; giant birds (thirteen
feet high) of New Zealand; now extinct, but apparently contemporaneous
with man. The Zpiornis, a gigantic bird of Madagascar, possibly belonged
near these. Skeletons and eggs holding two gallons found in alluvium.

Order IV. Carinate.

The name refers to the presence of the keel to the sternum,
which is correlated with the powers of flight possessed by most
species. Other characters of the class are the presence of rectrices
and remiges on tail and wings, and the fusion of clavicles to a
furcula. There are strong fliers, like the raptores and albatrosses,
which have but a small carina; in many poor fliers the carina may
be entirely absent. The furcula is not always present, the clavicles
not uniting (many parrots and toucans) or being absent (Jesi/es).
The remiges are also degenerate in some carinates, as in the pen-
guins (which are flightless, although they have a strong carina),
where they take the shape of small scales. Thus the distinctions
between ratite and carinate birds vanish in places.

Sub Order I. GALLINACEA. The hen-like birds are praecoces with
compact bodies and well-developed wings and legs, so that they run and
fly well without excelling in either direction. The feet have three toes in
front, usually connected by a membrane at the base (fig. 648, c); the fourth
toe is behind and at a higher level. Above this in the male is usually the
614 OHORDATA.

spur, a process of the tarso-metatarsus, covered with horn. The margins
of the upper jaw overlap the lower; the beak is bent downward at the tip
and is about as long as the head. Naked, richly vascular lobes form comb
and wattles which are specially large in the more elegantly plumaged
males.

The PHASIANIDZ are polygamous; Phasianus, with many species of
pheasants; Gallus bankiva of the Sunda Islands, the ancestors of domestic

 

Fia.643.—Foot forms. (From Schmarda.) a,semi-palmate, wading of Ciconia ; b,perch-
ing of Turdus;c, rasorial of Phasianus; d, raptorial of Falco; e, adherent of
Cypselus; f, cursorial of Struthio ; g, zygodactyl (scansorial) of Picus ; h, lobate of
Podiceps ; i, lobate and scalloped of Fulica ; k, palmate of Anas ; 1, totipalmate of

cz Phaethon.

fowl. Meleagris,* the turkeys. The TETRAONIDE are partly polygamous,
partly monogamous. Coturnix,* quail; Perdix,* partridge; Bonasa,*
grouse. The incubation of the Megapodes has been referred to (p. 611).

Sub Order II. COLUMBIN.©®. The pigeons are distinguished from
the Gallinaceze by the more slender bodies, shorter legs, the toes free, and
the longer wings capable of prolonged flight. They are altrical; the crop
produces a milky secretion used in feeding the young. The CoLUMBID£
are the most widely distributed and are represented in the tropics by
numerous beautifully colored species. Colwmba.* According to Darwin
the domestic pigeons come from C. livia, the blue rock pigeon ; Eetopistes
migratorius,* passenger pigeon, practically exterminated. Allied was the
dodo, Didus ineptus, of Madagascar, exterminated in the eighteenth
century,

Sub Order III. NATATORES. <A number of families, while differing
much in structure, are united by their inclination for an aquatic life.
They are called swimming birds (Natatores) because, thanks to their
IV. VERTEBRATA: AVES, CARINAT. 615

webbed feet, they are excellent swimmers and divers. Either all four toes
are connected by the web (totipalmate, fig. 643, 7), or only the three anterior
toes are webbed (palmate, fig. 648, i), or the three toes are each bordered
with a swimming membrane (lobate, fig. 643, 2). Thus the foot struc-
ture gives distinctions which forbid a closer association of the families, and
this is strengthened by differences of wing and beak. On the other hand
palatal structures show that here, as in the Grallatores, very diverse forms
are associated.

Section I. LAMELLIROSTRES (Anseriformes), feet palmate; the beak soft-
skinned up to the hard tip, its margins with transverse horny plates.
Anas boschas,* wild duck, source of domestic breeds. A. mollissima, eider;
Anser,* goose (domestic derived from A. ferus). Cygnus,* swans. Sec-
tion II. TUBINARES (Longipennes), predaceous birds with strong beak,
tubular nostrils, palmate feet, and long wings capable of rapid and pro-
longed flight. Diomedea, albatross; Lavus,* gulls; Sterna,* terns. Sec-
tion III. Urinatores. Birds with small wings, sometimes reduced to
flippers, and upright position owing to position of the legs far back. The
Aucip& (Alea impennis,* the great auk, exterminated in the nineteenth
century), which are northern and are related to the gulls, and the antarctic
IMPENNES (Aptenodytes — fig. 644,
penguin) agree in having palmate
feet, but otherwise differ greatly in
structure. Some of the COLYMBIDZ
(Urinator,* loons) have palmate
feet, others (Colymbus,* grebes) have
lobate feet. Section IV. STEGANO-
PODES, With totipalmate feet. Pele-
canus,* pelicans; Phalarocorax,*
cormorants; Phaethon,* tropic birds.

Sub Order IV. GRALLATORES.
The wading birds affect swampy
lands and the shores of the sea,
ponds and streams, their legs being
lengthened, chiefly by elongation of
the tarso-metatarsus, the feet semi-
palmate (fig. 648, a), and the feath- Sg
ers only on the upper parts, the ae
lower with horny plates, all feat-  ~ SS a
ures adapted to the wading life, Fic. 644.—Aptenodytes patagonica, penguin.

7 ¢ a nes (From Brehm.)

Correlated is the striking length of

neck and beak. These features have appeared in groups which are very
different in anatomical characters.

Section I. Ciconrrormes. Beak with a strong horny coat. <Ardea,*
herons; Ibis; Ciconia, storks ; Pheenicopterus,* flamingo. Section II.
GruirorMes. Beak always with soft skin at the base, often extending to
the tip. @rus,* cranes; Rallus,* rails; Otis, bustards, terrestrial. Section
III. CHARADRIFORMES. Allied to the auksand gulls. Scolopax,* woodcock ;
Charadrius,* plover.

 
616 CHORDATA.

Sub Order V. SCANSORES. The climbing birds are readily recog-
nized by their zygodactyle feet (fig. 648, y), in which two toes (2 and 3)
are directed forwards, the other two (Zz and 4) backwards. The forms
united under this head differ much in structure and their association does
not rest on blood-relationship.

Section I. CucuLiFoRMES. The PsITTAct, or parrots, are brightly colored
mostly tropical birds with short, high, compressed, and strongly bent beak
and fleshy tongue. But one species (Conwrus carolinensis *) in the United
States. Cacatua, Plictolophus, cockatoos; Melopsittucus, Psittacus, parrots,
CucuLt, bill slightly arched or straight; outer toe usually versatile;
Cuculus, Coccygus,* cuckoos. Section II. Picarr#. The woodpeckers
have a long, straight, conical beak and long, protrusible tongue; Picus,*
Nearly allied are the toucans (Rhamphastos) of the tropics.

Sub Order VI. PASSERES. This is by far the richest in species of all
the groups of birds. They are altrices of moderate size, with slender feath-
ered tarsi and strong, horny beak without cere. Of the three anterior
toes the two outer are either united or separated to the base (fig. 648, 6),
while the hind toe is at. a level with the rest. In some, which are usually
but not invariably noticeable for the powers of song of the males, there
are special muscles to the syrinx which are lacking in other birds, These
are called Oscines, in contrast to the other Passeres, the crying birds, or
Clamatores. These groups are further distinguished by a large, freely
movable hind toe in the Oscines, while in the Clamatores it is restricted
in its motions.

Section I. OSCINES. All our song birds belong here: FRINGILLIDS,
finches ; Passer domesticus,* English sparrow; Zowxia,* crossbills ; IcTER-
Dz; Icterus,* orioles; Dolichonya, bobolink; ALAUDIDH, Alauda,* sky-
lark; SYLVICOLIDZ, Dendreca,* Helminthophaga, warblers; TURDIDZ,
Turdus,* thrushes; Stala,* bluebirds; H1RUNDINIDZ, Hirwndo,* swallows;
TROGLODYTIDA, wrens; CoRVIDZ, Corvus,* crows; Cyanocitta,* jays.
The PAaRADISEIDA, or birds of paradise, with marked sexual dimorphism, are
closely related to the crows (fig. 15), Section II. CLAMATORES. Hereare
frequently included a few groups (COTINGID.®, TYRANNID.£) best developed
in South America and the lyre birds (MENURID#) of Australia. Earlier
other forms were regarded as allied, but now are separated as Cypselo-
morphe, or Coraciformes, and united with the owls and Picaria. CyYPSELE-
p&; Chetura,* chimney ‘swallow,’ with adherent feet (fig. 643, ©).
TROCHILIDZ, humming birds, best developed in tropical America; Tvochi-
dus.* CAPRIMULGIDE, night hawks; Antrostomus vociferus,* whippoor-
will. ALCEDINIDA, kingfishers, Ceryle.* BucERONTIDA, horn bills, tropical.

Sub Order VII. RAPTORES. The birds of prey are strong birds of
considerable size. They have the tarso-metatarsus feathered and four
strongly clawed toes of what is termed the raptatorial type (fig. 648, d).
The beak is strong, the upper half, strongly hooked at the tip, extending
over the lower. There are two groups recognized which probably are
not closely related,

Section I. FALCONIFORMES. Slender birds with close plumage aad
extraordinary sight; related structurally to the herons. CATHARTIDA,
IV. VERTEBRATA: MAMMALIA. 617

buzzards ; Cathartes aura,* turkey buzzard.  Panpiontpm®, Pandion
halietus,* fish hawk; FaLconip#®: Aquila,* Halietus,* eagles ; Buteo,*
buzzards; Falco,* faleons; Accipiter,* hawks. Section II. STRIGES,
owls; compact birds with loose, fluffy plumage, large eyes in a circle of
feathers; more closely related structurally to the Caprimulgid than to
the Falconiformes. Bwbv,* horned owls ; Scops,* screech owls; Stria,*
gray and brown owls ; Speotyto,* burrowing owls.

Class III. Mammalia.

The mammals occupy the highest place among the vertebrates,
and consequently in the animal kingdom; they also possess a
special interest for us, for man, in structure and development,
belongs to the group, although separated in intelligence from the
most highly organized of the members by a wide gap.

The most striking characteristics of the mammals are again
furnished by the skin. In fact one may, with Oken, call them
hair-animals, since hair is as diagnostic as feathers are for birds.
The hairs (fig. 645, H) are cuticular structures which are seated

    

5

OD
Loe TE, tee — aes
ey “ee 4

w LS

Fic. 645.—Section of skin of man. (From Wiedersheim.) Co, derma (corium); D,
oil gland; F, fat; G, blood-vessels; GP, vascular papilla; H, hair; V, nerves; VP,
nerve papilla; Sc, stratum corneum; SD, SD’, sweat gland and duct; SM,
stratum Malpighii.

on papille of the derma, and are nourished by blood-vessels in
these. The lower end, the root of the hair, lies in a pit in the
epidermis, the hair follicle, and is surrounded by a double envelope,
the epithelial root sheath, formed by an inpushing of the epidermis
and an outer connective-tissue follicular sheath. Small muscles
attached to the base of the larger hairs serve for their erection.
615 CHORDATA.

Since side branches are lacking, the structure of the hair is more
simple than that of feathers, and the forms fewer. Wool is char-
acterized by its spiral turns; then there is straight hair which, by
increase in size, forms the ‘ whiskers’ (vibrisse) on the upper lip
of many mammals, bristles (swine), and lastly the spines of hedge-
hogs and porcupines. In the pelts of many animals two kinds of
hair may occur, wool below and straight hair outside. Tistolog-
ically hair consists of cornified cells, often arranged in medullary
and cortical layers. On the outside there may be another layer
recalling the pseudocuticula of reptiles. In most mammals there
is a periodic shedding and renewal of the hair, the new hair aris-
ing from the old follicle (? from the old papilla). Ordinarily this
occurs only in spring. Besides hair some mammals have true
scales. Constant horny structures are the armatures of the tips of
the digits, which, according to form, are divided into claws (ungues),
hoofs (ungule), and nails (lamnz).

The old view that the hair, like feathers, corresponds to the scales of
reptiles has recently found both defenders and opponents, the latter think-
ing it probable that the hair has arisen from the nerve-end structures of
aquatic vertebrates. The claws, together with those of reptiles and birds,
must have come from horny scales, which indeed occur in many amphibia
as hollow cones capping the toes. The dorsal part of this scale, the claw
plate, becomes especially strong, its formation taking place at the base,
the root, from whence it is forced forward over the bed (in man the limit
of nail formation is shown by the lunule). The ventral part of the scale,
the subungua or solenhorn, is poorly developed in true claws because its
region is restricted by the arching of the claw plate in both directions, but
is more evident in hoofs, in which the plate is curved only horizontally.
In the horse it forms the ‘sole,’ lying between the frog and the hoof. It is
rudimentary or entirely lost in the nails of apes and man.

The skin of mammals is further characterized by its richness
in glands, of which, with few exceptions, there are two kinds,
sebaceous and sweat glands. The first are acinose glands, and usu-
ally open in the hair follicles, giving the hair the required oiliness
(fig. 645, D). The sweat glands, except in the monotremes, are
entirely independent of the hairs, and are simple tubes, coiled
at their deeper ends (S/), secreting a fluid sweat which is of great
value in the preservation of a constant temperature, its evapora-
tion cooling the body. Under the influence of sexuality the glands
in certain regions, and especially the sebaceous glands, develop
great activity and form considerable glandular pouches or pockets:
caudal and anal glands of many carnivores, hoof glands and sub-
orbital glands of ruminants, musk and castor glands of musk deer
IV. VERTEBRATA: MAMMALIA. 619

and beaver (fig. 652, a). More important than these are the modi-
fications of dermal glands into mammary or milk glands, which,
indeed are so characteristic that they have given rise to the name
mammalia. These are almost invariably sebaceous glands (in the
monotremes sweat glands) which empty in great numbers upon a
restricted area of the skin, which, except in monotremes, is elevated
into true nipple (fig. 646, A), or around which the adjacent skin
A

J. LO

Fie. 646.— A, true, B, false nipple. (After Gegenbaur.)
becomes elevated in tubular form (#) as in the cows. The mam-
me are always symmetrically arranged upon the ventral surface,
sometimes in the breast region, but more frequently in the inguinal
region. There are at least two, usually more (22 in Cenéetes). In
general the number corresponds to the maximal number of young
at a birth.

A dermal skeleton occurs in few species (e.g., the firm bony
plates of the armadillos); on the other hand the axial skeleton
shows many features not occurring elsewhere. In the skull many
ot the bones already referred to are evident only as centres of ossi-
fication, fusing early with their neighbors to form larger bones.
As the temporal bone shows, parts of diverse origin may fuse—parts
of the visceral skeleton and parts of the cranium; membrane and
cartilage bones—so that a sharp line between cranial and facial
portions cannot be drawn. So it becomes uecessary in describing
the skull not to follow exactly the model adopted so far, but to
take that of human anatomy.

In the hinder region of the skull is a large occipital bone (figs.
561, 562), jointed to the atlas by double occipital condyles, and
arising by the fusion of the four bones of the occipital region,
Besides it includes usually a membrane bone, the interparietal,
which occurs only in mammals. This is, strictly speaking, a
paired bone, arising in the angle between the parietal and the
supraoccipital and fusing with the latter. In front of it lie in the
roof of the cranium, as in other vertebrates, the parietals (fused
with the interparietals in many ruminants and rodents), the
trontals and nasals, the lachrymals being always associated with

 
620 CHORDATA.

them. In the floor of the cranium the sphenoid bone lies in front
of the basioccipital portion of the occipital. In many mammals
this consists of an anterior and a posterior portion throughout life;
in man this condition occurs at least in the embryo. Each of
these parts in development consists of three elements, the posterior
of the basisphenoid as the body, and the paired alisphenoids (great
wings); the anterior is similarly composed of the presphenoid and
the paired orbitosphenoids (lesser wings) (fig. 562, Spb, Ps, Als,
Ors). In front of the sphenoid lies the ethmoid, Zth, likewise
formed from three parts, the mesethmoid, which forms a partition
between the two nasal cavities, and the paired ectethmoids, which
form the lateral walls of the nasal cavities. These last have com-

 

Fia. 647.—Skull of pecans Tatusia. (After Parker, from Wiedersheim.) Cartilage
dotted, membrane and membrane bones lined. «, incus (quadrate): de, dentary;
fr, frontal; h, (above) membrane over anterior fontanelle, (below) hyoid bones;
im, premaxillary : ju, jugal (malar); kb, remnants of gill arch; la, lachrymal;
mk, Meckel’s cartilage; ma, maxillary; n, malleus (articulare); na, nasal: 0, oe
cipital cartilage; os, supraoccipital; pa, parietal; pe, petrosal ; sq, squamosal ;
stapes; ty, tympanic.

plicated folds on their inner surface, the superior and middle

turbinated bones, which support the olfactory membrane, thus

greatly increasing itssurface. With these is associated the os tur-
binale, a distinct bone, the inferior turbinated bone of human
anatomy.

The temporal bone, which is intercalated between the roof and
floor of the skull, can only be understood by its embryonic rela-
tions and its connexion with the first and second visceral arches
(fig. 647). Its centre is formed by the petrosal (pe), developed
in the walls of the otic capsule, to which, as elsewhere in the
vertebrates, are attached: (1) the cartilaginous jaw arches, the

quadrate (a), and the mandibular (and ms); (2) the cartilagious
IV. VERTEBRATA: MAMMALIA. 621

hyoid arch, the stapes (in part equalling the hyomandibular, sf),
and the hyoid proper () (compare with the visceral skeleton of
the selachian, fig. 588). To these are added the membrane bones,
the squamosal (sq), at the base of the quadrate, which increases as
the latter loses in size, and below the squamosal the tympanic (ty).
With ossification of the cartilaginous parts several centres form
the petrosum, which fuses with the squamosal, and frequently
with the tympanic, which in some forms enlarges to a conspicuous
bulla ossea. Petrosum and squamosal on the one side, tympanic
on the other, enclose a space, the tympanic cavity, into which the
upper parts of both visceral arches extend, ossifying into the ear
bones, the quadrate to the incus, the hyomandibular possibly to
stapes (fig. 577). :

The tympanic in uniting with the squamosal (forming Glaser’s
fissure) encroaches on the mandibular cartilage so that the upper
end (n), which is homologous with the articulare of other ver-
tebrates, is enclosed in the tympanic cavity and, along with a sec-
ond bone, the angulare, ossifies to form the malleus, while the
lower portion, Meckel’s cartilage proper (im/:), becomes pinched
off. Meckel’s cartilage gradually disappears; on the other hand
the surrounding membrane bone, the dentary (de) increases and
alone forms the lower jaw, which now forms a new articulation
with the squamosal. It will be noticed that the old articulation
was between cartilage bones, the new between membrane bones
developing around the cartilages. (There is, however, some evi-
dence to show that the mammalan lower jaw consists of several
bones, some of them preformed in cartilage, and that one of these
forms the articulation with the squamosal. )

The lower part of the hyoid arch, the hyoid, remains outside the
ear and often fuses with the petrosal. The upper end (styloid
process) may then become entirely separate from the lower, which
becomes attached to the copula (body of hyoid) as the anterior
horn, the connecting cartilage being reduced to a stylohyoid liga-
ment. In the hyoid of mammals there is also included a remnant
of a gill arch as the posterior horn.

As the quadrate, by its modification into the incus, becomes
strikingly reduced, the rest of the arch—vomer, palatine, and
pterygoid—is poorly developed in contrast to the large maxillary
bones. Premaxillaries and maxillaries (fused in man to a single
bone) form an important element in the face, and send backwards
and inwards palatine processes into the roof of the mouth. These
encroach upon the bones of the palatal series; the vomers of the
622 CHORDATA.

two sides are pressed together to a single bone lying vertically
entirely within the nasal partition; the palatine and pterygoid are
forced backwards. The palatines contribute to the hard palate,
the pterygoids only exceptionally (Cetacea, many edentates); the
latter usually lose their independence and fuse with the nearest
bone of the base of the cranium, the basisphenoid (more accurately
with a process of the basisphenoid, the lamina externa of the
pterygoid process, the pterygoid forming the lamina interna).
Thus the hinder sphenoid, hike the temporal, contains cranial and
visceral elements.

In the vertebral column the cervical and the rib-bearing
thoracic vertebra are always distinct, and the same, with the ex-
ception of the Cetacea and Sirenia, is true of lumbar, sacral, and
caudal vertebre. Of sacrals there is one in all embryos, and
throughout life in the marsupials, elsewhere from two to five,
rarely, as in edentates, as many as thirteen. The number of ver-
tebre in each group is rather restricted. Thus, except in Brady-
pus tridactylus (9), Cholepus hoffmanni and Manatus (6), the
number of cervicals is always seven.

Of the appendicular skeleton the girdles are most interesting.
The coracoid, which in mono-
tfemes reaches the sternum, is
reduced in all other mammals to
a small coracoid process of the
scapula. More rarely the clavicle
is lacking (rapid runners); in
the monotremes it extends to the
episternum (fig. 648, Cl, £p);
elsewhere it appears to articulate
with the sternum, in reality by
the intervention of interarticular

cartilages (once regarded as a
Fic. 648,—Sternum and shoulder girdle ~ .. > ( ey :
of Ornithorhynchus paradorus. (From rudimentary episternum, nhow
Wiedersheim.) Cl, clavicle; Co, Co’, v Ss ' ;
coracoid; Ep, episternum; @, glenoid called preclavie). In the pelvis
fossa for humerus: S, scapula; St,
manubrium sterni (anterior element all three elements are fused toa
of sternum). ‘ 2 é Ri
single os innominatum; pubis and
ischium unite ventrally with each other, enclosing between them
the obturator foramen (fig. 655). The pubes of the two sides
unite by a symphysis which can extend back to the ischia.
Since the mammals in general are distinguished from other
vertebrates by their intelligence, the brain is characterized by the
size of cerebrum and cerebellum (fig. 649). In contrast to birds

 
IV. VERTEBRATA: MAMMALIA, 623

and fishes, the cerebellum (/V) is differentiated into a median
vermis and lateral cerebellar hemispheres. In the cerebrum the
mantle comes first into consideration. Its frontal lobes grow for-
wards over the olfactory lobes, which consequently lie farther and
farther back on the lower surface. The temporal lobes extend
right and left over the optic lobes and down to the floor of the
cranium ; the occipital lobes cover successively the mid brain, cere-
bellum, and medulla oblongata. Since the greatest increase
of intelligence les within the mammals, the cerebra may be
arranged in an ascending series. In the monotremes, marsupials,
insectivora, and rodents (fig. 649, 4) the olfactory lobes are

   

V-
Fic. 649.—A, brain of rabbit (after Gegenbaur); B, of fish otter. C, of pavian
monkey (after Leuret and Gratiolet). J, cerebrum; I//, optic lobes; IV, cerebel-
lum; V, medulla oblongata; lo, olfactory lobes.
visible in front, usually the mid brain behind (///). In the lemurs,
carnivores (fig. 649, £), and ungulates the olfactory lobes are
completely, the cerebellum partly, covered. In man and the
anthropoid apes, on removing the roof of the skull, only the two
cerebral hemispheres are visible, all other parts being more or less
completely covered.

Further, it is to be noted that in the first group the surface of
the cerebrum is smooth, while in the others the cortex is increased
by infolding and the formation of convolutions (gyri and sulci)
which reach their greatest complication in the anthropoid apes
and especially in man. A consequence of the increase in size of
the brain is the great development of the connecting nerve tracts,
which become more and more prominent as parts of the brain.
Thus the two halves of the cerebrum are connected by a large
transverse tract, the corpus callosum; two solid cords, the crura
cerebri, run back from the cerebrum to the other parts, while a
transverse commissure, the pons Varolii, passes below, connecting
624 CHORDATA.

the two sides of the cerebellum. These connexions in the other
vertebrates are small, and even in the lower mammals, like mono-
tremes and marsupials, are but slightly developed.

The increase of cerebrum and cerebellum, which occurs chiefly in the
dorsal portion, has resulted in flexures in the axis of the brain, already in-
dicated in the reptiles, increased in the birds, and reaching their maximum
in the mammals. Instead of continuing in the course of the spinal cord,
the axis of the brain bends ventrally in the medullar region (cervical
flexure), then in the region of the pons again dorsally (pontal flexure), an
at the level of the optic lobes again ventrally (cephalic flexure). By its
increase in size the brain has influenced the skull in an interesting way ;
for, while even in birds the brain is almost entirely confined to the region
behind the eyes, in the higher mammals it has extended forward to the
olfactory region. Thus there comes an increase of the cranium at the ex-
pense of the face. The relative sizes of the two were adopted by Camper
as an index of intelligence, and were measured by ‘ Camper’s angle,’ a
method which has since undergone considerable improvements.

Of the sense organs the nose is characterized by three features.
An outer nose, supported by cartilage and often extended as a
proboscis, has been formed. Its cavity has been increased, since
by the formation of hard and soft palate a part of the primitive
mouth cavity has been included in it. Its upper portion, the
olfactory region, has been complicated by the formation of olfactory
folds, supported by the turbinated bones already referred to (p.
620). To increase the mucous surface there are extensions of the
nasal cavity, frontal, maxillary, and sphenoidal sinuses, into the
corresponding bones. The eye has the upper and lower lids, besides
the nictitating membrane in a more or less reduced condition.
The ear, except in monotremes, Cetacea, Sirenia, and some seals,
has a conch supported by cartilage, while the external auditory
meatus is always present. Internally the ear is much modified,
since the three bones, malleus, incus, and stapes (p. 544), occur
nowhere else, while the lagena has been greatly lengthened, coiled
into a spiral with two to four turns (figs. 80, 576), while inside
the wonderful organ of Corti has been developed.

Of digestive structures, the teeth—which are restricted to max-
illary, premaxillary, and dentary bones—need special mention,
because of the distinctions they afford from all other vertebrates,
and because of their importance in differentiating the various
orders. If we omit the monotremes, edentates, and whales, in
which there is marked degeneration in the dentition, there are
four particulars which show the dentition of mammals more de-
veloped than that of other vertebrates. (1) The number of teeth
IV. VERTEBRATA: MAMMALIA. 625

is constant for the species, usually for the genus, and often for
the family. As man normally has thirty-two teeth, so the dog
has forty-two, the anthropoid apes thirty-two, the platyrhine apes
thirty-six, etc. (2) The teeth are firmer. The body of dentine
is divided, by a slight constriction, into a crown covered with
enamel, and a root enveloped in cement (bony tissue). The roots
are placed in separate sockets (alveoli) in the jaws, and in those
cases where continuous growth is necessary the pulp persists and
the teeth, as in the incisors of rodents and the tusks of elephants
and pigs, grow indefinitely. (3) In consequence of their greater
firmness the teeth are not used up so fast and do not require rapid
replacement. There occurs only one change, in which the denti-
tion present at birth or developed soon after—the milk, or lacteal,
dentition or, better, first dentition—is replaced by the second or
permanent dentition (diphyodont mammals). In some cases
(monophyodont mammals) there is no change, the first dentition
being permanently retained (marsupials, perhaps toothed whales),
or the first dentition is more or less rudimentary (edentates, many
rodents, bats, seals, some insectivores). Besides the two typical
dentitions traces of a third or even of a fourth may occur. A
prelacteal dentition of calcified germs which are never functional
is best seen in marsupials, and is rare in placental mammals. A
dentition following the permanent one is outlined in many placen-
talia, and some of its teeth may exceptionally come into function.
(4) Among the teeth a division of labor has brought about change
of form (heterodont dentition). The teeth of the premaxillaries and
their antagonists in the lower jaw are
single-rooted and usually have more or
less a chisel shape, hence they are
called incisors even when, as in in-
sectivores, the crowns are needle-like
(fig. 661). Behind the incisors (in the
maxillary bone in the upper jaw) is
the canine tooth (fig. 650, c), which is
single-rooted and has usually a conical
crown (probably a modified premolar).
Following the canine come the mo-
lars, broad teeth mostly with two roots Fie. 650,—Permanent and milk

dentitions of the cat. (From
and tubercular crowns. Only the an- Boas.) c¢, canines; pp‘, pre-
: : s molars; m’, molar (the milk den-
terior ones appear in the milk den- tition darker and each letter
titi i ‘ preceded by d).
‘ition, while the others appear only in

the permanent dentition and are not replaced. On this develop-

 
626 CHORDATA.

mental basis the molars are divided into premolars (bicuspids of
dentists), which appear in both dentitions, and the true molars,
which occur only in the last.

From the foregoing it will be seen that every species of mam-
mal is characterized by its dentition, and these features may be
expressed by a short formula. It is only necessary to place the
number of each of the four kinds of teeth mentioned in their regular
order, those of the upper jaw separated from those of the lower by
a horizontal line, to express this. Since the two sides of the body
are symmetrical, only those of one side need be enumerated, and
in case that one kind be absent the deficiency is indicated by a
zero. The dental formula of man would thus be 31335 of the rein-
deer, in which in the upper jaw incisors and canines are absent,
00332 The different formule, by comparison, give us a funda-

3133°

mental formula from which they have been derived by reduction.
This was probably 4444.

The molars undergo, according to the food, the greatest modification
of form. Asa starting point the bunodont tooth may be taken which
occurs in omnivorous mammals and which has the crown with several
blunt projections or cones. With animal food (fig. 650, 657) the cones be-
come sharper and cutting (secodont dentition of carnivores and insec-
tivores), and when the cutting angle becomes very sharp, with a special
prominence on the inner side, it is spoken of as a flesh or carnasial tooth.
In vegetable feeders the cones become connected by crests (lophs) or are
half-moon-shaped (lophodont or selenodont). Since the cones and lophs
become in part worn away and the grooves between them are filled with
cement, there arise broad grinding surfaces strengthened by the harder and
more resistant enamel of the cones and lophs; this extends inwards as
folds from the outer enamel wall of the tooth ; the folds may become cut
off and form islands of enamel on the grinding surface (dentes complicati
of ungulates). When the folds extend in regular order from the outside
and inside and meet in the middle they form numerous successive leaves,
bound together by cement (compound teeth of elephants, fig. 667, and
many rodents).

Paleontological investigation, with which the more recent embryologi-
cal results are in accord, have shown that a great regularity prevails in
the formation of the cones of the molars. Triconodont and tritubercular
teeth are recognized, in which the three cones are either arranged in a
line or in a triangle, as well as multitubercular teeth with more numerous
cones irregularly arranged. The triconodont type develops farther by the
formation of secondary cones. The development of these occurs in dif-
ferent ways in molars and premolars. Since the latter are the more sim-
ple, their distinction from the molars does not rest alone upon the existence
of a milk dentition, but upon structure as well. This is important, because
it happens that there are premolars which are not replaced (marsupials,
IV. VERTEBRATA: MAMMALIA. 627

mary insectivores and rodents) and, on the other hand, beneath the molars
the anlagen of replacing teeth may be found, The latter fact shows
that the molars, strictly speaking, belong not to the permanent but to the
milk dentition. They are late in formation and are therefore parts of the
first dentition carried over into the second.

The mouth, which contains tongue and teeth, is separated from
the next division of the alimentary tract, the pharynx, by the
uvula. The pharynx narrows behind into the esophagus, the en-
trance of which into the stomach is marked by a constricting
cardia. At its other end the stomach has a similar constrictor,
the pylorus, separating it from the intestine. In the latter small
and large intestines (the latter consisting of colon and rectum)
are differentiated by the diameter of the lumen. The small in-
testine opens laterally into the colon and at the junction arises a
blind diverticulum, the ceeum, which is small in mammals with
animal food, but in herbivores (especially rodents) is always large
and forms a conspicuous part of the alimentary tract. The ver-
miform appendix (primates, rodents) is a narrower part of the
cecum. Three pairs of salivary glands empty into the mouth,
the liver and pancreas into the small intestine (duodenum).

Most important of respiratory peculiarities is the diaphragm,
which separates the body cavity into thoracic and abdominal cavi-
ties. This occurs only in its beginnings in other vertebrates
(perhaps even in Amphibia). In the thoracic cavity are the
esophagus, heart with its pericardium, and especially the trachea,
bronchi, and lungs; the remaining vegetative organs are in the
abdominal cavity. The diaphragm is a muscular dome, its con-
vex side towards the thoracic cavity; by contraction it flattens an
increases the size of the cavity, in consequence of which air is
drawn into the lungs (inspiration). On relaxation the lungs con-
tract from their own elasticity and force out a part of the air
(expiration). The intercostal muscles, which raise and lower the
framework of the chest, also play a part, as in birds. The respira-
tory ducts (fig. 579) begin with the larynx (with vocal cords),
which can be closed from the pharynx by the epiglottis; this is
followed by the trachea, which divides into right and left bronchi.
Each bronchus divides again and again, and the finest of these
divisions, the bronchioles, are continued as alveolar ducts to small
chambers, the infundibula, both these and the alveolar ducts being
lined with small respiratory pockets, the alveoli.

The heart, with two auricles and two ventricles, is completely
separated into systemic and pulmonary halves. In early embryonic
628 CHORDATA.

life the arterial trunk, which at first is simple, is divided into a
pulmonary artery, arising from the right half of the heart and
carrying venous blood, and an aorta ascendens, with arterial blood,
connected with the left half. In contrast with the reptiles, the
right aortic arch is entirely lost, the left persisting.

The urogenital system is of great importance in the separation
of the group into smaller divisions (fig. 651). In both sexes this
consists of practically the same parts in early embryonic life.
These are the early formed Wolffian body (W); the permanent
kidneys, which appear later and are not shown in the diagram;

 

Iiq. 651.

Fia. 651.—Diagram of embryonic mammalian urogenital system. (From Balfour,
after Thompson.) cl, cloaca; cp, genital process: go, genital cord; i, rectum ;
is, ridge for formation of labia or scrotum; m, Miillerian duct; ot, gonad; wg,
urogenital sinus; 17, Wolffian body; w, Wolffian duct; 3, ureter; 4, urinary blad-
der, 5, continuation of latter to allantois (urachus).

Fia. 652.—Urogenital system of male beaver. (From Blanchard.) a, castoreum
sacs ; b, openings of their ducts into preputial canal; c, tip of penis ; d, preputial
opening ; ¢, anal glands; f, their ducts; g, anus; h, base of tail; i, corpora caver-
nosa; k, Cowper’s glands; /, seminal vesicles; m, vasa deferentia; 7, testes; 0,
urinary bladder with ureters.

the urinary bladder (4), a part of the allantois which extends (2)
into the foetal appendages; the three ducts, the Millerian (m),
the Wolffian (wz), and the ureter (3). These ducts no longer
empty into the intestine, but into the allantoic structures, the
ureters into the base of the urinary bladder, the Wolffian and
IV. VERTEBRATA: MAMMALIA. 629

Millerian ducts into the urogenital sinus (wg), the lower continua-
tion of the bladder. The gonad is connected with the Wolffian
duct. In the anterior wall of the urogenital sinus is a mass of
highly vascular tissue (cp), from which and a surrounding fold the
external genitalia are developed. Since the urogenital sinus opens
from in front into the intestine, there is always a claoca (c/) in
the embryonic stages, which persists throughout life in the mono-
tremes, and to a considerable extent in the female marsupials: in
all other vertebrates it is divided by a partition, the perineum,
into a urogenital opening in front and an anal opening behind.
From this indifferent condition the male and female apparatus
are derived, the structures being closely similar in most males (fig.
652). The Miillerian duct vanishes, while the Wolffian duct be-
comes the vas deferens and its accessories, serving as a canal for
the genital products, while the external genitals arise from the
other parts mentioned, these forming an intromittent organ
(penis). In the female the Wolffian body and duct degenerate,
the Millerian ducts become the reproductive canals. The modi-
fications of these become of great systematic importance. In the
monotremes both ducts open separately and become differentiated
into two parts (fig. 653, A), anterior oviducts with wide openings

 

Fie. 653.—Female genitalia of (A) Echidna aculeata; (B) of Didelphys dorsigera; (C)
Phascolomys wombat. (B and C, after Wiedersheim.) cl, cloaca; d, rectum; Hh,
urinary bladder; n, kidney; 0, ovary; ed, oviduct; pu, month of ureters; su, uro-
genital sinus; ¢, ostium abdominale tube; u, uterus; uw’, opening into vagina;
ur, ureter; v, vagina; vb, vaginal blind sac.

into the body cavity (od, ¢) and the uterus (w). The ureters open
into the sinus (and not into the bladder) between the uterine
openings. In the marsupials (B and C) there are three divisions,
oviduct, uterus, and vagina; besides, the two Miilerian ducts may
approach, near the uterus (£), or fuse in this region (C’) in some
630 CHORDATA.

species, forming an unpaired blind sac (vb), which may even open
into the urogenital sinus as a third vagina. This partial fusion of
the vagine of the marsupials is completed in the placental mam-
mals, the single vagina and the sinus forming a single canal (fig.
654). Here the uterine portions may remain distinct (uterus

A B cr

Fie. 654.—A, uterus duplex; B, uterus bicornis; C, uterus simplex. (From Gegen-
baur.) od, oviduct; u, uterus; v, vagina.

 

duplex of rodents, A), or they may fuse partially (uterus bicornis
ot insectivores, whales, ungulates, and carnivores, 2), or they may
be completely fused (uterus simplex of apes and man, C’.

Thus there are three different types of the female genitalia, in
which the vagina is not differentiated (Ornithodelphia), or is
double (Marsupialia), or is single and unpaired (Monodelphia).
To these correspond three types of development. The Ornitho-
delphia are oviparous, the others viviparous, but are distinguished
by the duration of pregnancy. The eggs of the viviparous forms
are so small (about .01 inch) that they have a total, nearly equal
segmentation. Such eggs require nourishment from the mother
in order to produce an animal with the complicated structure of a
mammal. Since in the Didelphia the uterine nourishment is
usually very incomplete, the period of pregnancy is very short,
in comparison with the Monodelphia, in which a placenta, a com-
pleated apparatus for the nourishment of the young, appears;
hence the marsupials, with their small imperfectly formed young,
are often called Aplacentalia; the Monodelphia, Placentalia.

All mammals care for the young, this being chiefly or wholly done by
the mother, who not only supplies them with milk but protects them in
warm if rude nests. Most mammals are monogamous, some polygamous,
while in others there is no permanent association of the sexes. The body
temperature is constant and ranges from 36° to 41° C. (98° to 106° F.); in
Echidna it is only 26° to 84° C. (79° to 83° F.). In most, continual feeding
is necessary for existence; from this rule there are a few exceptions, like
the bears, marmots, badgers, ete., which hibernate during the winter,
taking no food. At this time there is a fall in the temperature due to the
diminished metabolism.
LV. VERTEBRATA: MAMMALIA, MONOTREMATA, 631

Sub Class I. Monotremata (Ornithodelphia, Prototheria).

A few mammals, confined to Australia and New Guinea,
divided among the genera Echidna, Proechidna, and Ornitho-
rhynchus, are the only living representatives of the group. They
are distinguished from all other mammals by laying eggs about
half an inch long, rich in yolk and with soft shells. These undergo
in the uterus a discoidal (meroblastic) segmentation and are then
incubated by Ornithorhynchus in a nest, by Lehidna in a tempo-
rary pouch (marsupium) on the ventral surface of the body. On
hatching the young are nourished by the secretion of enormously
enlarged sweat glands, which form two large masses to the right
and left of the mid-ventral surface, and which must not be con-
founded with the milk glands (sebaceous) of other mammals.
Each opens on a special region of the ventral surface, which is
slit-like in Ornithorhynchus, a flattened pocket in the others.

Other distinctions from other mammals, which are also points
of resemblance to reptiles and birds, are the strong development
of the episternum and the extension of the coracoid to the ster-
num (fig. 648), the termination of the ureters in the urogenital
sinus and not into the fundus of the bladder (fig. 653), the exist-
ence of a cloaca in both sexes, and the specifically bird-like char-
acter of the female sexual organs, in which the large left ovary is
alone functional, and uterus and
vagina are not differentiated. But
with all this it must not be forgot-
ten that the monotremes have the
hair, the skull, the urogenital sinus
of true mammals, and in the pres-
ence of marsupial bones (fig. 655,
Om) show a close relationship with
the marsupials. The upper end of
the hyoid is connected directly or by
a ligament with the cartilaginous
auditory opening, while a scarcely a [
visible external ear occurs. The jaws oe pee ;
are toothless and enclosed in horny '¢i pene puradorus, (From Wie:

: . dersheim.) Fo, obturator fora-
sheaths, yet in the young of Orni- men; 11, ilivmj Js, ischium; Om,
thorhynchus there are in each jaw  ™2Tsupial bones P. os pubis
three pairs of multitubercular molars, which are later replaced
by four broad horny plates.

   
632 CHORDATA,

EcHIDNID#. The spiny ant-eaters have the body covered with bristles,
snout with a worm-shaped tongue used in catching insects; Echidna
aculeata of Australia, feet five-toed, with digging claws; Proechidna
(Acanthoglossus) of New Guinea, three-toed. ORNITHORHYNCHIDZ. The
duckbills are toothless, close-haired animals with horny jaws which
resemble those of a duck; the five-toed feet with a swimming web especially
well developed on the fore feet. Ornithorhynchus paradoxus of Australia.

 

Fie. 656.—Ornithorhynchus paradozus, duckbill. (From Schmarda.)
The male has a spine with a gland on the hind feet which fits in a corre-
sponding pit on the thigh of the female and apparently plays a réle in
copulation,

The oldest fossil mammals are possibly to be regarded as belonging to
the monotremes, These appear in the trias and form a group, MULTITU-
BERCULATA (Allotheria), which is but imperfectly known (TZritylodon,
Microlestes, Plagiaulax), Their multitubercular teeth resemble the tempo-
rary ones of Ornithorhynchus, while there are indications that the cora-
coid existed as a distinct bone. Less certain are the PRoTOoDONTA (Droma-
thertum, Microconodon) of the American Jurassic, of which only the lower
jaws are known.

Sub Class IT. Marsupitalia (Didelphia).

These, like the remaining mammals, are viviparous. They
have small eggs which undergo a total segmentation in most species,
and develop in the maternal uterus, being nourished by a secre-
tion from its walls. In a few species there is a placenta which, in
Perameles, is allantoic in origin, in Dasyurus viverrinus possibly
also from the yolk sac. In most species there is no placenta. In
all there is insufficient nourishment and the young are born in a very
immature condition. They are therefore carried a long time by
the mother in the marsupium, a pouch formed by a fold of skin
on the posterior ventral surface, into which the nipples open.
The ventral surface is supported by the marsupial bones, slender
rods articulated, right and left, at the pubic symphysis. Other
characteristics of the marsupial skeleton are the inflected posterior
angle of the lower jaw (fig. 657, @) and the rudimentary replace-
ment of teeth. The milk teeth and molars (first dentition) are
as a whole retained, only premolar 3 being replaced by another
IV. VERTEBRATA: MAMMALIA, MARSUPIALIA. 6338

tooth; but it is in question whether this belongs to the second den-
tition or is a belated member of the first. The sexual apparatus
has already been described (p. 630).

Marsupials are known from the secondary (Jurassic) and tertiary strata
of Europe and both Americas. They were apparently then spread over
the whole earth, but were crowded out by the placental mammals and
persisted only as remnants (Cenolestes and the opossums) in America, but
as a richly developed fauna in Australia. In the latter region they con-

 

Fie. 657.—Lower jaw of Thylacinus cynocephalus (from Flower), showing (a) the
inflected angle characceristic of marsupials; cd, articular surface.

tinued because here, on account of the early separation of this continent
from the rest of the world, no development of Placentalia occurred. The
placentals are entirely lacking in Australia with the exception of those
introduced by man and such (mice, bats, seals) as easily pass from island
to island. In their present habitat, in adaptation to similar conditions
they have undergone a development analogous to that of the placentals in
other parts of the earth, so that they present groups parallel with the
carnivores, rodents, insectivores, and ungulates.
Order I. Polyprotodonta (Zoophaga).

Many marsupials, among them the oldest, have a dentition
adapted to animal food. They have numerous incisors (up to five
in each half-jaw), strong canines, and sharp-pointed molars (fig.
657). Some in teeth, as well as in body form, resemble the Insec-
tivora, others the carnivores.

The Dasyuride are carnivorous: Dasyurus ; Sarcophilus ursinus, the
Tasmanian ‘devil,’ dangerous to larger mammals; Thylacinws, pouched
wolf. The PERAMELIDA are insectivorous ; Perameles, bandicoot. The
DIDELPHYID, or opossums, which are confined to America (chiefly South)
are more carnivorous in dentition and recall the apes with their opposable
thumb. Didelphys virginiana.*

Order II. Diprotodonta (Phytophaga).

The herbivorous habits are correlated with the degeneration
of canines, which usually are lacking in the lower jaw and are at
least very small in the upper. There are also only two incisors,
of large size, in the lower jaw, while the middle two of the upper
are much larger than the one or two lateral which may be present.

The PHAscoLOMYID are the rodents of the marsupials with one chisel-
like incisor in each half of each jaw. Phascalomys, wombat. The Macro-
634 CHORDATA.

PODIDA, or kangaroos, resemble the ungulates in their large herds on the
grassy places. The fore legs being very small, the animals leap with the
strong hind legs and tail, Jacropus giganteus. The PHALANGISTIDE
have very variable teeth. They resemble in habits the squirrels, Petawrus
having the same parachute folds as does our flying squirrel. The Dipro-
todonta contain many fossil forms in Australia and a few in South
America. Some of the Australian fossils were very large, Diprotodon
australis larger than a rhinoceros,

Sub Class [1I. Placentalia (Monodelphia).

The first reason for associating the mammals of the Old World
and most of those of the New together as Placentalia is an embry-
ological one, the presence of a placenta. When serosa, amnion,
and allantois (p. 553) have developed in the embryo, the vessels
of the allantois spread out beneath the serosa and form with this
the chorion, which sends small processes or villi into the now
highly vascular mucous mem-
brane of the uterus in order to
obtain nourishment somewhat as
a tree obtains food by its roots.
These villi may be distributed
over the greater part of the sur-
face (fig. 658), producing the
chorion frondosum, or diffuse
placenta, which occurs in Cetacea,
perissodactyles and many artio-
dactyles (swine). On the other
hand the villi may be restricted
Firs. 658.—Diagram of mammalian em- £9 “contain BINGE: Pecos ee

bryo with chorion frondosum; ah, amni- strong there. This gives rise to

oe AGN atime emg en eae cotyledonary, discoidal, or zonary

sac 7, space (extraembryonic celomy, Placente. To these correspond

between chorion and amnion; sh, serosa. portions of the uterine lining
which are distinguished from the rest by becoming extremely vas-
cular (uterine placenta). The cotyledonary placenta (fig. 659)
consists of many small placentar patches, the cotyledons (most
ruminants). In the zonary placenta the villous area takes the
shape of a girdle or barrel (carnivores, Sirenia), while the discoidal
(other mammals) is, as its name indicates, disc-like.

Besides the placental structures the higher mammals are char-
acterized by the disappearance of the cloaca, the unpaired vagina,
and absence of marsupial bones and inflected angle of the jaw. The

 
IV. VERTEBRATA: MAMMALIA, EDENTATA. 635

dentition, on the other hand, has undergone a progressive, diver-
gent development, so that the distinctions are much more pro-
nounced than in the marsupials, and hence of importance in
differentiating the orders.

Order I. Edentata.

A few families, poor in species, are united under the name
Edentata because teeth are absent or, as is more usually the case,
are markedly degenerate. Persistent functional incisors are lack-

 

Fie. 659.—Cotyledonary placenta and embryo of cow. (From Balfour, after Colin.)
C!, cotyledons of uterine, C*, of fcetal placenta; Ch, chorion; U, uterus; V,
vagina.

ing, canines but rarely occur (Bradypus); molars may be present,

sometimes in great numbers (Priodon gigas, the large armadillo, has

about a hundred molars), but they are poorly rooted, prismatic,
without enamel, and usually monophyodont. Since the aardvark

(Orycteropus) and Tatusia have a heterodont milk dentition in

embryonic life in which incisors occur, and fossil edentates

(Entelops) with complete dentition are known, the absence of a

replacement of the teeth is to be explained by degeneration, which

may affect other parts, and is to a certain extent the reason for the
low position accorded these forms. The great number of sacral
vertebra is striking, being as many as thirteen in some armadillos.

The placenta is very variable, being diffuse, discoidal, or zonary in

different species. The group is essentially tropical, but one species
636 CHORDATA.

entering the United States. The oldest fossils occur in the Santa
Cruz beds of Patagonia (eocene or oligocene).

Sub Order I. NOMARTHRA. Old World edentates. FODIENTIA.
Animals with strong digging claws, long tail, and long, vermiform, sticky
tongue used in catching ants and other insects. Orycteropus capensis,
aardvark, with long snout, sparse bristly hair, five small molars, and
rudimentary milk dentition. SQUAMATA. Toothless, body covered with
overlapping scales. Janis, pangolins of Asia and Africa (fig. 660).

 

 

Fic. 660.—Manis longicaudata, pangolin. (From Monteiro.)

Sub Order II. XENARTHRA. Edentates of the New World. VER-
MILINGUIA, ant eaters. Resemble manids in toothless jaws, long ant-
catching tongues, and strong digging claws, but are hairy and lack scales.
Myrmecophaga. TARDIGRADA, sloths. Hairy, head short, rudimentary
tail, and few teeth, long strong claws by which they hang back down-
wards from limbs of trees. Bradypus tridactylus, nine cervical verte-
bree ; Cholepus, six cervicals. Fossils allied are Megatherium, as large
as an elephant, Mylodon, Megalonyx, these two extending north to Penn-
sylvania. LORICATA, armadillos. Body with armor of bony plates,
molars numerous ; insectivorous. In the extinct GLYPTODONTID# of South
America the plates fused to a continuous armor. One species twelve feet
long. One species may have occurred in Europe. Dasypopip#; dermal
armor in three or more movable transverse plates; nocturnal. Genera
based upon the number of bands: Dasypus, Tenurus ; Tacusia novem-
cincta * enters United States.

Order II. Insectivora.

These primitive forms have a complete dentition, all the differ-
ent kinds of teeth being present, although they vary in number.
The roots are developed early and consequently the teeth are small.
Since they end with sharp cusps, adapted for eating insects, they
resemble the carnivores, from which they may be distinguished by
IV. VERTEBRATA: MAMMALIA, CHIROPTERA. 637

the rudimentary condition or occasional absence of the canines
(Talpa $}+43. many shrews $123), There is great variability in
the matter of replacement of teeth; in the shrews, for instance,
the milk dentition is suppressed and
the second only is functional, while in
the hedgehog one incisor and one pre- {
molar in each jaw, a second premolar
and the canine of the lower jaw func- = a
tion in both dentitions. In many re- Fic. 661.—Skull of Sorex, (From
: ‘ Ludwig-Leunis.)

spects the insectivores resemble the

rodents: a clavicle is present; there are usually five toes furnished
with claws; there is a uterus bicornis, often divided its whole
length, and discoidal placenta.

Aside from the proboscis-like snout the insectivores resemble the
rodents in appearance, forming parallel groups to those of that order.
The ERINACID#, or hedgehogs, of the Old World are spined like the porcu-
pines ; the Soricip#, or shrews (Sorea,* Blarina*), are mouse-like, as are
the allied TaLpmz, or moles (Scalops,* Condylura,* star-nosed mole),
which burrow in the earth and have the eyes more or less rudimentary.
Some authors place here Galeopithecus of the East Indies, which has a
similar membrane and similar sailing powers as the flying squirrels. It
also presents resemblance to the bats and to the lemurs. The earliest
known insectivores date from the eocene.

  

 

Fia. 662.Skeleton of bat. (After Brehm.)

Order III. Chiroptera.
The bats are the only mammals which actually fly, and this at
once characterizes them. The flying membrane (patagium) is a
thin fold of skin, richly supplied with nerves, which begins at the
638 CHORDATA.

tail, includes the lower extremities to the foot, and extends thence
to the fingers, leaving the thumb free. Fingers 2-5 are enormously
elongated and support the membrane. Since flight requires
strong muscles, the sternum develops a small keel, recalling that
of birds, for the attachment of the large pectoral muscle. In con-
nexion with the flying powers the clavicle is strong. The patagium
is the seat of a very acute tactile sense, by means of which blinded
bats can fly among all kinds of obstacles without disturbing them.
The enormous ear conchs and a noticeable nose leaf, widely dis-
tributed through the group, also have marked tactile powers. In
the pectoral position of the mammary glands and in the discoidal
placenta these animals resemble the primates. In temperate
regions bats hibernate during the winter. The dentition is vari-
able, often 2133, Fossils occur in the eocene.

Sub Order I. MICROCHIROPTERA, with insectivorous dentition,
only the thumb of the fore limbs clawed. VESPERTILIONIDA, tail long, no
nose leaf : Vesperugo,* Atalapha.* PHYLLOSTOMID#, with nose leaf, trop-
ical America ; Desmodus, the blood-sucking or vampyre bat.

Sub Order Il. MACHROCHIROPTERA (Frugivora), with smooth-
crowned molars, claws on thumb and first two fingers. Includes the flying
foxes, Pteropus, of the East Indies.

Order IV. Rodentia.

The rodents unite great similarity in appearance with a char-
acteristic dentition. The canines are absent, and the molars are
separated by a large gap (diastema) from the incisors (fig. 663).
The latter are strong, chisel-like,
have persistent pulps and grow at
the lower end as they are worn
away at the cutting edge. Since
only the front surface has enamel,
wear keeps them constantly sharp.
Usually there is but a single in-
cisor, and only in the Duplici-
dentata is a second present in the
The molars are cus-

f kL

 

car upper jaw.
Fia. 663.—Skull of porcupine. (From

Schmarda.) f, frontal; im, premaxil-
lary; k, temporal fossa continuous in
front with orbit; 0, infraorbital fora-
men, enormous on account of the por-
tion of the masseter muscle which
passes through it.

varying between 1008

3 and

 

T002°
angle of the jaw like that of marsupials.

pidate or have enamel folds and
frequently continue to grow
throughout life. Their number is
frequently reduced, the: formule

Many species have an inflected
The infraorbital canal

is a striking feature in Muride and Hystricide (fig. 663, 0), a
IV. VERTEBRATA: MAMMALIA, UNGULATA. 639

large opening in front of the orbit in which a part of the masseter
muscle is attached.

The rodents are distinguished from the ungulates, which, like
them, are herbivorons, by the usually smaller size, the possession
of claws, five toes (sometimes reduced to three), the occurrence
usually of a clavicle, and a discoid placenta. The mamme are
inguinal in position and, corresponding to the great fertility, are
very numerous. The occurrence of glands with a strong-smelling
secretion, which open near the anus, is common. About nine
hundred living species are known, occurring in all regions except
the Australian. The order appears in the eocene.

Sub Order I. DUPLICIDENTATA (Lagomorpha), two upper incisors,
includes the hares, Zepus,* and the picas, Lagomys.*

Sub Order II. SCIUROMORPHA. The squirrels, ScluriD&, are distin-
guished by the soft fur and bushy tail. Setiwrus,* squirrels ; Cynomys,*
prairie dogs ; Scdwropterus,* flying squirrels. The CAsTorID#& have soft fur
and sealy tail. Castor fiber,* beaver of Europe and America.

Sub Order II]. MYOMORPHA, rats and mice. Mus musculus,*
common mouse; Jfus rattus,* house rat, once abundant but now replaced
by the gray rat, Jf decemanus,* an immigrant from Asia. White rats are
albinos of M. rattus. Fiber zibethicus,* musk rat; Arvicola,* field mice.

Sub Order III]. HYSTRICOMORPHA. The porcupines (HysTRICID#)
have spines; the Old World forms, Hystriz, are terrestrial, ours (Hrethyzon)
arboreal. The Caviip& of South America have hoof-like claws. Cavia
cobaya, guinea pig. Hydrocherus, capybara, the largest existing rodent.

Order V. Ungulata.

Under the heading of Ungulata, or hoofed animals, are bere
included two groups of living animals in which the body weight
is supported on hoofs on the tips of the toes, and which are sharply
marked off from other forms. If, however, the fossils are in-
cluded, the limits of the group must be extended so that it includes
the elephants and conies of the existing fauna as well as several
extinct forms, for these so interlock and intergrade that sharp
lines cannot be drawn.

The ungulates, which arise from common ancestors, the Con-
dylarthra, the representatives of which occur in the eocene of
America (Phenacodon), are preeminently herbivorous; the canines
are rarely well developed, the molars numerous and adapted to
grinding the food, more or less flattened and frequently with
folded enamel. The mamme are inguinal, the uterus bicornuate,
and the placenta either diffuse or (most ruminants) cotyledonary
(fig. 659). The legs are exclusively locomotor structures and, to
640 CHORDATA.

permit freer motion, the clavicles are absent; the feet touch but
the tips of the toes, enclosed in hoofs, to the ground (unguligrade).
Since the metacarpals and metatarsals are greatly elongate, the
wrist and ankle are raised high from the ground so that they are
frequently confounded with elbow and knee. With this exclu-
sively supporting character of the limbs there is the same tendency
to reduction and fusion of bones which was noticed in birds (p.
606). There is a constant increase in the development of radius
and tibia to the chief supports of the body, the fibula becoming
rudimentary, the ulna being developed sometimes throughout its
whole extent, sometimes only in its upper part, which serves for the
attachment of muscles (olecranon), and is more or less fused with
the radius. The same tendency to simplification prevails in the
feet, but is expressed differently in the odd-toed (perissodactyle)

and even-toed (artiodactyle) forms. In the Perissodactyla the
A B c D E F

 

Fia. 664.—Fore feet of ungulates. (After Flower.) 4-C, perissodactyle; D-F, artio-
dactyle. A, tapir; B, rhinoceros; C, horse; D, pig: E, deer; F, camel. __c, trique-
trum (ulnare); 1, lunatum (intermedium); m, capitatum; m?-m5, rudiments of

metacarpals II and V; p, pisiforme; R, radius; s, seaphoid (radiale); td, trapezoid;
tm, trapezium; U, ulna; u, hamatum; II-V, digits.

axis of pressure passes through the middle toe (fig. 664, 4-C,
III), while the other toes disappear symmetrically around this.
Since the first toe is early lost, toe V is next to disappear (2), and
then toes IT and IV (C), so that at last there remain only the
skeleton and hoof of the middle toe (horse), the rudiments of toes
IJ and IV persisting as the small splint bones.

In the Artiodactyla the axis of pressure falls between toes HI
and IV (fig. 664, D), which both unite in supporting the body
and are equally developed and frequently fuse, at least so far as
the metacarpals are concerned (£, #’). The figures D-F show

 
IV. VERTEBRATA: MAMMALIA, UNGULATA. 641

how the other digits disappear, digit I being lost still earlier.
Since the weight of the body rests more upon the hind legs than
upon the front ones, the former are the first to become modified.
Since we are able, by using abundant paleontological material, to
follow in detail the lines of descent of both artiodacvyles and peris-
sodactyles, the conclusion is certain that these form diverging
series, distinct from the beginning. In each series most of the
common characters enumerated above have been independently
acquired so that the uniformity in appearance of the various
groups of ungulates is in great part the result of convergence.
The discussion of the fossils will be given under a separate head.

Sub Order J. PERISSODACTYLA (Solidungula). The dentition is
peculiar in having molars and premolars (with more or less pronounced
enamel folds) of equal size; a second character is the predominant devel-
opment of the middle toe, the others in the three existing families reduced
to different degrees. Tapirip2, fore feet four-toed, hind feet three-toed;
teeth 2148; nose elongate into a proboscis. Tapirus, tapirs, tropical Amer-
ica and India. RHINOCEROTIDA, three toes on all feet, teeth 3943; one or
two horns on the nasal bones, these without skeleton; skin thick, hairless,
hence these were formerly united with elephants as Pachydermata.
Rhinoceros, a single horn, India; Ceratorhinus (Asia), Atelodus (Africa),
have two horns. Equip, a single functional toe, toes II and IV forming
splint bones (fig. 664, ¢); teeth 2132; Zgz2s caballus,* horse, a native of
Asia; E. asinus, ass; E. zebra, Hybrids between jackass and mare are
called mules; between stallion and she-ass, hinnies.

Sub Order II. ARTIODACTYLA. Besides the features of the feet,
these forms have the premolars, three or four in number, smaller than the
molars. The species are much more numerous than the perissodactyles
and may be divided into three sections. Section I, NON-RUMINANTIA
(Bunodontia); omnivorous and have correspondingly a bunodont dentition,

ae the canines frequently developed into tusks; the stomach is

usually simple, but is occasionally divided into three chambers (Dicotyles,
Hippopotamus), although rumination does not occur. The leg skeleton is
little modified (fig. 664, D), ulna and fibula not being reduced, and meta-
carpals and metatarsals separate. HIPPOPOTAMID®, all four toes reach the
ground; skin thick (‘ pachyderm’), body heavy; living species all African,
Hippopotamus. Suip#; two functional toes, skin with bristles, snout
proboscis-like. Sus scrofa, swine; Dicotyles,* peccaries of warmer
America.

Section II]. RUMINANTIA (Pecora); teeth and stomach are adapted to
the exclusively herbivorous diet. The stomach (fig. 665) is divided into
two portions, each again subdivided. The first of these, the rumen, or
paunch (ru), receives the food as it is eaten; then at a time of quiet it is
regurgitated into the mouth and ground by the molars (‘ chewing the cud’).
It then passes back, this time into the secoud division, the honeycomb, or
642 CHORDATA.

reticulum (re), thence to the manyplies or omasum (0), and lastly to the
abomasum, or true stomach (a). Usually not only the canines but the in-
cisors of the upper jaw are degenerate, while the incisors of the lower jaw
are strong and the canines have taken the form and position of incisors.
The molars are selenodont (have crescent-shaped cusps). With few excep-
tions they are of large size and many bear horns on the frontal bones. These
are larger in the males and may occur exclusively in that sex. In the sim-
plest case (giraffes) these are cones of horn free from the frontals and coy-
ered with skin. In others (Cavicornia) the horn cores fuse secondarily with

 

Fia. 665.—Stomach of sheep. (After Carus and Otto.) a, abomasum (true stomach),
o, omasum (manyplies) ; 7e, reticulum (honeycomb); 7u, rumen (paunch).

the frontals and are covered with a firm sheath of horn. Lastly, the horns
are outgrowths of the frontal bone, in which usually the outer coats are
lost and only the bone projects freely (antlers). These are shed yearly,
the new antler which takes its place being larger and consisting of a larger
number of branches or tines, thus constituting an index of age (Cervicornia).
CAMELOPARDALID& (Devexa), giraffes, long-legged forms (two genera) from
Africa with persistent horns; teeth 9938, Giraffa. CERVIDS, deer, with
deciduous horns in the male. Cervus,* common deer; Alces,* moose;
Rangifer,* reindeer; Moscnip&, horns lacking, males with enlarged upper
canines and with a musk gland (the source of the familiar perfume) on the
ventral surface; Moschus, central Asia. The TRAGULID, primitive Asiatic
and African forms, includes the chevrotain, Tragulus javanicus, the small-
est living ungulate. The CAVICORNIA include a large number of forms,
some of great economic importance; teeth $988. Bovipas: Bos taurus,
domestic cattle, probably descended from three distinct stocks (B. primi-
genius, the aurochs, B. longifrons and B. frontosus); Bison,* including
B. evropeus, the bison proper, and B. americanus* our ‘ buffalo,’ so near
extinction; Bubalus, the true buffalo of the Old World. Ovipx: Ovis aries,
sheep; O. montana,* big horn; Capra hircus, goat; Ovibos moschatus,*
musk ox, ANTILOPIDAS: including a host of Old World forms (Antilope,
Gazella, Rupicapra tragus, the chamois, etc.) and Antilocapra americana,*
IV. VERTEBRATA: MAMMALIA, PROBOSCIPIA. 6438

the prong horn, which sheds its horns, and Hoploceras montanus,* the
Rocky mountain sheep,

Section III. TYLOPODA, stomach without manyplies, no frontal
horns, diffuse placenta. Camelus, the camels of the Old World; C. drome-
darius, one hump; C. bactrianus, two humps. Auchenia lama, A. alpaca
of South America.

Paleontology of the Ungulata.

Extensive paleontological material, especially from the tertiary rocks
of our western states, has cleared up many lines of ungulate descent
and has rendered it probable that the CONDYLARTHRA of the eocene,
with five-toed plantigrade feet, well-developed ulna and fibula, and an
omnivorous dentition, formed the stoek from which descended the artio-
dactyles and perissodactyles, and possibly carnivores and primates as well,
the ungulate line extending through the Amblypoda. From one group of
these (the PHENACODONTID#) the lines of rhinoceros and tapir have come,
and in an almost complete series we know the ancestry of the horse,
Hyracotherium (EHohippus) and Orohippus of the eocene had the fore feet
four-toed (fig. 666, 2); Paleotherium and Mesohippus (2) of the lower

1 2 3 4 6 6

vg wy

   

i

 

oe
Fic. 666.—Evolution of fore foot of horse. (From Wiedersheim.) 1, Orohippus

(eocene): 2, Mesohippus (lower miocene); 3, Miohippus (miocene), 4, Protohippus

(upper pliocene); 5, Pliohippus (pleistocene); 6, Equus.
miocene and Miohippus of the later miocene were three-toed, while Mery-
hippus and Hipparion (Pliohippus, 4) of the pliocene were near the horse
in tooth structure. The single-toed horses appeared in the pleistocene
with Pliohippus (5) and then Hgwus itself (6). It is a peculiar fact that
the horse entirely died out in America, although the chief part of its his-
tory was enacted here.

The AMBLYPODA, mentioned above, were semi-plantigrade penta-
dactyle forms, appearing in the lowest eocene, and reaching, in Uinta-
therium (Dinocerus) an elephantine size. The TOXODONTIA of the
South American tertiaries combined perissodactyle, rodent, hyracoid, and
proboscidian features, while the TILLODONTIA of the eocene recall both
carnivores and rodents.

Order VI. Proboscidia.

The elephants and their allies, with their hoofs and herbivorous
dentition, are closely related to the ungulates. They are charac-
terized by their thick skin (*pachyderm’), the large, massive,
five-toed legs, and especially by the nose drawn out into a
644 CHORDATA.

long proboscis with a finger-like process at the tip, lastly by
the dentition. Canines are entirely lacking, but the incisors of
the upper jaw have pulps and therefore continue to grow
throughout life, forming the well-known tusks. In the living
elephants there are but a single pair of tusks, but in some extinct
Mastodons there were a second smaller pair in the lower jaw, while
in Dinotherium only the lower in-
cisors were developed, these pro-
jecting downwards. The molars
(in Mastodon and Dinotherium
with normal replacement and
cusps) consist of numerous plates
of enamel and dentine united
Fig obt—Inede, of ett lower jave ot NY cement, and madergo a Mater

(After Owen.) 1, functional molar; 2, displacement. Of the three large

Tes SUCerssor. molars and premolars only one
at a time is functional (fig. 667, 7); when worn out the next one
behind (2) takes its place. Further features are a uterus bicornis,
a zonary placenta, and two pectoral mamme.

ELEPHANTIDE: Elephas indicus, small ears; FE. africanus, large
ears. HE. primegenius, mammoth, in the pleistocene; specimens found
frozen in ice in Siberia have close woolly hair, in some places three feet
long. Mastodon, with tuberculate teeth, range from miocene through the
pliocene. DINOTHERID#, only lower incisors; Dinotherium, Old World
miocene.

 

Order VII. Hyracoidea.

The single genus Hyraz, including species from western Asia
and Afrida, with four-toed front feet, hind feet with three toes,
the digits with nails, the placenta zonary, and the dentition $343,
forms this group, no fossils being known. Hyrax syriacus is sup-
posed to be the ‘coney’ of the Bible.

Order VIII. Sirenia.

This order consists of a few aquatic mammals which are whale-
like in form, with the fore limbs fin-like, the hind legs lacking, and
a horizontal caudal fin. They live in shallow seas or in the
mouths of rivers, where they feed on the tang, which they chew
with jaws covered with horny plates. The teeth (in the fossil
Prorostomus 3444) are reduced or entirely lacking. The fore legs
are pentadactyle and often have rudimentary nails and always a
flexible elbow. The two pectoral mammz have possibly caused
these animals to furnish the germ of truth in the mermaid myth.
Manatus anericanus,* the manatee, six cervical vertebra, eight to
IV. VERTEBRATA: MAMMALIA, CETACEA. 645

ten molars; Halicore dugong, Indo-Pacific; fehytina stelleri of the
northern Pacific, exterminated in 1768.

Order IX. Cetacea.
In external form the whales resemble the sirenians, a result of
an aquatic life, but the resemblance ends here. The whales are

 

Fira. 668.—Restoration of skeleton of Halitherium, an extinct sirenian. (After Miss
Woodward.)

so fish-like that they are commonly included by the laity in that
group, and every one speaks of the whale fishery. Head and trunk
are scarcely distinguished, the cervical vertebre being very short
and more or less completely fused. The hinder limbs are absent,
and of the pelvic girdle only a small ilium remains, and no sacral
vertebre are developed. The caudal fin is two-lobed and differs
from that of a fish in being horizontally flattened; the skin is thick
and is sparsely haired or completely naked, in some hair being
lacking even in the embryo. Most of the species inhabit the high
seas, Inia boliviensis and Platanista gangetica occur in rivers.

The fore limbs are modified into flippers, the bones of which are of
nearly equal size and are jointed only at the shoulder. A dorsal fin ( ‘ fin
backs’) occurs in some, The lack of hair is compensated by the thick
layer of subcutaneous fat (blubber) which, like the fat penetrating the
spougy bones, tends to lessen the specific gravity. In order that the ani-
mals may breathe while feeding, the larynx is prolonged into a tube
which extends up through the pharynx to the choane, from which the
nostrils extend directly upwards to the single (Denticetes) or paired (Mys-
ticetes) external opening. Since the air driven out with great force con-
tains much moisture and this is condensed on contact with the cooler
external air, the impression was natural that the animals in ‘ blowing’
spouted water. Since the olfactory membrane is degenerate and the
olfactory lobes are reduced, the nose is an organ of respiration only.

The eyes are small, cxternal ears are lacking, the mamme are close to
the sexual opening. The teeth are either present in large numbers, similar
and conical, and, since the second dentition is rudimentary, are mono-
phyodont (Denticetz) or they are outlined early and then resorbed and
replaced by plates of baleen (Mysticetz). This is composed of large horny
plates (whalebone) in large animals a dozen feet long (fig. 669, w), of
which several hundred are arranged in close succession extending inward
to the tongue. They correspond to the transverse palatal folds which
646 CHORDATA.

occur in other mammals. As they are fringed on the inner edges they
form a strainer which retains the small marine animals (plankton, Ceto-
chilus septentrionalis, a copepod, and Clione borealis, a pteropod) on which
these whales feed. The cesophagus is too narrow for the passage of
much larger animals,

The origin of the whales is one of the unsolved problems, That they
came from some terrestrial, quadrupedal forms is beyond question, and the

 

Fig. 669.—Section through jaws of whalebone whale. (After Delage.) c, septum of
nose; m, mouth cavity; mx, maxillary bone; p, premaxillary (hinder end); %,
vomer ; w, baleen.

little evidence would seem to point to an ungulate or a carnivore ancestry.

It is possible that the toothed and whalebone whales may have had differ-

ent ancestries, and their resemblances may be the result of convergence.

Sub Order I. ZEUGLODONTA. Extinct (eocene) forms with hetero
dont dentition, the posterior teeth two-rooted.

Sub Order II. DENTICET.A, toothed whales, carnivorous, some hav-
ing but two teeth. Delphinus, dolphins ; Globiocephalus,* black fish ;
Monodon, narwal, with, in the male, a long maxillary tusk (possible origin
of the ‘unicorn’), Physeter macrocephalus, sperm whale, pursued for
the spermaceti, an oily mass situated in the ‘chair’ between the cranium
and the snout, as well as for ambergris, formed in the intestines.

Sub Order III. MYSTACETI, whalebone whales, with baleen. Bale-
noptera,* rorquals and fin backs. B. sibbaldi,* the largest whale, reach-
jvg a length of eighty-five feet. Balwna, right whale.

Order X. Carnivora.

The carnivores live chiefly on the flesh and blood of other ver-
tebrates, which they catch by craft, by coursing, or by pouncing
upon them, overpowering their prey by their sharp claws and
cutting teeth. With this mode of life correspond the high devel-
opment of the brain (fig. 649, B) and sense organs, as well as
IV. VERTEBRATA: MAMMALIA, CARNIVORA. 647

structure of teeth and claws. Since this predacecous character in-
creases within the order from the bears to the cats, and again tends
to disappear in the aquatic species, there are few constant charac-
ters, but a great variation in structure. In interest of greater
mobility the clavicle is reduced or lost, ulna and radius well de-
veloped. In the structure of the feet there is a gradual transition
from the plantigrade bears, in which the whole sole of hand and
foot rest upon the ground, to the digitigrade cats, which tread on
the tips of the toes. In the latter the claws, which occur in all
carnivores, are kept from injury, when not in use, by being re-
tracted by an elastic ligament into pockets on the penult joint,
from which they are extended by strong muscles. In dentition
( g. 650) the striking features are the almost constantly three
incisors, and the great size of the canines; the molars, on the other
hand, vary with the different families, the cusps assuming more of
the shearing character (secodont teeth). The last premolar of the
upper jaw and the first molar of the lower jaw become carnassial
teeth (sectorial teeth), and acquire a dominating position in the
jaw, while the others become smaller and tend to disappear at
cither end of the series. Further characters are the possession of
a penis bone in the males, the abdominal position of the milk
glands and the uterus bicornis in the females; the placenta is
zonary. Anal glands, furnishing a strong, even offensive smelling
secretion, are of wide occurrence.

Sub Order I. FISSIPEDIA. These are the typical members of the
order and are preeminently terrestrial animals with well-developed toes
usually cleft to the base. The number of digits is frequently five on all
feet, but is often reduced to four on the hind feet (Felide, Canide), rarely
on the fore feet (Hywnide); but in these cases, as in the domestic dog, the
reduced first toe may bear a claw. Unsipas, plantigrade; Ursus,* bears;
Procyon lotor,* raccoon. MUSTELIDA; many species of Mustela * and Puto-
rius,* which include minks, martens, sable, ermines, and weasels, are
yaluable for their fur; Lwéra,* otter; Hniydris,* sea otter ; Mephitis,*
skunk; Taaidea,* badger ; Gulo,* glutton ; anal glands common in this
family. Fossils (Avctotherium, ete.) connect the bears and the CANIDAz
with five toes in front, four behind, claws not retractile; which includes
in the genus Cants* dogs, foxes, and wolves. The FELID« have toes as in
the dogs, but with retractile claws. Felis domestica, our domestic eat.
F. leo, lion; F. tigris, tiger; F. concolor,* puma or cougar. Hyanip.,
all feet freer-toed; Hyena of Africa.

Sub Order I]. PINNIPEDIA. These are aquatic carnivores with the
limbs flattened to broad flippers, the five toes long and webbed, the nails
frequently rudimentary ; the dentition differs from that of the terrestrial
forms in the similarity of molars and premolars (absence of carnassial) ;
648 CHORDATA.

the milk dentition degenerates early, without being functional. Puoctws,
seals, without external ears; Phoca vitulina,* harbor seal. OTaRIIDm,
with external ears ; Otaria,* sea lions ; Callorhinus ursinus, fur seal of
Alaska. TRICHECHIDZ; incisors reduced, upper canines developed into
large tusks ; T’richechus, walrus.

The first carnivores appear in the eocene in the order CREODONTA,
plantigrade forms with slightly differentiated dentition (no carnassial) ;

 

Via. 670.—Phoca vitulina, harbor seal. (After Elliott.)

they present marked resemblances to marsupials, insectivores, as wellas to
the Condylarthra, the ancestral ungulates. True carnivores appear in
the upper eocene and become abundant in the miocene.

Order XI. Prosimiz.

Linné united with the true apes a small group of animals
known as lemurs (from India and the adjacent islands, and
especially from Africa), because of similarity in body form and
climbing habits, because they had grasping hands and feet (oppos-
able thumb and great toe), and at least frequently nails on some
of the toes. To-day many set them aside as a separate order on
account of their lower organization. They have a less-developed
cerebrum, uterus bicornis, and a diffuse placenta. Further
peculiarities are the peculiar and variable dentition (Chiromys
rei3, Lemur $433) and the presence of claws, which always occur
on the second and frequently on the third finger of the hind feet,
and in Chiromys replace the nails on all the digits of all the feet
except the great toe. Their nocturnal habits have resulted in
IV. VERTEBRATA: MAMMALIA, PRIMATES. 649

large eyes, which give these animals a most striking appearance.
A distinction from the primates is the connexion of orbital and
temporal cavities beneath the osseous postorbital ring. Usually
there are a pair of pectoral mamme, to which are added in many

 

Fia. 671.—Stenops gracilis, slender loris. (From Brehm.)
species a pair in the abdominal or inguinal region, the latter alone
occurring in Chiromys.

CHIROMYID#, digits long, allexcept the great toe with claws; Chiromys
madagascarensis, aye-aye. TARSIIDA, second and third hind toes
clawed. Yarsius spectrum of the East Indies differs from all Prosimiz
in having the orbits closed and a discoidal placenta like that of man,
LeMURID, second hind toe alone clawed. Lemur; Stenops, loris. The
old tertiary PACHYLEMURIDE and ANAPTOMORPHID are close to the most
primitive mammals and to the creodonts and insectivores. The GALEo-
PITHECID (p. 637) are often referred here.

Order XII. Primates.

The most highly organized mammals, the monkeys, apes, and
man, are united in a single order because among them there is <
great agreement in features of classificatory value. If we here, as
elsewhere, ignore grades of intelligence and regard alone greater
or lesser anatomical resemblances, we are forced to the conclusion
that the anthropoid apes are much closer to man than to the lower
monkeys. ‘

The primates have in common nails on all the fingers and toes
(except the Hapalidw), orbits separated from the temporal fosse
by a bony wall, and a cerebrum which covers the other parts of
650 CHORDATA.

the brain (fig. 649, ¢). They have a single pair of pectoral
mammx, uterus simplex, and a discoidal placenta. The dentition
is essentially the same throughout; in the Platyrrhinew 34123, in the
Hapalide 3433, in the Catarrhine and in man 3423. Yet there is
a tendency to variation, since in the chimpanzee and in man the
third molar (wisdom tooth) is in process of degeneration, while in
the orang a fourth molar often occurs. In all the molars are
bunodont.

The skeleton of the hand and foot has played an important
réle in classification. As in the lemurs and opossums, the thumb
and great toe can be opposed to the other digits, so that an ape can
grasp objects with either hand or foot. In man this opposability
of the thumb is increased, but that of the great toe, in consequence
of the upright position, is only retained to a slight degree by chil-
dren and primitive people. On this peculiarity rest the names often
given of Bimana, for man, and Quadrumana, for the apes and
monkeys. In contradiction of this it must be emphasized that the
apes do not have a hand, but rather a grasping foot, on the hinder

extremilies. In the grasping foot (fig. 672) are the same bones,

 

Fia. 672.—Hand and foot of gorilla. c, capitatum; ca, calcaneus; cu, cuboid; h, ha-
matum ; l, lunatum ; mc, metacarpals ; at, metatarsals ; 2, naviculare ; p, pisi-
forme; ph, phalanges ; s,scaphoid; t, triquetrum; ta, talus ; td, trapezoid; t”, tra-
pezium; J-V, digits; 1-3, cuneiformia.

similarly arranged and of about the same shape as in the foot of
man, while the musculature is essentially the same. On the other
IV. VERTEBRATA: MAMMALIA, PRIMATES. 651

hand the same distinctions between hand and foot (4 and B)
occur as are found in the hand and foot of man. The separation
of Quadrumana and Bimana is without anatomical basis; it rests
solely upon functional peculiarities and egotism.

Sub Order I. PLATYRRHIN_A, New World monkeys. Nostrils separated
by a wide septum so that they are visible from in front; teeth 2123, the
tympanum not extended by an outer bony meatus. CEBID#, tail fre-
quently prehensile, long. Cebus, sapajous; Ateles, spider monkeys. The
HAPALID#, or marmosets, are an aberrant group with teeth 2132 and claws
on all the digits except the relatively small great toe, thumbs not opposable.
Hapale, Midas.

Sub Order II, CATARRHINA, Old World apes; internasal septum
small, the nostrils directed in front and downwards; teeth 2122; since the

2123 0

large canines are interlocked in the opposite row of teeth, hens is a more
or less evident diastema in each jaw; the tympanum is prolonged as in
man into a bony meatus. Section I. CYNOMORPHA, with naked places
on the buttocks (ischial callosities), usually a long tail and hairy face,
and only two sacral vertebrae. Cynocephalus, baboons, drills, and man-
drils; Macacus, macaques; J. ecaudatus, with stumpy tail, entering Europe
at Gibraltar. Section Il. ANTHROPOIDA! (Simiide), man-like apes,
usually without ischial callosities, face, fingers and toes without hair, no
tail, five sacral vertebrze (three in Hylobates) fused to an os sacrum. Hylo-
bates, gibbons, with very long arms; Stimia satyrus of Sumatra and
Borneo, the orang-utan; Gorilla eugena; Troglodytes niger, the chim-
panzee, of Africa.

Sub Order III], ANTHROPINA, man. Degeneration of the hair on
most parts of the body; upright position and as a result slight mobility
of the great toe (non-opposable) ; development of articulate speech ; high
intelligence ; strong development of the cerebrum and consequent increase
of the cranium at the expense of the face, are the most prominent char-
acters of mankind. The dentition is the same as in the Catarrhine, only
the canines are smaller and there is no diastema. It was long a question
whether there was a single species of man (Homo sapiens) with several
races or whether there were several species. Since crosses between the
different races are fertile, the first view receives general acceptance,
although the differences which are actually present are constant and point
to the second alternative. The answer to these questions, which in the
light of evolution have lost most of their significance, and the characteriza-
tion of the various races, belong to a special branch of science, anthro-
pology. Here will only be mentioned the three great groups (each of
which has several subdivisions) recognized by Flower. I. Woolly-haired
men or Negroes, with blackish skin and strongly curled hair (elliptical in
section). The subdivisions are Papuans, Australians, Hottentots, Kaffirs,
and Sudan negroes. II. Straight-haired men, or Mongolians, with yellow-
ish-brown skin and straight hair (circular in section). The subdivisions
Eskimos, Malays, Mongols, and Indians belong here. III. A group called

  
652 CHORDATA.

for want of a better name Caucasians, with straight or wavy hair and
lighter complexion. Here belong the Hamosemites, the Aryans or Indo-
Germans, Nubians, and Dravidians (primitive inhabitants of India),

Since an arboreal life was unfavorable for fossilization, the paleontolog-
ical material for the history of the primates is so far very scanty. Of
these the greatest weight has been laid on a ‘find’ in the upper pliocene
of Java. This consisted of a top of a skull, a femur, and a molar tooth
which were found at some distance from each other, making it doubtful
whether they belonged together. These fragments were regarded on one
side as a connecting link, Anthropopithecus erectus, between apes and man,
on another as belonging to a true ape, and from the third as true man.
The latter is now to be regarded as out of the question. Most probable is
the view that these pieces belonged to an extinct gibbon-like animal of
extraordinary size, an enormous cranial capacity and correspondingly a
very large brain. In these respects no Anthropoid now living could com-
pare with Anthropopithecus.

Summary of Important Facts.

1. The CHORDATA are united by the possession of an axial
skeleton, the notochord, lying between the nervous system and the
alimentary tract; a central] nervous system entirely on one side of
the digestive canal, and gill slits extending from the pharynx to
the exterior.

2. The Chordata are subdivided into Leptocardii, Tunicata,
Enteropneusta, and Vertebrata.

3. The LEPTOCARDI] are fish-like in form, have a notochord
extending the length of the body, but lack skull and vertebral
column; the brain is rudimentary, the gill slits numerous.

4. The TUNICATA have a notochord only in the caudal
region. The young is tadpole-like, but in most forms there is a
metamorphosis in which tail and notochord are lost.

5. The body is usually enclosed in a tunic or mantle containing
cellulose, gill slits and an endostyle are present in the pharynx,
the heart changes in the direction of the flow of blood. The nerv-
ous system In its development is tubular and connects with the
digestive tract by a neurenteric canal. In the Salpide there is a
typical alternation of generations between a solitary asexual and a
sexual chain form.

6. The ENTEROPNEUSTA are worm-like, with collar and
proboseis; a diverticulum of the digestive tract is compared to the
notochord; gill slits occur in the pharynx; some undergo a meta-
morphosis in development, the larva resembling those of Echino-
derms. The pertinence of the Enteropneusta to the Chordata is
not certain.
CHORDATA. SUMMARY OF IMPORTANT FACTS. 6538

7. The VERTEBRATA are segmented animals without ex-
ternal ringing of the body, but with metameric arrangement of
internal parts (myotome, neurotome, sclerotome).

8. A cuticular skeleton is absent, but there may be cornifications
of the epithelium or ossifications in the derma (scales of fishes, etc. ).

9. An axial skeleton is present, consisting of a notochord or
skull and vertebral column, which more or less completely replace
the notochord.

10. There are two kinds of appendages supported by an axial
skeleton, the unpaired fins, occurring only in fishes and Amphibia,
and the paired appendages (anterior and posterior), which are
usually present.

11. The central nervous system (brain and spinal cord) are dor-
sal in position. The brain consists of five parts—cerebrum, ’twixt
brain, optic lobes, cerebellum, and medulla oblongata.

12. Of the sensory organs the eyes and ears are the most highly
developed.

13. The respiratory organs arise from the entoderm (pharynx);
gill slits are present at least in the embryo, extending from the
pharynx to the exterior. In all terrestrial groups these are later
replaced by lungs, developed from the hinder end of the pharynx.

14. The heart, consisting of auricle and ventricle, lies ventrally
ina pericardium. In gill-breathing species it contains only venous
blood, but with pulmonary respiration it is divided into venous
and arterial halves. The circulation is closed.

15. The sexes are usually separate. In most species the ex-
cretory (nephridial) system forms the ducts for the reproductive
products (urogenital system).

16. The reproduction is strictly sexual.

17. In the CycntosromaTa there is a primitive skull; but ver-
tebre, paired fins, true scales, and teeth are lacking. The gills
are saccular and the nose is unpaired. There is no skeleton to
the mouth (no jaws).

18. The true fishes (Pisces), like all other forms, have true
jaws (Gnathostomata). The fishes are further distinguished from
the Cyclostomes by the vertebral column (amphicwle vertebre),
by paired pectoral and ventral fins, scales, and paired nostrils.
They breathe by gills, and have a venous heart with auricle and
ventricle.

19. The fishes are divided into Elasmobranchii, Ganoidei,
Teleostei, and Dipnoi.

20. The Llasmobranchii have a cartilaginous skeleton, usually a
654 CHORDATA.

heterocercal tail, placoid scales, gills covered, heart with arterial
cone, spiral valve in the intestine, no swim bladder.

21. They are divided into Selachii (subdivided into Squali,
sharks, and Raiz, skates) and Holocephali.

22. The Telcostei have bony skeleton, usually a homocercal
tail, usually cycloid or ctenoid scales, comb-like gills and operculum,
bulbus arteriosus, usually pyloric appendages, and a swim blad-
der; no spiral valve.

25. They are subdivided into Physostomi, Pharyngegnathi,
Acanthopteri, Anacanthini, Lophobranchii, and Plectognathii.

24. The Ganoided form a connecting group; they resemble the
elasmobranchs in the presence of a conus arteriosus and _ spiral
valve, and usually in the heterocercal tail; they are like the teleosts
in operculum and comb-formed gills, swim bladder, and pyloric
appendages. They usually have fulcra and ganoid scales.

25. The ganoids are subdivided into Chondrostei, with carti-
laginous skeletons, and Crossopterygii and Holostei, with bony
skeletons.

26. The Dipnot have branchial respiration, occasionally the
swim bladder serves as lungs; heart with beginning division: nose
with choana.

27. The Ampnrsta, in contrast to the fishes, have pentadactyle
appendages; in contrast to the reptiles, double occipital condyles.
They have bushy external gills, and lungs either persisting together
or succeeding each other, the young (larve) breathing by gills,
the adult by lungs (metamorphosis!). The heart consists of two
auricles and one ventricle.

28. The Amphibia are subdivided into Gymnophiona, Urodela,
and Anura; to these are added the extinct Stegocephali (Laby-
rinthodonts).

29. The Gymnophiona are blind and have lost the limbs.

30. The Urodeles have many vertebrx and a well-developed tail.
They retain the gills permanently (Perennibranchia), or at least a
gill shit (Derotrema), or they lose the branchial apparatus com-
pletely in development (Salamandrina); the metamorphosis is not
pronounced.

31. The Anura have few vertebra, no tail nor gills in the adult,
and a marked metamorphosis (the larve, tadpoles, are furnished
at first with external, then with internal, gills, and with swimming
tail, but at first lack appendages and lungs).

32. Cyclostomes, fishes, and Amphibia are grouped as Anamina
because of the lack of amnion and allantois; they are also called
CHORDATA. SUMMARY OF IMPORTANT FACTS. 655

Ichthyopsida, because of their branchie and aquatic habit. They
are poikilothermons (cold-blooded),

33. The reptiles, birds, and mammals are called Amniota on
account. of the embryonal organs, the amnion and allantois. They
never respire by gills (although gill clefts occur in the embryo),
and the appendages are based on the pentadactyle type.

34. The REpriLia are poikilothermous, have a strongly ossi-
fied skeleton, with unpaired occipital condyle and usually an
os transversum in the skull; a strongly cornified skin, two auricles,
and usually two incompletely separated ventricles in the heart.

35. Recent reptiles are divided among the Chelonia, Rhyncho-
cephalia, Squamata (including Lacertilia and Ophidia), and
Crocodilia. To these are added the extinct groups Theromorpha,
Plesiosauria, Ichthyosauria, Dinosauria, and Pterodactylha.

36. The Chelonia are compact, have a skeletal capsule (carapace
+ plastron) composed of bone and horny plates, an immovable
quadrate and hard palate, no os transversum or teeth, but horny
plates in the place of the latter; the cloacal opening elongate, with
an unpaired penis in front.

37. The Sqguwamata have horny scales periodically renewed, a
transverse cloacal opening, with behind it paired penes and a
movable quadrate.

38. The Lacertilia have usually movable eyelids, tympanic
membrane, four appendages or their rudiments, and all but invari-
ably a sternum.

39. The Ophidia lack appendages, sternum, and tympanum;
the eyelids are fused to a false cornea; the mouth is usually exten-
sible; poison fangs are frequently present.

40. The Rhynchocephalia resemble the Lacertilia in form, but
differ in having a fixed quadrate.

41. The Crocodilia are elongate, have bony plates in the skin,
elongate cloacal opening, fixed quadrate, teeth placed in separate
alveoli, and a long swimming tail.

42, The Aves (birds) are closely related to the reptiles (Sau-
ropsida) and share with them the single occipital condyle. They
are distinguished by the feathers, and by having the heart com-
pletely divided into right and left halves.

43. Other characters are homoiothermy (warm-blooded), pneu-
maticity of bones, fusion of bones of manus, formation of tibio-
tarsus and tarso-metatarsus (intertarsal joint).

44. The birds are divided into fatite, which lack a furcula and
a keel to the sternum, and the Carinate, in which the sternum is
keeled and the clavicles are united to a furcula. To these are
656 CUORDATA.

added two extinct groups, Saurure and Odontornithes, which had
teeth.

45. The Mammarra have a double occipital condyle, hairy skin,
and milk glands in the female for the nourishment of the young.

46, Other characters are the homoiothermous condition, the
complete separation of the heart, the modification of parts of the
visceral arches into the ear bones, high development. of the denti-
tion (formation of roots, usually heterodont and diphyodont).

47. The mammals are divided into Monotremata, Marsupialia,
and Placentalia.

48. The Monotremata are egg-laying mammals with persistent
cloaca; they have a distinct coracoid and an episternum.

49, The Marsupialia are viviparous, but the young, on account
of imperfect nourishment (usually no placenta), are born early and
usually carried in a marsupium (marsupial bones).

50. In the skeleton the inflected angle of the lower jaw is char-
acteristic. The urogenital apparatus is separated from the anus
by the perineum; uterus and vagina are double.

51. The Placentalia produce well-developed young which are
nourished in the uterus by a placenta; they have no marsupium
nor marsupial bones. The vagina is single (Monodelphia), the
uterus simple or paired.

52. The clawed Edentata and the Cetacea and Sirenia, which
have flippers, have a degenerate dentition (teeth monophyodont or
lacking).

53. The hoofed ungulates (Perissodactyla and Artiodactyla),
the Proboscidia, and the small clawed Rodentia are preeminently
herbivorous.

54. The Chiroptera, which have a flying membrane (patagium),
are partly herbivorous, partly insectivorous.

55. The small Insectivora (with small canines and no earnas-
sial) and the Carnivora (with strong canine and carnassial molar)
are carnivorous. The Carnivora are subdivided into the terrestrial
Fissipedia and the aquatic Pinnipedia.

56. The Prosimie and Primates have a more or less indifferent
dentition. They have largely or entirely replaced claws by nails,
and are largely provided with grasping hands and feet. The
Prosimie are lower, the Primates more highly organized.

57. The Primates are subdivided, according to the position of
the nostrils, the development of the tail, as well as the character of
the dentition and the feet, into the Platyrrhiny, or monkeys of the
New World, the Catarrhiney, or apes of the Old World, and the
Anthropinew, or man.
Aard vark, 636
Abalone, 379
Abdominal cavity, 546
Abdominales, 562
Abdominal fin, 562
Abdominalia, 426
Abducens nerve, 536
Abomasum, 642
Acantharia, 196
Acanthia, 489
Acanthias, 569, 571
Acanthiide, 489
Acanthin, 195
Acanthobdella, 318
Acanthoderus, 48
Acanthobothrium, 286
Acanthocephala, 304
Acanthocotyle, 274
Acanthodes, 572
Acanthodidz, 572
Acanthoglossus, 632
Acanthometra, 194
Acanthophracta, 196
Acanthopteri, 574, 577
Acanthopterygii, 577
Acaride, 454
Acarina, 453
Accessory nerve, 536
Accessory tissue, 99
Accessory yolk, 80
Accipiter, 617
Acephala, 358
Acerata, 442
Acetabula, 280
Acheta, 317
Achatina, 383
Achoreutes, 477
Achromatin, 65
Achtheres, 36, 422

 

INDEX.

Aciculum, 308
Acineta, 212
Acinetaria, 212
Acinous glands, 77
Acipenser, 573
Acipenseride, 573
Acmza, 378
Acmeide, 378
Accela, 269
Acontia, 253
Acorn barnacle, 423
Acrania, 502
Acraspedia, 246
Acraspedota, 246
Acridiide, 481
Acridium, 481
Acris, 588
Acrodont teeth, 599
Actinaria, 259
Actinian, section of, 136
Actinophrys, 191, 192
Actinopoda, 349
Actinospherium, 190, Ig2
Actinotrichia, 527
Actinotrocha, 325
Actinozoa, 251
Aculeata, 486
Aculeus, 472, 476
Acustic nerve, 536
Adambulacral plate, 335
Adamsia, 254
Adhesive cells, 264
Adipose fin, 576
Adoral band, 209
Adradius, 246
‘Ega, 441, 442
/Egina, 242
‘Eolidz, 382
/Eolidia, 382

657
658

A‘piornis, 613
Afquoria, 240, 242
Atschna, 479
Atsthetes, 357
fEthalium, 199
Aftershaft, 603
Agalmia, 244
Agamide, 599
Agassiz, 21
Agelacrinoidea, 342
Agelacrinus, 342
Agkistrodon, 601
Aglaophenia, 242
Aglossa, 588
Aglypha, 601
Agnatha, 555
Agrion, 479

Air bladder, 567
Air pipes, 609

Air sacs of birds, 608
Ale, 466

Alze cordis, 470
Alauda, 616
Alaudidz, 616
Albatross, 615
Albertus Magnus, 9
Alca, 615
Alcedinidz, 616
Alces, 642

Alcidz, 615
Alciopidze, 313
Alcippe, 424
Alcyonaria, 258
Alcyonidz, 259
Alcyonidium, 324
Alcyonium, 255, 259
Aletia, 495

Alima, 429
Alimentary tract of vertebrates, 546
Alisphenoid bone, 522
Allantoidea, 553
Allantois, 553
Alligator, 602
Alligator turtle, 596
Allobophora, 316
Alloposus, 395
Allotheria, 632
Alosophila, 494

 

INDEX.

Alpaca, 643

Alpheus, 434
Alternation of generations, 144, 512
Altrices, 612

Alula, 604

Alveolar duct, 548
Alveoli, 625

Alveolus, 548

Alytes, 585
Amaroucium, 510
Ambergris, 646
Amblyopsidee, 576
Amblypoda, 643
Amblystoma, 587
Amblystoma, larva of, 36
Ambulacra, 331
Ambulacral grooves, 335
Ambulacral plates, 335
Ambulacral system, 122, 330
Ambulacral vessels, 331
Ametabolous, 473

Amia, 574

Amia, tail of, 41
Amicula, 357

Amiide, 574
Ammoceetes, 557
Ammonitide, 394
Amnion, 472, 553
Amniota, 553, 588
Ameeba, 61, 62, 187, 189
Ameebina, 189

Ameeboid motion, 187
Amphacanthe, 574
Amphiaster, 70
Amphibia, 580
Amphibiotica, 479
Amphicceele, 518
Amphiccelias, 596
Amphidiscs, 227
Amphigony, 142
Amphilina, 286
Amphineura, 356
Amphioxus, 502, 504
Amphioxus, cleavage of, 151
Amphioxus, gastrula of, 156
Amphiphorus, 291
Amphipoda, 438
Amphisbzena, 599
Amphistomum, 278
Amphithe, 428
Amphitrite, 312, 313
Amphiuma, 585, 587
Amphiura, 338
Amphoride, 342
Ampullaridz, 380
Ampullee, 223, 331, 542
Amyda, 596

Anabrus, 481
Anacanthini, 577
Anaconda, 601

Anal glands, 106

Anal fin, 526, 562
Anallantoidia, 553, 555
Analogy, 14, 100
Anamnia, 553, 555
Anaplocephalus, 287
Anaptomorphide, 649
Anas, 615

Anaspides, 437
Anatomy, 57, 58
Anatomy, comparative, 2
Anaxial form, 135
Androctonus, 400
Anelasma, 423, 424
Angiostoma, 601
Anguillidz, 576
Anguillula, 300
Anguillulidz, 300
Anguis, 599

Angulare, 582

Animal morphology, 57
Animal organs, IOI, 121
Animal pole, 147, 151
Animals and plants, 171
Anisomyaria, 367
Anisopoda, 442
Annelida, 305
Annulata, 599
Anodonta, 361, 367
Anolis, 599
Anomodontia, 594
Anopheles, 217, 492
Anser, 615
Anseriformes, 615
Antarctic province, 178
Ant eaters, 636

 

INDEX. 659

Antedon, 339, 342
Antenna, 401, 410, 430, 463
Antennal gland, 411
Antennulz, 430
Antheomorpha, 251
Anthomastus, 259
Anthomedusz, 239, 241
Anthomyia, 492
Anthozoa, 251
Anthropinz, 651
Anthropoide, 651
Anthropopithecus, 652
Antilocapra, 642
Antilope, 642
Antilopide, 642
Antimeres, 137
Antipatharia, 259
Antipathes, 259
Antlers, 642

Ant lion, 481, 483
Antrostomus, 616
Antrum of Highmore, 539
Ants, 487

Ants, white, 478
Anura, 588

Anurida, 477

Aorta, 548

Aorta ascendens, 504, 549
Aorta, descendens, 504
Apes, 651
Aphaniptera, 493
Aphidz, 490
Aphrodite, 313
Apiariz, 487

Apical plate, 306
Apis, 487
Aplacophora, 358
Aplysia, 381
Aplysilla, 226
Aplysina, 224, 226
Apoda, 349, 425, 587
Apodidz, 416
Apophysis, 517
Aporosa, 260
Appendicularia, 506
Aprophora, 489
Aptenodytes, 615
Aptera, 491
660

Apteria, 604
Apteryges, 613
Apterygota, 477
Apteryx, 613

Apus, 416

Aquatic faune, 179
Aqueduct of Sylvius, 534
Aqueous humor, 131
Aquila, 617
Arachnida, 444
Araneina, 451
Arbacia, 345

Arcella, 198

Archean era, 180
Archeopteryx, 33, 612
Archegony, 139
Archenteron, 103, 104, 156
Archianellide, 313
Archigetis, 286
Archiptera, 477
Archipterygium, 529
Architeuthes, 384, 395
Arcide, 367

Arcifera, 588

Arctic province, 178
Arctogzea, 177
Arctotherium, 647
Arcyria, 199

Ardea, 615
Arenicolidz, 313
Areolar connective tissue, 85
Argas, 454

Argina, 367

Argiope, 327, 453
Argonauta, 392, 395
Argonautidz, 395
Argulide, 422
Argulus, 421, 422
Ariolimax, 383

Arion, 383

Arista, 493

Aristotle, 7

Aristotle’s lantern, 345
Armadillidium, 442
Armadillo, 636
Armata, 317

Army worm, 495
Artemia, 416

 

INDEX.

Arterial arches, 549
Arteries, 112

Arthrodira, 579
Arthrogastrida, 447
Arthropoda, 398
Arthrostraca, 438

Articular bone, 525
Articular process, 519
Articulata, 342, 398
Artificial impregnation, 147
Artificial selection, 43
Artiodactyla, 640, 641
Arvicola, 639

Ascalabote, 598

Ascaridz, 301

Ascaris, 145, 299

Ascaris, fertilization in, 150
Ascidizeformes, 508
Ascidians, 505

Ascones, 222, 225

Ascyssa, 224

Asellide, 442

Asellus, 440

Asexual reproduction, 140, 143
Asilidze, 493

Asp, 601

Aspergillum, 368
Aspidobranchia, 378
Aspidonectes, 596
Aspidotus, 490

Ass, 641

Assimilation, organs of, 102
Astacide, 435

Astacoidea, 435

Astacus, 431, 432, 435
Astarte, 367

Astartide, 368

Asterias, 337

Asterias, early development, 146, 148
Asteridz, 337

Asterinid, 337

Asteriscus, 335, 337, 564
Asteroidea, 333

Astra, 261

Astrangia, 260, 261
Astroides, 261
Astrophyton, 338
Asymmetrical form, 135
Asymmetron, 504
Atalapha, 638
Atax, 454

Ateles, 651
Atelodus, 641
Atheca, 595
Atlantide, 380
Atlas, 581, 590
Atoke, 310

Atoll, 258

Atolla, 250

Atrium, III, 506, 508
Atrypa, 328

Attus, 453

Atypus, 453
Auchenia, 643
Auditory meatus, 545
Auditory nerve, 536
Auditory organs, 127
Auk, 615
Aulacantha, 196
Aulophorus, 315
Aulosphera, 196
Aurelia, 245, 250
Auricle, 111, 548
Auricularia, 332
Aurochs, 642
Australian region, 176
Autoflagellata, 200
Autoinfection, 215
Autolytus, 313, 314
Aves, 603
Avicularia, 323
Aviculidz, 367
Aye-Aye, 649

Axial skeleton, 526
Axiothea, 313

Axis, 590

Axis cylinder, 96
Axolotl, 587

Axons, 94

Azoic era, 180
Azygobranchia, 379

Babvons, 651
von Baer, 17
Beetisca, 479
Badger, 647

 

INDEX. 661

Baleena, 646
Balznoptera, 646
Balancers, 491
Balaninus, 485
Balanoglossus, 513
Balantidium, 209, 210
Balanus, 423

Baleen, 645
Bandicoot, 633

Barbs 603

Barbules, 603
Barnacles, 423
Basalia, 330, 340, 527
Bascanion, 601
Basioccipital bone, 522
Basiopodite, 410
Basisphencid bone, 522
Bass, black, 577
Bassomatophora, 383
Bath sponges, 227
Bats, 637
Bdellodrilus, 315
Bdellostoma, 557
Bdelloura, 271

Bears, 647

Beaver, 639

Bedbug, 489

Bee, larva of, 105
Bees, 487

Beetles, 483

Bela, 380

Belemnites, 388
Bellovacensis, 9

Bell’s law, 536
Belosepia, 388
Belostoma, 489
Belostomide, 489
Beroe, 264

Beroide, 264
Bestiarius, 9
Bicidium, 259
Bicoseca, 201
Bicuspid teeth, 626
Pig horn, 642
Bilateral symmetry, 131
Bilharzia, 277
Bimana, 650
Biogenesis, fundamental law of, 34
662

Biology, 4, 57
Bipalium, 271
Bipinnaria, 332
Biradial symmetry, 136
Bird lice, 479

Birds, 603

Birds of paradise, 50, 616
Birgus, 432, 436
Bison, 642

Bittacus, 483

Bivium, 334

Black bass, 577

Black fish, 646

Black flies, 493

Black snake, 601
Bladder, urinary, 552
Bladder worm, 278, 284
Blarina, 637
Blastoderm, 153
Blastodermic vesicle, 155
Blastoidea, 342
Blastomeres, 151
Blastopore, 156
Blastostyle, 242
Blastula, 151, 155
Blatta, 480

Blattidze, 480

Blind fish, 576
Blissus, 489

Blister beetle, 484
Blood, 88, 111

Blood corpuscles, 88
Blood vessels, 110, III
Blow flies, 493

Blue birds, 616

Boa, 601

Bobolink, 616

Body cavity, 109
Bojanus, organ of, 363
Bolina, 264
Bombycina, 495
Bombyx, 495

Bonasa, 614

Bone, 86

3onellia, 317, 318
Book lice, 479
Bopyridz, 442
Bopyrus, 442

 

INDEX.

Bos, 642

Bosmina, 417

Botall’s duct, 550

Bot flies, 493
Bothriocephalidz, 287
Bothriocephalus, 281, 283, 287
Bothrops, 601
Botryllus, 510
Bougainvillea, 144, 241
Bovide, 642

Bow fin, 574

Box turtle, 596
Brachialia, 340
Brachiolaria, 332
Brachiopoda, 325
Brachycera, 493
Brachyura, 437
Braconide, 486
Bradypus, 636

Brain coral, 261
Branchial arch, 524
Branchial chamber, 352, 506
Branchial clefts, 501
Branchial heart, 391
Branchial tree, 348
Branchiata, 408
Branchiomerism, 523
Branchiopoda, 416
Branchiostegal membrame, 56
Branchiostegal rays, 562
Branchiostegite, 431
Branchipide, 416
Branchipus, 416
Branchiura, 422
Braula, 493

Breast bone, 518
Brevilinguia, 599
Brissus, 346

Bristles, 311

Bristles. tactile, 126
Bristle tails, 477

Brittle stars, 337
Bronchiole, 548
Bronchus, 547
Bryozoa, 321

Bubalus, 642

Bubo, 617

Buccal cavity, 106
Buccal ganglion, 390
Buccinide, 380
Buccinum, 379
Bucerontide, 616
Budding, 141
Budding and germ layers, 159
Buffalo, 642

Buffalo leaf hopper, 490
Buffon, 21

Bufo, 588

Bufonide, 588

Bugs, 489

Bugula, 324

Bulbus arteriosus, 568
Bulbus olfactorius, 534
Bulimus, 383

Bulla, 381

Bulla ossea, 621
Bunodes, 259
Bunodontia, 641
Bunodont teeth, 626
Burbot, 578

Bursa, 331, 338
Bursa copulatrix, 471
Bustard, 615

Buteo, 617

Buthus, 448

Butrinus, 575
Butterflies, 494, 495
Butterflies, leaf, 47
Buzzard, 617

Byssus, 363

Cabbage worm, 496
Cacatua, 616
Cacospongia, 22
Caddis flies, 483
Ceecidotea, 442
Cecilia, 587
Czecum, 106, 461, 627
Czenolestes, 633
Czenosarc, 231
Calamoichthys, 573
Calandra, 485
Calanide, 421
Calappa, 437
Calcispongiz, 225
Caligid, 422

 

INDEX. 663

Caligus, 422
Callianira, 263
Calliphora, 493
Callorhinus, 648
Calosoma, 484
Calycella, 242
Calyconecte, 244
Calycophore, 244
Calycozoa, 250
Calyptoblastea, 242
Camarasaurus, 596
Cambarus, 435
Cambrian, 180
Camelopardalide, 642
Camels, 643

Camelus, 643
Campanella. 242
Campanula Halleri, 564
Campanularia, 232, 233
Campanularix, 239, 242
Camper’s angle, 624
Campodea, 400, 477
Canal, radial, 331
Canal, ring, 331
Canal, semi-circular, 128
Canals of sponges, 223
Cancer, 437
Cancridz, 437
Candona, 423
Canidz, 647

Canine teeth, 625
Canis, 647

Canker worms, 494
Cannostome, 250
Cantharidz, 484
Canthocamptus, 421
Capillaries, 111
Capillitium, 199
Capra, 642

Caprella,, 440
Caprimulgide, 616
Capsule, central, 193
Capybara, 639
Carabide, 484
Carapace, 410, 594
Carboniferous, 180
Carcharinus, 571
Carcharodon, 571
664

Carchesium, 210, 211
Cardiidz, 367
Cardinal teeth, 359
Cardinal vein, 549
Cardium, 367

Cardo, 463

Caridea, 434

Carina, 424

Carina sterni, 605
Carinaria, 380
Carinariidz, 380
Carinella, 291
Carinatee, 613
Carnassial teeth, 626
Carnivora, 646
Carotid artery, 549
Carp, 576

Carpal bones, 529
Carpocapsa, 494
Cartilage, 86
Cartilage bone, 519
Cartilaginous cranium, 519, 520
Caryogamy, 184
Caryophylleus, 285, 286
Caryophylleide, 285
Caryophyllia, 257, 260
Cassowary, 613
Castor, 639
Castoridze, 639
Casuarina, 613
Casuarius, 613
Cataclysm theory, 20
Catallacta, 220
Catarrhinze, 651
Caterpillars, 494, 495
Catfish, 576
Cathartes, 617
Cathartide, 616
Catocala, 495
Catometopa, 437
Catostomidz, 576
Cats, 647

Cattle, 642

Caudal fin, 526, 562
Caudina, 347, 349

Causal foundation of theory of evolu-

tion, 43
Cavia, 639

 

INDEX.

Caviare, 573

Cavicornia, 642

Caviidze, 639

Cavolinide, 382

Cebide, 651

Cebus, 651

Cecidomyia, 492

Cell, 58

Cell complexes, 71

Cell division, 68

Cell, nature of, 60

Cell organs, 183
Cell-reticulum, 61

Cell theory, 17, 58

Cells, adhesive, 264

Cells, blood, 88

Cells, contractile fibre, 92
Cells, division of, 68

Cell, egg, 80

Cells, ganglion, 94

Cells, gland, 76

Cells, goblet, 77

Cells, muscle, 92

Cells, multiplication of, 68
Cells, nettle, 229

Cells, sexual, 143

Cells, somatic, 143

Cells, supporting, 83

Cells, thread, 229

Cells, yellow, 195

Cellular connective tissue, 84
Cellularia, 324

Cellulose, 172, 505
Cenozoic era, 181
Centipedes, 459, 461
Central capsule, 193
Central nervous system, 122
Centrifugal nerve tracts, 94
Centripetal nerve tracts, 94
Centrodorsal, 338
Centrolecithal eggs, 152, 153
Centrosome, 190

Centrum, 518

Centrurus, 448
Cephalaspis, 557
Cephalochordia, 502
Cephalodiscus, 514
Cephalopoda, 384
Cephalothorax, 399
Cephalothrix, 291
Cerambycidz, 485
Ceraospongiz, 227
Ceratium, 203
Ceratodus, 579
Ceratorhinus, 641
Cercaria, 268, 276
Cercomonas, 202
Cercopide, 489
Cercus, 477

_ Cere; 604
Cerebellar hemispheres, 535
Cerebellum, 535
Cerebral flexures, 624
Cerebral ganglion, 123
Cerebral hemispheres, 534
Cerebratulus, 292
Cerebrum, 534
Ceresa, 490
Cereus, 253
Cerianthus, 255
Cervicornia, 642
Cervide, 642
Cervus, 642
Ceryle, 616
Cestide, 264
Cestoda, 278
Cestum, 264
Cetacea, 645
Cetochilus, 421
Chelura, 439
Chetz, 311
Chetiferi, 317
Chetoderma, 358
Cheetognathi, 296
Cheetonotus, 295
Cheetopoda, 306
Cheetura, 616
Chain salps, 512
Chalcis, 486
Chalcididz, 486
Chalina, 227
Chameleon, 599
Chamois, 642
Charadriformes, 615
Charadrius, 615
Charybdea, 250

 

INDEX. 665

Chelicera, 445
Chelifer, 450

Chelone, 596
Chelonia, 594
Chelonide, 596
Chelydra, 596
Chelydridz, 596
Chermes, 450
Chevron bones, 518
Chevrotain, 642
Chiastoneury, 373
Chigoe, 494
Chilognatha, 496
Chilomonas, 201
Chilomycterus, 578
Chilopoda, 459, 460
Chilostomata, 324
Chimeera, 572
Chimney swallow, 616
Chimpanzee, 651
Chinch bug, 489
Chiromys, 649
Chiroptera, 637
Chirotes, 599

Chitin, 398

Chiton, 357
Chitonide, 356
Chlamydosaurus, 599
Chlamydoselachus, 570
Chloragogue cells, 116
Choanoflagellata, 202
Cholcepus, 636
Chondrilla, 224
Chondrin, 86
Chondrioderma, 199
Chondrocranium, 520
Chondropterygii, 569
Chondrostei, 573
Chone, 313

Chorda dorsalis, 501
Chordata, 501
Chordodes, 304
Chordotonal sense organs, 406
Chorion, 148, 634
Choroid, 540
Choroidea, 131
Choroid coat, 130, 131
Choroid gland, 564
666 INDEX.

Chromatin, 65
Chromatophores, 387
Chromomonadina, 202
Chrysalis, 494
Chrysomelidz, 485
Chrysomitra, 244
Chrysopa, 483

Chyle, 550

Chyle vessels, 114
Cicada, 488, 489, 490
Cicadide, 489
Cicindelide, 484
Ciconia, 615
Ciconiformes, 615
Cidaridea, 345
Ciliata, 204

Ciliated epithelium, 75
Cilioflagellata, 203
Cimbex, 486
Cinclides, 253

Ciona, 507
Circulatory apparatus, 109
Circulatory organs of vertebrates, 548
Cirolana, 442

Cirri, 312

Cirripedia, 423
Cirrus, 120, 272, 308
Cirrus pouch, 272
Cistenides, 314
Cistudo, 596
Citigrada, 453
Cladocera, 417
Cladocora, 260, 261
Cladoselache, 572
Clamatores, 616
Clams, 368

Class, 10
Classification, difficulties in, 30
Clathrulina, 191, 192
Clava, 241
Clavellinidz, 510
Clavicle, 528

Claws, 618

Clear wings, 495
Cleavage cavity, 155
Cleavage planes, I51
Cleavage of eggs, 149, 151
Cleavage process, 151

 

Cleavage, types of, 153
Cleon, 479
Clepsidrina, 213, 215
Clepsinide, 321
Clibanarius, 436
Clidastes, 600
Climbing birds, 616
Clione, 382
Clisiocampa, 495
Clitellio, 315
Clitellum, 315
Cloaca, 106, 223, 506, 546
Clothes moth, 494
Clupeidze, 576
Clymene, 313

Cly peaster, 343, 346
Clypeastroidea, 346
Clypeus, 462

Clytia, 242
Cnemidophorus, 599
Cnidz, 229

Cnidaria, 228
Cnidocil, 229

Cobra, 601

Coccide, 490
Coccidiz, 215
Coccidium, 215, 216
Coccinellide, 485
Coccus, 490
Coccygus, 616
Cochineal, 490
Cochlea, 128, 543
Cockatoos, 616
Cockroach, 480

Cod, 577, 578

Codlin moth, 494
Codosiga, 201, 202
Coelenterata, 228
Ceelhelminthes, 295
Ceelom, 109, 158
Ccelom of Vertebrates, 545
Ccelomic pouches, 158
Ceelenteron, 228
Ccelodendron, 196
Coeloplana, 264
Ceelopleurus, 343, 345
Ceeloria, 260, 261
Ceenurus, 285
INDEX.

Coiter, 13

Cold rigor, 63
Cold-blooded animals, 114
Coleoptera, 483
Collaterals, 94

Collembola, 477
Collozoum, 194
Coloborhombus, 49

Colon, 461

Colony, 164

Coloration, sympathetic, 46
Colossendeis, 456
Colossochelys, 596
Colubriformia, 601
Columba, 614

Columbidz, 614
Columella, 199, 256, 370, 525, 544, 598
Columns of cord, 533
Colymbidz, 615
Colymbus, 615
Comatrichia, 199
Comatulide, 342
Commissures, 123
Complemental males, 424, 442
Compound eye, 403, 404
Conchiolin, 352

Conch of ear, 545
Condylarthra, 639, 643
Condylura, 637

Cones of eye, 540

Coney, 644

Conide, 380

Conjugation, 184, 206
Connective tissues, 83
Conocephalus, 481
Conocladium, 200, 202
Conotrachelus, 485
Contractile fibre cells, 92
Contractile vacuole, 183
Conurus, 616

Conus, 380

Conus arteriosus, 567
Convergent development, 169
Cope, 24

Copelatee, 506

Copepoda, 417
Copperhead, 601

Copula, 524

 

667

Copulation, 147
Coraciformes, 616
Coracoid, 528

Coral, brain, 260, 261
Coral, deer’s horn, 261
Coral, organ pipe, 259
Coral, precious, 259

Coral, red, 256

Coral reefs, 258

Coral snake, 601
Corallium, 256, 259
Cordylophora, 239
Coregonus, 576

Corium, 514

Cornea, £30, 131, 541
Cornacuspongia, 227
Coronula, 423, 424
Corpora bigemina, 534
Corpora quadrigemina, 534
Corpus callosum, 623
Corpus striatum, 534
Corpuscles, Miescher’s, 218
Corpuscles, Meissner’s, 126
Corpuscles, Rainey’s, 218
Corpuscles, Vater-Pacinian, 126
Correlation of parts, 14
Corrodentia, 478

Corti, organ of, 543
Corvus, 616

Corvide, 616

Coryceide, 421

Corydalis, 482
Corymorpha, 241

Costee, 256

Costal plates, 594
Cotingidz, 616

Cotton worm, 495

Cottus, 577

Cotyledonary placenta, 634
Coturnix, 614

Cougar, 647

Covering scale, 243
Coverts, 604

Cowries, 380

Coxa, 463

Coxal glands, 445

Crab louse, 491

Crabs, 437
668

Crab stones, 433
Crangon, 434, 435
Crane flies, 492
Cranes, 615

Crania, 328

Cranial nerves, 536
Craspedon, 235
Craspedota, 235
Crassatella, 359
Crassilinguia, 599
Crayfish, 435
Creodonta, 648
Crepidula, 379
Crepidula, cleavage of egg, 154
Cretaceous, 180
Cribillina, 324
Cribrellum, 452
Crickets, 481
Crinoidea, 338
Crisia, 324

Crista acustica, 127, 542
Crista sterni, 605
Crocodilia, 601
Crocodilus, 602
Crop, 106, 467
Crossbill, 616
Crosses, 27
Crossopterygii, 573
Crotalide, 601
Crotalus, 601

Croton bug, 480
Crows, 616

Crura cerebri, 623
Crustacea, 408
Cryptobranchus, 587
Cryptocephala, 313
Cryptochiton, 357
Cryptodira, 596
Cryptoniscus, 442
Cryptopentamera, 484
Crystalline cone, 405
Crystalline style, 364
Ctenidia, 353
Cteniza, 453
Ctenobranchia, 379
Ctenodiscus, 337
Ctenoid scale, 558
Ctenolabrus, 576

INDEX.

Ctenophora, 261
Ctenoplana, 264
Ctenostomata, 324
Cubomedusz, 250
Cuckoos, 616
Cuculiformes, 616
Cuculus, 616
Cucumaria, 349
Culcita, 334, 337
Culicide, 492
Cumacea, 437
Cunina, 242
Cunocantha, 239, 242
Curculio, 485
Currant worm, 494
Cursorial foot, 614
Cursoria, 480
Cuspidaria, 368
Cutaneous artery, 585
Cuticle, 75

Cutis, 514

Cuttle bone, 388, 395
Cuttle fish, 395
Cuvier, 14, 15
Cuvierian ducts, 548, 567
Cuvierian organs, 348
Cyamus, 440
Cyanea, 250
Cyanocitta, 616
Cycladidz, 368
Cyclas, 368

Cycloid scale, 558
Cyclometopa, 437
Cyclostomata, 324, 555
Cyclostomide, 380
Cyclopide, 421
Cyclops, 37, 421
Cydippide, 264
Cygnus, 615
Cymbulide, 382
Cymothoa, 440, 442
Cynipide, 486
Cynocephalus, 651
Cynomorphe, 651
Cynomys, 639
Cynthia, 510
Cynthiidz, 510
Cyprzidz, 380

 
Cypridide, 423
Cypridina, 423
Cypridinide, 423
Cyprinide, 576
Cypris, 423
Cypselidz, 616
Cypselomorphe, 616
Cyrtidz, 196
Cyrtophilus, 481
Cysticercoid, 285
Cysticercus, 278, 284
Cystid, 322
Cystidea, 342
Cystoflagellata, 203
Cystonectee, 244
Cytoblast, 58
Cytopharynx, 183
Cytopyge, 183
Cytosporidz, 213
Cytostome, 183

Dactylethra, 588
Daddy long-legs, 450
Daphnia, 417, 418
Daphnidz, 417

Dart sac, 376
Darwin, 23
Darwinian theory, 25
Dasyatis, 572
Dasypodide, 636
Dasypus, 636
Dasyuride, 633
Dasyurus, 633
Datames, 450
Decapoda, 394, 429
Deer, 642
Degeneration, 167
Delamination, 157
Delphinus, 646
Demibranch, 566
Demodex, 454
Dendrites, 94
Dendroceelum, 271
Dendreeca, 616
Dendronotus, 382
Dental formula, 626
Dentalium, 369
Dentary bone, 525, 582

 

INDES. 669

Denticete, 645, 646
Dentine, 515
Derma, 514

Dermal teeth, 515
Dermanyssus, 454
Dermatobia, 493
Dermatoptera, 480
Dermochelys, 595
Dero, 315
Derotrema, 587
Desmodont hinge, 359
Desmodus, 638
Desor’s larva, 291
Deutocerebrum, 462, 468
Deutomerite, 214
Deutoplasm, 80
Devexa, 642
Devonian, 180
Diaphragm, 546
Diapophysis, 518
Diaptomus, 421
Diastema, 638
Diastictis, 494
Diapheromera, 480
Diastylis, 437
Dibranchia, 394
Dicotyle, 641
Dicyemida, 220
Didelphia, 632
Didelphys, 633
Didus, 614
Differentiation of tissues, 71
Difflugia, 198
Diffuse nervous system, 122
Diffuse placenta, 634
Digger wasps, 486
Digenea, 274
Digestive tract, 103
Digitigrade, 647
Dimorphodon, 602
Dimyaria, 367
Dinichthys, 579
Dinobryon, 200, 202
Diomedia, 615
Dinoflagellata, 203
Dinoceras, 643
Dinornithide, 613
Dinosauria, 596
670

Dinotheride, 644
Dinotherium, 644
Dicecious, 118
Diopatra, 313
Diotocardia, 378
Diphasia, 242
Diphycercal fin, 41, 562
Diphyes, 244, 245
Diphyodont, 625
Diploblastica, 230
Diplocardia, 316
Diplopoda, 459, 496
Diploria, 261
Diplospondyli, 570
Diplozoon, 273
Diplozoon, development of, 165
Dipneumones, 453
Dipneumonia, 579
Dipneusti, 579

Dipnoi, 579

Diporpa, 165, 274
Diprotodon, 634
Diprotodonta, 633
Diptera, 491

Dipurena, 240, 242
Dipylidium, 287, 289
Direct development, 160
Directive corpuscles, 140
Directive spindle, 146
Directives, 254

Discina, 328
Discodermia, 226
Discodrilidz, 315
Discoidal placenta, 634
Discomedusz, 250
Disconanthe, 245
Discophori, 318
Dispermy, 148
Distaplia, 510
Distichalia, 340
Distichopus, 315
Distomiz, 274
Distomum, 116, 272, 275, 276
Vistribution, 40

Disuse, 55

Division of labor, 72, 165
Dobsons, 482
Docoglossa, 378

INDEX.

Docophorus, 479
Dodo, 614

Dog-day harvest fly, 489
Dogfish, 569, 571
Dog, prairie, 639
Dog sharks, 571
Dogs, 647
Dolichonyx, 616
Doliolum, 512
Dolomedes, 453
Dolphins, 646
Dondersia, 358
Doridiide, 382

Doris, 382
Doryphora, 485
Dorsal aorta, 548
Dorsal fin, 526, 562
Dorsal organ, 341
Draco, 599

Dragon flies, 479
Dranculus, 303
Dreissenia, 367
Drepanidotzenia, 289
Drills, 651

Dromeus, 613
Dromatherium, 632
Drum of ear, 544
Duck, 615

Duckbill, 632

Ducts, genital, 120
Ductus Botallii, 550
Ductus choledochus, 546
Ductus cochlearis, 543
Ductus ejaculatorius, 120
Dugong, 645
Duplicidentata, 639
Dynastes, 484
Dysmorphosa, 241
Dysodont hinge, 359
Dytiscide, 484

Eagles, 617

Ear bones, 525

Ear of vertebrates, 542
Earth worms, 315

Ear wig, 480
Ecardines, 328

 

Ecdysis, 399
Echeneis, 577

Echidna, 631, 632
Echidnidz, 632
Echinarachnius, 345, 346
Echinobothrium, 286
Echinocardium, 34°
Echinococcus, 288
Echinoderidz, 295
Echinoderma, 329
Echinoidea, 343
Echinorhynchus, 304
Echinospheerites, 342
Echiuroidea, 317
Echiurus, 317

Eciton, 488

Ecology, 4

Ectethmoid bone, 522
Ectochondrostoses, 519
Ectocyst, 322

Ectoderm, 103, 156
Ectoparasites, 169
Ectopistes, 614
Ectoprocta, 322
Ectosarc, 189

Edentata, 635
Edriophthalmata, 438
Edrioasteroidea, 342
Edwardsia, 253, 255, 259
Edwardsiella, 259

Egg cell, 80

Egg of bird, 153

Egg, cleavage of, 149, 151
Egg, fertilization of, 147
Egg, maturation of, 146
Egg nucleus, 146, 149
Egg, segmentation of, 149, 151
Egg tooth, 593
Eichhorn, 13
Eiderduck, 615

Eimeria, 213

Elaps, 601

Elasipoda, 349
Elasmobranchii, 569
Elastic cartilage, 86
Elastic tissue, 85
Elastica externa, 516
Elastica interna, 516
Elastin, 85

 

INDEX. 671

Elater, 199

Electric catfish, 576
Electric eel, 576
Electric organs, 563
Elephantiasis, 304
Elephantidz, 644
Elephants, 643
Elephas, 644

Elytra, 312, 466
Embiotocidz, 577
Embryo, 160
Embryology, 3, 139, 160
Emu, 613

Enamel, 515
Enchylema, 62
Enchytreide, 315
Encyrtidium, 193, 196
Encystment, 184
Endite, 410
Endocyst, 322
Endolymph, 127, 543
Endolymphatic duct, 542
Endopodite, 410
Endostyle, 506
English sparrow, 616
Enhydris, 647
Enopla, 290
Ensatella, 368
Entalis, 369
Entelops, 635
Enteroceele, 109
Enteroponeusta, 512
Entoconcha, 349
Entochondrostoses, 519
Entoderm, 103. 156
Entomobrya, 477
Entomostraca, 414
Entoniscus, 441
Entoniscidz, 441, 442
Entoparasites, 169
Entophaga, 486
Entoplastron, 594
Entoprocta, 321
Entosarc, 189
Entovalva, 349
Environment, 54
Eocene, 181
Eohippus, 643
672

Eozoon, 180, 198
Epaxial muscles, 518
Epeira, 451, 453
Ependyma, 124, 532
Ephemera, 479
Ephemerida, 479
Ephippium, 417
Ephydatia, 227
Ephyra, 246, 248, 249
Epiblast, 156
Epibdella, 274
Epididymis, 320, 552
Epigenesis, 16
Epiglottis, 547
Epimerite, 214
Epiotic bone, 522
Epipharynx, 463
Epiphragm, 372
Epiphysis, 535
Epiplastron, 595
Epipleural bones, 574
Epipodite, 410
Epipodium, 369
Epipterygoid bone, 598
Episternum, 528
Epistropheus, 590
Epistylis, 208, 211
Epitheca, 256
Epithelial tissues, 73
Epithelium, germinal, 118
Epitoke, 311
Epizoanthus, 170, 259
Equatorial furrow, 151
Equatorial plate, 69
Equidz, 641
Equilibration, organs of, 128
Equus, 641, 643

Erax, 493

Erethyzon, 639
Eretmochelys, 596
Erichthus, 429, 430
Erigone, 453
Frinacidz, 637
Eristalis, 493

Ermine, 647
Errantia, 313
Erythroblasts, 89
Erythroneura, 490

 

INDEX.

Eschara, 324

Esocide, 576

Essence of pearl, 558
Estheria, 417

Estheriidz, 417
Ethiopian region, 176, 178
Ethmoidalia, 521
Ethmoid bone, 523, 620
Euchilota, 240
Eucopepoda, 421
Eucratea, 324
Eucrinoidea, 342
Eudendrium, 232, 242
Eudoxia, 166, 244
Euflagellata, 202
Euglena, 200, 202
Euglenide, 202
Euglypha, 198
Euisopoda, 442

Eumeces, 599

Eunectes, 601

Eunice, 311

Eunicide, 313
Eupagurus, 170, 435, 436
Euphausia, 429
Euplectella, 226
Euplexoptera, 480
Eupolia, 292

Euryalide, 338
Eurypauropus, 497
Eurypterida, 444
Eurypterus, 444
Euselachii, 571
Euspongia, 221, 227
Eustachian tube, 544
Eustachius, 12

Eusuchia, 602

Eutainia, 601

Eutima, 240

Evadne, 417, 419
Everyx, 495

Evolution, 16

Evolution, Theory of, 19, 25
Evolution vs. Creation, 22
Excreta, 73, 103
Excretory organs, 115
Excretory organs of Vertebrates, 550
Exite, 410
Exoccipital bone, 522
Exoceetidee, 576
Exoccetus, 576
Exopodite, 410

Eugyra, 510
Extracapsulum, 193
Exumbrella, 235

Eyes, 129

Eyes of Vertebrates, 539
Eye spot, 183

Fabricius, 13

Face, bones of, 525
Faceted eye, 403, 404
Facial nerve, 536

Factors of evolution, 44
Fairy shrimp, 417
Falciform spores, 215
Falco, 617

Falcons, 617
Falconiformes, 616
Fasciolaria, 276

Fat body, 407, 468
Faunal provinces, 175
Favia, 260, 261
Favositidz, 259

Feather tracts, 603
Feathers, 603

Feathers, molting of, 611
Fecundation, 147

Felidz, 647

Felis, 647

Femoral pores, 589
Femur, 463, 529

Fenestra ovalis, 544
Fenestra rotunda, 544, 593
Fertility of hybrids, 2
Fertilization of eggs, 147, 148
Fertilization in Protozoa, 206
Fibre, 639

Fibres, nerve, 94

Fibula, 529

Fibulare, 529

Fiddler crab, 437

Field mice, 639

Fierasfer, 349

Filaments, mesenterial, 252
Filar substance, 61

 

INDEX. 673

Filaria, 303
Filibranch, 362
Filibranchiata, 365
Fin backs, 646
Finches, 616

Fins, 526, 562
Fireflies, 484
Firmisternia, 588
Fish hawk, 617
Fishes, 557

Fishes, tails of, 41
Fishes, circulation in, 112
Fissilinguia, 599
Fissipedia, 647
Fissurella, 378
Fissurellidze, 379
Fissures of the cord, 532
Flabellum, 410
Flagellata, 200
Flagellate epithelium, 75
Flagelluin, 376
Flame cell, 116, 280
Flamingo, 615

Flat worms, 267
Flea, snow, 477
Fleas, 493

Flesh flies, 493
Flies, 491

Flies, black, 493
Flies, blow, 493
Flies, bot, 493
Flies, caddis, 483
Flies, crane, 492
Flies, dragon, 479
Flies, fire, 484
Flies, flesh, 493
Flies, gall, 486
Flies, harvest, 489
Flies, Hessian, 492
Flies, horse, 493
Flies, house, 492, 493
Flies, May, 479
Flies, robber, 493
Flies, saw, 485, 486
Flies, Spanish, 484
Flies, stone, 479
Fluke, 276

| Flustra, 324
674

Flustrella, 324

Flying fish, 576

Flying foxes, 635
Flying squirrel, 639
Fodientia, 636
Fontanelles, 521

Food vacuole, 183
Food yolk, $0

Foot, 351

Foramen magnum, 522
Foramen Panizze, 592
Foraminifera, 196
Fore brain, 533

Fore gut, 104
Forficula, 480
Formative yolk, $0
Formicariz, 487

Fossa rhomboidalis, 535
Fossores, 486

Fowl, 613

Fowl, digestive tract of, 105
Fowl, egg of, 153
Foxes, 647

Frenulum 494
Fringillidee, 616
Fritillaria, 506

Frogs, 588

Frons, 462

Frontal bone, 523
Frontal sinus, 624
Frontoparietal bone, 581
Frugivora, 638

Fulcra, 573

Function, change of, 100
Function, community of, 165
Fungia, 261

Fungiacea, 261
Funiculus, 322

Furca, 420

Furcula, 605

Fur seal, 648

Gadide, 578
Gadus, 577, 578
Galea, 463
Galeide, 571
Galen, 12
Galeodes, 450

 

INDEX.

Galeopithecide, 649
Galeopithecus, 637, 649
Galeus, 571

Gall flies, 486
Gallinacea, 613

Galls, 486

Gallus, 614

Gamaside, 454
Gamasus, 400, 454
Gammarina, 439
Gammarus, 439
Ganglion, buccal, 390
Ganglion cells, 94
Ganglion, cerebral, 123
Ganglion, optic, 129
Ganglion, stellate, 390
Ganglion, supracesophageal, 123
Ganglionic nervous system, 122
Ganoid scale, 572
Ganoidei, 558

Ganoin, 558

Gapes, 302

Garpike, 574

Garter snake, 601
Gasteropoda, 369
Gasterosteus, 577

Gastral tentacles, 246
Gastrochzena, 368
Gastrophilus, 493
Gastrotricha, 295
Gastrovascular space, 228
Gastrovascular system, 109
Gastrula, 156
Gastrulation, 156
Gavialis, 602

Gazella, 642

Gecarcinus, 437

Gecko, 598

Gegenbaur, 18
Gelasimus, 437
Gemmaria, 241
Gemmellaria, 324
Gemmulx, 227
Gemmularia, 241

Gena, 414, 462
Generation, asexual, 140
Generation by parents, 140

Generation, sexual, 142
Generation, spontaneous, 139
Generations, alternation of, 144
Genital ducts, 120

Genital plates, 344

Genus, 10

Geocores, 489

Geodia, 227

Geographical distribution, 174
Geological distribution, 180
Geometrina, 494
Geonemertes, 291
Geophilidz, 461

Geophilus, 461

Gephyrza, 316

Gerardia, 259

Germinal disc, 152

Germinal epithelium, 118
Germinal vesicle, 81, 146
Germ layer theory, 17

Germ layers and budding, 159
Germ layers, formation of, 156
Geryonia, 242

Geryonid, delamination in, 157
Geryonid, germ layers, 157
Giant cells, 71

Gibbons, 651

Gigantostraca, 443

Gila monster, 599

Gill arch, 524

Gill arteries, 504, 548

Gill clefts, 501, 547

Gill leaves, 361

Gill slits, 501, 547

Gill, tracheal, 469

Gills, 108

Gills of fishes, 565

Gills of vertebrates, 547

Gipsy moth, 119, 495

Giraffa, 642

Girdles, 527

Gizzard, 106, 461

Glabella, 414

Gland cells, 76

Glands, 77

Glands, castor, 618

Gland, choroid, 564

Glands, germinal, 118

Gland, Harder’s, 542

INDEX. 6

 

=f

Gr

Glands, hoof, 618
Gland, lachrymal, 542
Glands, lymph, 550
Gland, lymphoid, 331
Glands, mammary, 619
Glands. milk, 619
Glands, musk, 618
Gland, nidamental, 392
Gland, ovoid, 331
Gland, paraxon, 331
Gland, parotid, 534
Glands, sexual, 80, 117
Glands, sweat, 618
Gland, subneural, 509
Glands, suborbital, 618
Gland, thymus, 547
Gland, thyroid, 547
Glandular epithelium, 73, 76
Glaser’s fissure, 621
Glass crab, 436

Glass snake, 599

von Gleichen, 13
Globiceps, 241
Globigerina, 197, 198
Globiocephalus, 646
Glochidium, 364
Glomeridz, 497
Glomerulus, 117, 552°
Gloss, 464
Glossopharyngeal nerve, 536
Glottis, 547

Glugea, 218

Glutin, 85

Glutton, 647

Gly ptodontidze, 636
Gnathobdellidz, 321
Gnathochilarium, 496
Goat, 642

Goblet cells, 77

Goblet organs, 307
Goethe, 14, 21

Geeze, 13

Gomphus, 479
Gonads, 117
Goniatites, 394.
Goniodes, 479
Gonochorism, 118
Gonodactylus, 429
676

Gonophore, 238
Gonotheca, 242
Gonys, 604

Goose barnacle, 425
Gopher turtle, 596
Gordiacea, 304
Gordius, 304
Gorgonidee, 259
Gorilla, 651
Gradientia, 587
Grallatores, 615
Grantia, 225
Grasshoppers, 48, 481
Gray matter, 124, 532
Grebes, 615

Green gland, 411
Green turtle, 596
Gregarina, 213, 215
Gressoria, 480
Gribble, 442

Gromia, 62, 198
Ground substance, 62
Grouse, 614
Gruiformes, 615
Grus, 615

Gryllide, 481
Gryllotalpa, 481
Gryllus, 481

Guanin, 558

Guard, 389

Guinea pig, 639
Guinea worm, 303
Gula, 462

Gulls, 615

Gulo, 647

Gunda, 271
Gymnoblastea, 241
Gymnodonti, 578
Gymnolemata, 323
Gymnonoti, 576
Gymnophiona, 587
Gymnosomata, 382
Gynzcophoral canal, 119
Gynandromorphism, 277
Gyri, 535
Gyrodactylus, 273, 274

Tlabrocentrum, 453

 

INDEX.

Haddock, 578
Hadeneecus, 481
Haeckel, 18, 24
Hemadipsa, 321
Heemal arch, 516
Heemal ribs, 518
Heemal spine, 517
Heemapophysis, 517
Heemoccele, 109, 113
Hemoglobin, 89
Heemosporida, 216
Hemuntaria, 321
Hagfishes, 555
Hair, 617
Hair necks, 302
Hair worm, 304
Hairs, auditory, 127
Hairs, tactile, 126
Halcampa, 259
Haleremita, 241
Haliztus, 617
Halibut, 578
Halicore, 645
Halicryptus, 317
Haliomma, 135
Haliommide, 196
Haliotidz, 379
Haliotes, 379
Halisarea, 226
Halitherium, 645
von Haller, 17
Halowises, 239
Halteres, 491
Halyclystus, 250
Hammerhead shark, 571
Hapale, 651
Hapalidz, 651
Harder’s gland, 542
Hares, 639
Harpactida, 421
Hatteria, 596
Haustellum, 465
Haversian canals, 87
Haversian lamell, 87
Tawks, 617
Head kidney, 310, 550
Head, segments of, 536
Hearing, organs of, 127

a)
Heart, 111

Heart shells, 367

Heat rigor, 63
Hectocotylus, 393
Hedgehogs, 637
Heliaster, 337

Helicide, 383
Heliopore, 259
Heliozoa, 190

Helix, 383

Hell-bender, 587
Hellgrammite, 482
Helminthes, 169
Helminthophaga, 616
Heloderma, 599
Helodermatide, 599
Hemelytra, 489
Hemerobiide, 482
Hemibranchii, 575. 577
Tlemichordia, 512
Hemimetabolous, 473
Hemiptera, 489
Hemitripterus, 577

Hen, 613

Hen clam, 368
Hepatopancreas, 106, 411
Hepatus, 437
Heptanchus, 570
Heredity, 67, 150
Hermaphroditism, 118
Hermit crabs, 436
Herons, 615

Herring, 576
Hesperornis, 612
Hessian fly, 492
Heterakis, 301
Heteraxial symmetry, 136
Heterocercal tail, 41, 562
Ileteroconchiz, 367
Heterocotylea, 273
Heterodera, 300
Heterodont dentition, 625
Heterodont hinge, 359
Heterogony, 144, 145, 486
Heteromera, 484
Heteromyaria, 367
Heteronemertini, 292
Heteronereis, 311

 

INDEX. 677

Heteronomy, 138
Heteropleuron, 504
Heteropoda, 380
Heteroptera, 489
Heterosyllis, 311
Heterotricha, 209
Hexacoralla, 259
Hexactinellidz, 226
Hexamita, 201
Hexanchus, 570
Hexapoda, 461

Hind brain, 533

Hind gut, 104

Hinge, 358

Hinny, 641
Hipparion, 643
Hippasterias, 337
Hippide, 437
Hippocampus, 578
Hippocrates, 12
Hippocrene, 241
Hippoglossus, 578
Hippolyte, 434
Hippopotamidz, 641
Hippopotamus, 641
Hippospongia, 227
Hirudinei, 318
Hirudo, 321
Hirundinidz, 616
Hirundo, 616
Holoblastic cleavage, 153
Holoblastic eggs, 152, 153
Holocephali, 572
Holocystites, 342
Holometabolous, 473
Holostei, 573
Holostomate, 371
Holothuria, 349
Holothuria, gastrula of, 158
Holothuridea, 346
Holotricha, 209
Homarus, 435
Homaxial animals, 135
Homo, 651
Homocercal tail, 41, 563
Homoiothermous, 115
Homology, 14, 100
Homonomy, 138
678

Homoptera, 489
Honey ant, 488
Honeycomb, 641
Hoofs, 618

Hooker, 24
Hoploceras, 643
Hoplorhynchus, 213
Hop worm, 495
Hormea, 324
Hormiphora, 262, 264
Horn bills, 616
Horns of cord, 533
Horned toad, 599
Horn tails, 486
Horse flies, 493
Horse mackerel, 577
Horses, 641
Horseshoe crab, 444
House fly, 492, 493
Human embryo, 35
Humerus, 529
Humming birds, 616
Huxley, 18, 24
Hyena, 647
Hyeenide, 647
Hyalea, 381
Hyaleide, 382
Hyaline cartilage, 86
Hyalonema, 226
Hyalopus, 198
Hyalospongia, 226
Hyas, 437

Hyatt, 24

Hybrids, 28
Hydnophyton, 488
Hydra, 230, 240
Hydra, section of, 141
Hydrachna, 454
Hydrachnide, 454
Hydractinia, 241
Hydranth, 231
Hydraria, 239, 240
Hydrichthys, 240, 242
Hydrobatide, 489
Hydrocaulus, 231
Hydrocheerus, 639
Hydrocorallina, 239, 241
Hydrocores, 489

 

INDEX.

Hydroides, 313
Hydromeduse, 230
Hydrophilide, 484
Hydropolyp, 230
Hydropsyche, 483
Hydrorhiza, 231
Hydrosauria, 594
Hydrotheca, 233
Hydrozoa, 230

Hyla, 588

Hylesinus, 485
Hylide, 588
Hylobates, 651
Hylodes, 586
Hymenolepis, 287, 288
Hymenoptera, 485
Hyocrinus, 340

Hyoid arch, 524
Hyoid bone, 524
Hyoid cartilage, 524
Hyomandibular, 524, 525
Hypeena, 495
Hypaxial muscles, 518
Hyperia, 439
Hyperina, 439
Hyperoartia, 557
Hyperotretia, 557
Hypoblast, 156
Hypobranchial groove, 503
Hypoderma, 493
Hypodermis, 398
Hypogeophis, 587
Hypoglossal nerve, 536
Hypopharynx, 463
Hypophysis, 535
Hypoplastron, 595
Hyporachis, 603
Hypotricha, 211
Hyracoidea, 644
Hyracotherium, 643
Hyrax, 644
Hystricide, 639
Hystricomorpha, 639
Hystrix, 639

Tapyx, 477
Ibis, 615
Ichneumonidze, 486
Ichthydium, 295
Ichthyobdella, 321
Ichthyodolurites, 570
Ichthyophis, 585, 587
Ichthyopsida, 555
Ichthyosauria, 594
Ichthyotomi, 572
Icteride, 616

Icterus, 616
Idiothermous, I15
Idotea, 441, 442
Idoteidz, 442

Idyia, 264

Iguanide, 599

Tlium, 528

Ilyanassa, 379

Imaginal discs, 476
Impennes, 615
Impregnation, 147
Inbreeding, 29

Incisor teeth, 625

Incus, 525, 544
Indirect cell division, 68
Indirect development, 160
Inermes, 317
Infrabasalia, 340
Infundibulum, 534
Ingluvies. 106, 467

Inia, 645

Inorganic bodies, 133
Inquilines, 486

Insecta, 458

Insectivora, 637

Insects, cleavage of egg, 155
Integripalliata, 367
Interambulacral platg, 335
Intercalaria, 516
Interfilar substance, 62
Interhyal bone, 561
Intermaxillary bone, 525
Intermedium, 529
Interorbital septum, 560
Interparietal bone, 619
Interradius, 246
Intervertebral ligament, 519
Intestine, 106
Invagination, 156
Inversion of retina, 541

 

INDEX. 679

Tris, 130, 131
Irritability, 62
Ischial callosities, 651
Ischium, 528
Isinglass, 573
Isis, 259

Isodont hinge, 359
Isopoda, 440
Isoptera, 478
Itch, 454

Iter, 534

Tulidz, 497

Tulus, 497

Ixodes, 454
Ixodidee, 454

Jacobson’s organ, 539
Jassidze, 490

Jays, 616

Jigger, 494

Jugal arch, 526
Jugal bone, 526
Jugulares, 562
Jugular vein, 549
June bug, 484
Jurassic, 180

Kallima, 47
Kangaroos, 634
Karyokinesis, 68
Katydid, 481
Keyhole limpets, 379
Kidneys, 116, 550
Kielmeyer, 15
Kinetoskias, 324
King crab, 444
King fishers, 616
Kinosternon, 596
Kiwi, 613
Keenenia, 449
Kolliker, 18
Kowalewskia, 506
Kowalewsky, 18

Labial cartilage, 534
Labial palpi, 362
Labidura, 480
Labium, 463, 464
680

Labor, division of, 165
Labride, 576

Labrum, 463
Labyrinth, 128, 542
Labyrinthodonta. 586
Lac, 490

Lacerta, 599
Lacertilia, 598
Lacertilidz, 599

Lace wings, 482
Lachrymal bone, 590
Lachrymal gland, 542
Lacinia, 473

Lacteal dentition, 625
Lacuna, 379

Lacunar blood system, 113
Ladder nervous system, 124
Lady bird, 485

Lady crab, 436
Lemodipoda, 439
Lagena, 543

Lagomys 639

Lama, 643

Lamarck, 14, 22
Lamarckism, 53
Lamblia, 202

Lamelle, Haversian, 87
Lamelle bone, 87
Lamellibranchiata, 358
Lamellicornia, 484
Lamellirostres, 615
Lamna, 571

Lamne, 618

Lamprey eels, 555, 557
Lampyridz, 484

Land crab, 437
Lanistes, 372

Lantern of Aristotle, 345
Larus, 615

Larva, 160

Larval organs, 161
Laryngeal cartilages, 524
Lateralia, 424

Lateral line, 537. 564
Lateral teeth, 359
Latrodectes, 451
Laurer’s canal, 273
Leaf butterflies, 47

INDEX.

 

Leaf hoppers, 489
Leatherback tortoise, 595
Leather turtle, 596
Leda, 367

Leeches, 318
Leeuwenhoek, 13
Lemniscus, 304
Lemuride, 649
Lemurs, 648, 649
Lens of eye, 130, 131, 541
Lepadide, 425
Lepas, 172, 425
Lepidonotus, 313
Lepidoptera, 494
Lepidosauria, 594, 597
Lepidosiren, 579
Lepidosteide, 574
Lepidosteus, 574
Lepidurus, 416
Lepisma, 477
Lepralia, 324
Leptalis, 48
Leptasterias, 337
Leptocardii, 502
Leptocephalus, 575
Leptochela, 441, 442
Leptoclinum, 510
Leptodiscus, 204
Leptodora, 417
Leptomedusz, 239, 242
Leptoplana, 110, 271
Leptostraca, 427
Lepus, 639

Lerneea, 422
Lernzidz, 422
Lernzeocera, 421, 422
Lernzopodide, 422
Leucania, 495
Leucetta, 225
Leuckart, 18
Leucocytes, 88
Leucon, 223
Leucones, 226
Leucortis, 226
Leucosoidea, 437
Leucosolenia, 225
Libellula, 479
Libellulidz, 479
Libinia, 436, 437
Lice, 491

Lice, bird, 479

Lice, book, 479

Life, origin of, 140
Ligula, 286
Ligulide, 286
Limacide, 383
Limacinidee, 382
Limax, 383

Limbs of vertebrates, 527
Limicola, 315
Limitans cxterna, 540
Limitans interna, 540
Limnadia, 417
Limnea, 383
Limneide, 3383
Limnocnida, 239
Limnocodium, 239
Limnoria, 441, 442
Limnothrips, 479
Limpets, 379
Limulus, 444
Linckia, 334

Linear nervous system, 122
Linerges, 250

Lines of growth, 358
Lineus, 289, 292
Lingual ribbon, 355, 373
Linguatulida, 454
Lingula, 328

Linin, 65

Linnzean system, 10
Linneeus, Io
Liobunum, 451

Lion, 647

Liriope, 239, 242
Lithistidze, 226
Lithobiidz, 461
Lithobius, 461
Lithodidze, 437
Lithodomus, 367
Littorina, 379
Littorinidz, 380
Liver, 106

Liver fluke, 276
Lizards, 598

Lizzia, 240, 242

 

INDEX. 681

Lobate, 264

Lobate foot, 614, 615
Lobi inferiores, 563
Lobosa, 189

Lobster, 435

Lobster, spiny, 436
Locomotion, 121
Locustide, 481
Locusts, 481, 489
Loggerhead, 596
Loligo, 384, 395
Loligo, cleavage of, 155
Longipennes, 615
Loons, 615
Lophobranchii, 578
Lophodont teeth, 626
Lophogastridz, 429
Lophophore, 324
Lophopoda, 324
Lophopus, 324
Lophs of teeth, 626
Lorica, 200

Loricata, 435, 577, 601, 636
Loris, 649

Lota, 578

Love dart, 376
Loven’s larva, 309
Loxia, 616
Loxosoma, 322
Lucernariz, 250
Luciz, 510
Lumbricus, 315, 316
Lumbricus, anatomy of, 118
Lunatia, 379, 380
Lung book, 443

Lung fishes, 579
Lungs, 109, 547
Lung sac, 443

Lung sacs of birds, 609
Lutra, 647

Lycosa, 453

Lyell, 23, 24
Lygzide, 489
Lymph, 88, 90
Lymph corpuscles, 90
Lymph glands, 550
Lymph system, 550
Lymph vessels, 114
682 INDEX.

Lymphoid gland, 331
Lyonet, 13

Lyre birds, 616
Lyriform organs, 445
Lytta, 484

Macacus, 651
Macaques, 651
Machilis, 458, 477
Mackerel, 577
Mackerel shark, 571
Macoma, 368
Macreesthete, 357
Macrobdella, 321
Macrobiotus, 455
Macrochelys, 596
Macrochiroptera, 638
Macrodrila, 315
Macrogamete, 185
Macronucleus, 206
Macropodide, 633
Macropus, 634
Mactra, 359
Mactride, 368
Macrura, 434
Madrepora, 261
Madreporaria, 260
Madreporite, 330
Maioidea, 437
Malaclemmys, 596
Malacobdella, 291
Malacoderma, 259
Malacopoda, 456
Malacopteri, 574
Malacostraca, 426
Malagassy region, 178
Malapterurus, 576
Malar bone, 526, 620
Malaria, 217, 492
Maldanidz, 313
Malleus, 525, 544
Mallophaga, 479
Malpighi, 13
Malpighian body, 117
Malpighian tubes, 438, 445, 459, 461
Mammalia, 617
Mammals, 617
Mammoth, 644

 

Man, 651

Manatee, 644
Manatus, 644
Mandible, 401
Mandibles, 463, 464
Mandibular arch, 524
Mandibular cartilage, 524
Mandrils, 651
Manicina, 261

Manis, 636

Manna, 489
Manubrium, 235
Mantidz, 480

Mantis, 480

Mantis shrimp, 429
Mantle, 351, 505
Mantle cavity, 352
Manyplies, 642
Manyunkia, 313
Margarita, 379
Margelis, 144, 241
Marginal plates, 595
Marine faunz, 179
Marmosets, 651
Marsipobranchii, 555
Marsupialia, 632
Marsupial bones, 631, 632
Marsupium, 632
Marten, 647

Mastax, 294
Mastigameceba, 188, 201
Mastigophora, 200
Mastodon, 644
Maturation of egg, 146
Maturation and Fertilization, 147
Matuta, 437

Maxilla, 401, 463
Maxillary bone, 525
Maxillary sinus, 624
Maxillipeds, 401

May flies, 479

Measly meat, 284, 285
Measuring worms, 494
Meckel, 14

Meckelia, 292
Mecoptera, 483
Mediastinum, 546
Medulla oblongata, 534
Medullary plate, 501
Medullary sheath, 96
Medusez, 144, 230, 234
Megalops, 434
Megalonyx, 636
Megalospheres, 198
Megatherium, 636
Megascolex, 316
Megastoma, 202
Meissner’s corpuscles, 126
Melanoplus, 481
Meleagrina, 367
Meleagris, 614

Melitta, 346

Meloide, 484
Melolontha, 484
Melonites, 345
Melophagus, 493
Melopsittacus, 616
Membracide, 490
Membrane bones, 515
Membranipora, 324
Membranella, 209
Membranous cranium, 519
Menopoma, 587

Mentum, 464

Menuride, 619

Mephitis, 647

Meridional furrows, 151
Mermithide, 304
Meroblastic cleavage, 153
Meroblastic eggs, 152, 154
Meryhippus, 643
Mesectoderm, 222
Mesencephalon, 533
Mesenchyme, 157
Mesenterial filaments, 252
Mesenteries, 109, 545
Mesenteron, 105
Mesethmoid bone, 522
Mesites, 613

Mesoblast, 157
Mesobronchus, 609
Mesoderm, 104, 157
Mesogloea, 230
Mesohippus, 643
Mesonemertini, 291
Mesonephros, 550

INDEX. 683

 

Mesonephric duct, 550
Mesopterygium, 529
Mesorchium, 546
Mesotroche, 309
Mesosternum, 462
Mesothelium, 158
Mesothorax, 462
Mesozoic era, 180
Mesovarium, 546
Metabolism, 172
Metacarpal bones, 529
Metagenesis, 144
Metameres, 137, 305
Metamerism, 137
Metamorphosis, 161
Metamorphosis of insects, 473
Metanemertini, 291
Metanephric duct, 550
Metanephros, 550
Metapodium, 369
Metapterygium, 529
Metastoma, 430
Metatarsal bones, 529
Metathorax, 462
Metazoa, 221
Metencephalon, 533
Methona, 48
Metridium, 259
Miastor, 492

Mice. 639
Micrzesthete, 357
Microcentrum, 481
Microchiroptera, 638
Microconodon, 632
Microcotyle, 274
Microdrilze, 315
Microgametes, 185
Microlepidoptera, 494
Microlestes, 632
Micronucleus, 206
Micropterus, 577
Micropylar apparatus, 148
Microspheres, 198
Microstomidee, 271
Microthelyphonida, 448
Micrura, 292

Midas, 651

Mid brain, 533
684

Middle Ages, Zoology in, 9
Mid gut, 105
Miescher’s corpuscles, 218
Migration of birds, 612
Migration theory, 52
Miliola, 197, 198
Milk teeth, 625
Millepora, 233, 241
Mimicry, 46

Mink, 647

Miocene 181
Miohippus, 643
Miracidium, 276
Mites, 453

Mitosis, 68
Mixipterygium, 570
Mnemiopsis, 264
Moccasin, 601
Modiola, 366, 367
Molar teeth, 625
Mole cricket, 481
Moles, 637

Molgula, 510
Molgulidz, 510
Mollusca, 351
Molpadia, 349
Menactinellide, 227
Monadina, 202
Monascidiz, 510
Monaxial symmetry, 135
Monera, 189
Moniezia, 287, 289
Monitor, 599
Monkeys, 651
Monocaulis, 240, 241
Monocystis, 215
Monodelphia, 634
Monodon, 646
Monogenea, 273
Monogony, 140
Monomyaria, 367
Monops, 271
Monophyodont, 625
Monopneumonia, 579
Monopylea, 196
Monorhina, 556
Monoscelis, 271
Monospermy, 148

INDEX.

Monostomum, 275
Monothalamia, 198
Monotocardia, 379
Monotremata, 631
Moose, 642
Morphology, 2
Morphology, development of, 12
Mosaic vision, 406
Mosasaurus, 600
Moschide, 642
Moschus, 642
Mosquitos, 492
Mosquitos and malaria, 217
Moths, 494

Mouse, 639

Mud crab, 437
Mud puppy, 587
Mud turtle, 596
Miiller, Fritz, 24
Miiller, J., 18
Miiller, O. F., 13
Miillerian duct, 551
Miiller’s fibres, 540
Mule. 641
Multicellular glands, 77
Multicellularity, 70
Multinuclearity, 70
Multituberculata, 632
Muricidz, 380
Mus, 639

Musca, 492, 493
Muscariz, 493
Muscide, 493
Muscle cells, 92
Muscle fibres, 91
Muscular tissue, 91
Musculature, 121
Musk deer, 642
Musk ox, 642
Musk rat, 639
Mussels, 367
Mustela, 647
Mustelidee, 647
Mustelus, 571

Mya, 368
Mycetozoa, 198
Myctodera, 587
Myelin, 96

 
Mygale, 453
Mygalide, 453
Mygnimia, 49
Myidz, 368
Mylodon, 636
Myocommata, 531
Myomerism, 523
Myomorpha, 639
Myopsida, 395
Myosepta, 531
Myotomes, 531
Myrianida, 310, 313
Myriapoda, 408, 459, 496
Myriothelia, 241
Myriotrochus, 349
Myrmecocystus, 488
Myrmecophaga, 636
Myrmecophily, 169
Myrmeleo, 481, 483
Mysidide, 429
Mysis, 428, 429
Mysticetz, 645, 646
Mytilus, 363
Myxicolida, 313
Myxidium, 213, 217
Myxine, 557
Myxobolus, 217
Myxomycetes, 198
Myxospongiz, 225, 227
Myxosporida, 217
Myzobdella, 315
Myzontes, 557

Nacre, 361

Nageli. 24, 54, 55
Naiadz, 367

Naididze, 315

Nails, 618

Nais, 307

Naja, 601

Nandu, 613

Nanomia, 244. 245
Narcomeduse, 239, 242
Narwal, 646

Nasal bone, 523

Nassa, cleavage of, 154
Nassellaria, 196
Natatores, 614

 

INDEX. 685

Naticidz, 380
Natural selection, 44
Nauplius, 37, 413
Nauplius eye, 412
Nausithot, 134, 250
Nautilidz, 394
Nautilus, 387, 394
Nearctic region, 176, 178
Nebalia, 427
Nectonema, 304
Nectonemertes, 291
Necturus, 587
Nectocalyx, 243
Needham’s sac, 392
Nematocysts, 229
Nematoda, 298
Nemathelminthes, 298
Nematophora, 228
Nematus, 486
Nemerteans, 289
Nemertini, 289
Nemocera, 492
Nemognatha, 483
Nemopsis, 242
Neocrinoidea, 342
Neog zea, 177
Neomenia, 358
Neotropical region, 176, 177
Nephilis, 321
Nephridia, 116, 308
Nephrostome, 116
Nepidz, 489
Neptunus, 437
Nereidx, 313
Nereis, 311, 312, 313
Nerve-end buds, 537
Nerve fibres, 94
Nerve hillock, 537
Nerve roots, 533
Nerves of vertebrates, 535
Nervous system, 122
Nervous tissue, 94
Nettle bodies, 205
Nettle cells, 229
Neural arch, 516
Neural plates, 594
Neural spine. 517
Neurapophysis, 517
686

Neurenteric canal, 502, 532
Neurites, 94
Neuropodium, 308, 312
Neuropore, 503, 532
Neuroptera, 481

Neverita, 380

New Zealand, 177
Nictitating membrane, 541
Nidamental glands, 392
Night hawks, 616

Nipple, 619

Nirmus, 479

Noctiluca, 201, 203
Noctuina, 495

Nodes of Ranvier, 96
Nomarthra, 636
Nomenclature, binomial, 10
Non-Ruminantia, 641
Nosema, 218

Nothria, 313

Notochord, 501, 515
Notochordal sheath, 516
Notodelphys, 585
Notodelphidz, 422
Notogzea, 176
Notonectidz, 489
Notopodium, 308, 312
Nototrema,. 585

Notum, 462

Nuclear plate, 595
Nuclear fragmentation, 70
Nuclear spindle, 68
Nuclear substance, 65
Nuclein, 65

Nucleolus, 66

Nucleus, 58, 64

Nucleus, cleavage, 149
Nucleus, egg, 146
Nucleus in fertilization, 149
Nucleus of Salpa, 511
Nucleus, significance of, 67
Nucleus, somatic, 208
Nucleus, sperm, 149
Nucleus, substance of, 65
Nucleus, structure of, 65
Nucula, 365, 367
Nuculidze, 367

Nuda, 264

 

INDEX.

Nudibranchia, 382
Nummulites, 198

Nurse, 144

Nutrition and reproduction, 64
Nyctotherus, 210

Nymphon, 456

Obelia, 240, 242
Obisium, 450
Obturator foramen, 622
Occipitalia, 521
Occipital bone, 521, 619
Occiput, 462
Ocellate, 239

Ocellus, 129, 403
Ocneria, 119, 495
Octocoralla, 258
Octopoda, 395
Octopodide, 395
Octopus, 384, 390, 394, 395
Ocular plate, 335
Oculina, 261
Oculomotor nerve, 536
Odonata, 479
Odontoholce, 612
Odontophore, 373
Odontorme, 612
Odontornithes, 612
CEcanthus, 481
(Ecology, 457, 164
CEdipoda, 481
CEdogonium, 173
CEgopsida, 394
(Esophageal ring, 124
(Esophagus, 106, 546
CEstridze, 493

Estrus, 193
Oikopleura, 506, 507
Oil bottle, 484
Olfactory organs, 126
Olfactory organs of vertebrates, 538
Olfactory lobe, 534
Olfactory nerve, 536
Oligocene, 181
Oligochzetee, 314
Oligosoma, 599
Oligotrochus, 349
Olividze, 380
INDEX. 687

Olynthus, 222

Omasum, 642
Omentum, 546
Ommastrephes, 388, 395
Ommatidium, 405
Oncosphera, 283
Oniscide, 442

Oniscus, 442

Ontogeny, 3, 160
Oospore, 185

Ootype, 272

Opalina, 209

Opercular bones, 562, 566
Opercularella, 242
Operculum, 210, 323, 371, 440, 566
Ophidia, 600
Ophidiaster, 334
Ophiocoma, 338
Ophiocnida, 338
Ophioglypha, 338
Ophiopholis, 338
Ophiothelia, 338
Ophisaurus, 599
Ophisthotic bone. 522
Ophiuroidea, 337
Opisthobranchia, 381
Opisthoccelous, 519
Opisthopatus, 458
Opossums, 633
Opossum shrimp, 428
Opoterodonta, 601

Optic ganglion, 129
Optic lobes, 534

Optic nerve, 536

Optic stalk, 541

Optic thalami, 534
Optic vesicle, 541
Oralia, 330, 340
Orang-utan, 651

Orange scale insect, 490
Orbitelarize, 453
Obitosphenoid bone, 522
Orchestia, 438, 439
Order, Io

Organic bodies, 133
Organisms, origin of, 139
Organ-pipe coral, 259
Organs, 99

 

Organs, animal, IoI, 121
Organs, of assimilation, 102
Organs, auditory, 127
Organs, of Bojanus, 363
Organs, circulatory, 109
Organs, of Corti, 543
Organs, digestive, 103
Organ, dorsal, 341
Organs, electric, 563
Organs of equilibrium, 128
Organs, excretory, 115
Organs, excretory, of vertebrates, 550
Organ of Jacobson, 539
Organs of heariny, 127
Organs, lateral line, 537
Organs, olfactory, 126
Organs, pearl, 558
Organs, respiratory, 107
Organs, sensory, 125
Organs, sexual, 117
Organs, sexual, of vertebrates, 550
Organs of smell, 126
Organs, systems of, 100
Organs, tactile, 125
Organs, of taste, 126
Organs, of touch, 537
Organs, vegetative, 101, 102
Oriental region, 176, 178
Orioles, 616
Ornithodelphia, 631
Ornithorhynchidz, 632
Ornithorhynchus, 631, 632
Orohippus, 643

Oronasal groove, 538
Orthis, 328

Orthoceras, 394
Orthonectida, 220
Orthoneurous, 374
Orthopoda, 597
Orthoptera, 480
Orycteropus, 636
Oscarella, 227

Oscines, 616

Osculum, 222, 225

Os en ceinture, 581

Ossein, 86

Os transversum, 590

Os turbinale, 620
688

Osmerus, 576
Osphradium, 354
Ossicle, auditory, 127
Ostariophysi, 575
Osteoblasts, 88
Ostracoda, 422
Ostracodermi, 557, 578
Ostracoteuthis, 388
Ostreeidee, 367
Ostium tube, 552
Ostrich, 613
Otaria, 648

Otic ganglion, 537
Otica, 521

Otis, 615

Otocysts, 236
Otoliths, 127
Otter, 647

Ovibos, 642
Ovicells, 322
Ovide, 642
Oviducts, 120
Oviparous, 161
Ovis, 642

Ovoid gland, 331
Ovoviviparous, 161
Owen, 18

Owlet moths, 495
Owls, 617

Ox warble, 493
Oxy-hemoglobin, 89
Oxyrhyncha, 437
Oxystomata, 437
Oxyuris, 301
Oyster crab, 437
Oysters, 367

Pachydermata, 641
Pachydrilus, 315
Pachylemuridz, 649
Paddle fish, 573
Padogenesis, 142, 472
Paguridea, 436
Palearctic region, 176, 178
Palzemon, 400, 434
Paleemonetes, 434
Palemonide, 434
Paleocrinoidea, 342

 

INDEX.

Palzotherium, 643
Paleozoic era, 180
Palate, 539
Palatine bone, 525
Paleacrita, 494
Palechinoidea, 345
Paleontology, 4
Paleozoology, 4
Pali, 256
Palinuridee, 435
Palinurus, 436
Pallial line, 359
Pallial sinus, 360
Pallium, 351, 534
Palmate foot, 614, 615
Palm crab, 436
Palolo, 311

Palpi, labial, 362
Palpus, 430, 463
Paludicella 324
Paludinide, 380
Pancreas, 106
Pandalus, 434, 435
Pandion, 617
Pandionide, 617
Pangolin, 636
Panopeus, 437
Panorpa, 483
Panorpide, 483
Pantopoda, 456
Paper nautilus, 395
Papilio, 496
Parachordals, 520
Paractinopoda, 349
Paradidymis, 552
Paradisea, 50
Paradiseide, 616
Paradoxides, 415
Paraglossa, 464
Paragnath, 430
Paramecium, 206, 207, 209
Paranuclein, 66
Paranucleus, 206
Parapodium, 312, 369
Parapophysis, 518
Parapterium, 604
Paraquadrate bone, 581
Parasita, 422
Parasitism, 167
Parasphenoid bone, 523
Parasuchia, 602
Paraxon gland, 331
Parietal bone, 523
Parietal foramen, 590
Parietal ganglia, 354
Parietal organ, 535
Parostoses, 519
Parotid gland, 534
Parrots, 616
Parthenogenesis, 142, 145, 472
Partial cleavage, 152, 153
Partridge, 614
Parypha, 241

Passer, 616

Passeres, 616
Patagium, 637
Patellidz, 378
Pathetic nerve, 536
Paunch, 641
Pauropida, 497
Pauropus, 497

Pearl organs, 558
Pearl oysters, 367
Pearls, 361

Pearls, artificial, 558
Pébrine, 218
Peccaries, 641
Pecora, 641

Pecten, 366

Pecten of eye, 611
Pectinatella, 324
Pectines, 447
Pectinibranchia, 379
Pectinidz, 367
Pectoral fin, 562
Pectoral girdle, 527
Pedal cords, 354
Pedal gangha, 353
Pedata. 349

Pedes spurii, 475
Pedicellina, 322, 330
Pediculati, 575
Pediculus, 491
Pedipalpi, 448
Pedipalpus, 445
Pelagia, 246, 250

 

INDEX. 689

Pelecanus, 615
Pelecypoda, 358
Pelicans, 615
Pelmatozoa, 338
Pelobatide, 588
Pelomyxa, 189
Peltogaster, 426

Pelvic fin, 526, 562
Pelvic girdle, 527

Pen, 389

Peneide, 434

Penella, 422

Peneus. 434

Penguin, 615

Penis, 120

Pennaria, 241
Pennatula, 259
Pennatulidz, 259
Pentacrinus, 339, 342
Pentacta, 349
Pentadactyle appendage, 529
Pentamera, 484
Pentamerus, 328
Pentastomum, 169, 445
Pentatomidz, 489
Pentatoma, 489
Pentremites, 342
Perameles, 633
Peramelidz, 633
Perca, 574, 577

Perch, 577

Percidz, 577

Perdix, 614
Pereiopoda, 401
Perforata, 197, 198
Peribranchial chamber, 503, 505
Pericardial sinus, 470
Pericardium, I11, 546
Pericheta, 316
Perichondrium, 86
Pericolpa, 250
Peridinium, 203
Perilymph, 543
Periosteum, 87
Teripatide, 456
Peripatopsis, 458
Peripatus, 456, 458
Peripharyngeal band, 506
690

Peripheral nervous system, 122
Periphylla, 250
Periplaneta, 480
Periproct, 343
Peripylea, 195
Perisarc, 233
Perissodactyla, 640, 641
Peristome, 209, 343
Peritoneal cavity, 546
Peritoneum, I09, 546
Periwinkle, 380

Perla, 479

Perlide, 479

Permian, 180
Perennibranchiata, 587
Peromedusz, 250
Perophora, 510
Peropoda, 601
Perradius, 246
Petaurus, 634

Petiole, 485

Petoscolex, 315
Petromyzon, 557
Petromyzon, cleavage of egg, 154
Petromyzontes, 557
Petrosal bone, 522
Phacellz, 246
Pheenicopterus, 615
Pheodaria, 196
Pheeodium, 196
Phaethon, 615
Phagocata, 271
Phalanges, 529
Phalangida, 450
Phalangistida, 634
Phalangium, 451
Phallusia, 509
Pharyngeal bones, 560, 576
Pharyngognathi, 576
Pharynx, 106, 506, 546
Phascalosoma, 316, 317
Phascolomyide, 633
Phascalomys, 633
Phascolion, 317
Phasianella, 379
Phasianidz, 614
Phasianus, 614
Phasmidz, 480

 

INDEX.

Phasmomantis, 480
Pheasants, 614
Phenacodon, 639
Phenacodontide, 643
Phidippus, 453
Philichthys, 38
Philine, 381
Philonexidze, 395
Phlegethontias, 495
Phoca, 648
Phocide, 648
Pholadidz, 368
Phoronidea, 325
Phoronis, 325
Phoxichilidium, 456
Phragmocone, 389
Phronima, 439
Phryganea, 483
Phrynicus, 448
Phrynoidea, 448
Phrynosoma, 599
Phrynus, 448
Phthirius, 491
Phylactolaemata, 324
Phyllium, 48
Phyllodactylus, 598
Phyllopoda, 415
Phyllosoma, 434, 436
Phyllostomidz, 638
Phylloxera, 491
Phylogeny, 4, 31
Physalia, 245
Physeter, 646
Physiological character of species, 27
Physiologus, 9
Physiology, 3
Physoclisti, 567
Physonectz, 244
Physophora, 244
Physophore, 244
Physopoda, 479
Physostomi, 567, 575
Phytoflagellata, 202
Phytophaga, 633
Picarix, 616

Picas, 639

Pickerel, 576

Picus, 616
Pieris, 496

Pieris, cleavage of. 155
Pigeons, 26, 27, 614
Pigmented epithelium, 540
Pike, 576

Pilidium, 290, 291

Pill bug, 442

Pineal eye, 535
Pinealis, 535
Pinnipedia, 647
Pinnotheres, 437
Pinnulz, 340

Pin worm, 301

Pipa, 585, 588

Pipe fish, 578

Pisces, 557

Piscicola, 321
Pisidium, 368
Pituitary body, 535
Placenta, 634
Placentalia, 634
Placoid scale, 515, 558
Placophora, 356
Plagiaulax, 632
Plagiotremata, 597
Plagiostomi, 569
Planaria, 271
Planarians, 268
Planipennia, 482
Plankton, 179
Planorbis, 383
Plantigrade, 647
Plant lice, 490

Plants and animals, 171
Planula, 237

Plasma, blood, 88
Plasmic products, 64, 72
Plasmodium, 198, 216
Plastin, 66
Plastogamy, 184
Plastron, 594
Platanista, 645
Plathelminthes, 267
Platyonichus, 436, 437
Platyrrhine, 651
Plecoptera, 479
Plectognathi, 578
Pleistocene, 181

 

INDEX. 691

Pleopoda, 401, 402
Plesiosauria, 594
Plethodon, 587
Pleura, 414, 462, 546
Pleuracanthus, 572
Pleural cavity, 546
Pleural cords, 354
Pleural ribs, 518
Pleurobrachia, 262, 264
Pleurocercoid, 283
Pleurodira, 596
Pleurodont teeth, 599
Pleuronectide, 578
Pleuroperitoneal cavity, 546
Plictolophus, 616
Pliocene, 181
Pliohippus, 643

Pliny, 8

Plover, 615
Plumularia, 242
Plumatella, 324
Pluteus, 332
Pneumatic duct, 567
Pneumaticity of bones, 608
Pneumatophore, 243
Pneumodermon, 382
Pneumogastric nerve, 536
Podocoryne, 241
Podophrya, 68, 212
Podophthalmia, 427
Podura, 477
Poikilothermous, 115
Polar bodies, 146

Pole field, 263

Poles of egg, 147, I51
Polian vesicles, 331
Polistotrema, 557
Polybostrichus, 311
Polycheetee, 311
Polychcerus, 269, 271
Polycladidea, 269, 271
Polyclinum, 510
Polyclonia, 247, 251
Polycystide, 214
Polydesmidz, 497
Polyergus, 488
Polygordius, 309, 314
Polymorphism, 165
692

Polynesia, 177
Polynoe, 313
Polynoide, 313
Polyodon, 573
Polyodontide, 573
Polyp, 230
Polyphemide, 417
Polypid, 322
Polypodium, 239, 241
Polyprododonta, 633
Polypterus, 573
Polypterus tail, 41
Polyscelis, 271
Polyspermy, 148
Polystomez, 273
Polystomella, 198
Polystomum, 273, 274
Polythalamia, 196, 198
Polytroche, 309
Polyzoa, 321
Poneride, 487

Pons Varolii, 623
Pontohdella, 321
Pontodrilus, 308
Pontella, 421
Porcellanide, 437
Porcellain crabs, 437
Porcellio, 442
Porcellio, nervous system of, 124
Porcupines, 639
Porella, 324

Pori abdominales, 546
Porifera, 221

Porites, 261

Porpita, 245

Portal vein, 548
Portuguese man-of-war, 245
Portunidz, 437

Porus branchialis, 503
Postabdomen, 401
Postfrontal bone, 526, 590
Postorbital bone, 590
Postpermanent dentition, 625
Potato beetle, 485
Powder down, 603
Praeclavia, 528, 622
Priecoces, 612

Prairie dogs, 639

INDEX.

Praya, 166, 244
Prefrontal bone, 526, 590
Prelacteal dentition, 625
Premaxillary bone, 525
Premolar teeth, 626
Presphenoid bone, 522
Priapuloidea, 317
Priapulus, 317
Primaries, 604
Primary bone, 519
Primary yolk, 80
Primates, 649

Primnoa, 259
Primordial cranium, 521
Principal tissue, 99
Priodon, 635

Pristidee, 572

Pristis, 572
Proboscidia, 643
Proboscis, 373
Proceelous, 519
Procoracoid, 528
Proctodzeum, 104
Procyon, 647
Preechidna, 631, 632
Proglottids, 278
Profeet, 4.66
Progression, principle of, 55
Prolegs, 475
Promorphology, 133
Pronephric duct, 550
Pronephros, 550

Prong horn, 643
Pronotum, 462
Pronucleus, 149

Proofs of phylogeny, 32
Proostracum, 389
Prootic bone, 522
Propodium, 369
Propterygium, 529
Prorostomus, 644
Prosencephalon, 533
Prosimiz, 648
Prosobranchia, 378
Prosternum, 528
Prostoma, 156
Protameeba, 189

 

Proteroglypha, 601
Proteroglyphic tooth, 600
Proteus, 587

Prothorax, 462
Protobranchiata, 365
Protista, 186

Protocaris, 416
Protocerebrum, 462, 468
Frotoconchiz, 365
Protodonta, 632
Protohydra, 239, 241
Protomerite, 214
Protonemertini, 291
Protonephridia, 115
Protoplasm, 61, 80
Protoplasm, discovery of, 59
Protoplasm, movement of, 62
Protopterus, 579
Prototheria, 631
Protovertebre, 531
Protozoa, 183
Prortacheata, 408, 456
Protula, 313
Proventriculus, 467
Psammonyx, 198
Pseudelectric organs, 563
Pseudobranch, 570
Pseudocuticula, 597
Pseudolamellibranchiata, 365
Pseudonavicellz, 215
Pseudoneuroptera, 477
Pseudopodia, 187
Pseudoscorpii, 450
Pseudosuchia, 602
Psittaci, 616

Psittacus, 616

Psocidee, 479

Psolus, 349

Psorosperms, 217
Pteranodon, 602
Pteraspis, 557
Pterichthys, 557
Pterodactylia, 602
Pteronarcys, 479
Pteropoda, 382

Pteroped ooze, 382
Pteropus, 638
Pterosauria, 602

Pterotic bone, 522, 560

 

INDEX. 693

Pterotracheidz, 380
Pterygoid bone, 525
Pterygoid process, 622
Pterygoquadrate, 524
Pterygotus, 444
Pterylae, 603

Pubic bone, 528
Pugettia, 437

Pulex, 493, 494
Pulmonata, 383
Pulmonary artery, 549
Pulmonary circulation, 549
Pulmonary vein, 549
Pulp cavity, 515
Pulvilla, 493

Puma, 647

Pupa, 383

Pupee, 474

Pupipara, 493
Purpura, 379, 380
Putorius, 647
Pycnogonida, 456
Pygidium, 414
Pyloric czeca, 565
Pylorus, 546
Pyrosoma, 510
Pyrula, 373

Python, 601
Pythonaster, 337
Pythonomorpha, 600

Quadrula, 196, 198
Quadrumana, 650
Quadrate bone, 525
Quahog, 368
Quail, 614
Quaternary, 181

Raccoon, 647

Rachis, 603

Racemose glands, 77
Radial canals, 235, 331
Radial symmetry, 135
Radiale, 529

Radialia, 330, 340, 527
Radiata, 228, 329
Radiolaria, 192
Radius, 529
694

Radula, 355, 373

Raia, 571, 572

Raid, 572

Rail, 615

Rainey’s corpuscles, 218

Rallus, 615

Rana, 588

Ranatra, 489

Rangifer, 642

Ranvier, nodes of, 96

Raptores, 616

Raptorial foot, 614

Rasorial foot, 614

Rasor clam, 368

Rathke, 18

Ratite, 612

Rats, 639

Rat-tail larva, 493

Rattlesnake, 601

Ray, I0, 20

Réamur, 13

Receptaculum seminis, 120, 471

Rectrices, 604

Rectum, 461

Red coral, 256

Redia, 276

Reduviide, 489

Regulares, 345

Reindeer, 642

Remak, 13

Remiges, 604

Remora, 577

Renilla, 258, 259

Reproduction, asexual, 140, 143

Reproduction, sexual, 142

Reptilia, 588

Respiratory organs, 107

Respiratory
547

Reticularia, 196

Reticulum, 642

Retina, 129, 131

Retina of vertebrates, 540

organs of

Retinaculum, 494
Retinula, 405
Retitelariz, 453
Rhabdites, 270
Rhabditis, 300

vertebrates,

 

INDEX.

Rhabdoceelida, 269, 271
Rhabdom, 129, 405
Rhabdonema, 145, 300
Rhabdopleura, 514
Rhachiglossa, 380
Rhachis, 414
Rhamphastos, 616
Rhea, 613
Rhegmatodes, 242
Rhinoceros, 641
Rhinocerotide, 641
Rhinoderma. 585
Rhizocephala, 426
Rhizocrinus, 342
Rhizopoda, 187
Rhizostomez, 250
Rhopalocephalus, 213
Rhopalocera, 495
Rhopalonema, 234
Rhynchobdellidz, 321
Rhynchobothrium, 286
Rhynchocephalia, 596
Rhynchonella, 325, 328
Rhynchophora, 485
Rhynchota, 489
Rhytina, 645

Rib, 517, 518

Right whale, 646

Ring canal, 331

Rocky Mountain sheep, 643
Rodentia, 638

Rods and cones, 129, 540
Root barnacles, 426
Rorqual, 646

Résel von Rosenhofen, 13
Rossia, 395

Rostellum, 280
Rostrum, 389, 424, 465
Rotalia, 188, 198
Rotatoria, 293
Rotifera, 293

Round worms, 298
Rove beetles, 484
Rudistidz, 368
Rugosa, 258

Rumen, 641
Ruminantia, 641
Rupicapra, 642
Sabellidze, 313
Sabinea, 435

Sable, 647
Sacconereis, 311
Sacculina, 426
Sacculus, 128, 542
Saccus vasculosus, 563
Sacral ribs, 528
Sagartia, 259

Sagitta, 296

Sagitta (ear bone), 564
Sagitta, development of, 158
St. Hilaire, 14, 22
Salamandra, 585, 587
Salamindrina, 587
Salinella, 220
Salivary glands, 106
Saltatoria, 480
Saltigrada, 453
Salmo, 576

Salmon, 576
Salmonidz, 576
Salpa, 510, 512
Salpzeformes, 510
Sand dollar, 345
Sand saucers, 380
San José scale insect, 490
Sapajous, 651
Sapphirina, 421
Sarcocystis, 213, 218
Sarcode, 60
Sarcolemma, 93
Sarcophaga, 493
Sarcophilus, 633
Sarcopsylla, 494
Sarcoptes, 454
Sarcosepta, 255
Sarcosporida, 218
Sarsia, 241

Saurii, 598
Sauropsida, 588
Saururee, 612
Savigny, 14
Savigny’s law, 401
Sawfish, 572

Sawflies, 485, 486
Saxicava, 367
Saxicavide, 368

 

INDEX. 695

Scala media, 543
Scala tympani, 543
Scala vestibuli, 543
Scale insects, 490
Scale, placoid, 515
Scales of fishes, 515, 558
Scales of reptiles, 597
Scallops, 637
Scalpellum, 424
Scansores, 616
Scansorial foot, 614
Scape, 603
Scaphander, 381
Scapharca, 367
Scaphiopus, 588
Scaphognathite, 431
Scaphopoda, 369
Scapula, 528
Scarabeeidee, 484
Schaffer, 13
Schizodont hinge, 359
Schizopoda, 428
Schizopodal appendages, 409
Schizosomi, 437
Sclerophylla, 261
Schleiden-Schwann theory, 58
Schwann, sheath of, 96
Scincidze, 599
Sciuridz, 639
Sciuromorpha, 639
Sciuropterus, 639
Sciurus, 639

Sclera, 131. 539
Scleral bones, 611
Sceleporus, 599
Scelrophyllia, 257
Sclerosepta, 255
Sclerotic, 539
Sclerotic bones, 593
Sclerotic coat, 130, 131
Sclerotomes, 531
Scolex, 278

Scollops, 367

Scolopax, 615
Scolopendra, 460, 461
Scolopendrella, 497
Scolopendride, 461
Scomber, 577
696

Scombridz, 577

Scops, 617

Scorpionida, 447

Sculpin, 577

Scutellum, 489
Scutibranchia, 378
Scutigera, 461
Scutigeridz, 461

Scutum, 424
Scyphomeduse, 245
Scyphopolyp, 230
Scyphostoma, 245, 246
Scyphozoa, 245

Sea anemones, 251, 259
Sea cucumbers, 346

Sea fans, 259

Sea horse, 578

Sea lion, 648

Sea pens, 259

Sea otter, 647

Sea snakes, 601

Sea squirts, 505, 508

Sea urchin, fertilization of, 149
Sea urchins, 343

Sea whips, 259

Seals, 648

Secodont teeth, 626
Secondaries, 604
Secondary bones, 515
Secreta, 73

Sedentaria, 313, 453
Segmental organs, 116, 308
Segmentation cavity, 155
Segmentation of egg, 149, I51
Segments of head, 536
Selachii, 570

Selection, artificial, 43
Selection, natural, 44
Selection, sexual, 46
Selenodont teeth, 626
Semeeostome, 250
Semicircular canals, 128, 542
Semilunar valves, 567
Semipalmate foot, 614
Semiplumes, 604.
Sensations, 125

Sense organs of vertebrates, 537
Senses, 125

 

INDEX.

Sensory epithclium, 73, 82
Sensory organs, 125
Sepia, 386, 388, 395
Septibranchiata, 368
Septum, 306
Sericteria, 494

Serosa, 473
Serpulidz, 313
Serranidee, 577
Serripes, 368
Sertularia, 242

Serum, blood, 88
Sesiidee, 495
Seventeen-year locust, 489, 490
Sexual cells, 143
Sexual epithelium, 78
Sexual glands, 78, So
Sexual organs, 80, 117
Sexual organs of vertebrates, 550
Sexual reproduction, 142, 145
Sexual selection, 49
Shad, 576

Shagreen, 569

Sharks, 571

Sheath of Schwann, 96
Sheep, 642

Sheep tick, 493

Shell gland, 411

Shell, layers of, 361
Ship worms, 368
Shore crab, 437
Shoulder blade, 528
Shoulder girdle, 527
Shrews, 637

Shrimp, mantis, 429
Shrimp, opossum, 428
Siala, 616

Sialidze, 482

Sialis, 482

Sicyonia, 434
Siderone, 47

von Siebold, 18
Silenia, 368
Silicispongiz, 226
Siliqua, 367
Silkworms, 495
Silurian, 180
Siluridze, 576
Silverfish, 477
Simia, 651

Simiidee, 651
Simuliide, 493
Sinupalliata, 368
Sinus frontalis. 539
Sinus, sphenoid, 539
Siphon, 345, 387, 360
Siphonaptera, 493
Siphonophora, 240, 243
Siphonophores, 166
Siphonostomate, 371
Siphonostomata, 422
Siphuncle, 338
Sipunculoida, 317
Sipunculus, 317
Siredon, 36

Siren, 586

Sirenia, 644

Sirex, 485

Siricidz, 486

Sixth sense, 125, 538
Skalis, 571

Skin, 76

Skull, 519

Skull of mammals, 619
Skunk, 647

Skylark, 616

Slime animals, 198
Slime eels, 557
Slime moulds, 198
Sloths, 636

Smell, organs of, 126
Smelt, 576

Snakes, 600
Snapping turtle, 596
Snout beetles, 485
Snow flea, 477

Social animals, 167
Soft-shell crab, 437
Soft-shelled turtle, 596
Solasteride, 337
Sole, 578
Solemyide, 367
Solen, 368
Solenoconchz, 366
Solenogastres, 358
Solenoglypha, 601

 

INDEX. 697

Solenoglyphic tooth, 600
Solenide, 368
Solidungula, 641
Solifugze, 449

Solpuga, 450

Solpugida, 449

Somatic cells, 143
Somatic layer, 159
Somatopleure, 159
Somites, 305

Song birds, 616

Sorex, 637

Soricidz, 637

Sowbug, 442

Spadella, 298

Spadix, 238

Spanish flies, 484

Span worms, 494
Spatangoidea, 346
Species, 10

Species, nature of, 19, 25
Species, physiological characters of, 27
Spelerpes, 587
Speotyto, 617
Spermaceti, 646
Spermatophore, 392
Spermatozoa, 81
Spermatozoids, 202
Sperm nucleus, 149
Sperm whale, 646
Spheeridia, 330
Spheerogastrida, 451
Sphzeroma, 442
Spheeromidee, 442
Spherophrya, 212
Spheerozoidz, 195
Sphargis, 595
Sphenethmoid bone, 581
Sphenodon, 596
Sphenoidalia, 521
Sphenoid bone, 523, 620
Sphenoid sinus, 539, 624
Sphenopalatine ganglion, 537
Sphenotic bone, 522
Spherical animals, 135
Sphingina, 495
Sphyranura, 274
Spicula, 300
698

Spicules of sponges, 225
Spider crab, 436, 437
Spider monkeys, 651
Spiders, 451, 452

Spinal canal, 517

Spinal ganglion, 533
Spindle, directive, 146
Spindle fibres, 69
Spindle, nuclear, 68
Spinnerets, 452

Spinous process, 517
Spiny ant eaters, 632
Spiny lobster, 436
Spiracle, 459, 524, 544
Spiral valve, 565
Spirifer, 328

Spirorbis, 313
Spirobolus, 497

Spirula, 388, 394
Spirulide, 394

Spittle bug, 489
Splanchnic layer, 159
Splanchnopleure, 159
Spleen, 550

Splenial bone, 582
Splint bones, 640
Spondylidee, 367
Sponge, fresh-water, 133
Sponges, 221

Spongida, 221

Spongilla, 133, 221, 227
Spongillidz, 227
Spongioplasm, 61
Spontaneous generation, 31
Sporangia, 199

Spores, 213, 215
Sporoblasts, 185, 213, 215
Sporocyst, 276
Sporosacs, 238
Sporozoa, 213
Sporozoites, 185, 213, 215
Springtails, 477
Sprinkling-pot shell, 368
Spumellaria, 195

Squali, 571

Squalus, 571

Squamata, 597, 636
Squamosal bone, 526

 

INDEX.

Squid, 395

Squilla, 429

Squirrels, 639
Staggers, 493

Stapes, 525
Staphylinidee, 484
Starfish, 333
Statoblasts, 227, 323
Statoliths, 128
Stauromeduse, 250
Steganopodes, 615
Stegocephali, 586
Stegosaurs, 597

Stellate ganglion, 390
Stelmatopoda, 323
Stemma, 403

Stenops, 649

Stenson’s duct, 539
Stentor, 209

Stephalia, 244
Stephanocyphus, 250
Sterna, 615

Sternaspis, 314
Stercoral pocket, 445
Sternum, 462, 518
Sticklebacks, 577
Stigmata, 459

Sting, 472, 486

Sting rays, 572

Stipes, 463

Stink bug, 489

Stolo prolifer, 512
Stomach, 106
Stomatopoda, 429
Stomodzum, 104
Stomolophus, 251

Stone canal, 330

Stone flies, 479

Storks, 61 5

Stratified epithelium, 73, 74
Stratum corneum, 76, 514
Stratum Malpighi, 76, 514
Streaming of protoplasm, 62, 188
Strepsiptera, 483
Streptoneury, 373
Stridulating organs, 469
Striges, 617

Strix, 617
Strobila, 249, 278
Strongylidz, 301
Strongyloides, 30c
Strongylocentrotus, 345
Strongylosoma, 497
Struggle for existence, 44
Struthio, 613
Struthiones, 613
Sturgeon, 573

Sturgeon, tail of, 41
Stylaster, 241

Style, crystalline, 364
Stylochus, 271
Stylohyoid ligament, 621
Styloid process, 621
Stylommatophora, 383
Stylonychia, 211, 212
Stylopide, 483

Stylops, 483
Subcutaneous tissue, 514
Subintestinal ganglion, 374
Subintestinal vein, 548
Submentum, 464
Subneural gland, 509
Subumbrella, 234
Suckers, 576

Suck fish, 577

Suctoria, 212

Suidz, 641

Sulci, 535

Summer eggs, 416

Sun animalcules, 190
Supporting cells, 83
Supporting layer, 230
Supraintestinal ganglion, 374
Supraoccipital bone, 522
Supracesophageal ganglion, 123
Suprascapula, 528

Surf perch, 577

Sus, 641

Swallows, 616

Swallow tails, 496
Swammerdam, 13
Swans, 615

Swarm spores, 185, 195
Sweat glands, 618

Swell fish, 578

Swim bladder, 547

 

INDEX. 699

Swimming bell, 243
Swimming birds, 614
Swine, 641

Sword fish, 577
Sycandra, 222, 225
Sycon, 223, 225
Sycones, 225

Syllide, 313

Syllis, 311

Sylvicolide, 616
Sylvius, 12

Symbiosis, 169
Symmetry, 134
Sympathetic coloration, 46
Sympathetic system, 537
Symplectic bone, 561
Symphyla, 497
Synapta, 349
Synapticula, 257
Synascidiz, 510
Syncitia, 71
Synceelidium, 269, 271
Syncoryne, 241
Synentognathi, 575, 576
Syngamus, 301
Syngnathus, 578
Syringopora, 259
Syrinx, 608

Syrphide, 493

Systems of organs, 100
Systemic circulation, 549
Systemic heart, 391

Tabanide, 493
Tabulz, 257
Tabulate, 257
Tactile bristles, 126
Tactile corpuscle, 537
Tactile organs, 125
Tadpole, 586
Tadpoles of Rana temporaria, 35
Tenia, 169, 279, 282
Teniadz, 287
Teniolx, 246
Tenioglossa, 380
Talpa, 637

Talpidee, 637

Tanais, 442
700

Tanystoma, 492
Tapetum nigrum, 131, 540
Tape worms, 285
Tapiridx, 641

Tapirs, 641

Tapirus, 641
Tarantula, 453
Tardigrada, 455, 636
Tarsal bones, 529
Tarsus, 463

Tarsiidz, 649

Tarsius, 649
Tarso-metatarsus, 607
Tasmanian devil, 633
Taste, organs of, 126
Taste organs of vertebrates, 538
Tatusia, 636

Tautoga, 576

Taxidea, 647
Taxodont hinge, 359
Tectibranchia, 381
Tectrices, 604

Teeth, dermal, 515
Teeth of mammals, 624
Teeth of vertebrates, 547
Tejidee, 599

Tejus, 599

Telea, 495

Teleostei, 574
Teleostomi, 569
Tellina, 368

Tellinidze, 368
Telolecithal eggs, 152
Telson, 427

Telotroche, 309
Temporal bone, 620
Temperature of mammals, 630
Tendinous tissue, 85
Tenebrionide, 484
Tentacles, gastral, 246
Tentaculata, 264

Tent caterpillars, 495
Tenthredinidx, 486
Terebella, 108, 313
Terebellida, 313
Terebra, 486
Terebrantia, 486
Terebratulina, 328

 

INDEX.

Teredo, 368
Teredide, 368
Tergum, 424
Termes, 478
Termitida, 478
Terns, 615
Terrapin, 596
Terricola, 315
Tertiary, 181
Tessellata, 342
Tesseridx, 250
Testicardines, 328
Testudo, 596
Testudinata, 594
Testudinide, 596
Tethyoidea, 508
Tetrabothrium, 286
Tetrabranchia, 394
Tetracoralla, 258
Tetractinellide, 227
Tetramera, 484
Tetraonide, 614
Tetrapneumones, 453
Tetrapoda, 555
Tetrarhynchidz, 286
Tetrarhynchus, 281, 286
Tetrastemma, 290, 291
Tetrasticta, 453
Tetraxonia, 226
Tettix, 481
Thalamophora, 196
Thalamus, 534
Thalassicola, 192
Thalassicolidie, 195
Thalassima, 317
Thalassochelys, 596
Thaliacea, 510
Thamnocnida, 242
Thaumantia, 242
Theca, 255, 338
Thecasomata, 382
Thelepus, 313
Thelyphonida, 448
Thelyphonus, 448
Theridium, 453
Theriodonta, 594
Theromorpha, 594
Thoracici, 562
Thoracic fin, 526, 562
Thoracostraca, 427
Thread cells, 229
Thrips, 479
Thrushes, 616
Thylacinus, 633
Thymus gland, 547, 577
Thyone, 349

Thyroid gland, 547
Thysanoptera, 479
Thysanozoon, 271
Thysanura, 477
Tiara, 236

Tiaris, 242

Tibia, 463, 529
Tibiale, 529
Tibio-tarsus, 607
Ticks, 454

Tick, sheep, 493
Tiedemann’s vesicles, 331
Tiger, 647

Tiger beetles, 484
Tillodontia, 643
Tima, 242

Tinea, 494

Tineidz, 494
Tipulide, 492
Tissues, 71

Tissues, accessory, 99
Tissues, classification of, 72
Tissues, connective, 83
Tissues, elastic, 85
Tissues, epithelial, 73
Tissues, muscular, 91
Tissues, nervous, 94
Tissues, principal, 99
Tissues, tendinous, 85
Toad, horned, 599
Toads, 588

Tobacco worm, 495
Tocogony, 140
Tomato worm, 495
Tongue bone, 524
Toothed birds, 612
Tooth shells, 369
Tornaria, 513
Torpedinid, 572
Torpedo, 572

 

INDEX. TO1

Tortoises, 594

Tortoise shell, 596
Tortricidz, 494

Total cleavage, 152
Totipalmate foot, 614, 615
Toucans, 616
Toxiglossa, 380
Toxodontia, 643
Toxopneustes, 345
Trabeculz, 520
Trachea, 109, 443, 458, 547
Tracheal gills, 469
Trachydermon, 357
Trachymeduse, 239, 242
Trachynema, 242
Tractus olfactorius, 534
Tragulide, 642
Tragulus, 642
Transverse commissure, 123
Transverse process, 518
Trapdoor spider, 453
Tree cricket, 481

Tree hoppers, 490

Tree toads, 588
Trematoda, 271
Triarthus, 415

Triassic, 180
Triaxonia, 226
Trichechide, 648
Trichechus, 648
Trichina, 302
Trichocephalus, 302
Trichocysts, 205
Trichodectes, 479
Trichomonas, 202
Trichoplax, 220
Trichoptera, 483
Trichotrachelidz, 302
Triclalidz, 269, 271
Triconodont teeth, 626
Tridacna, 367
Trigeminal nerve, 536
Trilobite, 414

Trimera, 485
Trionychia, 596
Tristicta, 453

Tristoma, 274
Tritocerebrum, 462, 468
702

Triton, sections of embryo, 39
Tritonide, 382
Tritubercular teeth, 626
Tritylodon, 632
Trivium, 334
Trochal disc, 293
Trochanter, 463
Trochide, 379
Trochilidx, 616
Trochilus, 616
Trochlear nerve, 536
Trochophore, 306
Trochosa, 453
Trochus, 379
Troctes, 479
Troglodytidz, 616
Troglodytes, 651
Trombidide, 454
Trophi, 294

Tropic birds, 615
Tropidonotus, 601
Trout, 576

Trunk fish, 578
Trutta, tail of, 41
Trygonide, 572
Tubicola, 313
Tubificide, 315
Tubinares, 615
Tubifex, 315
Tubiporidz, 259
Tubitelariz, 453
Tubular glands, 77
Tubular nervous system, 124
Tubulariz, 239, 241, 242
Tubulipora, 324
Tunic, 505

Tunicata, 505
Turbellaria, 268
Turbinated bone, 620
Turbinide, 379
Turbo, 379

Turbot, 578
Turbide, 616
Turdus, 616
Turkey, 614

Turkey buzzard, 617
Turritopsis, 240, 242
Turtles, 594

 

INDEX.

Twixt brain, 534
Tylenchus, 300
Tylopoda, 643

Tympanal organ, 128, 468
Tympanic annulus, 544
Tympanic bone, 526
Tympanic cavity, 621
Tympanic membrane, 544
Tympanum, 544

Type theory, 15
Typhline, 599

Typhlops, 601

Tyrian purple, 380
Tyrannide, 616

Uca, 437
Uintatherium, 643
Ulmaris, 247

Ulna, 529

Ulmare, 529
Umbilicus, 370, 554
Umbo, 358

Umbrella, 234
Uncinate process, 605
Ungues, 618

Ungule, 618
Ungulata, 639
Unguligrade, 640
Unicellular glands, 77
Unicorn, 646

Unio, 367

Unionidz, 367
Ureter, 550

Urinary bladder, 552
Urinator, 615
Urinatores, 615
Urnatella, 322
Uroceride, 486
Urochorda, 505
Urodela, 587
Urogenital sinus, 553
Urogenital system, 120
Urogenital system of vertebrates, 550
Urosalpinx, 379, 380
Urside, 647

Ursus, 647

Use and Disuse, 55, 99
Uterus, 120, 629
Utriculo-saccular duct, 542
Utriculus, 128, 542

Varanus, 599

Vacuole, contractile, 183
Vacuole, food, 183
Vagabunde, 453

Vagina, 120, 629

Vagus nerve, 536
Valkeria, 324

Vampyre, 638

Vanessa, 496

Varanidze, 599

Variation, 25

Vas deferens, 120

Vasa Malpighii, 461
Vascular arches, 504
Vater Pacinian corpuscles, 126
Vegetative organs, IOI
Vegetative pole, 147, 151
Veins, 112

Velella, 245

Veliger, 355, 364

Velum, 235, 356
Veneridz, 368

Ventral aorta, 548
Ventral fin, 562

Ventral nerve cord, 123
Ventricles of brain, 534
Ventricle of heart, 111, 548
Venous sinus, 567

Venus, 368

Venus’ flower basket, 226
Venus’ girdle, 264
Vermiform appendix, 627
Vermilinguia, 599, 636
Vermes, 535

Vertebra, 518

Vertebra (of ophiuroids), 337
Vertebral column, 516
Vertebrata, 514

Vertex, 462

Vesal, 12

Vesicle, auditory, 127
Vesicle, blastodermic, 155
Vesicle, germinal, 146
Vesicle, Polian, 331
Vesicle, Tiedemann’s, 331

 

INDEX. 703

Vesicularia, 324
Vesicula seminalis, 120
Vespariz, 487
Vesperlilionidze, 638
Vesperugo, 638
Vibracularia, 323
Vibrissze, 618
Viperidz, 601
Visceral ganglia, 353
Visceral sac, 351
Visceral skeleton, 523
Vitellarium, 267
Vitreous body, 130
Vitrodentine, 558
Viviparous, 161
Vogt, 24

Volutidz, 380
Volvocina, 202
Volvox, 202

Vomer, 525

Vortex, anatomy of, 120
Vorticella, 211
Vorticellidz, 210

Wading birds, 615
Wagner, 24
Waldheimia, 326
Walking stick, 480
Wallace, 24

Wallace’s line, 177
Walrus, 648

Warblers, 616
Warm-blooded animals, 114
Wasps, 487

Water bears, 455
Water beetles, 484
Water scorpion, 489
Water snake, 601
Water-vascular system, I15, 330
Weasel, 647

Weevils, 485
Weismann, 24
Whalebone, 645
Whales, 645

Whelks, 380

White ants, 478

White fish, 576

White matter, 124, 532
704

White rats, 639
Windpipe, 547
Winter eggs, 416
Wisdom tooth. 650
Wish bone, 605
Wolff, 17

Wolffian body, 550
Wolffian duct, 550
Wolves, 647
Wombat, 633
Woodcock, 615
Woodpeckers, 616
Wool, 618
Worms, 182
Wotton, 9

Wrens, 616
Wrisberg, 13

Xenarthra, 636
Xenos, 483
Xenurus, 636
Xerobates, 596
Xiphiide, 577
Xiphisternum, 595
Xiphosura, 444
Yellow cells, 195

Yellow fever, 492
Yoldia, 366, 367

 

INDEX.

Yolk, 80

Yolk and segmentation, 151
Yolk granules, 81

Yolk membrane, 148

Yolk sac, 553

Zebra, 641
Zeuglodonta, 646
Zoantharia, 259
Zoanthez, 259

Zoea, 413

Zonary placenta, 634
Zonuride, 599
Zoology, I, 5

Zoology, history of, 6
Zoology, purpose of, I
Zoophaga, 633
Zoophytes, 17£, 228
Zoospore, 185, 188, 195
Zoothamnion, 211
Zootomy, beginning of, 13
Zooxanthelle, 170, 195
Zygeena, 571
Zygapophysis, 519
Zygobranchia, 378
Zygodactyl foot, 614
Zygomatic arch, 526
Zygomatic bone, 526
 

 

 

 

 

 

 

 

 

 

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