G . ' > OUTLINES OF ^ tlt. '~U~T1^ Lj-<~**~*~t.4^Ct t-*+djU.. -ZOOLOGY. -' ^ ; ^ ^K__ J. ARTHUR THOMSON, MA., F.R.S.E. LECTURER ON ZOOLOGY AND BIOLOGY IN THE SCHOOL OF MEDICINE, EDINBURGH; JOINT-AUTHOR OF "THE EVOLUTION OF SEX"; AUTHOR OF "THE STUDY OF ANIMAL LIFE." SECOND EDITION, REVISED AND ENLARGED WITH 266 ILLUSTRATIONS. p* I $ ; *, ; ;*'," . . NEW YORK: D. APPLETON & COY. 1895. vi PREFACE TO THE SECOND EDITION. also written the chapter on Comparative Physiology. To my assistant, Mr. R. A. Staig, I am indebted for the index. I wish to express my thanks to my artist friend, Mr. William Smith, for the carefulness with which he has executed many of the illustrations ; and I am indebted to Dr. Traquair for allowing me to figure some of the specimens in the Edinburgh Museum of Science and Art. In regard to these illustrations, I may say that in almost every case they have either been derived from original memoirs and works of reference, or drawn from specimens. Of course, no one who has worked with such excellent practical books as that by Marshall and Hurst or Parker's Zootomy, can help being assisted by them in preparing analogous diagrams ; but I have refrained from incurring any but an absolutely necessary debt to such books, except in the case of Figure 215, which Messrs. Macmillan have kindly permitted me to make use of. J. A. T. SCHOOL OF MEDICINE, EDINBURGH, March 1895. CONTENTS. GENERAL. PAGE CHAPTER I. GENERAL SURVEY OF THE ANIMAL KINGDOM, . . i CHAPTER II. PHYSIOLOGY, ... . 14 CHAPTER III. MORPHOLOGY, ... ... 30 CHAPTER IV. EMBRYOLOGY, ..... 49 CHAPTER V. PAL/EONTOLOGY, .... .74 CHAPTER VI. DOCTRINE OF DESCENT, . . . . .81 x CONTENTS. GENERAL. PAGE CHAPTER XXVII. COMPARATIVE PHYSIOLOGY, ..... 731 CHAPTER XXVIII. DISTRIBUTION, ....... 753 CHAPTER XXIX. ETIOLOGY, ....... 765 APPENDIX ON BOOKS, . . 773 INDEX, ... -779 LIST OF ILLUSTRATIONS. 1 . Diagrammatic expression of classification in a genealogical tree, 1 1 2. Structure of the cell. (CARNOY), .... 44 3. Fertilised ovum of Ascaris. (BovERi), ... 44 4. Diagram of cell division. (BovERi), ... 45 5. Karyokinesis. (FLEMMING), .... 47 6. Diagrammatic expression of alternation of generations, . 55 7. Diagram of ovum, showing diffuse yolk granules, . . 57 8. Forms of spermatozoa, ..... 5^ 9. Diagram of maturation and fertilisation. (GEDDES and THOMSON), ...... 59 10. Spermatogenesis and polar bodies. (HERTWIG and WEISMANN), ...... 61 11. Fertilisation in Ascaris megalocephala. (BovERi), . . 62 12. Modes of segmentation, ..... 64 13. Life history of a coral, Monoxenia Darwinii. (fLECKEL), 66 14. Embryos ( I ) of bird ; (2) of man. (His). ... 69 15. Gradual transitions between Paludina Neumayri and Palu- dina Hcernesi. (NEUMAYR), . . . 77 16. Life history of A mceba, ..... 87 17. End-to-end union of Gregarines. (FRENZEL), . . 88 18. Life history of Gregarina. (BtJTSCHLi), ... 89 19. Life history of Monocystis. (BiJTSCHLl), ... 90 20. Paramcecium. (BuTSCHLi), . . . .91 21. Conjugation of Paramctciiun aurelia four stages. (MAUPAS), ...... 92 22. Diagrammatic expression of process of conjugation in Para- mcecittm aurelia. ( MAUPAS), .... 93 23. Vorticella. (BiJTSCHLT), ..... 95 24. Volvox globator. (COHN), . , . . . 96 25. Diagram of Protomyxa aurantiaca. (H^CKEL), . . 99 26. Formation of shell in a simple Foraminifer. (DREYER), . 100 xii LIST OF ILLUSTRATIONS. FIG. PAGE 27. Polystomella. (SCHULTZE), ..... 101 28. A pelagic Foraminifer Hastigerina ( Globigerina) Murrayi. (BRADY), ....... 102 29. Optical section of a Radiolarian (Actinomma]. (H^CKEL), 103 30. A colonial flagellate infusorian Proterospongia Hceckelii. (SAVILLE KENT), ..... 104 31. Simple sponge Ascetta primordialis. (ILECKEL), . 117 32. Section of a sponge. (F. E. SCHULZE), . . 118 33. Diagram showing types of canal system of sponge. (KORSCHELT and HEIDER), . . . .119 34 Development of Sycandra raphamis. (F. E. SCHULZE), . 123 35. Diagrammatic representation of development of Oscarella lobularis. (HEIDER), ..... 124 36. A. Young Dicyema. B. Female Orthonectid (Rhopalura Giardii}. (WHITMAN and JULIN), , . . 127 37. Salinella. (FRENZEL), . . . . .128 38. Diagram of Coelenterate structure, . . . .132 39. Hydra, hanging from water- weed. (GREENE), . . 134 40. Minute structure of Hydra. (PARKER and JICKELI), . 137 41. Development of Hydra. (BRAUER), . . . 139 42. Surface view of A tirelia. (ROMANES), . . . 144 43. Vertical section of A urelia. (CLAUS), . . . 145 44. Diagram of life history of A urelia. (H^CKEL), . . 146 45. Lucernaria. (KOROTNEFF), .... 148 46. Structure of sea anemone. (ANDRES), . . .150 47. Section through sea anemone. (ANDRES), . . . 151 48. Diagrammatic sections of Zoantharian and Alcyonarian. (CHUN), ....... 153 49. Diagram of a Gymnoblastic Hydromedusa. (ALLMAN), . 156 50. Diagrammatic figure of a simple Turbellarian, . . 163 51. Diagrammatic expression of part of the structure of a simple Turbellarian, . . . . . .163 52. Structure of liver fluke. (SOMMER), . . . 165 53. Reproductive organs of liver fluke. (SOMMER), , . 166 54. Life-history of liver fluke. (THOMAS), . . .168 55. Diagram of reproductive organs in Cestode joint. (LEUCKART), . . . . . .172 56. Life history of Tania solium. (LEUCKART), . . 174 57. Transverse section of the Nemertean Drepanophorus latus. (BURGER), ... .177 58. Transverse section of a Nemertean (Carinella}. (BURGER), 178 59. Diagram of structure of a Nematode (Oxyziris}. (CALEB), 182 LIST OF ILL USTRA TIONS. xiii FIG. PAGE 60. Anterior region of earthworm. (HERING), . . 190 6 1. Transverse section of earthworm. (CLAPARDE), . . 193 62. Reproductive organs of earthworm. (HERING), . . 195 63. Stages in the development of earthworm. (WlLSON), . 198 64. Arenicola piscatorum. (CUNNINGHAM and RAMAGE), . 202 65. Anterior part of nervous system in Arenicola. (VoGT and YUNG), ....... 203 66. Dissection of anterior region of A renicola. (CosMOVici), . 204 67. Cross section of A renicola. (CosMOVici), . . . 205 68. Development of Polygordius. (FRAIPONT), . . 207 69. Parapodium of a marine Polychsete, Heteronereis. (QuATRE- FAGES), ....... 210 70. Transverse section of leech. (A. G. BOURNE), . . 217 71. Dissection of leech, . . . . . .219 72. Development of Sagitta. (O. HERTWIG), . . . 222 73. Interior of Brachiopod shell, showing calcareous support for the "arms." (DAVIDSON), .... 226 74. Pluteus larva with rudiment of adult. (JOHANNES MULLER), 228 75. Alimentary system of starfish. (MULLER and TROSCHEL), 231 76. Diagrammatic cross section of starfish arm. (LuDWio), . 233 77. Ventral half of sea urchin. (TiEDEMANN), . . 238 78. Dissection of Holothurian. ( HUNTER), . . . 242 79. Diagrammatic vertical section through disc and base of one of the arms of Antedon rosaceus. (MiLNES MARSHALL), 245 80. Stages in development of Echinoderms. (SELENKA), . 247 81. Forms of Echinoderm larva. (MULLER), . . . 249 82. Appendages of Norway lobster, .... 258 83. A single eye element or ommatidium of the lobster. (PARKER), ..... 260 84. Longitudinal section of lobster, showing some of the organs, 262 85. Female reproductive organs of crayfish. (SucKOw), . 265 86. Section through the egg of Astacus after the completion of segmentation. (REICHENBACH), . . . 266 87. Longitudinal section of later embryo of Astacus. (REICH- ENBACH), ...... 267 88. Embryo of crayfish, flattened out, with removal of yolk. (REICHENBACH), ...... 268 89. Acorn shell (Balanus tintinnabuluni}. (DARWIN), . 273 90. Development of Sacculina. (DELAGE), . . . 274 91. Zoaea of common shore crab ( Carcinus mcsnas}. (FAXON), 281 92. External form of Peripatus. (BALFOUR), . . . 286 93. Dissection of Peripatus capensis. (BALFOUR), . . 288 xvi LIST OF ILLUSTRATIONS. FIG. PAGE 159. Spiral valve of skate. (T.J.PARKER), . . .487 160. Upper part of the dorsal aorta in the skate. (MONRO), . 488 161. Heart and adjacent vessels of skate. (MoNRO), . . 489 162. Urinogenital organs of male skate, . . . 491 163. Urinogenital organs of female skate. (MoNRO), . . 492 164. Elasmobranch development. (BALFOUR), . . 494 165. External characters of a Teleostean a carp. (LEUNis), . 496 1 66. Caudal vertebra of haddock, .... 497 167. Disarticulated skull of cod, '. . . . 498 1 68. Pectoral girdle and fin of cod, .... 499 169. Diagram of Teleostean circulation. (NuHN), . . 501 170. Young skate. (BEARD), ..... 506 171. Outline of Acanthodes sulcatus. (TRAQUAIR), . . 507 172. Pterichthys milleri. (TRAQUAIR), . . . 509 173. Skeleton of Ceratodus fin. (GEGENBAUR), . . 513 174. Head region of Protopterus. (W.N.PARKER), . . 514 175. Vertebral column and pelvic girdle of bull frog, . . 532 176. Skull of frog upper and lower surface. (W. K. PARKER), 533 177. Pectoral girdle of frog. (EcKER), .... 535 178. Side view of frog's pelvis. (ECKER), . . . 536 179. Brain of frog. (ECKER), ..... 537 180. Nervous system of frog. (ECKER), . . . 538 1 8 1. Arterial system of frog. (EcKER), . . . 542 182. Venous system of frog. (ECKER), .... 544 183. Urinogenital system of male frog. ( ECKER), . . 548 184. Urinogenital system of female frog. (ECKER), . . 548 185. Division of frog's ovum. (EcKER), . . . 550 186. Gastrula stage of newt. (HERTWIG), . . . 551 187. Dissection of tadpole. (MILNES MARSHALL and BLES), . 552 188. Life history of frog. (BREHM), .... 554 189. Gecilian (Ichthyophis] with eggs. (SARASIN), . . 557 190. External appearance of tortoise, . . . 563 191. Carapace of tortoise, . . . . 564 192. Internal view of skeleton of turtle, . . . 565 193. Dissection of Chelonian heart. (HUXLEY), . . 566 194. Heart and associated vessels of tortoise. (NuHN), . 567 195. Hatteria or Sphenodon. (HAYEK), . . . 568 196. Side view of skull of Lacerta. (W.K.PARKER), . 572 197. Heart and associated vessels of lizard. (NuHN), . 574 198. Lung of Chamaleo vulgaris, showing air sacs. (WlEDER- SHEIM), ....... 576 199. Snake's head. (NuHN), ..... 579 LIST OF ILLUSTRATIONS. xvii FIG. PAGE 200. Skull of grass snake. (W. K. PARKER), . . . 581 201. Lower surface of skull of a young crocodile, . . 584 202. Crocodile's skull from dorsal surface, . . . 585 203. Half of the pelvic girdle of a young crocodile, . . 586 204. Origin of amnion and allantois. (BALFOUR), . . 590 205. Comparison of pelvic girdles of cassowary, Iguanodon, and crocodile, . . . . . . 593 206. Position of organs in a bird. (SELENKA), . . 597 207. Diagrammatic section of young bird. (GADOW), . " 598 208. Disarticulation of bird's skull. (GADOW), . . 602 209. Under surface of gull's skull, .... 603 210. Wing of dove, ...... 604 211. Side view of pelvis of cassowary, .... 605 212. Bones of hind leg of eagle, .... 606 213. Brain of pigeon. (BRONN), .... 607 214. Diagrammatic section of cloaca of male bird. (GADOW), . 608 215. Circulation of pigeon. (PARKER), .. . . 609 216. Urinogenital organs of pigeon, .... 612 217. Diagrammatic section of egg. (ALLEN THOMSON), . 613 218. Stages in development of chick. (MARSHALL), . . 615 219. Diagrammatic section of embryo within egg. (KENNEL), 616 220. Pectoral girdle and sternum of swan, . . . 622 221. Position of wings in pigeon at maximum elevation. (MAREY), ....... 623 222. Wings coming down. (MAREY), .... 624 223. Wings completely depressed. (MAREY), . . . 624 224. Segmentation of rabbit's ovum. (VAN BENEDEN), . 648 225. Development of hedgehog. Three early stages. (HUBRECHT), 649 226. Two stages in segmented ovum of hedgehog. (HUBRECHT), 650 227. Development of foetal membranes. (HERTWIG), . . 652 228. Diagram of foetal membranes. (TURNER), . . 653 229. View of embryo of rabbit, with its foetal membranes. (KENNEL), ...... 657 230. Side view of rabbit's skull, , ... . . 662 231. Dorsal view of rabbit's skull, . . . .663 232. Under surface of rabbit's skull, .... 664 233. Rabbit's fore leg, ...... 665 234. Rabbit's hind leg, ...... 665 235. Dorsal view of rabbit's brain. (KRAUSE), . . 667 236. Under surface of rabbit's brain. (KRAUSE), . . . 667 237. Diagram of caecum in rabbit, . . . 670 238. Duodenum of rabbit. (CLAUDE BERNARD), . 671 b xviii LIST OF ILLUSTRATIONS. FIG. PAGE 239. Circulatory system of rabbit. (PARKER and KRAUSE), . 672 240. Vertical section through rabbit's head, . . . 675 241. Urinogenital organs of male rabbit, . . . 677 242. Urinogenital organs of female rabbit, . . . 677 243. Pectoral girdle of Echidna, .... 681 244. Pelvis of Echidna, ...... 682 245. Urinogenital organs of male duckmole. (OWEN), . 683 246. Urinogenital organs of female duckmole. (OwEN), . 683 247. Lower jaw of kangaroo, ..... 684 248. Foot of young kangaroo, ..... 688 249. Foot of ox, ...... 694 250. Fore leg of pig, ...... 694 251. Side view of sheep's skull, with roots of back teeth exposed, 695 252. Stomach of sheep. (LEUNIS), .... 696 253. Side view of calf's fore leg, .... 697 254. Side view of lower part of pony's fore leg, . . 699 255. Side view of ankle and foot of horse, . . . 699 256. Side view of horse's skull, roots of back teeth exposed, . 700 257. Feet of horse and its progenitors. (NEUMAYR), . . 701 258. Fore limb of Balcsnoptera, .... 705 259. Fore limb of whale (Megaptera longimand]. (STRUTHERS), 706 260. Pelvis and hind limb of Greenland whale. (STRUTHERS), 707 261. Vertebra, rib, and sternum of Balanoptera, . . 708 262. Lower surface of dog's skull, . . . .712 263. Skeleton of fox bat, . . . . . 720 264. Skeleton of male gorilla, . . . . .723 265. Skull of orang-utan, ..... 726 266. Skull of gorilla, ...... 727 OUTLINES OF ZOOLOGY. BIRDS. Placentals. VERTEBRATES. H 8 MAMMALS. Marsupials. Flying Birds. Running Birds. Monotremes. r h Snakes. Lizards. REPTILES. Crocodiles. Tortoises. Dipnoi. FISHES. B ny FishS - Ganoids. Elasmobranchs. AMPHIBIANS. Newt. Frog. CYCLOSTOMATA. Lamprey. Hagfish. LANCELET. TUNICATES. Insects. Arachnids. BALANOGLOSSUS. Cuttlefish. Gasteropods. ANNELIDS. Myriopods. Peripatus. MOLLUSCS. c c/5 W ARTHROPODS. Crustaceans. Bivalves. "WORMS." Feather stars. H Brittle stars. HH Ctenophores. Jellyfish. Sea Anemones. Corals. COELENTERA. Medusoids and Hydroids. Infusorians. Rhizc SIMPLEST SPONGES. >pods. Gregarines. ANIMALS. N o OUTLINES OF ZOOLOGY. CHAPTER I. GENERAL SURVEY OF THE ANIMAL KINGDOM. AT the outset of our study of Zoology it is useful to take a general survey of the " animal kingdom." Without some such bird's eye view necessarily very superficial one is apt not to see the wood for the trees. Mammals. We naturally begin a survey with the animals which are most like man the monkeys. But neither we nor the monkeys are separated by any structural gulf from the other four limbed, hair bearing animals, to which Lamarck gave the name of Mammals. For although there are many different types of Mammals such as monkeys and men ; horses, cattle, and other hoofed quadrupeds ; cats, dogs, and bears ; rats, mice, and other rodents ; hedgehogs, shrews, and moles, and so on the common possession of certain characters unites them all in one class, readily dis- tinguishable from Birds or Reptiles. Among these characters we rank the milk giving of the mother mammals, the growth of hair on the skin, the general presence of convolutions on the front part of the 1 2 GENERAL SURVEY OF THE ANIMAL KINGDOM. brain, the occurrence of a muscular partition or diaphragm between the chest and the abdomen, and so on, as we shall afterwards notice in detail. Most mammals are suited for life on land, but diverse types such as seals, whales, and sea cows have taken to the water, while the bats are as markedly suited for aerial life. Among the mammalian characteristics of great import- ance .are those which relate to the bearing of young, and even a brief consideration of these enables us to see that some mammals are distinguished from others by differences deeper than those which separate whales from carnivores, or rodents from bats. These deep differences may be stated briefly as follows : (a) Before birth most young mammals are very closely united (by a complex structure called the placenta) to the mothers who bear them. (b) But this close connection between mother and unborn young is only hinted at in the kangaroos and other pouched animals or Marsupials, which bring forth their young in a peculiarly helpless condition, as it were prematurely, and in most cases place them in an external pouch, within which they are sheltered and nourished, (c) In the Australian duck- mole and its two relatives, the placental connection is quite absent, for these animals lay eggs as birds and most reptiles do. Besides these differences in the bearing of young, there are others relating to structure which are of great importance, and which seem to warrant the division of Mammals into three sub-classes : 1. Prototheria, Ornithodelphia, or Monotremes the egg laying duckmole (Omithorkytukus], Echidna and Proechidna. 2. Metatheria, Didelphia, or Marsupials the prematurely bearing, usually pouch possessing kangaroos and their relatives. 3. Eutheria, Monodelphia, or Placentals those in which there is a close (placental) union between the unborn embryo and its mother, e.g.. Ungulates, Carnivores, Monkeys. Birds. There can be no hesitation as to the class which we should rank next to Mammals. For Birds are in most respects as highly developed as Mammals, though in a divergent direction. They are characterised by their feathers and wings, and many other adaptations for flight, by their high temperature, by the frequent sponginess and hollow- VERTEBRATES. 3 ness of their bones, by the tendency to fusion in many parts of the skeleton, e.g., backbone and ankle, by the absence of teeth in modern forms, by the fixedness of the lungs and their association with numerous air sacs, and so on. But here again different grades must be distinguished (i) there is the vast majority the flying birds, who have a breast-bone keel or carina, to which the muscles used in flight are in part attached (Carinatae) ; (2) there is the small minority of running birds (ostriches, emu, cassowary, and kiwi), whose wings are incapable of flight, and who have no keel (Ratitae) ; and (3) there is an extinct type, Archceopteryx, with markedly reptilian affinities. Reptiles. There are no close relationships between Birds and Mammals, but the old-fashioned Monotremes have some markedly reptilian features, and so have some aberrant living birds, such as the Hoatzin and the Tinamou. More- over, when we consider the extinct Mammals and Birds, we perceive other resemblances linking the two highest classes of animals to Reptiles. Reptiles do not form a compact class, but rather an assemblage of classes. In other words, the types of Reptile differ much more widely from one another than do the types of Bird or Mammal. Nowadays, there are five distinct types : the crocodilians, the unique New Zealand "lizard" (Hatterid), the lizards proper, the snakes, and the tortoises. But the number of types is greatly increased when we take account of the entirely extinct saurians who had their golden age in the inconceivably distant past. The Reptiles which we know nowadays are scaly-skinned animals, they resemble Birds and Mammals in having a practically or really four chambered heart, in never having gills, and in having during embryonic life two important "fcetal membranes," known as the amnion and the allantois. Amphibians. The Amphibians, such as frogs and newts, were once regarded e.g., by Cuvier as naked Reptiles, but a more accurate classification has linked them rather to the Fishes. 4 GENERAL SURVEY OF THE ANIMAL KINGDOM. Thus Professor Huxley grouped Birds and Reptiles together as Sauropsida; Amphibians and Fishes together as Ich- thyopsida for reasons which shall be afterwards stated. Amphibians mark the transition from aquatic life, habitual among Fishes, to terrestrial life, habitual among Reptiles, for while almost all Amphibians have gills in their youth at least all the adults have lungs, and some retain the gills as well. In having limbs which are fingered and toed, and thus very different from fins, they resemble Reptiles. But the two foetal membranes characteristic of the embryonic life of higher Vertebrates are not present in Amphibian embryos, and the general absence of an exoskeleton in modern forms is noteworthy. Fishes. The members of this class are as markedly adapted to life in the water as birds to life in the air. The tail usually forms the locomotor organ, and the limbs are fins. There are also unpaired median fins supported by fin rays. All have permanent gills borne by bony or gristly arches. There is an exoskeleton of scales, and the skin also bears numerous glandular cells and sensory structures. In many ways Fishes are allied to Amphibians, especially if we include among Fishes three peculiar forms, known as Dipnoi, which show hints of a three chambered heart, and have a lung as well as gills. Other Fishes have a two chambered heart, containing only impure blood, which is driven to the gills, whence, purified, it passes directly to the body. Apart from the divergent Dipnoi, there are three orders of Fishes : the cartilaginous Elasmobranchs, such as shark and skate ; the Ganoids, such as sturgeon and bony pike ; and the Teleosteans or bony fishes, such as cod, herring, salmon, eel, and sole. Primitive (?) Vertebrates. Under this title we include (i) the class of Round- mouths or Cyclostomata ; (2) the class of which the lancelet or Amphioxus is the only adequately known type; (3) the class of Tunicates, some of which are called sea-squirts ; and (4), with much hesitation, several strange formSj VER TEBRA TES. 5 especially Balanoglossus, which exhibit structures suggestive of affinity with Vertebrates. The Cyclostomata, represented by the lamprey (Petro- myzon) and the hag (Myxine), and some other forms, probably including an interesting fossil known as Palceo- spondylus, are sometimes ranked with Fishes under the title Marsipobranchii. But they have no definitely developed jaws, no paired fins, no scales, and are in other ways more primitive. The lancelet (Amphioxus), for which the class Cephalo- chorda has been erected, is even simpler in its general structure. Thus there is an absence of limbs, skull, jaws, well-defined brain, heart, and some other structures. The vertebral column is represented by an unsegmented (or un- vertebrated) rod, called the notochord, which in higher animals (except Cyclostomata and some fishes) is a transitory organ replaced by a backbone. The Tunicata or Urochorda form a class of remarkable forms, the majority of which degenerate after larval life. In the larvae of all, and in the few adults which are neither peculiarly specialised nor degenerate, we recognise some of the fundamental characters of Vertebrates. Thus there is a dorsal supporting axis (a notochord) in the tail region, a dorsal nervous system, gill clefts opening from the pharynx to the exterior, a simple ventral heart, and so on. Of Balanoglossus and its allies, for which the class Hemi- chorda or Enteropneusta has been established, it is still difficult to speak with confidence. The possession of gill clefts, the dorsal position of an important part of the nervous system, the occurrence of a short supporting struc- ture on the anterior dorsal surface and other features, have led some to place them at the base of the Vertebrate series. At this stage, having reached the base of the Vertebrate series, we may seek to define a Vertebrate animal, and to contrast it with Inverte- brate forms. The distinction is a very old one, for even Aristotle distinguished mammals, birds, reptiles, amphibians, and fishes as "blood holding," from cuttlefish, shell bearing animals, crustaceans, insects, &c., which he regarded as " bloodless." He was, indeed, mistaken about the bloodlessness, but the distinctiveness of the higher animals first men- tioned has been recognised by all subsequent naturalists, though it was first precisely expressed in 1797 by Lamarck. 6 GENERAL SURVEY OF THE ANIMAL KINGDOM. Yet it is no longer possible to draw a boundary line between Verte- brates and Invertebrates with that firmness of hand which characterised the early or, indeed, the pre-Darwinian classifications. For we now know ( I ) that Fishes and Cyclostomata do not form the base of the Vertebrate series, for the lancelet and the Tunicates must also be in- cluded in the Vertebrate alliance ; (2) that Balanoglossus and Cep halo- discus have several Vertebrate-like characteristics ; (3) that some of the Invertebrates, especially Chaetopods and Nemerteans, show some hints of affinities with Vertebrates. The limits of the Vertebrate alliance have been widened, and though the recognition of their characteristics has become more definite, not less so, the apartness of the sub-kingdom has disappeared. It does not matter much whether we retain the familiar title Verte- brata, or adopt that of Chordata, provided that we recognise (i) that it is among Fishes first that separate vertebral bodies appear in the supporting dorsal axis of the body; (2) that, as a characteristic, the back- bone is less important than the notochord, which precedes it in the history alike of the race and of the individual. Nor need we object to the popular title backboned, provided we recognise that the adjective "bony" is first applicable among Fishes, and not even to all of them. The essential characters of Vertebrates may be summed up in the following table, where they are contrasted, somewhat negatively, with what is true of Invertebrates : " BACKBONKLES," INVERTEBRATE OR NON-CHORDATE. The greater part of the nervous system is on the ventral surface. No corresponding structure is known. No corresponding structures are known with any certainty. The eye is usually derived directly from the skin. The heart, if present, is dorsal. " BACKBONED," VERTEBRATE OR CHORDATE. The central nervous system brain and spinal cord is dorsal, and t^tbular. There is a dorsal supporting axis or noto- chord, which is in most cases replaced by a backbone. Gill-slits or visceral clefts open from the sides of the pharynx to the exterior. In fishes, and at least young amphi- bians, they are associated with gills and are useful in respiration ; in higher forms they are transitory and functionless, except when modified into other structures. The essential parts of the eye are formed by an outgrowth from the brain. The heart is ventral. INVERTEBRATES. Molluscs. This series of forms includes Bivalves, such as cockle and mussel, oyster and clam ; Gasteropods, such as snail and slug, periwinkle and buckie ; Cephalopods, such as octopus INVER TEBRA TES. 7 and pearly nautilus. They may be placed highest among Invertebrates since many of them exhibit a concentration of the nervous system greater than occurs elsewhere. Unlike Vertebrates, and such Invertebrates as Insects and Crustaceans, Molluscs are without segments and without appendages. A muscular protrusion of the ventral surface, known as the " foot," serves in the majority as an organ of locomotion. In most cases, a single or double fold of -skin, called the "mantle," makes a protective shell. The nervous system has three chief pairs of nerve centres or ganglia. In many cases, the larval stages are very char- acteristic. Arthropods. This large series includes Crustaceans, Myriopods, Insects, Spiders, and other forms, which have segmented bilaterally symmetrical bodies and jointed appendages. The skin produces an external cuticle, the organic part of which con- sists of a substance called chitin, associated in Crustaceans with carbonate of lime. The nervous system consists of a dorsal brain, connected, by a nerve ring around the gullet, with a ventral chain of ganglia. Echinoderms. This is a well-defined series, including starfishes, brittle stars, sea urchins, sea cucumbers, and feather stars. The r symmetry of the adult is usually radial, though that of the i larva is bilateral. A peculiar system, known as the water-; vascular system, is characteristic, and is turned to various uses, as in locomotion and respiration. There is a marked tendency to deposition of lime in the tissues. The develop- ment is strangely circuitous or " indirect." Segmented " Worms" It is hopeless at present to arrange with any definiteness those heterogeneous forms to which the title " worm " is given. For this title is little more than a name for a shape, assumed by animals of varied nature who began to move head foremost and to acquire sides. There is no class of " worms," but an assemblage a mob not yet reduced to order. It seems useful, however, to separate those which 8 GENERAL SURVEY OF THE ANIMAL KINGDOM. are ringed or segmented, from those which are unsegmented. The former are often called Annelids, and include : Chaetopoda, or Bristle-footed worms, e.g., earthworm and lobworm ; and Hirudinea, or Leeches ; and some smaller classes. Unsegmented " Worms" These differ from the higher " worms " in the absence of true segments and appendages, and resemble them in their bilateral symmetry. The series includes Turbellarians or Planarians ; the parasitic Trematodes or Flukes ; the para- sitic Cestodes or Tapeworms ; the Nemerteans or Ribbon- worms ; the frequently parasitic Nematodes or Thread- worms ; and several smaller classes. As to certain other forms, such as the sea mats (Polyzoa or Bryozoa), the lamp shells (Brachiopoda), and the worm- like Sipunculids, it seems best, at this stage, to confess that they are incertce sedis. But the general fact is not without interest that in the midst of the well-defined classes of Invertebrates there lies, as it were, a pool from which many streams of life flow, for among the heterogeneous " worms " we detect affinities with Arthropods, Molluscs, Echinoderms, and even Vertebrates. At this stage we may notice that in all the above forms the typical symmetry is bilateral (in Echinoderms, the radial symmetry belongs only to the adults) ; that in most types a body cavity or coelome is developed ; that the embryo consists of three germinal layers (external ectoderm or epiblast, internal endoderm or hypoblast lining the gut, and a median mesoderm or mesoblast lining the body cavity). In the next two classes (Coelentera and Sponges) the conditions are different, as may be expressed in the following table, though it is open to question whether the contrast is quite so great as it seems : SPONGES AND COELENTERA. HIGHER ANIMALS (CCELOMATA). There is no body cavity. There is but one cavity, that of the food canal. There is no definite middle layer of cells (mesoderm), but rather a middle jelly (mesogloea). The radial symmetry of the gastrula em- bryo is retained in the adult, and the longitudinal (oral-aboral) axis of the adult corresponds to the long axis of the gastrula. There is a body cavity or coelome between the food canal and the walls of the body. But this is often incipient, or degenerate. There is a distinct middle layer of cells (mesoderm) between the external ectoderm and the gut lining endo- derm. The longitudinal axis of the adult does not correspond to the long axis of the gastrula embryo. INVERTEBRA TES. Cozlentera. This series includes jelly fish, sea anemones, corals, zoo- phytes, and the like, most of which are equipped with stinging cells, by means of which they paralyse their prey. . All but four or five are marine. The body may be a tubular polype, or a more or less bell-like "medusoid," and in some cases the two forms are included in one life cycle. Budding is very common, and many of the sedentary forms " corals " have shells of lime. Porifera. Sponges, or Porifera, are the simplest many celled animals. In the simplest forms, the body is a tubular, two layered sac, with numerous inhalent pores perforating the walls, with a central cavity lined by cells bearing lashes or flagella, and with an exhalent aperture. But budding, folding, and other complications arise, and there is almost always a skeleton, calcareous, siliceous, or "horny," or both siliceous and horny at once. Water passes in by the small inhalent pores and out by the exhalent aperture. With few exceptions they are marine. All the animals hitherto mentioned have bodies built up of many cells or unit masses of living matter, but there are other animals, each of which consists of a single cell. These simplest animals are called Protozoa. / Every animal hitherto mentioned, from mammal or bird to sponge, [ develops, when reproduction takes its usual course, from a fertilised ^ egg cell. This egg cell or ovum divides and redivides, and the daughter cells are arranged in various ways to form a "body." But the Protozoa form no "body," they remain single cells, and when they divide, the daughter cells almost invariably go apart as independent organisms. Here, then, is the greatest gulf which we have hitherto noticed that between multicellular organisms (Metazoa) and unicellular organisms (Protozoa). But the gulf was bridged, and traces of the bridge remain. For (a) there are a few Protozoa which form loose colonies of cells, and (b] there are multicellular organisms of great simplicity. Protozoa. The Protozoa remain single cells, with few exceptions. Thus they form no " body ; " and necessarily therefore they io GENERAL SURVEY OF THE ANIMAL KINGDOM. have no organs, nor sexual reproduction in the ordinary sense of the phrase. The series includes . (a) Infusorians, with actively moving lashes of living matter ; (fr) Rhizopods, with outflowing threads or processes of living matter ; (c) Gregarines, parasitic forms, without either lashes or outflowing processes. Note on Classification. We naturally group together in the mind those impressions which are like one another. In this lies the beginning of all classification, whether that of the child, the savage, or the zoologist. For there are many possible classifications, varying according to their purpose, according to the points of similarity which have been selected as important. Thus we may classify animals according to their habitats or their diet without taking any thought of their structure. But a strictly zoological classification is one which seeks to show the natural relationships of animals, to group together those which resemble one another in their real nature or structure. It must therefore be based on the results of comparative anatomy, technically speaking, on " homologies," or real resemblances of structure. Whales must not be ranked with fishes, nor bats with birds. To a classification based on structural resemblances, two corrobora- tions are necessary, from embryology and from palaeontology. On the one hand, the development of the forms in question must be studied ; thus no one dreamed that a Tunicate was a Vertebrate until its life- history was worked out ; on the other hand, the past history must be inquired into, thus the affinity between Birds and Reptiles is confirmed by a knowledge of the extinct forms. In classification it is convenient to recognise certain grades or degrees of resemblance, which are spoken of as species, genera, families, orders, classes, and so on. To give an illustration, all the tigers are said to form the species Felis tigris, of the genus Felis, in the family Felidae, in the order Carnivora, within the class Mammalia. The resemblances of all tigers are exceedingly close ; well-marked, but not so close, are the resem- blances between tigers, lions, jaguars, pumas, cats, etc., which form the genus Felis ; broader still are the resemblances between all members of the cat family Felidse ; still wider those between cats, dogs, bears, and seals, which form the order Carnivora ; and lastly, there are the general resemblances of structure which bind Mammals together in contrast to Birds or Reptiles. It must be understood that the real things are the individual animals, and that a species is a subjective conception within which we include all those individuals who resemble one another so closely that we feel we need a specific name applicable to them all. And as resemblances which seem important to one naturalist may seem trivial to others, there CLASSIFICA TION. II are often wide differences of opinion as to the number of species which a genus contains. In a handful of small shells the "splitters" may recognise 20 species where the " slumpers " see only 3. Thus Hseckel says of calcareous sponges that, as the naturalist likes to look at the problem, there are 3 species, or 21, or 289, or 591 ! But while no rigid definition can be given of a species, seeing that FIG. i. Diagrammatic expression of classification in a genealogical tree. B indicates possible position of Balano- glossus, D of Dipnoi, S of Sphenodon or Hatteria. the conception is one of practical convenience and purely relative, there are certain common-sense considerations to be borne in mind I. No naturalist now believes, as Linnoeus did, in the fixity of 12 GENERAL SURVEY OF THE ANIMAL KINGDOM. species ; we believe, on the contrary, that one form has given rise to another. At the same time, the common characteristic on the strength of which we deem it warrantable to give a name to a group of individuals must not be markedly fluctuating. The specific character should exhibit a certain degree of constancy from one generation to another. 2. Sometimes a minute character, such as the shape of a tooth or the marking of a scale, is so constantly characteristic of a group of individuals that it may be safely used as the index of more important characters. On the other hand, the distinction between one species and another should always be greater than any difference between the members of a family (using the word family here to mean the progeny of a pair). For no one would divide mankind into species according to the colour of eyes or hair, as this would lead to the absurd conclusion that two brothers belonged to different species. Thus it is often doubly un- satisfactory when a species is established on the strength of a single specimen, (a) because the constancy of the specific character is undeter- mined ; (b] because the variations within the limits of the family have not been observed. Indeed, it has happened that one species has been made out of a male and another out of its mate. But the characters of a single specimen are sometimes so distinctive that the zoologist is safe in making it the type of a new species, or even of a new genus. 3. While cases are known where members of different species have paired and brought forth fertile hybrids, this is not common. The members of a species are fertile inter se, but not usually with members of other species. In fact, the distinctness of species has largely depended on a restriction of the range of fertility. To sum up, a species is but a relative conception, convenient when we wish to include under one title all the members of a group of individuals who resemble one another in certain characters. There is no absolute constancy in these specific characters, and one species often melts into another with which it is connected by intermediate varieties. At the same time, the characters, on account of which the naturalist gives a specific name to a group of individuals, should be greater than those which distinguish the members of any one family, should show a relative constancy from generation to generation, and should be associated with reproductive peculiarities which tend to restrict the range of mutual fertility to the members of the proposed species. It will be enough now simply to state some of the more important grades of classification : Individuals. Varieties among these individuals. . Species, e.g., Felis tigris. Genus, Felis. Family, Felidae. Order, Carnivora. Class, Mammalia. Phylum or Series, Vertebrata. [TABLE. TABULAR SURVEY OF CLASSES. (For Future Reference.} METAZOA CHORDATA. MAMMALIA. AVES. REPTILIA. AMPHIBIA. PISCES. Eutheria. Placentals. Metatheria. Marsupials. Non-pla- cental. Prototheria. Monotremes. Oviparous. {Carinatae. Keeled flying birds. Ratitae. Keel-less running birds. Extinct reptile-like birds. ,-Crocodilia. Crocodiles and alligators. I Ophidia. Snakes. J Lacertilia. Lizards. I Rhyncocephalia. Sphenodon. I Chelonia. Tortoises and turtles. ^Extinct Classes. f Anura. Tail-less frogs and toads. I Urodela. Tailed newts. < Gymnophiona, e.g., C&cilia. I Labyrinthpdpnts and other extinct V Amphibians. Dipnoi. Mud fishes. Teleostei. Bony fishes. Ganoidei, e.g., Sturgeon. Elasmobranchii. Cartilaginous fishes.; ), and Lamprey if II /Hagfish (Myxine), " I (Petromyzon). CYCLOSTOMATA. ~\ CEPHAI.OCHORDATA. A mphioxus. UROCHORDATA. Tunicates. HEMICHORDATA. Balanoglossus, Cephalodiscus. METAZOA NON-CHORDATA. MOLLUSCA. T Cephalopoda. Cuttle fishes. -f Gasteropoda. Snails. ^Lamellibranchiata. Bivalves. TArachnoidea. Spiders, scorpions, mites. J Insecta. ARTHROPODA. -; Myriopoda. Centipedes and millipedes. I Protracheata. Peripatus. ^Crustacea. TCrinoidea. Feather stars. (Cystoids and Blastoids, extinct.) I Ophiuroidea. Brittle stars. \ Asteroidea. St?- ^ oVl I F.chinoidea. Se v Holothuroidea. ECHINODERMA. \ Asteroidea. Star fish. Sea urchins. Sea cucumbers. " J Chaetopoda. 1 Discophora. ' WORMS." CCELENTERA. PORIFERA. Bristle-footed worms. Leeches. Annelids. f Brachiopoda. Lamp shells. -| Pplyzoa, e.g., Sea mat (Flustra). V.Sipunculoidea, e.g. , Sipunculus. Nematoda. Nemertea. Thread-worms. Ribbon -worms. {Cestoda. Tape-worms. ^ Trematoda. Flukes. VPlathelmin Turbellaria. Planarians. J ( Ctenophora, e.g. , Beroe. \ Scyphozoa. Jellyfish and sea anemones. VHydrozoa. Zoophytes and medusoids. Sponges. PROTOZOA. INFUSORIA. RHIZOPODA. GREGARINIDA. Simplest forms of animal life. thes. CHAPTER II. THE FUNCTIONS OF ANIMALS. (PHYSIOLOGY.) MOST animals live a conscious and active life, busied with the search for food, the wooing of mates, the building of homes, and the tending of young. These and other forms of activity depend upon internal changes within the body. For the movements of all but the very simplest animals are due to the activity of contractile parts known as muscles, which are controlled by nervous centres and by impulse- conducting fibres. But as the work done means expenditure of energy, and is followed by muscular and nervous exhaustion, the necessity for fresh supplies of energy is obvious. This recuperation is obtained from food, but before this can restore the exhausted parts to their normal state, or keep them from becoming, in any marked degree, exhausted, it must be rendered soluble, diffused throughout the body, and so chemically altered that it is readily incorporated into the animal's substance, t In other words, it has to be digested. We may say then that there are two master activities in the animal body, those of muscular and those of nervous parts, to which the other internal activities are subsidiary conditions, turning food into blood and thus repairing the waste of matter and energy, keeping up the supply of oxygen and the warmth of the body, sifting out and removing waste products. Besides the more or less constantly recurrent activities or functions, which are summed up under the general term " metabolism," there are the processes of growth and repro- DIVISION OF LABOUR. 15 duction. When income exceeds expenditure in a young animal, growth goes on, and the inherited qualities of the organism are more and more perfectly developed. At the limit of growth, when the animal has reached " maturity," it normally reproduces, that is to say, liberates parts of itself which give rise to new individuals. / It is this power of growing and reproducing which most distinguishes an organism from an inanimate thing. Division of Labour. All the ordinary functions of life are exhibited by the simple unicellular animals or Protozoa. Take the Amoeba for example. It moves by contracting its living substance, it draws back sensitively from hurtful influences, it engulfs and digests food, it gets rid of waste, and it absorbs oxygen, without which its living matter cannot continue active or indeed alive. ('* For activity implies, in part, an oxidation, a combustion of material, and respiration in plants and animals alike consists in absorbing oxygen, and in liberating the car- bonic acid gas which is one of the waste products both of life and burning. x , But all these activities occur in the Amoeba within the compass of a unit mass of living matter, a single cell, physiologically complete in itself. There is no division of labour, there are as yet no parts. In all other animals, from Sponges onwards, there is a " body " consisting of hundreds of unit masses or cells. It is impossible for these to remain the same, for some are internal and others external, nor would it be well for the organism that all its units should retain the primitive and many sided qualities of Amoebae. <' Division of labour, con- sequent on diversity of conditions, is thus established in the organism. In some cells one kind of activity predominates, in others a second, in others a third. And this division of labour is followed by that complication of structure which we call differentiation. Thus, in the fresh water Hydra, which is one of the simplest many celled animals, the units are arranged in two layers, and form a tubular body. Those of the outer layer are protective, nervous, and muscular ; those of 1 6 THE FUNCTIONS OF ANIMALS. the inner layer absorb and digest the food, and are also muscular. In worms and higher organisms, there is a middle layer in addition to the other two, and this middle layer becomes, for instance, predominantly muscular. ^Moreover, the units or cells are not only arranged in strands or tissues, each with a predominant function, but become compacted into well-defined parts or organs. None the less should we remember that each cell remains a living unit, and that, in addition to its principal activity, it usually retains others of a subsidiary character. History. Physiologists, or those who study the activities of organisms and of their parts, were at first content to speak of these as the result of " animal and vital spirits," of moods and temperaments. Stimulated, however, by the anatomists' disclosure of organs, the physiologists soon began to explain the organism as a complex engine of many parts. The muscles were recognised as the mechanism which produced movement, the heart pumped the blood through the body, the brain was the seat of thought, and so on. This was an exceedingly necessary and natural step in analysis. Nor has it yet been thoroughly taken in every case, for there are many organs, especially in backbone- less animals, about whose predominant use we are uncertain. But the physiologists of this school sometimes finished their work too quickly. That the liver was an organ for secreting bile was deemed a completely satisfactory statement, until it began to be seen that this organ is the seat of many other activities. Moreover, some thought that it was possible to deduce the function of an organ from its visible structure, as one might infer the use of a piston from its shape. To a certain extent this is true, as when we show how an image is formed on the retina of the eye. {But we cannot, in terms of visible structure, explain another function of the eye that of distinguishing the " colours " of things. In fact, it must be clearly understood that each organ is far more than a piece of mechanism in a living engine, that it is a complicated factory of living units, each with sujbtle and manifold powers, J In 1801, Bichat analysed the animal body into its component tissues muscular, nervous, glandular, c., and being a physiologist as well as an anatomist, sought to explain the activities of the organism in terms of the contractile, irritable, secretory, or other properties of its tissues. This was a further step in the analysis, and one of great importance. About forty years later, however, it began to be recognised that the body was a great city of cells, each with a life of its own. The functions were not merely the activities of organs of various construction, or of tissues with various properties, they were the results of the life of the component units or cells. Finally, in thfcse last days, the physiologists have touched the bottom PLANTS AND ANIMALS. 17 in their analysis, for they are endeavouring to discover the physical and chemical changes associated with the living stuff or protoplasm itself. These are obviously at the foundation of the whole matter. Plants and Animals. Before we give a sketch of the chief functions in a higher animal, let us briefly consider the resemblances and differ- ences between plants and animals. (a.) Resemblance in Function. The life of plants is essentially like that of animals, as has been recognised since Claude Bernard wrote his famous book, Phenomenes de la vie communs aux animaux et aux vegetaux. The beech tree feeds and grows, digests and breathes, as really as does the squirrel on its branches. In regard to none of the main functions is there any essential difference. Many simple plants swim about actively ; young shoots and roots also move; and there are many cases in which even the full- I grown parts of plants exhibit movements. Moreover, the ' tendrils of climbers, the leaves of the sensitive plant, the tentacles of the sun-dew, the stamens of the rock rose, the stigma of the musk, are but a few instances of the numerous plant structures which exhibit marked sensitiveness. (b.} Resemblance in Structure. The simplest plants (Pro- tophyta) like the simplest animals (Protozoa) are single cells ; the higher plants (Metaphyta) and higher animals (Metazoa) are built up of cells and of various modifications A of cells. ' In short, all organisms have a cellular structure, 1 This general conclusion is known as the Cell Theory or Cell Doctrine (see p. 41). (c.} Resemblance in Development. When we trace the beech tree back to the beginning of its life, we find that it arises from a uniLjelement or egg cell, which is fertilised by intimate union with a male element derived from the pollen- grain. When we trace the squirrel back to the beginning of its life, we find that it also arises from a unit. element or egg cell, which is fertilised by intimate union with a male cell or spermatozoon. Thus all the many celled plants and animals begin as fertilised egg cells, except in cases of virgin birth (parthenogenesis) or of asexual reproduction. From the egg cell, which divides and redivides after fertilisa- tion, the body of the plant or animal is built up by con- 9 1 8 THE FUNCTIONS OF ANIMALS. tinued division, arrangement, and modification of cells. Thus, plants and animals resemble one another in their essential functions, in their cellular structure, and in their development. But while there is no absolute distinction between plants and animals, they represent divergent branches of a V-shaped tree of life. It is easy to distinguish extremes, like bird and daisy, less easy to contrast sponge and mushroom, well nigh impossible to decide whether some very simple forms, which Haeckel called "protists," have a bias towards plants or towards animals. / But the food which most plants absorb is cruder or chemically simpler than that which animals are able to utilise. Thus plants derive the carbon they require from the carbonic acid gas of the air, whereas only a few (green) animals have this power. Almost all animals depend for their carbon supplies on the sugar, starch, and fat already made by other animals, or by plants. As regards nitrogen, most plants derive this from nitrates and the like, absorbed along with water by the roots ; whereas animals obtain their nitrogenous supplies from the complex proteids formed within other organisms. Most plants, therefore, feed at a lower chemical level than do animals, and it is characteristic of them that, in the reduction of carbonic acid, and in the manufacture of starch and proteids, the kinetic energy of sunlight is transformed by the living matter into the potential chemical energy of complex food stuffs. Animals, on the other hand, get their food ready-made ; they take the pounds which plants have, as it were, accumulated in pence, and they spend them. For it is characteristic of animals that they convert the potential chemical energy of food stuffs into the kinetic energy of locomotion and other activities. In short, the great distinction an average one at best is that most animals are more active than most plants. Let us condense in tabular summary the time- honoured " distinctions between plants and animals." [TABLE. PLANTS AND ANIMALS. a ^g 12 S.S 8 - 3 TJ 5 **3 'S "5 S O g O rt Qi'S 5 o 33 S > o ^ . ...I o '5 w >> *a.S c bf) c s rt a; C^ bo c "^ 3 l> Q ^ t/5 o "c "5 " < 3 fe rt CO ^ "5 1 rt O w fo eat! 2 i2 o 1 o c t/T g =T^ .- c c^>> oT -^OJ3J3 "> X.S Cl3 ?/" cL'"" 2s c .t! ^ J '3 3 O ej CJ 1 JJ& fll.-d , g ' c 1 ^ i . rti c -f.2 3* rt C *g ~ ^.~ 1 1 S "* 8 ;- ,a l-a .S c u 4J <2 'g |^'^S Li g| S S J S?l 1 1 |j g'P s^-l j 1 oj '^'O, C T3 '^'ftS m o x g w cS S*^ 2^2o'8S !3 fl3S8 rt >,l o S^o g s s.s.s"^ S3S i ^ S o-'i^ ^ Is ^ll D -G Jjj -G -*0 >>^ ^ bO ^ 4-T g O ^"8-g ^if -c "2*eL'-3 ^ ""' -^'ri .2 >> c* 2 *a H H S jj H aJ3 ^ H vp^J^ ^ ^ ' 'i H H . |. 3 J.u g o "= J 3 o ^ ^ ^ c: i^^ Saaus 3 ^j3 1 M O c >, jg- 5 Si g s - o . S s 2 &i ? g o isljf li>st 6 c n O 4J *J G rt C u^ll rt ^fe n ^ a, S 2 ^ 1*2 2 ^boo^x; ,/^ u o^ 5^ ^.'H Is ^ ^^l^o !! 11 S o_o S -^ if ill M?s A 3, ^-2 grt &4*i'i^ ^ M s^ S o U -S-^ ^ ^ G ^ *i s ^ J^ O^ So bo rt " v^ ^ Jv g '^ 5 S 2 S V C/3 ^ r5 *^ O rt *^ l> k W) S u w c -'3 7. v^3- S.'E 5 ^ 'SJ2 .2 S ^ 20 THE FUNCTIONS OF ANIMALS. CHIEF FUNCTIONS OF THE ANIMAL BODY. We have seen that there are two master activities in animals, those of muscular and of nervous structures, and that the other functions, always excepting reproduction, are subservient to these. Let us now consider the various functions, as they occur in some higher organism, such as man, reserving comparative treatment for a subsequent chapter. Nervous Activities. Life has been described as consisting of action and reaction between the organism and its environment, and it is evident that an animal must in some way feel, or become aware of surrounding influences. In a higher animal we find parts which are specially excitable. These are the sensory end-organs : the retina of the eye for light, certain parts of the ear for sound, papillae on the tongue for taste, part of the lining of the nasal chamber for smell, tactile corpuscles of the skin for pressure and temperature. All these end-organs are associated with nerves which are stimulated by the excitation of the end-organ, and conduct the stimulus inwards to what are called centres or ganglia. In Vertebrate animals the brain and spinal cord contain a series of such centres, some of which serve for the per- ception of the changes produced in the end-organs by the stimulus, while others preside over the activities of the muscles. I As we ascend in the scale we find that in addition the brain possesses, to an increasing extent, the power of , correlating present and past experiences, and originating or inhibiting action in accordance with the judgment formed. J Thus, nervous activities involve (a) end-organs or sense organs ; () centres or ganglia ; and (c) the conducting nerves, some of which are afferent (or sensory) passing from end- organs to ganglia, while others are efferent (or motor) passing from centres to muscles. And in whatever part there is activity there is necessarily waste of complex sub- stances and some degree of exhaustion. / It is interesting to notice, as a triumph of histological technique, that Hodge, Gusfav Mann, and others have succeeded in demonstrat- MUSCULAR ACTIVITY. 21 ing in nerve cells the structural results (cellular collapse, c.) of fatigue, and that in such diverse types as bee, frog, bird, and dog. Muscular Activity. The movements of a unicellular animal are due to the contractility of the living matter, or of special parts of the cell such as cilia (see p. 106). In sponges, there are often specially contractile cells ; in most higher animals such cells are aggregated to form the muscles on whose activity all movement depends, In many of the lower animals, e.g., sea-anemones and sea- squirts, the contractile strands consist of long spindle-shaped L cells which appear almost homogeneous ; these are called , smooth muscle fibres. They occur in certain parts of the body in higher Vertebrates, e.g., on the wall of the urinary ) bladder. A more specialised kind of muscle, prevailing in active animals, consists of fibres which show alternate light and dark cross bands ; these are called striped muscle fibres. The two kinds, unstriped and striped, may be seen to pass into one another in the same animal, and in a general way one may think of the former as slowly con- tracting, the latter as rapidly contracting. A piece of living muscle consists of fine transparent tubes or fibres, each invested by a sheath or sarcolemma, and the whole muscle is surrounded by connective tissue. It usually runs from one part of the skeleton to another and is fastened to the skeleton by tendons or sinews. It is stimulated by motor nerves, and is richly supplied with blood. When a muscle contracts, usually under a stimulus propagated along a motor nerve, there is of course a change of shape it becomes shorter and broader. The source of" 1 the energy expended in work done is the " chemical ex- plosion " which occurs in the fibres, for the oxygen stored up (intramolecularly) in the muscle enters into rapid union with a carbohydrate. Heat, CO 2 , and water are produced as the result of this combustion, and lactic acid is also formed as a bye-product. Besides the chemical change and the change of shape, there are also changes of electrical potential associated with each contraction. 22 THE FUNCTIONS OF ANIMALS. Digestion. The energy expended in doing work or in growth is balanced by the potential energy of the food stuffs taken into the body. These consist of proteids, carbohydrates, fats, water, and salts in varying proportions according to the diet of the animal. 1 Oxygen may also be regarded as form- ing part of the food. In some of the lower animals, such as sponges, the food particles are directly engulfed by some of the cells with which they come in contact. Within these cells they are dissolved ; this is known as intracellular digestion. In most cases, however, the food is rendered soluble and diffusible within the food-canal by the action of certain ferments made by the cells which line the gut or form the associated glands. The great peculiarity of these fermenting substances is that a small quantity can act upon a large mass of material without itself undergoing any apparent change. But however digestion be effected, it means making the food soluble and diffusible. In a higher vertebrate, there are many steps in the process. (a) The first ferment to affect the food, masticated by the teeth and moistened by the saliva, is ihzptyalin of the salivary juice, which changes starch into sugar. The juice is formed or secreted by various salivary glands around the mouth. (b) The food is swallowed, and passes down the gullet to the stomach, where it is mixed with the gastric juice secreted by glands situated in the walls. These walls are also muscular, and their contractions churn the food and mix it with the juice. In the juice there is some free hydro- chloric acid and a ferment called pepsin ; these act together in turning proteids into peptones. The juice has also a slight solvent effect on fat, and the acid on the carbohydrates. (c) The semi-digested food, as it passes from the stomach into the small intestines, is called chyme, and on this other juices act. Of these the most important is the secretion of the pancreas, which contains various ferments, e.g.) trypsin, and affects all the different kinds of organic food. It continues the work of the stomach, changing proteids into peptones ; it continues the work of the salivary juice, changing starch into sugar ; it also emulsifies the fat, dividing the globules into extremely small drops, which it tends to split into fatty acids and glycerine. (d) Into the beginning of the small intestine, the bile from the liver also flows, but this is not of great digestive importance, being rather of the nature of a waste product. It seems to have a slight solvent, emul- sifying, and saponifying action on the fats ; in some animals it has a slight power of converting starch into sugar ; by its alkalinity it helps the action of the trypsin of the pancreas (which, unlike pepsin, acts in a ABSORPTION. 23 neutral fluid) ; it affects cell membranes, so that they allow the passage of small drops of fat and oil ; and it is said to have various other qualities. (e) In addition to the liver and the pancreas, there are on the walls of the small intestine a great number of small glands, which secrete a juice which probably seconds the pancreatic juice. The digested material is in part absorbed into the blood, and the mass of food, still being digested, is passed along the small intestine by means of the muscular contraction of the walls, known as peristaltic action. It reaches the large intestine and its reaction is now distinctly acid by reason of the acid fermentation of the contents. The walls of the large intestine contain glands similar to those of the small intestine, and the digestive processes are completed, while absorption also goes on ; so that by the time the mass has reached the rectum, it is semi-solid, and is known as fseces. These contain all the indigestible and undigested remnants of the food and the useless products of the chemical digestive processes. Absorption. But the food must not only be rendered soluble and diffusible, it must be carried to the different parts of the body, and there incorporated into the hungry cells. It is carried by the blood-stream, and in part also by what are called lymph vessels, which contain a clear fluid resembling blood minus red blood corpuscles. Absorption begins in the stomach by direct osmosis into the capillaries or fine branches of blood vessels in its walls, and a similar absorption, especially of water, takes place along the whole of the digestive tract. But lining the intestines there are special hair-like projections called villi ; they contain capillaries belonging to the portal system (blood vessels going to the liver), and small vessels known as lacteals connected with lymph spaces in the wall of the intestine. The lacteals lead into a longitudinal lymph vessel or thoracic duct, which opens into the junction of the left jugular and left subclavian veins at the root of the neck. The contents of the duct in a fasting animal are clear ; after a meal they become milky ; the change is due to the matters discharged into it by the lacteals. It is probable that nearly all the fat of a meal is absorbed from the intestines by the lacteals, but it is not certain in what measure, if at all, this is true of the other dissolved food stuffs ; the greater part certainly passes into the capillaries of the portal system, which are con- tained in each villus. The peptone or digested proteid, as it passes through the cells of the villi, is changed into other proteids nearly related to those of the blood, for no peptone is found in the portal vein. Function of the Liver. We now know the fate of the fats, and of the proteids of the food, and the manner in which they pass into the blood ; but we must follow the starchy material, or carbo- 24 THE FUNCTIONS OF ANIMALS. hydrates, a little further. The starch, we know, is converted into sugar, and this, with the sugar of the food, passes into the capillaries of the villi, and is carried to the liver. Dur- ing digestion there is an increase of sugar in the blood vessel going to the liver from the intestine, that is, in the portal vein, but no increase in the hepatic veins, the vessels leav- ing the liver. The increase must, therefore, be retained in that organ, and we recognise as one of the functions of the liver, the regulation of the amount of sugar in the blood. There is no special organ for the regulation of the amount of fat ; the drops pass through the capillary walls, and are retained in the connective tissue. We must remember that all the products of digestion, except the fat, pass through the liver, which receives every- thing before it is allowed to pass into the general circula- tion. Thus, many poisons, especially metals, are arrested by the liver, and many substances which result from digestive processes and would be harmful, are there altered into harm- less compounds. The excess of sugar, we have already noted, is stored in the liver. It is converted there into a substance called glycogen, which can be readily retrans- formed into sugar according to the needs of the system. Glycogen is stored in the muscles also, and is the material chiefly useful as the fuel for the supply of muscular energy and of the warmth of the body. Thus, if an animal be subjected to a low temperature, the glycogen of the liver disappears just as it does during the performance of muscular work. Another of the many functions of the liver is that in it nitrogenous waste products begin to be prepared for their final elimination by the kidneys. Respiration. There is another most important food stuff to be noticed, namely, the oxygen which is absorbed from the air by the lungs. We may picture a lung as an elastic sponge-work of air chambers, with innumerable blood capillaries in the walls, enclosed in an air-tight box, the chest, the size of which constantly and rhythmically varies. When we take in a breath the size of the chest is increased, the air pressure within is lowered, and the air from without rushes down EXCRETION. 25 the windpipe until the pressure is equalised. The oxygen of this air combines with a substance called haemoglobin, contained in the red corpuscles of the blood, and is thus carried to all parts of the body. The protoplasm of the tissues having a stronger affinity for oxygen than has the haemoglobin, takes as much as it requires. The carbonic acid gas formed as a waste product is absorbed by the serum of the blood, and so in time reaches the lungs. But as the partial pressure of the carbonic acid in the air is lower than it is in the serum, the gas escapes from the latter into the air chambers of the lungs. When the size of the chest is decreased, the pressure is increased, and the gas escapes by the mouth until the pressure is equalised. By the constant repetition of the breathing movements, oxygen is constantly being taken in, and carried to the tissues which are in a marvellous way "hungry" for it, while the waste carbonic acid gas is as constantly being removed. Excretion. We have seen that the blood carries the digested food to the various parts of the body, and that it is also the carrier of oxygen and of the waste carbonic acid gas. But there is much waste resulting from tissue changes, which is not gaseous. It is cast into the blood stream by the tissues, and has to be got rid of in some way. This is effected by the kidneys, which are really filters introduced into the blood stream. But they are the most marvellous filters imaginable, and give us a good example of the in- tricacy of life processes. For the kidneys not only cast out of the blood all the waste products that result from the metabolism of proteids, and contain nitrogen ; but they maintain the composition of the blood at its normal, reject- ing any stuffs that vary from that normal, either qualitatively or quantitatively, doing this work according to laws quite different from the simple ones of diffusion or solubility ; thus, sugar and urea are about equally soluble, and yet the sugar is kept in the body, while the urea is cast out. Even substances as insoluble as resins are removed from the blood by the living cells of the kidneys. A considerable quantity of water, and traces of salts, fats, &c., leave the body by the skin, but its chief use is to pro- 26 THE FUNCTIONS OF ANIMALS. tect and to regulate the temperature by variations in the size of its blood vessels. This completes our sketch (a) of the process by which the food becomes available for the organism as fuel for the maintenance of its life energies, and (b] of the removal of the waste products which are formed as the ashes of life. There are indeed some organs which we have not men- tioned, such as the spleen, which seems to be an area for the multiplication of blood corpuscles, and the thyroid gland, which seems to have to do with keeping the blood at a certain standard of efficiency, but what we have said is perhaps enough to convey a general idea of the processes of life in a higher animal. In conclusion, it is perhaps useful to remark that when in the course of further studies the student meets with organs which are called by the same name as those found in man or in Mammals, as, for example, the "liver " of the Molluscs, he must be careful not to suppose that the function of such a " liver " is the same as in Mammals, for comparatively little investigation into the physiology of the lower types of animal life has as yet been made. At the same time, he must clearly recognise that the great internal activities are in a general way the same in all animals ; thus, respiration, whether accomplished by skin, or gills, or air tubes, or lungs, by help of the red pigment (haemoglobin) of the blood, or of some pigment which is. not red, or occurring without the presence of any / blood at all, always means that oxygen is absorbed almost like a kind of ) food by the tissues, and that the carbonic acid gas which results from the oxidation of part of the material of the tissues is removed. Modern Conception of Protoplasm. The activities of animals are ultimately due to physical and chemical changes associated with the living matter or protoplasm. This is a mere truism. We do not know the nature of this living matter ; in fact, our most certain know- ledge of it is that in our brains its activity is expressed as thought. When more is known in regard to the chemistry and physics of living matter, it may be possible to bring vital phenomena more into a line with the changes- which are observed in inorganic things. At present, however, it is idle to deny that vital phenomena are things apart. / Not even the simplest of them can be explained in terms of chemistry and physics. > Even the passage of digested food from the gut to the blood vessels is more than ordinary MODERN CONCEPTION OF PROTOPLASM. 27 physical osmosis ; it is modified by the fact that the cells are living. From the point of view of a student of physics Dr. J. Joly draws the following contrast between an animate and an inanimate body : " While the transfer of energy into any inanimate material system is attended by effects retardative of the transfer and conducive to dissipation, the transfer of energy into any animate material system is attended by effects conducive to the transfer and retardative of dis- sipation." But though we cannot analyse living matter, nor thoroughly explain the changes by which the material of the body breaks down or is built up, we can trace, by chemical analysis, how food passes through various transformations till it becomes a useable part of the living body, and we can also catch some of the waste products formed when muscles or other parts are active. Apart from any theory, it is certain that waste products are formed when work is done, and that living animals have a marvellous power of rapid repair, of ceaselessly changing, and yet remaining more or less the same. Theory begins when we attempt to make the general idea of waste and repair more precise. In the study of "protoplasm," both morphologist and physiologist have reached their strict limits. Further analysis becomes physical and chemical, ! and ends in the confession that protoplasm is a marvellous form of matter in motion or a subtle kind of motion of which we can form only a very vague conception. What is known in regard to the structure of protoplasm does not help the physiologist very much. As we shall afterwards see, the micro- scopists discover an intricate network which pervades each unit of living matter, but no physiologist dreams of explaining the life of a cell in terms of its microscopically visible structure. Yet, as Burdon Sander- son says, "We still hold to the fundamental principle that living matter acts by virtue of its structure, provided the term structure be used in a sense which carries it beyond the limits of anatomical investigation, ?'.- d Division of Astroid and its loops J>- (metakinesis). (3) We are far from being able to give even an approximate account of the " mechanism " of cell division. Rapidly progressive research has disclosed many mysteries, but it does not explain them. The nucleus is resolved into a chromatin framework and an achromatin matrix, but we know the nature of neither. The longitudinal division of each loop shows how thorough is the partition of substance and implied qualities. The " central corpuscles," recently discovered, act like centres of force, and the indescribably fine threads, which pass from around these to the chromatin loops, have been credited with motive powers. Siriiilarly the threads of the nuclear spindle are believed by some to draw or drive the chromosomes, But we do not know. The whole process is vital, and therefore inexplicable in terms of matter and motion, so long at least as we do not know the secret of protoplasm. (4) On the other hand, Leuckart and Spencer have given a general rationale of cell division. Why do not cells grow much larger, why do they almost always divide at a definite limit of growth ? Their answer is as follows : Suppose a young cell has doubled its original mass, that means that there is twice as much living matter to be kept alive. But the living matter is fed, aerated, purified through its surface, which, in growing spherical cells for instance, only increases CELL DIVISION. 47 as the square of the radius, while the mass increases as the cube. The surface growth always lags behind the increase of mass. Therefore, when the cell has, let us say, quadrupled its original mass, but by no means quadrupled its surface, difficulties set in, waste begins to gain on repair, anabolism loses some of its ascendancy over katabolism. At the limit of growth, then, the cell divides, halving its mass and gaining new surface. Of course surface may be increased by out- flowing processes, just as that of leaves by many lobes ; and FIG. 5. Karyokinesis. (After FLEMMING.) 1. Coil stage of nucleus ; cc, central corpuscle. 2. Division of chromatin elements into U-shaped loops, and longitudinal splitting of these (astroid stage). 3-4. Recession of chromatin elements from the equator of the cell (diastroid). 5. Nuclear spindle, with chromatin elements at each pole, and achromatin threads between. 6. Division of the cell completed. division may occur before the limit of growth is reached, but as a general rationale, applicable to organs and bodies as well as to cells, the suggestion of Leuckart and Spencer is very helpful. (5) Protoplasm. Morphological as well as physiological analysis passes from the organism as a whole to its organs, thence to the tissues, thence to the cells, and finally to the protoplasm itself. But although 48 THE ELEMENTS OF STRUCTURE. we may define protoplasm as genuinely living matter as " the physical basis of life " we cannot definitely say how much or what part of an Amoeba, or an ovum, or any other cell is really protoplasm. We are able to make negative statements, e.g., the yolk of an egg is not protoplasm, but we cannot make positive statements, or say, This is protoplasm and nought else. Thus, what is spoken of as the structure of protoplasm is really the structure of the cytoplasm. In regard to this structure, we know that it is very complex, but we are not sure of much more. For different experts see different appear- ances, even in the same cells. Thus some, e.g., Frommann, see an intricate network or reticulum with less stable material in the meshes ; others, e.g., Flemming, see what looks like a manifold coil of fibrils; and others, e.g., Biitschli, see a foam-like or vacuolar structure. It seems likely that the structure is different at different times, or in different cells. Professor Biitschli's belief that the cytoplasm has a vacuolar structure is corroborated by his interesting experiments on microscopic foams. Finely powdered potassium carbonate is mixed with olive oil which has been previously heated to a temperature of 5O-6o C., an acid from the oil splits up the potassium carbonate, liberates carbon dioxide, and forms an extremely fine emulsion. Drops of this show a structure like that of cytoplasm, exhibit movements and streamings not unlike those of Amoeboe, and are, in short, mimic cells. Just as a working model may help us to understand the circulation, so these oil emulsions may help us to understand the living cell, by bringing the strictly vital pheno- mena into greater prominence. CHAPTER IV. THE REPRODUCTION AND LIFE HISTORY OF ANIMALS. I. REPRODUCTION. IN the higher animals the beginnings of individual life are hidden, within the womb in mammals, within the egg shell in birds. It is natural, therefore, that early preoccupation with those higher forms should have hindered the recog- nition of what seems to us so evident, that almost every animal arises from an egg cell or ovum which has been fertilised by a male cell or spermatozoon. The exceptions to this fact are those organisms which multiply by buds or detached overgrowths, and those which arise from an egg cell which requires no fertilisation. Thus Hydra may form a separable bud, much as a rose bush sends out a sucker ; thus drone bees " have a mother but no father," for they arise from parthenogenetic eggs which are not fertilised. Apart from these and similar cases, the "ovum theory," which Agassiz called " the greatest discovery in the natural sciences in modern times," is true, that each organism begins from the division of a fertilised egg cell. History. We can realise this discovery better if we consider its history. For a long time, on into the present century, what was called the doctrine of prefonnation prevailed. According to this theory, development was merely an unfolding ("evolution") of a preformed miniature which lay within the germ. The " ovists " found this minia- ture model of the future organism in the egg; the " animalculists " found and even figured it within the spermatozoon. "There is no becoming," said Haller, " no part of the body is made from another, all are created at once." But this was not all. The germ was more than a marvellous bud-like miniature of the adult, it included the next 4 50 REPRODUCTION AND LIFE HISTORY OF ANIMALS. generation, and the next, and the next, and all future generations. Germ lay within germ, preformed in transparency, and in successively smaller miniature, after the fashion of an infinite juggler's box. We laugh at this, but we need not laugh too much, for the preformationists, though wrong and crude in their facts, were right in two of their ideas, that the germ contains the potentiality of a future organism, and that it has relations, not only to the animal into which it develops, but also to generations following. (See p. 71.) In the middle of the seventeenth century, however, Harvey had reached conclusions which might have saved much blundering. Study- ing the development of the chick, as Greek naturalists had tried to do wellnigh two thousand years before, as we are doing still in our embryo : logical laboratories, Harvey maintained that every animal was produced from an ovum (OVUM esse primordium commune omnibus animalibtts], and that organs arose by new formation (epigenesis), not by the expansion or " evolution " of some invisible preformation. But the great champion of epigenesis was Caspar Friedrich Wolff, who, in his doctorial dissertation of 1759, traced the chick back to a layer of organised particles (the familiar cells of to-day), in which there was no likeness of the future embryo, far less of the adult. Wolff was long in finding successors, but in 1824 Prevost and Dumas described the division of the ovum ; in 1827 Von Baer discovered the mammalian ovum ; while Wagner, Von Siebold, and others elucidated the real nature of the spermatozoon. A great step was made in 1838-9, when Schwann and Schleiden formulated the "cell theory," according to which every organism is made up of cells, and starts from a cell. From this date modern em- bryology began. Sexual Reproduction. There is apt to be a lack of clearness in regard to sexual reproduction, because the process which we describe by that phrase is a complex result of evolution. It involves two distinct facts : (a) the liberation of special germ cells from which new individuals arise ; (&} the occurrence of two different kinds of germ cells ova and spermatozoa, which come to nothing unless they unite (fertilisation). Further- more, these dimorphic reproductive cells are produced by two different kinds of individuals (females and males), or from different organs of one individual or at different times within the same organ (hermaphroditism). It is conceivable that organisms might have gone on multiplying asexually, by detaching overgrown portions of themselves which had sufficient vitality to develop into complete forms. But a more economical method is the liberation of special germ cells, in which the qualities of the THE LIBERATION OF SPECIAL GERM CELLS. 51 organism are inherent. This is the primary characteristic of sexual, as opposed to asexual multiplication. It is also conceivable that organisms might have remained approximately like one another in constitution, and at all times very nearly the same, and that they might have liberated similar germ cells capable of immediate develop- ment. Such a race would have illustrated the one charac- teristic of sexual reproduction, the liberation of special germ cells, but it would have been without that other characteristic of sexual reproduction, the existence of dimorphic germ cells, of different kinds of sexual organs, or of male and female individuals. The Liberation of Special Germ Cells. One must think of this as an economical improvement on the method of starting a new life by asexual overgrowth or by the liberation of buds. Asexual reproduction, as Spencer and Haeckel point out, is a mode of growth in which the bud, or whatever it is, becomes discontinuous from the parent. The buds of a sponge, of a coral, of a sea mat (Polyzoon), or of many Tunicates, remain attached to the parent. If there be a keen struggle for subsistence, this may be disadvantageous ; but in some cases, doubtless, the colonial life which results is a source of strength. In the case of Hydra, however, the buds are set adrift ; the same is true of not a few worms. This liberation of buds takes us nearer the sexual process of liberating special germ cells. But unless the organism is in very favourable nutritive conditions, in which overgrowth is natural, the liberation of buds is evidently an expensive way of continuing the life of a species. Not only so, but we can hardly think of budding even as a possibility in very complex organisms, like snails or birds, in which there is much division of labour. More- over, the peculiarity of a true germ cell is, that it is un- specialised, continuous in quality with the original germ cell from which the parent arose, and not very liable to be tainted by the mishaps which may befall the " body " which bears it. And, finally, in the mixture of two units of living matter which have had different histories, the possibility of permutations and combinations, in other words, of variation 52 REPRODUCTION AND LIFE HISTORY OF ANIMALS. is evidently supplied (see p. 63). Thus it is not surprising to find that the asexual method of liberating buds has been replaced in most animals by the liberation of special germ cells, by the more economical and advantageous process of sexual reproduction. SUMMARY OF MODES OF REPRODUCTION. A. In single celled Animals (Protozoa). (1) The almost mechanical rupture of an amoeboid cell, which has become too large for physiological equilibrium (e.g., Schizogenes}. (2) The discharge of numerous superficial buds at once (e.g., Arcella and Pelomyxa]. (3) The formation of one bud at a time (very common). (4) The ordinary division into two daughter cells at the limit of growth. (5) Repeated divisions within limited time and within limited space (a cyst). This results in what is called spore formation, "free cell formation," "endogenous multiplication" (e.g., in Gre- garines). B. In many celled Animals (Metazoa). (Asexual.) (a) The separation of a clump of body cells, e.g., from the surface of some Sponges. (A crude form of budding.) (b] The formation of definite buds which may or may not be liberated ; and other forms of asexual multiplication. (Sexual.) (a) The liberation of cells from a simple Metazoon in which there is so little division of labour that the distinction between body cells and reproductive cells is not marked. (Hypothetical.) (b] The liberation of special reproductive or germ cells, which have not taken part in the formation of the body, and which retain, more or less unaltered, the inherent qualities of the original germ cell from which the parent arose. These special reproductive cells the ova and spermatozoa are normally united in ferti- lisation, but some animals have (parthenogenetic) ova which develop without being fertilised. The Evolution of Sex. A further problem is to account for the two facts (a) that most animals are either males or females, the former liberat- ing actively motile male elements or spermatozoa, the latter THE EVOLUTION OF SEX. 53 forming and usually liberating more passive egg cells or ova ; and (ft) that these two different kinds of reproductive cells usually come to nothing unless they combine. The problem is partly solved by a clear statement of the facts. Begin with those interesting organisms which are on the border line between Protozoa and Metazoa, the colonial Infusorians of which Volvox is a type (see p. 95). The adults are balls of cells, and the component units are connected by protoplasmic bridges. From such a ball of cells repro- ductive units are sometimes set adrift, and these divide to form other individuals without more ado. In other con- ditions, however, when nutrition is checked, a less direct mode of reproduction occurs. Some of the cells become large well fed elements, or ova; others, less successful, divide into many minute units or spermatozoa. The large cells are fertilised by the small. Here we see the formation of dimorphic reproductive cells in different parts of the same organism. But we may also find Volvox balls in which only ova are being made, and others with only sper- matozoa. The former seem to be more vegetative and nutritive than the latter; we call them female and male organisms respectively ; we are at the foundation of the differences between the two sexes. All through the animal series, from active Infusorians and passive Gregarines, to feverish Birds and more sluggish Reptiles, we read antitheses between activity and passivity, between lavish expenditure of energy and a habit of storing. The ratio between disruptive (katabolic) processes and con- structive (anabolic) processes in the protoplasmic metabolism varies from type to type. We believe that the contrast between the sexes is another expression of this fundamental alternative of variation. This theory may be confirmed in many ways, e.g., by contrasting the characteristic products of female life, passive ova, with the characteristic products of male life, active spermatozoa ; or by comparing the complex con- ditions (such as abundant food, favourable temperature) which tend to produce female offspring, with the opposite conditions which tend to produce males ; or by contrasting the secondary sexual characters of males (e.g., bright colours and smaller size), with the opposite characteristics of females. 54 REPRODUCTION AND LIFE HISTORY OF ANIMALS. Stages in the History of Fertilisation. While it is not difficult to see the advantage of fertilisation as a pro- cess which helps to sustain the standard or average of a species and as a source of new variations, we can at present do little more than indicate various forms in which the process occurs. (a) Formation of Plasmodia, the flowing together of numerous feeble cells, as seen in the life history of those very simple Protozoa called Proteomyxa, e.g., Protomyxa, and Mycetozoa, e.g., flowers of tan (ALthalium septicuui). (b) Multiple Conjugation, in which more than two cells unite and fuse together, as in some Gregarines and in the sun animalcule (Actinosphtzrmm). (c) Ordinary Conjugation, in which two similar cells fuse together, observed in Gregarines and Rhizopods. In ciliated Infusorians, the conjugation may be merely a temporary union, during which nuclear elements are interchanged. (d) Dimorphic Conjugation, in which two cells different from one another fuse into one, a process well illustrated in Vorticella and related Infusorians, where a small, active, free swimming (we may say, male) cell unites with a fixed individual of normal size, which may fairly be called female (see Fig. 23, p. 94). (e) Fertilisation, in which a spermatozoon liberated from a Metazoon unites intimately with an ovum liberated from another individual normally of the same species. Divergent Modes of Sexual Reproduction. (a) Hermaphroditism is the combination of male and female sexual functions in varying degrees within one organism. It may be demonstrable in early life only, and disappear as maleness or femaleness predominates in the adult. It may occur as a casualty or as a reversion ; or it may be normal in the adult, e.g., in some Sponges and Ccelentera, in many " worms," e.g., earthworm and leech, in barnacles and acorn shells, in one species of oyster, in the snail, and in many other Bivalves and Gastropods, in Tuni- cates and in the hagfish. In most cases, though these animals are bisexual, they produce ova at one period and spermatozoa at another (dichogamy). It rarely occurs (e.g., in some parasitic worms) that the ova of a hermaphrodite are fertilised by the spermatozoa of the same animal. Certain facts, such as the occurrence of hermaphrodite organs as a transitory stage in the development of the embryos of many unisexual animals (e.g., frog and bird), make it likely that hermaphroditism is the primitive condition, and that the ALTERNATION OF GENERATIONS. 55 unisexual condition of permanent maleness or femaleness is a secondary differentiation. The cases which we have cited above may be interpreted as due to persistence of the primi- tive condition, or as reversions to it. (b) Parthenogenesis, as we know it, is a degenerate form of sexual reproduction, in which ova produced by female organisms develop without being fertilised by male elements. It is well illustrated by Rotifers, in which fertilisation is not known to occur, while in some genera males have never been found ; by many small Crustaceans whose males are absent for a season ; by aphides, from among which males may be absent for the summer (or in artificial conditions for several years) without affecting the rapid succession of female FIG. 6. Diagrammatic expression of alternation of generations. I. Hydromedusae. ov. Fertilised ovum gives rise to asexual form A , which, by budding,, produces sexual form or forms 6" ; in case of Hydromedusae A is re- presented by hydroid (//) and .9 by medusoid (M). II. Liver Fluke. ov. Fertilised ovum gives rise to asexual stages (A) which, from special spore-like cells (Tv), produce eventually the sexual fluke (S). generations ; by the production of drones in the bee hive, for the eggs which give rise to drones are unfertilised (see p. 60). (c) Alternation of Generations. A fixed asexual hydroid or zoophyte (campanularian or tubularian) often buds off and liberates sexual medusoids or swimming bells, whose fertilised ova develop into embryos which become fixed and grow into hydroids (Fig. 49, p. 156). This is the simplest illustration of alternation of generations, which may be 56 REPRODUCTION AND LIFE HISTORY OF ANIMALS. defined as the alternate occurrence in one life cycle of two (or more) different forms differently produced. The liver fluke (Distoma hepaticum} of the sheep produces eggs which when fertilised grow into embryos. Within the latter, certain cells (which can hardly be called eggs) grow into numerous other larvae of a different form. Within these the same process is repeated, and finally the larvae thus produced grow (in certain conditions) into sexual flukes (Fig. 54, p. 1 68). In this case, reproduction by special cells like undifferentiated precocious ova, alternates with reproduc- tion by ordinary fertilised egg cells. So, too, the vegetative sexless " fern plant " gives rise to special spore cells, which develop into an inconspicuous bisexual " prothallus," from the fertilised egg cell of which a "fern plant" springs. Various kinds of alternation are seen in the life cycle of the fresh water sponge, in the stages of the jelly fish Aurelia, in the history of some " worms " and Tunicates. They illustrate a rhythm between asexual and sexual multiplica- tion, between parthenogenetic and normally sexual reproduc- tion, between vegetative and animal life, between a relatively " anabolic " and a relatively " katabolic " preponderance. II. EMBRYOLOGY. The Egg Cell or Ovum. Apart from cases of asexual re- production and parthenogenesis every multicellular organism begins life as an egg cell with which a male cell or sperma- tozoon has entered into intimate union. The most important characteristic of the reproductive cells, whether male or female, is that they retain the essen- tial qualities of the fertilised ovum from which the parent animal was developed. The ovum has the usual characters of a cell; its sub- stance is traversed by a fine protoplasmic network ; its nucleus or germinal vesicle contains the usual chromatin elements ; it has often a distinct sheath representing a cell wall. In Sponges, the ova are well nourished cells in the middle stratum of the body ; in Coelentera they seem to arise in connection with either outer or inner layer (ectoderm or endoderm) ; in all other animals, they arise in connection THE EGG CELL OR OVUM. 57 with the middle layer or mesoderm, usually on an area of the epithelium lining the body cavity. In lower animals they often arise somewhat diffusely ; in higher animals their for- mation is restricted to distinct regions, and usually to definite organs the ovaries. The young ovum is often amoeboid, and that of Hydra retains this character for some time (Fig. 41, p. 139). The ovum grows at the expense of adjacent cells, or by absorbing material which is contributed by special yolk glands or sup- plied by the vascular fluid of the body. The yolk or nutritive capital may be small in amount, FIG. 7. Diagram of ovum, showing diffuse yolk granules. g.v. Germinal vesicle or nucleus ; chr. chromatin elements. and distributed uniformly in the cell as in the ova of Mammals, earthworm, starfish, and sponge ; or it may be more abundant, sinking towards one pole as in the egg of the frog, or accumulated in the centre as in the eggs of Insects and Crustaceans ; or it may be very copious, dwarf- ing the formative protoplasm, as in the eggs of Birds, Rep- tiles, and most Fishes. Round the egg there are often sheaths or envelopes of various kinds, (a) made by the ovum itself, and then very delicate (e.g., the vitelline membrane) ; (b) formed by adja- cent cells (e.g., the follicular envelope) : or (c) formed by 58 REPRODUCTION AND LIFE HISTORY OF ANIMALS. special glands or glandular cells in the walls of the oviducts (e.g., the "shells" of many eggs). The envelope is often firm, as in the chitinous coat around the eggs of many Insects, and in these cases there is often a little aperture (micropyle) through which alone the spermatozoon can enter. The hard calcareous shells round the eggs of Birds and Tortoises, or the mermaid's purse enclosing the egg of a skate are of course formed after fertilisation. Egg shells must be distinguished from egg capsules or cocoons, e.g., of the earthworm, in which several eggs are wrapped up together. The Male Cell or Spermatozoon is a much smaller and usually a much more active cell than the ovum. In its FIG. 8. Forms of Spermatozoa (not drawn to scale). i and 2. Immature and mature spermatozoa of snail ; 3. of bird ; 4. of man (,&, head; /;z, middle portion ; t, tail); 5. of salamander, with vibratile fringe (_/") ; 6. of Ascaris, slightly amoeboid with cap (c); 7.* of crayfish. minute size, locomotor energy, and persistent vitality, it resembles a flagellate monad, while the ovum is comparable to an amoeba or to one of the more encysted Protozoa. A spermatozoon has usually three distinct parts : the essential " head," consisting mainly of nucleus, and the mobile " tail " which is often fibrillated, and a small middle portion between head and tail, which is regarded by some as the centrosome. The spermatozoa of Threadworms and Crustaceans are sluggish, and inclined to be amoeboid (Fig. 8 (6, 7) ). Both ova and spermatozoa are true cells, and they are MATURATION OF THE OVUM. 59 complementary, but the spermatozoon has a longer history behind it. The homologue of the ovum is the mother sperm cell or spermatogonium. This segments much as the ovum does, but the cells into which it divides have little coherence. They go apart, and become spermatozoa. There is a striking resemblance between the different ways in which a mother sperm cell divides and the various kinds of segmen- tation in ova. In most cases the spermatogonium divides into spermatocytes, which usually divide again into spermatides or young spermatozoa. Maturation of the Ovum. When the egg cell attains its definite size or limit of growth, it bursts from the ovary or C D FIG. 9. Diagram of maturation and fertilisation. (From " Evolution of Sex.") A. Primitive sex. cell, supposed to be amoeboid. B. Ovum ; C. formation of first polar body (i. /..); D. formation of second polar body (2. /.). Bi. Mother sperm cell; Ci. the same divided (sperm-morula or polyplast, or spermatogonium). Di. Ball of immature spermatozoa or spermatides ; sp. liberated spermatozoa. E. Process of fertilisation ; F. approach of male and female nuclei within the ovum. from its place of formation, and in favourable conditions meets either within or outside the body with a spermatozoon from another animal. Before this union between ovum and spermatozoon is effected, generally indeed before it has begun, the nucleus or germinal vesicle of the ovum moves to the periphery and divides twice. This division results in the formation and extrusion of two minute cells or polar 60 REPRODUCTION AND LIFE HISTORY OF ANIMALS. bodies, the first containing half, the second necessarily a quarter of the nuclear material which composed the germinal vesicle. The nucleus is thus reduced to a quarter of its original chromatin content. It is noteworthy that the second division follows close on the first without the inter- vention of the "resting stage," which usually succeeds a nuclear division. Moreover, there is this important differ- ence between the formation of polar bodies and ordinary cell division, that the number of nuclear rods or chromosomes suffers reduction, whereas in ordinary karyokinesis the daughter nuclei have as many nuclear rods as the original cell. The extruded polar bodies come to nothing, though they may linger for a time in the precincts of the ovum, and may even divide. The extrusion of polar globules from mature ova seems to be almost universal; but observations are lacking in regard to Birds and Reptiles. Moreover, Weismann and Ischikawa have shown that in all partheno- genetic ova which they have examined, only one polar body is formed. It is said, however, that in the parthenogenetic eggs which become drones (Blochmann), and in those of a moth called Liparis (Platner), two polar bodies are formed. But in neither of these two exceptional cases is the partheno- genesis habitual ; thus many of the eggs which the queen bee lays are fertilised, and give rise to queens and workers. One of the most important results of recent investigations as to polar bodies is due to O. Hertvvig and others. It may be briefly stated, with particular reference to the ova of Ascaris megalocephala the thread- worm of the horse. In one variety of this worm (var. bivalens] the germinal vesicle of the ovum contains four nuclear rods, chromosomes, or idants. By doubling these increase to eight (Fig. 10, B) ; the first polar body goes off with four (Fig. 10, C), and the second with two (Fig. 10, D) ; leaving two. Two "reducing divisions" have thus occurred. Similarly, the homologue of the ovum, the sperm mother cell contains four chromosomes in its nucleus (Fig. 10, A 1 ). By doubling these increase to eight (Fig. 10, B 1 ), and by division the cell forms four spermatozoa, each with two. When fertilisation takes place, the nucleus of the spermatozoon, with two chromosomes, unites with the reduced nucleus of the ovum, also with two chromosomes ; and the number is thus raised to four, which is the normal number in the cells of this variety of Ascaris megalocephala. There is thus a striking parallelism in the history of the two nuclei which unite in fertilisation ; both have been subjected to reducing divisions. If this did not occur, each fertilisation would involve a doubling of the number of chromosomes. Weismann interprets the whole process as an arrangement by which the corn- PER TILISA TION. 6 1 binations and permutations of nuclear rods and their vital qualities are increased so as to give rise to new variations. There are, indeed, other interpretations, and the facts are difficult to understand on any theory. Thus Minot, Balfour, Van Beneden, and others have suggested that the polar bodies are extrusions of male substance from the ovum. Biitschli, Giard, and others interpret the premature division of the ovum as the survival of an ancient habit, and regard the polar bodies as rudimentary or abortive ova. It may be possible to combine various interpretations : (l) the ovum divides, like any other cell, like the Protozoon ancestors, at its limit of growth ; (2) the extrusion does in some way differentiate the ovum and renders fertilisation possible or more profitable ; (3) the peculiar reduction involved in the process makes the origin of new variations more certain. Fertilisation. In the seventeenth and eighteenth cen- turies, some naturalists, nicknamed " ovists," believed that A 1 B' FIG. 10. Spermatogenesis and Polar bodies. (After HERTWIG and WEISMANN.) Ai. Primitive germ cell of A scan's megalocephala var. bivalens (4 chromosomes). Bi. Sperm mother cell (8 chromosomes). Ci. Two spermatocytes formed, each with 4 chromosomes (first reducing division). Di. Four spermatozoa formed, each with 2 chromosomes (second reducing division). A. Primitive germ cell (4 chromosomes). B. Fully developed ovum (8 chromosomes). C. Formation of first polar body (/3.i) (first reducing division). D. Formation of second polar body (/.2) (second reducing division). First polar body may divide into two. the ovum was all-important, only needing the sperm's awakening touch to begin unfolding the miniature model which it contained. Others, nicknamed u animalculists," were equally confident that the sperm was essential, though it required to be fed by the ovum. Even after it was 62 REPRODUCTION AND LIFE HISTORY OF ANIMALS. recognised that both kinds of reproductive elements were essential, many thought that their actual contact was un- necessary, that fertilisation might be affected by an aura seminalis. Though spermatozoa were distinctly seen by Hamm and Leeuwenhoek in 1677, their actual union with ova was not observed till 1843, when Martin Barry detected it in the rabbit. Of the many facts which we now know about fertilisation, the following are the most important : (i.) Apart from the occurrence of parthenogenesis in a FIG. n. Fertilisation in Ascaris megalocephala. (After BOVERI.) 1. Spermatozoon (s.) entering ovum, which contains reduced nucleus (A^), having given off two polar bodies (/.. i and 2). 2. Sperm nucleus (n), and ovum nucleus (A r ), each with two chromatin elements or idants, with centrosomes (c.s.). 3. Centrosomes (c.s.) with " archoplasmic " threads radiating out- wards, in part to the chromosomes of the two approximated nuclei. 4. Segmentation spindle before first cleavage. few of the lower animals, an ovum begins to divide only after a spermatozoon has united with it. After one sper- matozoon has entered the ovum, the latter ceases to be receptive, and other spermatozoa are excluded. If, as rarely happens, several spermatozoa effect an entrance into the ovum, the result is usually pathological. It is said, however, SEGMENTATION. 63 that the entrance of numerous spermatozoa (polyspermy) is frequent in insects and Elasmobranch fishes. (2.) The union of spermatozoon and ovum is very intimate; the nucleus of the spermatozoon and the reduced nucleus of the ovum approach one another, combining to form a single nucleus. (3.) When this combined or segmentation nucleus begins the process of development by dividing, each of the two daughter nuclei which result consists partly of material derived from the sperm nucleus, partly of material derived from the ovum nucleus. In other words, the union is orderly as well as intimate, and the subsequent division is so exact, that the qualities so marvellously inherent in the sperm nucleus (those of the male parent), and in the ovum nucleus (those of the mother animal), are diffused through- out the body of the offspring, and persist in its reproductive cells. As to the interpretation of these facts, Weismann maintains the importance of the quantitative addition which the sperm nucleus makes to the diminished nucleus of the ovum. At the same time, he finds an important source of transmissible variations in the mingling of the two nuclear substances (amphimixis). Others believe that the mingling diminishes the risk of unfavourable idiosyncrasies being trans- mitted from parents to offspring. Others emphasise the idea that the sperm supplies a vital stimulus to the ovum, and this seems to be corroborated by the fact well known to breeders that impregnation by a male with certain marked characteristics influences the constitution of the female, and may have an effect on the progeny of subsequent years and by different males ("telegony "). Segmentation. The different modes of division exhibited by fertilised egg cells depend in great measure on the quantity and disposition of the passive and nutritive yolk material, which is often called deutoplasm in contrast to the active and formative protoplasm. The pole of the ovum at which the formative protoplasm lies, and at which the spermatozoon enters, is often called the animal pole ; the other, towards which the heavier yolk tends to sink, is called the vegetative pole. In contrasting the chief modes of segmentation, it should be recognised that they are all connected by gradations. 64 REPRODUCTION AND LIFE HISTORY OF ANIMALS. FIG. 12. Modes of segmentation. 1. Ovum with little yolk segments totally and equally into a blastosphere, e.g., Hydra. 2. Ovum with considerable yolk (y) at lower pole segments wholly but unequally, e.g., frog ; (y.s.) larger yolk, laden cells. 3. Ovum with much yolk segments partially and discordally, forming blastoderm (bl.), e.g., bird. 4. Ovum with central yolk (y) segments partially and peripherally, e.g., crayfish. BLASTOSPHERE AND MORULA. 65 A. COMPLETE DIVISION Holoblastic Segmentation. I. Eggs with little and diffuse yolk material divide completely into approximately equal cells, [or, Ova which are alecithal (i.e., without yolk) undergo approxi- mately equal holoblastic segmentation]. This is illustrated in most Sponges, most Ccelentera, some "worms," most Echinoderms, some Molluscs, all Tuni- cates, Amphioxus. and most Mammals. II. Eggs with a little yolk material accumulated towards one pole, divide completely, but into unequal cells, [or, Ova without very abundant deutoplasm, but with what they have lying towards one pole (telolecithal), undergo unequal holoblastic segmentation]. This is illustrated in some Sponges, some Ccelentera (e.g., Ctenophora), some " worms," many Molluscs, the lamprey, Ganoid Fishes, Ceratodus, Amphibians. B. PARTIAL DIVISION Meroblastic Segmentation. III. Eggs with a large quantity of yolk, on which the formative protoplasm lies as a small disc at one pole, divide partially, and in discoidal fashion, [or, Ova which are telolecithal, and have a large quantity of deutoplasm, undergo meroblastic and discoidal segmentation]. This is illustrated in all Cuttle fishes, all Elasmobranch and Teleostean fishes, all Reptiles and Birds, and also in the Monotremes or lowest Mammals. IV. Eggs with a considerable quantity of yolk, accumulated in a central core, and surrounded by the formative protoplasm, divide partially, and superficially or peripherally, [or, Ova which are centrolecithal undergo meroblastic and super- ficial segmentation]. This is illustrated by almost all Arthropods, and by them alone. Summarising the above, we have : C I T^V.,,,-,1 A. Complete Division. jr* B. Partial Division. / III. Discoidal. \ IV. Peripheral. Blastosphere and Morula. The result of the division is usually a ball of cells. But when the yolk is very abundant (III.) a disc of cells a discoidal blastoderm is formed at one pole of the mass of nutritive material which it gradually surrounds. As the cells divide and redivide, they often leave a large 5 66 REPRODUCTION AND LIFE HISTORY OF ANIMALS. central cavity the segmentation cavity and a hollow ball of cells a blastosphere or blastula results. FIG. 13. Life history of a coral, Monoxenia Darivinii. (From H/ECKKL.) A, B, Ovum. C, Division into two. D, Four cell stage. E, Blastula. F, Free swimming blastula with cilia. G, Section of blastula. H, Beginning of invagination. I, Section of completed gastrula showing ectoderm, endoderm, and archenteron. K, Free swimming ciliated gastrula. MESODERM. 67 But if the so-called " segmentation cavity " be very small or absent, a solid ball of cells or morula, like the fruit of bramble or mulberry, results. Gastrula. The next great step in development is the establishment of the two primary germinal layers, the outer ectoderm and the inner endoderm. or the epiblast and the hypoblast. One hemisphere of the hollow ball of cells may be appar- ently dimpled into the other, as we might dimple an india- rubber ball which had a hole in it. Thus, out of a hollow ball of cells, a two layered sac is formed a gastrula formed by invagination or embole. The mouth of the gastrula is called the blastopore, its cavity the archenteron. But where the ball of cells is practically a solid morula, the apparent in-dimpling cannot occur in the fashion described above. Yet in these cases the two layered gastrula is still formed. The smaller, less yolk laden cells, towards the animal pole, gradually grow round the larger yolk containing cells, and a gastrula is formed by overgrowth or epibole. In the course of our studies, we shall have opportunity to discuss various forms of gastrulation, and some other pro- cesses by which two layers are established, such as that called delaminatien. Mesoderm. We are not yet able to make general state- ments of much value in regard to the origin of the middle germinal layer the mesoderm or mesoblast. In Sponges and Ccelentera it is less distinct than in higher forms, and is usually represented by a gelatinous material (meso- gloea) which appears -between ectoderm and endoderm, and into which cells wander from these two layers. In the other Metazoa, the middle layer may arise from a few primary mesoblasts or cells which appear at an early stage between the ectoderm and endoderm (e.g., in the earth- worm's development) ; or from numerous " mesenchyme " immigrant cells, which are separated from the walls of the blastula or gastrula (e.g., in the development of Echino- derms) ; or as ccelome pouches outgrowths from the endo- dermic lining of the gastrula cavity (e.g., in Sagitta, Balano- glossus, Amphioxus) or by combinations of these and other modes of origin. The mesoderm lies or comes to lie be- 68 REPRODUCTION AND LIFE HISTORY OF ANIMALS. tween ectoderm and endoderm, and it lines the body cavity, one layer of mesoderm (parietal or somatic) clinging to the ectodermic external wall, the other (visceral or splanchnic) cleaving to the endodermic gut and its outgrowths. Origin of Organs. From the outer ectoderm and inner endoderm, those organs arise which are consonant with the position of these two layers, thus nervous system from the ectoderm, digestive gut from the endoderm. The middle layer, which begins to be developed in " worms," assumes some of the functions, e.g., contractility, which in Sponges and Ccelentera are possessed by ectoderm and endoderm, the only two layers distinctly represented in these classes. In a backboned animal the embryological origin of the organs is as follows : (a) From the Ectoderm or Epiblast arise the epidermis and epidermic outgrowths, the nervous system, the most essential parts of the sense organs, infoldings at either end of the gut (fore gut or stomatodasum and hind gut or proctodaeum), and perhaps the segmental or primary excretory duct. (U) From the Endoderm or Hypoblast arise the mid gut (mesenteron) and the foundations of its outgrowths (e.g., the lungs, liver, allantois, &c., of higher Verte- brates), also the axial rod or notochord. According to some authorities, the blood and the vascular system of Vertebrates is in the main endodermic in origin. (c) From the Mesoderm or Mesoblast arise all other struc- tures, e.g., dermis, muscles, connective tissue, bony skeleton, the lining of the body cavity, and perhaps the vascular system. This layer aids in the forma- tion of organs originated by the other two. With it the reproductive organs are associated. Physiological Embryology. Of the physiological conditions of develop- ment, we know relatively little. To investigate them, is one of the tasks of the future. Why does an egg cell form polar bodies, how is the sperm attracted to the ovum, why does the fertilised egg cell divide, how does the yolk affect segmentation, what are the conditions of the infolding which forms the endoderm, and of the outfolding which makes the ccelome pouches, and what do the numerous larval stages mean ? GENERALISATIONS. 69 Generalisations (i) The Ovum Theory or Cell Theory. All many celled animals, produced by sexual reproduction, begin at' the beginning again. "The Metazoa begin where the Protozoa leave off" as single cells. Fertilisation does not make the egg cell double ; there is only a more com- plex and more vital nucleus than before. All development takes place by the division of this fertilised egg cell and its descendant cells. (2) The Gastrcea Theory. As a two layered gastrula stage occurs, though sometimes disguised by the presence of much yolk, in the development of the majority of animals, H?eckel concluded that it represents the individual's recapitulation of an ancestral stage. He believes that the simplest stable, pi. FIG. 14. Embryos (i) of bird ; (2) of man. (After His.) The latter about twenty-seven days old. y.s. Yolk sac ; //. placenta. many celled animal, was like a gastrula, and this hypo- thetical ancestor of all Metazoa he calls a gastr&a. The gastrula is, on this view, the individual animal's recapitula- tion of the ancestral gastrasa. Rival suggestions have been made : perhaps the original Metazoa were balls of cells like Volvox, with a central cavity in which reproductive cells lay ; perhaps they were like the planula larvae of some Ccelenterates two layered, externally ciliated, oval forms without a mouth. (3) The Fact of Recapitulation. It is a matter of experi- ence that we recapitulate in some measure the history of 70 REPRODUCTION AND LIFE HISTORY OF ANIMALS. our ancestors. Embryologists have made this fact most vivid, by showing that the individual animal develops along a path the stations of which correspond to some extent with the steps of ancestral history. (1) The simplest animals are single cells (Protozoa). (2) The next simplest are balls of cells (e.g.) Volvox}. (3) The next simplest are two- layered sacs of cells (e.g., Hydra}. (1) The first stage of development is a single cell (fertilised ovum). (2) The next is a ball of cells (blastula or morula). (3) The next is a two layered sac of cells (gastrula). Von Baer, one of the pioneer embryologists, acknow- ledged that with several very young embryos of higher Vertebrates before him, he could not tell one from the other. Progress in development, he said, was from a general, to a special type. In its earliest stage, every organism has a great number of characters in common with other organisms in their earliest stages ; at each successive stage the series of embryos which it resembles is narrowed. The rabbit begins like a Protozoon as a single cell, after a while it may be compared to the young stage of a very simple vertebrate, afterwards to the young stage of a reptile, after- wards to the young stage of almost any mammal, afterwards to the young stage of almost any rodent, eventually it becomes unmistakably a young rabbit, Herbert Spencer expressed the same idea, by saying that the progress of development was from homogeneous to heterogeneous, through steps in which the individual history was parallel to that of the race. But Haeckel has illustrated the idea more vividly, and summed it up more tersely than any other naturalist. His "fundamental biogenetic law" reads, " Ontogeny, or the development of the individual, is a shortened recapitulation of phylogeny, or the evolution of the race." It is hardly necessary to say that the young mammal is never like a worm, or a fish, or a reptile. It is at most like the embryonic stages of these, and it may also be noticed that as our knowledge is becoming more intimate, the individual peculiarities of different embryos are becoming more evident. Thus Professor Sedgwick has recently said that a blind man could distinguish the early stages of HEREDITY 71 Elasmobranch and Bird embryos. But this need not lead us to deny the general resemblance. Moreover, the individual life history is much shortened compared with that of the race. Not merely does the one take place in days, while the other has progressed through ages, but stages are often skipped, and short cuts are dis- covered. And again, many young animals, especially those "larvae" which are very unlike their parents, often exhibit characters which are secondary adaptations to modes of life of which their ancestors had probably no experience. In short, the individual's recapitulation of racial history is gen- eral, but not precise. But we do not understand how the recapitulation is sustained. Has the protoplasm of the embryo some unconscious memory of the past ? Have the protoplasmic molecules, as Hceckel puts it, learned long since some rhythmic dance which they cannot forget ? And, to what extent must there be similarity of external conditions if the recapitulation, "the perigenesis of the plastidules," is to be sustained? For a careful state- ment of the problem, the student would do well to read the late Pro- fessor Milnes Marshall's British Association address on RECAPITULA- TION, now published in his collected papers. (4) Organic Continuity between Generations. Heredity. Every one knows that like tends to beget like, that off- spring resemble their parents, and sometimes their ancestors (atavism). Not only are the general characteristics trans- mitted, but minute features, idiosyncrasies, pathological conditions, innate or congenital in the parents, may be transmitted to the offspring. Many attempts have been made to explain this, but the first suggestion with any scientific pretensions was that the reproductive cells, which may become offspring, consist of samples accumulated from the different parts of the body. This was a very old idea, but Herbert Spencer and Charles Darwin gave it new life. According to Darwin's "provisional hypothesis of pangenesis," the reproductive cells accumulate gernmules liberated from all parts of the body. In development these gemmules help to give rise to parts like those from which they originated. This hypo- thesis has been repeatedly modified, but, except in the gen- eral sense that the body may influence its reproductive cells, " pangenesis " is discredited by most biologists. The idea which is now accepted with general favour is, 72 REPRODUCTION AND LIFE HISTORY OF ANIMALS. that the reproductive cells, which give rise to the offspring, are more or less directly continuous with those which gave rise to the parent. This idea, suggested by Owen, Haeckel, Rauber, Galton, Jager, Brooks, Nussbaum, and especially emphasised by Weismann, is fundamentally important. At an early stage in the development of the embryo the future reproductive cells of the organism are distinguishable from those which are forming the body. These, the somatic cells, develop in manifold variety, and, as division of labour is established, they lose their likeness to the fertilised ovum of which they are the descendants. The future reproductive cells, on the other hand, are not implicated in the formation of the " body," but remaining virtually unchanged, continue the protoplasmic tradition unaltered, and are thus able to start an offspring which will resemble the parent, because it is made of the same protoplasmic material, and develops under similar conditions. A fertilised egg cell with certain characters (a, b, c\ de- velops into an organism in which these characters are vari- ously expressed ; but if, at an early stage, certain cells are set apart, retaining the characters, a, b, c, in all their entirety, then each of these cells will be on the same footing as the original fertilised egg cell, able to give rise to an organism, almost necessarily to a similar organism. An early insulation of reproductive cells, directly con- tinuous and therefore presumably identical with the original ovum, has been observed in the development of some "worm types" (Sagitta, Threadworms, Leeches, Polyzoa), and of some Arthropods (e.g., Moina among Crustaceans, Chironomus among Insects, Phalangidae among Spiders), in Micrometrus aggregatus among Teleostean fishes, and with less distinctness in some other animals. In many cases, however, the reproductive cells are not recognisable until a relatively late stage in development, after differentiation has made considerable progress. Weis- mann gets over this difficulty by supposing that the con- tinuity is sustained by a specific nuclear substance the germ- plasm which remains unaltered in spite of the differen- tiation in the body. But it is perhaps enough to say that as all the cells are descendants of the fertilised ovum, the reproductive cells are those which retain intact the qualities HEREDITY. 73 of that fertilised ovum, and that this is the reason why they are able to develop into offspring like the parent. Finally, it may be noticed in connection with heredity, that there is great doubt to what extent the "body" can definitely influence its own reproductive cells. Animals acquire individual bodily peculiarities in the course of their life, as the result of what they do or refrain from doing, or as dints from external forces. The " body " is thus changed, but there is much doubt whether the reproductive cells within the " body " are affected by such changes. Weis- mann denies the transmissibility of any characters except those inherent or congenital in the fertilised egg cell, and therefore denies that the influences of function and environ- ment are, or have been, of any importance in the evolution of many celled animals. Such influences affect the body, but do not reach its reproductive cells, and are therefore non-transmissible. Many of the most authoritative biolo- gists are at present of this opinion. On the other hand, many still maintain that profound changes due to function or environment may saturate through the organism, and affect the reproductive cells, and thus the race. The whole question remains under discussion. CHAPTER V. PAST HISTORY OF ANIMALS. PALEONTOLOGY. IN the two preceding chapters we have noticed two of the great records of the history of animal life, that preserved in observable structures, and the modified recapitulation discernible in individual development ; in this we turn to the third the geological record. From Morphology many conclusions as to the course of evolution have been drawn ; the study of form must indeed, by itself, in time have led to the doctrine of evolution, that the present is the child of the past. In the early days of the evolution theory the modern science of Embryology was still in its infancy, and could furnish few arguments, and it was the opponents of the new theory rather than its supporters who appealed to Palaeontology. They asserted that the palaeontological facts refused to lend the support which the theory demanded. To their attacks the evolutionists then chiefly sought to reply by pointing out that the geological record was very incomplete. The numerous investigations which have since been carried on on all sides, now show conclusively that it was imperfection rather of knowledge than of the record which produced the negative results. We must, however, still acknowledge that, except in a relatively few cases, little is known of the ancestors of living animals, and seek for reasons to explain this. Reasons for the "Imperfection of the Geological Record" If we remember the rule of modern Geology that the past is to be interpreted by the aid of the present, there can be no difficulty in realising that the chances against the pre- PAST HISTORY OF ANIMALS. 75 servation of any given animal are very great. Many are destroyed by other living creatures, or obliterated by chemical agencies. Except in rare instances, only hard parts, such as bones, teeth, and shells, are likely to be preserved, and this at once greatly limits the evidential value of fossils. The primitive forms of life would almost certainly be with- out hard parts, and have left no trace behind them. A number of extremely interesting forms, such as many worms and the Ascidians, are, for the same reason, almost unrepre- sented in the rocks. Finally, we cannot suppose that such an external structure as a shell can always be an exact index of the animal within. Some shells, such as Nautilus and some of the Brachiopods, occur as fossils from remote Palaeozoic ages onwards, but it is impossible to believe that the animal within has never varied during this period, though we cannot now learn either the nature or the amount of the variation. After fossilisation has taken place, the rock with its con- tents may be entirely destroyed by subsequent denudation, or so altered by metamorphic changes that all trace of organic life disappears. Of these fossils which have been preserved only a small percentage are available, for vast areas of fossili- ferous rocks are covered over by later deposits, or now lie below the sea or in lands which have not yet been explored. With all these causes operating against the likelihood of preservation, and of finding those forms that may have been preserved, it is little wonder if the geological record is incomplete ; but such as it is, it is in general agreement with what the other evidence, theoretical and actual, leads us to expect as to the relative age of the great types of animal life. Further, those specially favourable cases which have been completely worked out have yielded results which strongly support the general theory. Probabilities of "fossils" in the various classes of animals, But it will be useful to note the probabilities of a good representation of extinct forms in the various classes of animals. Thus, among the Protozoa, the Infusoria have no very hard parts, and have therefore almost no chance of preservation, and the same may be said of forms like Amoebae ; while the Foraminifera and the Radiolaria, having hard structures of lime or silica, have been well preserved. The Sponges are well represented by their spicules and skeletons. Of the Coelenterates, except an extinct order known as Graptolites, only the various forms of coral had any parts readily capable of preservation, and remains of these 76 PAST HISTORY OF ANIMALS. are very abundant in the rocks of many ancient seas. But, strange as it may seem, some beautiful remains of jellyfish have been discovered. Of the great series of "worms," only the tube makers have left actual remains, the others are known only by their tracks, while of any that may have lived on the land there is no evidence. The Echinoderms, because of their hard parts, are well represented in all their orders except the Holothurians, where the calcareous structures characteristic of the class are at a minimum. The Crustacea, being mostly aquatic, and in virtue of their hard skin, are fossilised in great numbers. The Arachnida and the Insects, owing to their air breathing habit, are chiefly represented by chance individuals that have been drowned, or enclosed within tree stumps and amber. The Molluscs and Brachiopods are perhaps better preserved than any other animals, since nearly all of them are possessed of a shell specially suitable for preservation. Among the Vertebrates, some of the lowest are without scales, teeth, or bony skeleton ; such forms have therefore left almost no traces. Fishes, which are usually furnished with a firm outer covering, or with a bony internal skeleton, or with both, are well represented. The primitive Amphibians were furnished with an exoskeleton of bony plates, and are fairly numerous as fossils. The bones and teeth of the others have been fossilised, though more rarely. Those living in fresh water have left footprints as traces. The traces of Reptilia depend upon the habits of the various orders, those living in water being oftenest preserved, but the strange flying Reptiles have also left many skeletons behind them. Of the Birds, the wingless ones are best represented, and then those that lived near seas, estuaries, or lakes. The history of Mammals is very imperfect, for most of them were terrestrial. But the discoveries of Marsh, Cope, and others show how much may be found by careful search. The aquatic Mammals are fairly well preserved. ' l Palceon tologica I Series. ' ' In spite of the imperfection of the "geological record," in spite of the conditions unfavourable to the preservation of many kinds of animals, it is sometimes possible to trace a whole series of extinct forms through progressive changes. Thus a series of fossilised fresh water snails (Planorlns} has been worked out ; the extremes are very different, but the intermediate forms link them indissolubly by a marvellously gradual series of transitions. The same fact is well illus- trated by another series of fresh water snails (Paludind), and not less strikingly among those extinct Cuttlefishes which are known as Ammonites, and have perfectly preserved shells. Similarly, though less perfectly, the modern croco- diles are linked by many intermediate forms to their extinct EXTINCTION OF ANIMALS. 77 ancestors, for it is impossible not to call them by that name, and the modern horse to its entirely different progenitors. In short, as knowledge increases, the evidence from Palae- ontology becomes more and more complete. In a general way, it is true that the simpler animals pre- cede the more complex in history as they do in structural rank, but the fact that all the great Invertebrate groups are represented in the oldest distinctly stratified and fossiliferous rocks the Cambrian system shows that this correspon- dence is only roughly true. To account for this we must remember that the whole mass of the oldest rocks, known FIG. 15. Gradual transitions between Paludina Ne-umayri (a) and Paludina Hccrnesi (/). (From NEUMAYR.) as Archaean or Pre-Cambrian, have been so profoundly altered that, as a rule, only masses of marble and carbona- ceous material are left to indicate that forms of life existed when these rocks were laid down. What these early forms of life were, it seems impossible for us to find out, although recent discoveries, for instance, of " annelid tracks " in rocks of possible Pre-Cambrian age in N. W. Scotland, suggest that patient investigation may yet do much towards the solving of the problem. Extinction of Animals > Some animals, such as some of the lamp shells or Brachiopods, have persisted from almost the oldest ages till now, and most fossilised animals have modern representatives which we believe to be their actual 78 PAST HISTORY OF ANIMALS. descendants. That a species should disappear need not surprise us, if we believe in the " transformation " of one species into another. The disappearance is more apparent than real, the species lives on in its modified descendants, " different species " though they be. But, on the other hand, there are not a few fossil animals which have become wholly extinct, having apparently left no direct descendants. Such are the ancient Trilobites (perhaps remotely connected with our king crab), their allies the Eurypterids, two classes of Echinoderms (Cystoids and Blastoids), many giant Reptiles, and some Mammals. It is almost certain that there has been no sudden extinction of any animal type. There is no evidence of universal cataclysm, though local floods, earthquakes, and volcanic eruptions occurred in the past, as they do still, with disastrous results to fauna and flora. In many cases, the waning away of an order, or even of a class of animals, may be associated with the appearance of some formidable new competitors ; thus Cuttlefish would tend to exterminate Trilobites, just as man is rapidly and often inexcusably annihilating many kinds of beasts and birds. Apart from the struggle with competitors, it is conceivable that some stereotyped animals were unable to accommodate themselves to changes in their surroundings, and also that some fell victims to their own constitutions, becoming too large, too sluggish, too calcareous, in short too extreme. Illustrations of the Appearance of Animals in Time. Such tables as those given here are apt to be misleading, in that they convey the impression that the great types of structure have appeared suddenly. It must be noted that any apparent abruptness is merely due to incompleteness of knowledge or inaccuracy of expression. The table is a mere list of a few important historical events, but one must fully realise that they are not isolated facts, that the present lay hidden in the past and has gradually grown out of it. Of the relative length of the periods represented here we know almost nothing, and we are also ignorant of the earliest ages in which life began. But the general result is clear. We find that in the Cambrian rocks, before Pishes appeared, the great Invertebrate classes were represented, though as yet but feebly. As we pass upwards they increase in number and in differentia- tion. Again, Fishes precede Amphibians, Amphibians are historically older than Reptiles, and many types of Reptiles are much older than Birds. In short, in the course of the ages life has been slowly creeping upwards. TABULAR SURVEY. 79 ternary or ^t-Tertiary. Pliocene. * Miocene. g ranch s. Drdovician. Cambrian. Representa- tives of al the chie classes o I n v e r t e brates. 're-Cambrian )r Archaean. 8o PAST HISTORY OF ANIMALS. Coelentera. Echinoderma. Cephalopoda. Quaternary or Post- Tertiary. Pliocene. ^ c C ^o s , g Miocene. IS... c s ^ c Eocene. S ~^ % Cretaceous. fe r\'^ | Jurassic. S J3 s .. IS *o c "C CJ >r. TD 'o s /chinoids. TJ '2 ^ 'rS ft Linmlus. eptostrac^ immonite >elemnites Jj "* ' ', C, and D show the formation of a second. Note outflowing pseudopodia and the enclosure of the shell by a thin layer of protoplasm ; note also the nucleus in the central protoplasm. amoeboid in food catching. Compared with Lobosa, the Heliozoa are passive. The majority occur in fresh water. . Examples. Actinosphceriuin, Actinophrys sol (sun animalcules) ; Raphidiophrys, forming colonies ; Clathrulina^ stalked. 6. FORAMINIFERA. Predominantly amoeboid forms, with fine branching and interlacing processes issuing from the main mass, which is always within a shell, calcareous in the majority, arenaceous or chiti- SYSTEMATIC SURVEY. 101 nous in others. A nucleus is present, and often multiplies, apparently in association with reproduction. Vacuoles, contractile or otherwise, seem to be very rare. Conjugation has not been certainly observed. Multiplication may take place by division, but usually by the repeated division of the nucleus and the formation of internal bud spores. The great majority are marine, occurring at all depths. Those from great depths have usually shells of glued sand ; the limy forms are found at their best in the shallow water of warm seas, but some occur in the open sea, and sinking down as they die form ooze. They are common as fossils from Silurian strata onwards. Examples. Ground, in both fresh and salt water, with one or two openings to its shell, which is, however, virtually enclosed in the over- FIG. 27. Polystomella. (After SCHULTZE.) flowing protoplasm ; Microgroinia socialis, in fresh water, forming colonies ; Shepheardella^ with an opening at each end of a long mem- branous case ; Miliolina, with a chambered cell simply coiled, and a single aperture. Such forms are often called Imperforate, in contrast to those whose shells have many pores. Lagena, with a simple flask- shaped cell, with diffuse holes for the processes ; Globigerina, a pelagic limy form, with many chambers covered with pores, contributes very largely to the ooze ; Hastigerina^ a pelagic form, with bubbly protoplasm abundantly overflowing round the shell .which comes to be internal like a Radiolarian "central capsule" (q v.) ; Amwodiscus, from the depths, 102 PROTOZOA. with a flinty agglutinate shell ; Haliphysema, a form utilising sponge spicules to cover itself, once mistaken for a minute sponge, or for a very simple many celled animal. Most kinds of chalk consist mainly of the shells of Foraminifera, accumulated on the floor of ancient seas ; Nummulites and related fossil forms were as large as shillings or half-crowns. 7. RADIOLARIA. Marine Rhizopods, divided by a membrane into an inner central capsule (with one or more nuclei), and an outer portion, giving off radiating thread-like pseudopodia. The protoplasm of the FIG. 28. A pelagic Foraminifer Hastigerina (Globigerina) Murrayi. (After BRADY. ) Note central shell, projecting calcareous spines with a protoplasmic axis ; also fine curved pseudopodia and vacuolated protoplasm. two regions is connected by openings in the capsule membrane, and contains many vacuoles. No contractile vacuoles have been seen. There is usually a skeleton, in most cases siliceous and of complex architecture, in some cases of a horn-like substance, called acanthin. The skeleton may be quite outside the central capsule, or may invade it SYSTEMATIC SURVEY. 103 also. Most lead an isolated existence (Monocyttaria) ; a few form colonies by fusion (Poly cytt aria}. Most Radiolarians include unicellular Algae (yellow cells), with which they live in intimate mutual partnership (symbiosis). Division is pro- bably the commonest mode of multiplication, but flagellate spores sometimes of two sizes, small and large, as if male and female may be formed in the central capsule. Conjugation is still unknown. Professor Lankester notes that the central capsule of a Radiolarian may be com- pared with the enclosed shell of Hastigerina, and that the character of the protoplasm, which in contrast with that of Foraminifera is abundantly vacuolated, may be associated with the pelagic life, which is rare in the former class. Radiolarians form much of the ooze of the great depths, and occur abundantly as fossils from Palseozoic times. Examples. Thalassicola (no skeleton) ; Acanthometra (acanthin) ; Adinomma (flinty skeleton, central capsule with pores all over) ; FIG. 29. Optical section of a Radiolarian (Actinomma). (After H^CKEL. ) a. Nucleus ; b. Wall of central capsule ; c. Siliceous shell within nucleus ; cl. Middle shell within central capsule ; A Outer shell in extra-capsular substance. Four radial spicules hold the three spherical shells together. Eucyrtidium (flinty skeleton, with one perforate area in cone shaped central capsule) ; Atilosphcera (flinty skeleton, central capsule with more than one perforate area) ; Collozoum and Spharozoum, multicellular colonial forms. >. C. Predominantly Encysted Protozoa Sporozoa. 8. GREGARINIDA (or better, perhaps, SPOROZOA). Protozoa of parasitic habit, very passive in adult life, clothed by a definite rind, almost never with any locomotor processes. Found in almost all kinds 104 PROTOZOA. of animals ; often, especially when young, within the cells of their host ; deriving their food by absorbing diffusible juices. A single large nucleus ; no contractile vacuole. Reproduction by division in early life, but typically by spore formation. An encysted phase precedes the division into encased spores. The young forms escaping from a spore case may be flagellate or amoeboid ; but, except in a very few cases, passivity pre- vails, and the adults are much restricted in their contractile movements. Conjugation, followed by fusion, often precedes encystation ; and two forms often occur joined together but not fused. Examples. Monocystis, in earthworm ; Gregarina^ with a cross par- tition, in food canal of Arthropods ; Eimeria, remaining, except in young stages, within a cell of the host ; Drepanidium, and other forms, in blood corpuscles ; Myxiditun, with amoeboid adult ; Sarcocystis, in muscle fibres of Mammals and some other Vertebrates ; Coccidium ovifonne, a FIG. 30. A Colonial Flagellate Infusorian Proterospongia Hseckelii. (After SAVILLE KENT.) There are about 40 flagellate individuals. , nucleus ; 6, contractile vacuole ; c, amoeboid unit in gelatinous matrix ; d, division of an amoeboid unit ; , flagellate units with collars contracted ; f, hyaline outer membranes ; g, unit forming spores. permanent cell parasite, in many Vertebrates, common in the liver of rabbits, &c. jD. Predominantly Active Forms (ciliate and flagellate), generally called Infusorians. {Occurring in fresh and sea water ^ abundant in infusions.) 9. FLAGELLATA, units with a definite rind, with 1-3 actively undu- lating flagella, often with a distinct aperture for the entrance of food. Reproduction by division into two, or by multiple division within a cyst. Conjugation and encystation are common. Some forms are colonial, and suggest the transition of Metazoa. SYSTEMATIC SURVEY. 105 Examples. Mastigamceba, possessing a flagellum and amoeboid pro- cesses ; Euglena, very common in wayside pools, with green or variable colouring matter, probably feeding for the most part like a plant ; the colonial Volvox ; Codosiga, with stalked colonies, each individual with a collar round the base of the flagellum ; Proterospongia, colonial, like a detached piece of sponge. Many, e.g. , Monads, live parasitically or in putrid liquids. 10. DINOFLAGELLATA, very successful Protozoa, which combine activity and passivity, having two flagella and generally a cellulose coat. The one flagellum projects from a longitudinal groove, the other lies in a transverse groove. Mostly marine. Examples. Peridininm and Ceratium. 11. RHYNCHOFLAGELLATA, large forms, with firm rind and very spongy protoplasm, with two flagella, the larger one striated like a muscle, springing from a deep groove, the smaller one near the aperture for the food. Examples. The phosphorescent Noctiluca ; Leptodiscus medusoides, disc-like in form, swimming like a miniature medusoid. 12. CILIATA, provided with numerous cilia, which bend and straighten rapidly, driving the animals along or wafting food particles into the " mouth." There is a definite rind. Beside the large macronucleus there is in most a micronucleus or "paranucleus." There are usually two contractile vacuoles. Multiplication by rapidly succeeding divisions ; in rare cases spores seem to be formed. Conjugation has in some cases at least been shown to be associated with intimate interchange of micro - nuclear material. Parasitic forms, some mouthless, are not uncommon. Examples. (a) Peritricha, with a circle of cilia at one end or at both, e.g., Vorticella ; Trichodina, common on Hydra; (b) Hetero- tricha, with long and short cilia, e.g. , the large Stentor, about -^V^h inch in length ; Balantidium coli, in colon of man. (c) Holotricha, uni- formly ciliated, e.g. , Paramcecium ; Opalina, in intestine of frog, with numerous nuclei, and no contractile vacuoles. (d] Hypotricha, locomotor cilia confined to under surface, e.g., Stylonichia. 13. ACINETARIA, ciliated when young, and probably derived from the Ciliata, but more passive when adult. They are fixed in adult life, generally stalked, and bear tentacle-like processes often suctorial. The nucleus is sometimes branched. They have one or more contractile vacuoles. They multiply by division, or by the formation of buds which usually remain for a time partly enclosed by the parent. Their food consists of other Protozoa. They represent " an extreme modification of the Protozoon series, in which the differentiation of parts in a uni- cellular animal reaches its highest point" (Lankester). Examples. Adneta, suctorial ; Dendrosoma, forming branched colonies, suctorial ; Ophryodendron^ non-suctorial. While most Acinetse seize other Infusorians by means of their suckers, there are others of minute size, e.g., Sphcerophrya paramtxdorum^ which penetrate into their prey and become parasites. History. Of animals so small and delicate as Protozoa, we do not expect to find distinct relics in the much battered ancient rocks. But there are hints of Foraminifer shells even in the Cambrian ; more than hints in the Silurian and Devonian ; and an abundant representation in io6 PROTOZOA. rocks of the Carboniferous and several subsequent epochs. The famous Eozoon canadeuse of Cambrian rocks is regarded by most as a purely mineral formation. There seem at least to be sufficient relics to warrant Neumayr's generalisation in regard to Foraminifera, that the earliest had shells of irregularly agglutinated particles (Astrorhizidae), that these were suc- ceeded by forms with regularly agglutinated shells, exhibiting types of architecture which were subsequently expressed in lime. Relics of siliceous Radiolarian shells are also known from Silurian strata onwards, with, perhaps, the exception of the Devonian. Best known are those which form the later Tertiary deposits of Barbados earth, from which Ehrenberg described no fewer than 278 species. GENERAL NOTES ON THE PROTOZOA. Ordinary Functions Movement. The most obvious function of a Protozoon is movement, of which the simplest mode is that termed amoeboid. This is well illustrated by an Amoeba. In ordinary conditions it is continually chang- ing its shape, putting forth blunt lobes and drawing others in. With this is usually associated a streaming movement of the granules, while within the cell itself a somewhat similar streaming is often seen, as in many plant cells. Besides the local changes of form seen in the Amoeba, a defined contraction, like that of a muscle cell, is illustrated in the contractile filament of the stalk of Vorticella and similar Infusorians ; and not less definite are the movements of cilia and flagella, by means of which most Infusorians travel swiftly through the water. Cilia in movement are " bent and straightened alternately," while flagella, which are usually single mobile threads, " exhibit lashing movements to and fro, and are thrown into serpentine waves during these movements." Considered generally, the movements are of two kinds ; either (i) reflex, i.e., responses to external stimulus, as when the Protozoon moves towards a nutritive substance, or (2) automatic, i.e., such movements as appear to originate from within, without our being able to point to the immediate stimulus, e.g., the rhythmical pulsations of contractile vacuoles. While all vital activity or life must . remain inexplicable in lower terms until we know the chemical nature of protoplasm, it is useful to compare the movements of Amoebse with the movements of drops of fine emulsion, as Professor Biitschli has done in great detail. For in this ORDINARY FUNCTIONS. 107 way the strictly vital may be distinguished from what depends on known physical conditions. Dr. Verworn has speculatively suggested that the substance of the amoeboid cell is drawn out towards oxygen in the medium, that the chemically satisfied particles make way for their unsatisfied neighbour particles, that external stimulus provokes a molecular disruption, and that the exhausted particles have then to retreat to the nucleus which he regards as a trophic centre. Sensitiveness. The Amoeba is sensitive to external influ- ences. It shrinks from strong light and obnoxious materials, it moves towards eatable substances. This sensitiveness is, so far as we know, diffuse, a property of the whole of the cell substance, but the pigment spots of some forms are / specialised regions. Many Protozoa well illustrate a strange sensitiveness to (the physical and chemical stimuli of) objects or substances with which they are not in contact. Thus the simple amoeboid Vampyrella will, from a con- siderable distance, creep directly towards the nutritive substance of an Alga, and the plasmodium of a Myxomycete will move towards a decoction of dead leaves, and away from a solution of salt. The same sensitiveness, technically termed Chemotaxis, is seen when micro- organisms move towards nutritive media or away from others, when the spermatozoon (of plant or animal) seeks the ovum, or when the phago- cytes (wandering amoeboid cells) of a Metazoon crowd towards an intrud- ing parasite or some irritant particle. Nutrition. The energy which the Amoeba expends in movement, it makes up for by eating and digesting food particles. Most of the free Protozoa live in this manner upon solid food particles, whether plant or animal ; a few such as Volvox, in virtue of their chlorophyll, live entirely as do plants ; the parasitic forms usually absorb soluble and diffusible substances from their hosts. Respiration. Like all living creatures the Amoeba respires, that is, its complex substance is continually undergoing a process of oxidation, carbon dioxide being produced as a waste product. Without oxygen none of the activities can be efficiently performed, and if it is long withheld death ensues. In all Protozoa oxygen is simply taken up by the general protoplasm from the surrounding medium, into which the waste carbonic acid is again passed. The bubbles which enter with the food particles assist in respiration. In parasitic forms the method of respiration must be the same as that of the tissue cells of the host. io8 PROTOZOA. Excretion. Of the details of this process little is certainly known, but the contractile vacuoles are, without doubt, primitive excretory appliances. In the more specialised forms they appear to drain the cell substance by means of fine radiating canals, and then to burst to the exterior. \ Uric acid and urates are said to be demonstrable as waste products. ] Growth and Reproduction. In favourable conditions, when income exceeds expenditure, the Amoeba or other Protozoon grows ; in reverse conditions, or at the limit of growth, it reproduces. The phenomena of reproduction we will consider in greater detail later on. Colour. Pigments are not infrequently present in the Protozoa ; we have already noticed the presence of chlorophyll in some forms. With Radiolarians, the so-called "yellow cells" are found almost constantly associated. Each of these cells consists of protoplasm, surrounded by a cell wall, and containing a nucleus. The protoplasm, is impregnated with chlorophyll, the green colour of which is obscured by a yellow pigment. Starch is also present. The cells multiply by fission and continue to live after isolation from the proto- plasm of the Radiolarian. All these facts point to the conclusion that the cells are symbiotic Algae, so-called Zoochlorellce. According to some, the "chlorophyll corpuscles" seen in the primitive Archerina, in some flagellate forms, as Euglena, and in many Ciliata, as Stentor^ Sty Ionic hia, one species of Paramcecium, Volvox and the allied forms, are also symbiotic Algce, which have lost the power of independent existence. The evidence for this is, however, insufficient, and this explanation will not apply to cases like that of Vorticella viridis, where the green colouring matter is uniformly distributed through the protoplasm. In many cases there is, besides the chlorophyll, a brown pigment, identical with the diatomin of Diatoms. In many of the Flagellata there are one or more bright pigment spots at the anterior end of the cell ; these may be specially sensitive areas. In some of the simpler Gregarines the medullary protoplasm is coloured with pigment which is apparently a derivative of the haemoglobin of the host. Psychical Life. As to the psychical life of the Protozoa, we find that they often behave in a way which suggests con- scious effort and intelligence, but as cut-off fragments also act with apparent reasonableness, and as the nucleus cannot be regarded as a brain, there seems no reason to credit them with more than that diffuse consciousness which is possibly co-extensive with life. Verworn has decided, after much labour, that the Protozoa do not exhibit what even the most sanguine could call intelligence, but this is STRUCTURE. 109 no reason why he or any other evolutionist should doubt that they have in them the indefinable rudiments of thought. Structure. The Protozoa are sometimes called " struc- tureless," but they are only so relatively. For though they have not stomachs, hearts, and kidneys, as Ehrenberg supposed, they are not like drops of white of egg. Our eyes, when aided by the microscope, can distinguish struc- ture in these simplest animals. They are simple as an egg is simple when compared with a bird. The cell substance consists of a living network or foam, in the meshes or vacuoles of which there is looser material. Included with the latter are granules, some of which are food fragments in process of digestion, or waste products in process of excretion. The cell substance includes one or more nuclei, special- ised areas which are essential to the life and multiplication of the unit. In the Protozoa there are several conditions under which the nucleus may exist. (1) In some adult forms, and in many spores or young forms, no nucleus has yet been discovered. It is, however, unnecessary to preserve the term "Monera" for such simple forms, as it is probable that nuclear material does exist in some form even in these cases. (2) In some of the Ciliata the nucleus is diffuse, that is, it exists in the form of a powder scattered through the medullary protoplasm, and is only discernible after death by means of careful staining. In Opalin opsis the fine powder sometimes coalesces into a single nucleus. (3) In the majority of cases, notably in the Gregarines. the nucleus is single, often large, and placed centrally ; from a consideration of the cells of Metazoa we may call this the typical case. (4) In many of the Ciliata, e.g., Paramcecium, the nucleus is double. There is a large oblong nucleus and beside it a smaller spherical one. (5) In Opalina, from the intestine of the frog, and a few other forms, there are very numerous nuclei, arranged in a symmetrical manner in the cell substance. In some cases these isolated nuclei have been observed to unite to form one large nucleus just before binary fission takes place. Of these various cases the diffuse condition is apparently very primitive. The nucleus, when stained and examined under high powers, is observed to be complex in structure. It consists of a nuclear network, or a coil of chromatin threads. In the division of many Protozoa, as in the cells of higher animals it plays an important part. During division it passes from the resting to the active condition. The nuclear threads or " chromatin filaments " loosen themselves from their coiled state, and arrange themselves in a star at the equator of the cell, whence they I io PROTOZOA. divide into two groups, which retreat from one another, and become the daughter nuclei of two daughter cells. In short, karyokinesis has been observed here as elsewhere (see p. 45). While we cannot at present define the physiological import of the nucleus, we must recognise its importance. Thus, Bruno Hofer has shown that when an Amoeba is cut in two, the part with the nucleus lives and grows normally, while the part without any nucleus sooner or later dies ; and Balbiani has observed that in the case of Infusorians cut into pieces, those parts which have nuclei survive, while if no nucleus is present in the fragment, the wound may remain unhealed and death ensues. There seems no reason why one may not combine the view of Weismann that the nucleus bears the essential hereditary substances with the view that it is a trophic, or, at any rate, a vital centre in the cell. In naked Protozoa, the outer part of the cell substance (" ectoplasm ") is often clearer and less granular than the inner part (" endoplasm "), but this difference is a physical one of little importance. In corticate Protozoa there is a more definite rind or thickened margin of cell substance. Outside this there may be a "cuticle" distinct from the living matter, sometimes consisting of chitin, or gelatin, or rarely of cellulose. The cuticle may form a cyst, which is either a protection during drought, or a sheath within which the unit proceeds to divide into numerous spores. More- over, the cuticle may become the basis of a shell formed from foreign particles, or made by the animal itself of lime, flint, or " horny " material. In the cell substance there may be bubbles of water taken in with food particles (food vacuoles), contractile vacuoles, fibres which seem to be specially contractile (in Gregarines), spicules of flint or threads of horn-like material which may build up a connected framework, and the pigments already mentioned. Reproduction of Protozoa. Growth and reproduction are on a different plane from the other functions. Growth occurs when income exceeds expenditure, and when constructive or anabolic processes are in the ascendant. 'Reproduction occurs at the limit of growth, or sometimes in disadvantag- eous conditions when disruptive or katabolic processes gain some relative predominance. As it is by cell division that all embryos are formed from the egg, and all growth is effected, the beginnings of this process are of much interest. (a) Some very simple RE PROD UCTION. 1 1 1 Protozoa seem to reproduce by what looks like the rupture of outlying parts of the cell substance. () The production of a small bud from a parent cell is not uncommon, and some Rhizopods (e.g., Arcella, Pelomyxa) give off many buds at once, (c) Commoner, however, is the definite and orderly process by which a unit divides into two ordinary cell division, (d) Finally, if many divisions occur in rapid succession or contemporaneously, and usually within a cyst enclosing the parent cell, i.e., in narrowly limited time and space, the result is the formation of a considerable number of small units or spores. In the great majority of cases, each result of division is seen to include part of the parent nucleus. A many celled animal multiplies in most cases by liberat- ing reproductive cells ova and spermatozoa different from the somatic cells which make up the " body." A Protozoon multiplies by dividing wholly into daughter cells. This difference between Metazoa and Protozoa in their modes of multiplication is a consequence of the difference between multicellular and unicellular life. Each part of a divided Protozoon is able to live on, and will itself divide after a time, whereas the liberated spermatozoa and ova of a higher animal die unless they unite. By sexual reproduction, we mean (a) the liberation of special reproductive cells from a " body," and (b) the fertilisation of ova by spermatozoa. It is obvious that unicellular Protozoa can show nothing corresponding to sexual reproduction in the first sense. Moreover, Pro- tozoa can live on, dividing and multiplying, for prolonged periods without the occurrence of anything like fertilisation. So it is often stated as a characteristic of Protozoa that " they have no sexual reproduction." But if this mean that the unicellular Protozoa have no special reproductive cells, then it is a truism. If, however, the statement mean that the Protozoa are without anything corresponding to fertilisation, then it is not true. For in many of the Protozoa, there occurs at intervals a process of " conjuga- tion " in which two individuals unite either permanently or temporarily. This is an incipiently sexual process ; it is the analogue of the fertilisation of an ovum by a spermato- zoon. 112 PROTOZOA. It is one of the recurrent phases in the life history of some of the simplest Protozoa (Proteomyxa and Mycetozoa) (see p. 98), that a number of amoeboid units flow together into a composite mass, which has been called a " plasmodium" It is known that more than two individual Gregarines and other forms occasionally unite. To this the term " multiple conjugation " has been applied. Commonest, however, is the union of two apparently similar individ- uals, either permanently so that the two fuse into one, or temporarily so that an exchange of material is effected. Permanent conjugation has been observed in several Rhizopods, Infusorians, and Gregarines. Temporary conjugation is well known in not a few ciliated Infusorians, and it is possible that a curious end-to-end union of certain Gregarines is of the same nature, or it may be of the nature of a " plasmodium " formation. Fourthly, there are some cases where one of the conjugating individ- uals is larger and less active than the other. Thus in Vorticella, a small free swimmin'g form unites and fuses completely with a stalked individual of normal size. To call this "dimorphic conjugation " is hardly necessary, since it is evidently equivalent to the fertilisation of a passive ovum by an active spermatozoon, one of the well-known charac- teristics of reproduction in the Metazoa. In Volvox this is even more obvious, for the small and active cells, both in shape and method of formation, recall the spermatozoa of higher forms. The conjugation of ciliated Infusorians, such as ParamtE- duni, has been studied with great care by Gruber, Maupas, R. Hertwig, and others, and though their results are not quite harmonious, the main facts are secure. In many ciliated Infusorians there are two nuclear bodies, one large, the other small. The smaller or micronucleus lies by the side of the larger or macronucleus. The micronucleus divides into parts, while the macronucleus degenerates. Two individual Infusorians (A and B) lie side by side in close contact, a portion of the micronucleus of A passes into B, and fuses with a portion of the micro- nucleus of B, similarly a portion of the micronucleus of B passes into A, and fuses with a portion of the micronucleus of A. In short, mutual fertilisation occurs, the conjugating individuals separate, a new micro- nucleus and a new macronucleus are established in each. The precise interpretation of the process is to some extent a matter of mere opinion. We may regard it as a mutual rejuvenescence, each unit supplying some substances or qualities which the other lacks ; or we may regard it rather as a process by which the average character of the species is sustained, peculiarities or pathological variations of one individual being counteracted by other characters in the neighbour (apparently no near relation) with which it conjugates ; or we may see in it a source of variation as the result of new combinations among the essential hereditary substances. The researches of M. Maupas have thrown much light on the facts, and some of his results deserve summary. It has been often alleged that the subsequent dividing is accelerated by conjugation ; but Maupas finds that this is by no means the case. The reverse in fact is true. While a pair of Infusorians ( Onychodromus BIONOMICS. 113 grandis] were engaged in conjugation, a single individual had, by ordinary asexual division, given rise to a family of from forty thousand to fifty thousand individuals. Moreover, the intense internal changes preparatory to fertilisation, and the general inertia during subsequent reconstruction, not only involve loss of time, but expose the Infusorians to great risk. Conjugation seems to involve danger and death rather than to conduce to multiplication and birth. The riddle was, in part at least, solved by a long series of careful observations. In November 1885, M. Maupas isolated an Infusorian (Sty Ionic kia pustulata) and observed its generations till March 1886. By that time there had been two hundred and fifteen generations pro- duced by ordinary division, and since these lowly organisms do not con- jugate with near relatives, there had been no conjugation. What was the result? At the date referred to, the family was observed to have exhausted itself. The members were being born old and debilitated. The asexual division came to a standstill, and the powers of nutrition were lost. Meanwhile, before the generations had exhausted themselves, several of the individuals had been restored to their natural conditions, where they conjugated with unrelated forms of the same species. One of these was again isolated, and watched for five months. In this case up till the one hundred and thirtieth generation, it was found that on removal to fresh conditions the organisms were capable of conjugating with unrelated forms. Later this power was lost, and at the one hundred and eightieth generation the individuals of the same family were observed making a vain attempt to conjugate with each other. We thus see that without normal conjugation the whole family becomes senile, degenerates both morphologically and physiologically. Morphologically, the individuals decrease in size, until they measure only a quarter of their original proportions, the micronucleus atrophies com- pletely or partially, the chromatin of the macronucleus gradually disappears, other internal structures also degenerate. Physiologically, the powers of nutrition, division, and conjugation come to a standstill, and this senile decay of the isolated individuals or family inevitably ends in death. The general conclusion is evident. Sexual union in those Infusorians, dangerous, perhaps, for the individual life, and a loss of time so far as immediate multiplication is concerned, is absolutely necessary for the species. The life runs in strictly limited cycles of asexual division. Conjugation with allied forms must occur, else the whole life ebbs. Without it, the Protozoa, which some have called "immortal," die a natural death. Conjugation is the necessary condition of their eternal youth. Bionomics. Many Protozoa raise organic debris once more into the circle of life, and many form part of the food of higher animals. Thus, those pelagic Foraminifera and Radiolarians, which dying sink to the great oceanic depths, form along with more substantial debris the fundamental food supply in that plantless world. Fundamental, since it 8 H4 PROTOZOA. is plain that the deep sea animals cannot all be living on one another. Almost every kind of nutritive relation occurs among the Protozoa. Predatory life is well illustrated by most In- fusorians, and thorough going parasitism by the Gregarines ; Opalina in the rectum of the frog may serve as a type of those which feed on decaying debris, and Volvox of those which are holophytic. Radiolarians, with their partner Algae, exhibit the mutual benefits of symbiosis, the plants utilising the carbon dioxide of their transparent bearers, the animals being aerated by the oxygen which the plants give off in sunlight, and probably nourished by the carbohydrates which they build up. Some of the parasitic forms, especially among the Sporozoa, are of serious importance to higher animals. Though Protozoa may be seriously infected by Bacteria, Acineta parasites, some fungi, like Chytridium, &c., fatal infection is rare, because of the power of intracellular digestion which most Protozoa possess. "The parasite," Metchnikoff says, "makes its onslaught by secreting toxic or solvent substances, and defends itself by paralysing the digestive and expulsive activity of its host ; while the latter exercises a deleterious influence on the aggressor by digesting it and turning it out of the body, and defends itself by the secretions with which it surrounds itself." With this struggle should be compared that between phagocytes and Bacteria in most multicellular animals. Few Protozoa come into direct touch with human life, but man has several Protozoon parasites, e.g., Amoeba coli, associated with inflammation of the intestinal mucous mem- brane, Coccidium oviforme (Sporozoa), affecting the liver, and various Infusorians. On the other hand, the shells of Protozoa deposited as ooze in ancient days, have formed important deposits, such as chalk and Barbadoes Earth. General Zoological Interest. The Protozoa illustrate, in free and single life, forms and functions like those of the cells which compose the many celled animals. Typically, they show great structural or morphological simplicity, but great physiological complexity. Within its single cell, the Protozoon discharges all the usual functions, while in a higher animal distinct sets of cells have been specialised for various GENERAL ZOOLOGICAL INTEREST. 115 activities, and each cell has usually one function dominant over the others. The Metazoan cells, in acquiring an in- creased power of doing one thing, have lost the Protozoan power of doing many things. The Protozoa remain at the level represented by the reproductive cells of higher forms, and are comparable to \ reproductive cells which have not formed bodies. In the sexual colonies of Volvox, however, we see the beginning of that difference between reproductive cells and body cells which has become so characteristic of Metazoa. The Protozoa are self-recuperative, and in normal conditions they are not so liable to " natural death " as are many celled animals. Weismann and others maintain that they are physically immortal. They illustrate (a) the beginnings of reproduction, from mere breakage to definite division, either into two as in fission, or in limited time and space into many units, as in the formation of spores within a cyst ; (b) the beginnings of fertilisation, from " the flowing together of exhausted cells " and multiple conjugation to the specialised sexual union of some Infusorians, where two individuals become closely united ; (c) the beginnings of sex, in the difference of size and of constitution sometimes observed between two con- jugating units ; (d} the beginnings of many celled animals in the associated groups or colonies which occur in several of the Protozoan classes. These colonies show a gradation in complexity. Raphidiophrys and other Heliozoa form loose colonies, which arise by the want of separation of the products of fission. Among the Radiolarians, there are several colonial forms, in these the individuals are united by their extra-capsular protoplasm, but are all equivalent. In Pro- teropongia the cells show considerable morphological dis- tinctiveness, some are flagellate, some amoeboid, some encysted and spore forming. Again, in Volvox, as we noticed above, the cells of the colonies show a distinction into nutritive and reproductive units. Lastly, in their antithesis of passivity and activity, con- structive and destructive preponderance, anabolism and katabolism, the Protozoa illustrate the phases of the cell cycle, and so furnish a key to the variation of higher animals. CHAPTER VIII. PORIFERA - SPONGES. A. Calcarea (Calcispongise). B. Non-Calcarea. Hexactinellida. Demospongke. onaxona. ^ letractmellida. SPONGES seem to have been the first animals to attain marked success in the formation of a "body." For though their details are often complex, their general structure is simpler than the average of any other class of Metazoa, and some of the simplest forms do not rise high above the level of the gastrula embryo. A " body " has been gained, but it shows relatively little division of labour or unified life ; it is a community of cells imperfectly integrated. There are no definite organs, and the tissues are, as it were, in the making. Sponges are passive, vegetative animals, and do not seem to have led on to anything higher ; but they are successful in the struggle for existence, and are strong in numbers alike of species and of individuals. General Characters. Sponges are diploblastic (two layered) Metazoa, the middle stratum of cells the mesoglcea not attaining to the definiteness of a proper mesoderm. There is no cwlome or body cavity. The longitudinal axis of the body corresponds to that of the embryo : in other words, the general symmetry of tjie gastrula is retained. In these three characters the Sponges agree with the Ccelentera and differ from higher (triploblastic and cwlomate) Metazoa. The body varies greatly in shape, even within the same DESCRIPTION OF A SIMPLE SPONGE. 117 species. It is traversed by canals, through which currents of ivater bear food inwards and waste outwards. Numerous minute pores on the surface open into afferent canals, leading into a cavity or cavities lined by endoderm cells, many or all of which are flagellate. To the activity of the flagella the all- important water currents are due. The endodermic or gastric cavity may be a simple tube, or it may have radially outgrow- ing chambers, or it may be represented by branched spaces, from which efferent canals lead to the exterior. Where there is a distinct central cavity there is usually but one large exhalent aperture (osculum), but in other cases there are many exhalent apertures. The ectoderm is the least important layer; it covers the body, and is perhaps continued into the afferent canals; the endoderm lines most of the internal cavities, and is typically flagellate ; the intervening mesoglcea contains a skeleton of lime, flint, or spongin ; amoeboid cells or phagocytes, im- portant in digestion and excretion ; repro- ductive cells, and other elements. Budding is very common, and in a few cases buds are set adrift. Both herma- phrodite and unisexual forms occur. The sexually produced embryo is almost always developed within the mesoglcea, and leaves the sponge as a ciliated larva. With the exception of one family, all are marine. Description of a Simple Sponge. A very simple sponge, such as Ascetta, is a hollow vase, moored at one end to rock or seaweed, with a large exhalent aperture at the opposite pole, and with numerous minute inhalent pores through the walls. These walls consist of (i) a flat ectoderm ; (2) a mesoglcea containing triradiate calcareous spicules, phagocytes, and reproductive elements ; and (3) an endoderm lining the central cavity, and composed of collared flagellate cells, almost exactly like some of the monad Infusorians. This simple sponge is not much above the gastrula level ; it FIG. 31. Simple Sponge (Ascetta primordialis}. (After H^CKEL.) Note the vase-like form, the apical oscu- lum, the inhalent pores in the walls. PORIFERA SPONGES. agrees generally with a simple Ccelenterate, such as Hydra, but differs from it in the absence of tentacles and stinging cells, and in the greater development of the mesoglcea. More Complicated Forms. But a description of a simple sponge like Ascetta conveys little idea of the structure of a complex form such as the bath sponge (Euspongia). Let us consider the origin of complications: (a) Sponges long regarded as plants are plant-like in being sedentary and passive. They seem also to feed easily and well. Like plants, they form buds, the out- come of surplus nourishment. These buds, like the suckers of a rose bush, often acquire some apparent inde- pendence, and the sponge looks like many vases, not like one. More- over, as they grow these buds may fuse, like the branches of a tree tied closely together. Thus the structure becomes more intricate. (b) In the simple sponge the gastric cavity of the vase is com- pletely lined by the collared endo- derm cells (Ascon type). But the endoderm may grow out into radial chambers, and the walls of these may also be folded into side aisles (Sycon type). The outgrowing of the endoderm into the mesoglcea may be continued even further, and the cells may become pavement-like except in minute flagellate chambers, where the characteristic collared type is retained (Leucon type). (See Fig. 33.) [Speculatively it may be suggested that the characteristic folding or outgrowth of the endoderm is necessitated by the fact that the endoderm cells are better nourished and multiply more rapidly than those of the ectoderm, which thus fails to keep pace with the inner layer.] (c) By infoldings of the skin ectoderm and a subjacent FIG. 32. Section of a Sponge. (After F. E. SCHULZE.) Showing inhalent canals, flagellate chambers, a gastrula forming in the mesoglcea, &c. COMPLICATED FORMS OF SPONGES. 119 sheath of mesogloea subdermal FIG. 33. Diagram showing types of Canal System. (After KORSCHELT and HEIDER.) The flagellate regions are dark throughout, the mesogloea is dot- ted, the arrows show the direction of the currents. All the figures represent cross sections through the wall. A. Simple A scon type, EC. ectoderm, En. endoderm, Mg. mesogloea. B. Sycon type, with flagellate radial chambers (r.c.). C. Leucon type, with flagellate side aisles ^on the main radial chambers. D. Still more complex type, with small flagellate chambers,/; ch. chambers, by flat epithelium with spaces may be formed ; an outer cortex may be distinctly differentiated from the internal region in which the flagellate chambers occur ; the pores may collect into sieve -like areas which open into dome - like cavities; these and many other complications are common. (d} The ectoderm is usually described as a covering layer of flat epithelium, but flask shaped cells have also been observed (Bidder). It may be folded in- wards, as we have noticed, and, according to some, it also lines the incurrent or afferent canals in whole or in part. In a few cases, e.g., Oscarella lobularis, it is ciliated, and its cells may also exhibit con- tractility, as around the osculum of Ascetta cla- thrus, though the con- tractile elements usu- ally belong to the meso- glrea. The endoderm con- sists typically of collared flagellate cells, but in the more complex sponges these are replaced, ex- cept in the flagellate or without flagella. 1 20 PORIFERA SPONGES. The mesoglota contains very varied elements, and illus- trates the beginnings of different kinds of tissue. Thus there are migrant amoeboid cells (phagocytes) ; irregular connective tissue cells embedded in a little jelly ; spindle shaped connective tissue cells, united into fibrous strands ; contractile cells, e.g., those forming a sphincter around the oscula of some forms, such as Pachymatisma ; skeleton making cells ; pigment containing cells ; supposed nerve cells, projecting on the surface, and believed to be connected internally with multipolar (ganglion ?) cells ; and lastly, the reproductive cells, which are connected by transitional forms with the ordinary phagocytes. (e) The skeleton consists of calcareous or siliceous spicules, or of spongin fibres, or of combinations of the two last. A calcareous spicule is formed of calcite, with a slight sheath and core of organic matter ; a siliceous spicule is formed of colloid silica or opal ; the spongin is chemically somewhat like silk. Uniradiate, biradiate, triradiate, quadri- radiate, sexradiate, and multiradiate spicules occur, and in a general way it may be said that they are arranged so that they give most architectural stability. Each is formed within a single cell, and may be speculatively regarded as an organised excretion. " During its growth," Prof. Sollas says, " the spicule slowly passes from the interior to the exterior of the sponge, and is finally (in at least some sponges Geodia, Stelletta), cast out as an effete product." The fibres of spongin are formed as the secretions of mesoglcea cells known as spongioblasts. Ordinary Functions. Excepting the fresh water Spongillidae, all Sponges are marine, occurring from between tide marks to great depths. After embryonic life is past, they live moored to rocks, shells, seaweeds, and the like. Their motor activity is almost completely restricted to the lashing movements of the flagella, the migrations of the phagocytes, and the con- traction of muscular mesoglceal cells, especially around the exhalent apertures. In the closure of the inhalent pores, sponges show sensitiveness to injurious influences, but how far this is localised in specialised cells is uncertain. REPRODUCTION OF SPONGES. 121 The most important fact in the life of a Sponge is that which Robert Grant first observed, that currents of water pass gently in by the inhalent pores, and more forcibly out by the exhalent aperture or apertures. This may be demonstrated by adding powdered carmine to the water. The instreaming currents of water bear dissolved air and supplies of food, such as Infusorians, Diatoms, and particles of organic debris. The outflowing current carries away waste. When a sponge is fed with readily recognisable substances, such as carmine or milk, and afterwards sec- tioned, the grains or globules may be found (a) in the collared endoderm cells ; (b) in the adjacent phagocytes of the mesoglrea ; (c) in the phagocytes surrounding the sub- dermal spaces, if these exist. It is uncertain whether the epithelium of the subdermal spaces or the collared endoderm is the more important area of absorption, but it is certain that the phagocytes play an important part in engulfing and transporting particles, in digesting those which are useful, and in getting rid of the useless. In an extract of several sponges, Krukenberg found a (tryptic) digestive ferment, probably formed within the phagocytes. Many sponges contain much pigment, thus the lipochrome pigment (see Chap. XXIX.) zoonerythrin is common, and like some others, such as floridine, is regarded as helping in respiration. The green pigment of the fresh water sponge is closely analogous, if not identical, with chlorophyll, and probably renders some measure of holophytic nutrition possible. Reproduction. Sponge growers often cut a sponge into pieces, and bed these out in suitable places. The parts regenerate the whole a fact which illustrates the relatively undifferentiated state of the sponge body. It is possible that fission may also occur naturally. The frequent budding is merely a kind of continuous growth, but when buds are set adrift, as sometimes happens, we have discontinuous growth or asexual reproduction. In the fresh water Spongillidoe there is a peculiar mode of reproduc- tion by statoblasts or gemmules. A number of mesogloeal cells occur 122 PORIFERA SPONGES. in a clump, some forming an internal mass, others a complex protective capsule with capstan-like spicules, known as amphidiscs. According to W. Marshall, the life history is as follows : In autumn the sponge suffers from the cold and the scarcity of food, and dies away.. But throughout the moribund parent gemmules are formed. These survive the winter, and in April or May they float away from the dead parent, and develop into new sponges. Some become short lived males, others more stable females. The ova produced by the latter and fertilised by spermatozoa from the former, develop into a summer generation of sponges, which in turn die away in autumn and give rise to gemmules. The life history thus illustrates what is called alternation of generations (see p. 55). Interpreted from a utilitarian point of view, the formation of gemmules is a life saving expedient. As Prof. Sollas says, " the gemmules serve primarily a protective purpose, ensuring the persistence of the race, while, as a secondary function, they serve for dispersal." All Sponges produce sex cells, which seem to arise from amoeboid mesoglcea cells retaining an embryonic character. In the case of the ovum, the amoeboid cell increases in size, and passes into a resting stage ; in the case of the male elements, the amoeboid cell divides into a spherical cluster of numerous minute spermatozoa. The similar origin of the ova and spermatozoa is of interest. Most sponges are unisexual, but many are hermaphrodite. In the latter case, however, either the production of ova or the production of spermatozoa usually preponderates, probably in dependence on nutritive conditions. Development. It is not surprising to find that there is great variety of development in the lowest class of Metazoa ; it seems almost as if numerous experiments had been made, none attended with progressive success. The minute ovum, without any protective membrane, usually lies near one of the canals, and is fertilised by a spermatozoon borne to it by the water. It exhibits a certain power of migration as in some Hydroids. Previous to fertilisation, the usual extrusion of polar bodies has been observed in a few cases, and is doubtless general. Seg- mentation is total and usually equal, and results in a spherical or oval embryo more or less flagellate. This leaves the parent sponge, swims about for a time, then settles down, and undergoes a larval metamorphosis often difficult to understand. It is peculiarly difficult to bring the DEVELOPMENT OF SPONGES. 123 history of the germinal layers in Sponges into line with that in other Metazoa. (a) In the small calcareous sponge Sycandra raphanus, as described by F. E. Schulze, the segmentation results in a hollow ball of cells the blastula. A few cells at the lower pole remain large, and are filled with nutritive granules ; the other cells divide rapidly and become small, clear, columnar, and flagellate. The large granular cells become temporarily invaginated, form- ing what is called a "pseudo- ^0j^^ gastrula" This leaves the ; fE; parent and the granular cells $ right themselves, forming the posterior hemisphere of the ^^^^^ embryo, now called an amphi- blastula. It swims for a time xf^y^x actively, but the flagellate cells of the anterior hemisphere are invaginated into or overgrown by the large granular cells, and thus what is generally called the gastrula stage results. This soon settles down, on rock or seaweed, with the blastopore or gastrula mouth downwards, and is moored by amoeboid pro- cesses from the granular cells, which likewise obliterate the blastopore. The granular cells lose their granules, for the larva is not yet feeding ; the now in- ternal flagella disappear in the absence of the stimulating water ; a mesoglcea with spicules begins to be formed between the inner FIG. 34. Development of Sycandra raphanus. (After F. E. SCHULZE.) 1. Ovum. 2. Section of 16 cell stage. 3. Blastula with 8 granular cells (^r.c.) at lower pole. 4. Free swimming amphiblastula, En with upper hemisphere of flagellate cells (f.c.\ and lower hemisphere of granu- ' ar cells. 5. Gastrula stage settled down. ^^ EC. , outer layer (ectoderm ? ) ; En. , inner layer(endoderm ? ) ; bl., closing blastopore; am.p., mooring amoeboid processes. 124 PORIFERA SPONGES. and outer layer, probably by migrants from the latter. But this dis- advantageous state of affairs cannot last. Pores open through the walls, the entrance of water enables the inner cells to recover their flagella, and an exhalent aperture is ruptured at the upper pole. The young sponge is now in an Ascon stage, from which, by the out- growth (?) of the inner layer into radial chambers, it passes into the per- manent Sycon form, grows into a cylin- der, and becomes differentiated in detail (Fig. 34). (b] In Oscarella (Halisarca] lobularis, a sponge without any skeleton, the ovum segments equally into a blastula, which is flagellate all over. This free swimming stage may be invaginated from either pole to form a hemispherical gastrula, which settles mouth down- wards. Pores, an osculum, and the mesoglcea are formed as before, and the inner layer becomes folded into flagellate chambers. It may be sug- gested that this folding is due to the higher nutrition, and consequent dis- proportionate growth, of the inner layer, for a rapidly growing sac within one of less rapid growth must become folded on itself (HEIDER). (c] Another type, seen for instance in a horny sponge, Spongelia^ results in a flagellate larva, whose cavity is rilled up with what may be called gela- tinous connective tissue, from which mesogloea and endoderm are subse- quently differentiated. Such a larva is called a parenchymula. As these are not all the types of de- velopment which occur among sponges, the general fact is impressive that in this lowest class of Metazoa there has been considerable plasticity in development. FIG. 35. Diagrammatic representation of develop- ment of Oscarella lobularis. (After HEIDER.) Bl. Free swimming blastula with flagella ; G. gastrula settled down. Next figure shows folding of Endoderm (En). Lowest figure shows radial chambers (A\C.)- Mesoglcea (Mg) ; inhalent pore (/*.); exhalent oscu- lum (0.). Classification. A. Port/era Calcarea, with skeleton of calcareous spicules : Order I. Homocoela. Endo- derm wholly composed of collared flagellate cells, e.g., Ascetta^ Leucosolenia. Order II. Heterocoela. Endoderm consists of collared flagel- late cells in radial tubes or chambers, and of flat epithelium elsewhere, e.g., Grant ia, Sycon. BIONOMICS. 125 B. For if era non-Cakarea, skeleton of silica or of spongin, or of both. (1) Hexactinellida, with sexradiate siliceous spicules, canal system usually simple, with Sycon chambers. The members live chiefly in deep water, e.g., Venus' Flower Basket (Euplectella) and the Glass Rope Sponge (Hyalonenid]. (2) Monaxonida, with siliceous spicules (which are not quadri- or sex-radiate), or with "horny" skeleton, or with both. Order I, Monaxona, with spicules only, e.g., Mermaid's Gloves (Chalina oculata), Crumb of Bread Sponge (Halichondria or Amorphina paniced), FreshWater Sponge (Spongilla}. Order II. Ceratosa, " horny " sponges with or without spicules, e.g. , the Bath Sponge (Euspongia}. (3) Tetractinellida, mostly with quadriradiate spicules, or with tritenes, in which a main shaft bears at one end three branches diverging at equal angles, e.g., l^etilla, Geodia, Pachymatisma, Plakina. There are also a few sponges (Myxospongioe) without any skeleton, perhaps survivals of primitive types (Oscarella, Halisarca) or degraded for m s ( Chondrosia ) . History. Sponges, as one would expect, date back almost to the beginning of the geological record. Thus the siliceous Protospongia occurs in Cambrian rocks, and in the next series the Silurian the main groups are already represented. From that time till now they have continued to abound and vary. Bionomics. Sponges are living thickets in which many small animals play hide-and-seek. Many of the associations are practically constant and harmless, but some burrowing worms do the sponges much damage. The spicules and a frequently strong taste or odour doubtless save sponges from being more molested than they are ; the numerous phagocytes wage successful war with intruding micro- organisms. Some sponges, such as Clione on oyster shells, are borers, and others smother forms of life as passive as themselves. Several crabs, such as Dromia, are masked by growths of sponge on their shells, and the free transport is doubtless advantageous to the sponge till the crab casts its shell. A compact orange coloured sponge (Suberites domunculd) of peculiar odour often grows round a whelk shell tenanted by a hermit crab, and gradually eats into the shell substance. Within several sponges, minute Algae live, like the " yellow cells " of Radiolarians, in mutual partner- 1 26 PORIFERA SPONGES. ship or symbiosis. Finally, sponges deserve mention as factors in human civilisation. General Zoological Interest and Position. Sponges have /this great interest, that they form the first successful class Jof Metazoa. They illustrate the beginnings of a " body "- ' the beginnings of tissues. Along with the Ccelentera, from which it is the almost unanimous opinion that they must be held distinct, they differ markedly from the triploblastic, Coelomate Metazoa, which do not retain the radial symmetry of the gastrula. Their origin is wrapped in obscurity, though there is much to be said for the view that they are the non-pro- gressive descendants of primitive gastrula-like ancestors of sluggish constitution. It does not seem likely that they have led on to anything higher, they rather represent a by-road in Metazoan evolution. MESOZOA. 127 APPENDIX TO SPONGES. MESOZOA. The title Mesozoa was applied by Van Beneden to some very simple organisms which appear to occupy a very humble position in the Metazoan series. The name sug- gests a grade between the Protozoa and the Metazoa. Hseckel called some of them Gastrseadae, regarding them A B FIG. 36. A. Young Dicyema. (After WHITMAN.) B. Female Orthonectid (Rhopalura Giardii}. (After JULIN.) e. Ectoderm ; en, inner endoderm cell with nucleus (n) ; and embryo (em). as slight modifications of the hypothetical gastrula-like ancestors of the Metazoa, while Hatschek, comparing them to precociously reproductive planulae speaks, of them as Planuloidse. It is also possible that some of them may be 128 MESOZOA. It will parasitic degenerations of Turbellarian-like worms, be enough here merely to notice four types : (i.) Dicyemidre (type Dicyema] occur as parasites in Cephalopods ; the body con- sists of a ciliated outer layer, enclosing a single multinucleate inner cell, within which egg-like germs develop, apparently without fertilisation, into dimorphic em- bryos (see Fig. 36, A). (2.) Orthonectidne (type Rhopalura] occur as parasites in Turbellarians, Brittle stars, and Nemerteans ; the body is slightly ringed, and consists of a ciliated outer layer, a subjacent sheath of contractile fibres, and an internal mass of cells, among which ova and spermatozoa appear. The sexes are separate and dimorphic (see Fig. 36 B.). (3.) Professor F. E. Schulze has dis- covered a small marine organism Tricho- plax adh) retractor muscles ; (c) ridges of reproductive cells, almost always either ova or spermatozoa, rarely both ; and (d) in some cases offensive threads (acontia), rich in stinging cells, and extrusible through the body wall. The mesenteric filaments seem to be closely applied to the food and perhaps secrete digestive juice. Intracellular digestion also occurs. Sea anemones have no sense organs ; the sapphire beads, which are so well seen at 152 CCELENTERA. the bases of the outermost tentacles of the common Actinia mesembryanthemum, are batteries of stinging cells. The nervous system is uncentralised, and consists of superficial sensory cells connected with a plexus of sub-epithelial ganglion cells. 7^he Layers of the Body. The ectoderm which clothes the exterior is continued down the inside of the gullet. The endoderm lines the whole of the internal cavity, including mesenteries and tentacles. The meso- glcea is a supporting plate between these two layers, and forms a basis for their cells. The ectoderm consists of ciliated, sensory, stinging, and glandular cells, and also of sub-epithelial muscle and ganglion cells based on the mesoglcea, but mainly restricted to the circumoral region. The endoderm consists mainly of flagellate cells, with muscle fibres at their roots. These form the main muscle bands of the wall, the mesen- teries, and the gullet. Nor are glandular and even sensory cells wanting from the endoderm. The Mesenteries. In sea anemones and nearly related Anthozoa twelve primary mesenteries are first formed. These are grouped in pairs, and the cavity between the members of a pair is called intra-septal, in contrast to the inter-septal cavities between adjacent pairs. In these inter-septal chambers other mesenteries afterwards appear in pairs. Two pairs of mesenteries, however, differ from all the rest, those, namely, which are attached to each corner of the mouth and to the correspond- ing grooves of the gullet. These two pairs of mesenteries are called " directive," and they divide the animal into bilaterally symmetrical halves. Anatomically, a pair of directive mesenteries differs from the other paired mesenteries, because the retractor muscles which extend in a vertical ridge along them, are turned away from one another, and run on the inter-septal surfaces, whereas in the other mesenteries the retractor muscles run on the intra-septal surface, those of a pair facing one another. The arrangement of these muscles is of great importance in classifying Anthozoa. It is possible that the mesenteries are homologous with the taeniolge of jelly fish, and the mesenteric with the gastric filaments. From the above description, it will be noticed that the funda- mental radial symmetry of the Ccelentera has here become profoundly modified. Development. Comparatively little is known in regard to the early stages of development in sea .anemones. From the fertilised ovum, a blastosphere may result which by invagination becomes a gastrula. Or the two layers may be established by a process known as delamination, in which a single layer of cells is divided into an inner endodermic and an outer ectodermic layer. Related Forms. The sea anemones are classified in the sub-class Anthozoa or Actinozoa, and along with many corals are distinguished as Zoantharia or Hexacoralla from the Alcyonaria or Octocoralla, like Alcyonium and related corals. This contrast is not perfectly satisfactory, but it rests on such distinctions as the following : CORALS. ANTHOZOA OR ACTINOZOA. 153 ZOANTHARIA, HEXACORALLA, e.g., SEA ANEMONE. ALCYONARIA, OCTOCORALLA, e.g. DEAD MEN'S FINGERS Many are simple, many colonial. Tentacles usually simple, usually some multiple of six, often dissimilar. Mesenteries usually some multiple of six, complete and incomplete. Retractor muscles never as in Alcyonarla. Two gullet grooves or siphonoglyphes, or only one. Dimorphism only in some Antipatharia, and in one Madrepore coral. Calcareous skeleton if present is derived from the basal ectoderm. Types. Actiniaria. Sea anemones. Madreporaria. Reef building corals. Antipatharia. Black corals. All colonial, except a small family includ- ing Monoxcnia. and Haimea. Tentacles eight, feathered, uniform. Mesenteries eight, complete. Retractor muscles always on one (ventral) side of each mesentery. One (ventral) gullet groove or siphono- glyphe, or none. Occasional dimorphism among members of a colony. There are usually calcareous spicules (of ectodermic origin) in the mesoglcea. Examples. Alcyonium (Dead men's fingers), with diffuse spicules of lime. Tubipora (Organ pipe coral), with spicules fused into tubes and trans- verse platforms. Corallium rubruin (Red coral), with an axis of fused spicules. fszs, with an axis of alternately limy and " horny " joints. Pennatula (Sea pen), a free phosphor- escent colony, with a " horny" axis possibly endodermic. Heliopora, blue coral. > S Z ^ A FIG. 48. Z, Diagrammatic section of Zoantharian ; A> of Alcyonarian. (After CHUN.) The line 6" ^ in Z is through the siphonoglyphes (a). The retractor muscles are represented by dark thickenings on the mesen- teriesall on one (the ventral) side in Alcyonaria. Coral Making. We have already noticed that there are "corals "among the Hydrozoa, viz., the Millepores. Leaving these out of account, we have to recognise that both divisions of Anthozoa include many corals. 154 CCELENTERA. With the doubtful exception of the Sea pens and their allies, in which the axial skeleton is believed by some to be endodermic, the " coral " is due to ectoderm cells, which either remain in the ectoderm or wander into the mesogloea. Taking as a basis the hard parts only, corals may be classified in various ways : According to Composition (i.) Discontinuous calcareous spicules Alcyonium, &c. ; these may also occur along with some forms of (2). (2.) Continuous skeleton. (a) Organic and "horny," e.g., axis of many Gorgonids, axis of Pennatulids. (I)} " Plorny" and calcareous, e.g., axis of his. (c) Wholly calcareous, in the great majority. According to extent of the hard parts (l.) Diffuse spicules, e.g., Alcyonium. (2.) Paused in an external tube, e.g., 7\ibipora (Organ pipe coral). (3.) Fused in an axis, e.g., Corallium rubrum (Red coral). (4.) Invading the outer wall (theca), the base, and forming cal- careous septa between the mesenteries, and often, also, a centra] pillar (columella), e.g., massive reef building corals. The terms Sclerodermic and Sclerobasic were formerly much used in the description of corals. The former denoted corals in which the hard parts are laid down by the individual polypes themselves, and support their soft tissues, as in Tubipora, Fungia, and numerous others ; the latter was used in describing cases, like the Red Coral, the Sea Pens, &c., where there is a calcareous skeleton in the connecting substance of the colony. According to position of the hard parts (i.) " Exoskeletal," more ov less directly continuous with the ectoderm, e.g., in Madrepore corals (reef builders), like Astrcea, Fungia, Madrepora ; in Gorgonids, Gorgonia and Isis. (2. ) " Mesoskeletal," i.e., in the mesoglcea, e.g., spicules of Alcyonium, fused spicules of Tubipora, axis of Corallium. SYSTEMATIC CLASSIFICATION OF THE COELENTERA. The Ccelentera are often classified as follows : ( fV^npdntfl ( Hydromedusse. A. Hydrozoa, I \Siphonophor, (^Acraspeda. ( Alcyonaria. B. Actinozoa, (^Zoantharia. C. Ctenophora. CLASSIFICATION OF CCELENTERA. 155 The complex structure of the Acraspeda, or true jelly fishes, together with the special points already noticed, seems, however, to justify their association with the sea anemones rather than with the simpler Craspedote forms. The classes are then arranged thus : fHydrophora. f Order I. Hydromedusoe, -[ Hydrocorallinae. A. Class Hydrozoa, \ ( Trachymedusre. [Order 2. Siphonophone. ( Lucernariae. Sub-class i. Scyphome- ] Discomedusoe. dusse, or Acraspeda. 1 Conomedusse. B. Class Scyphozoa. - C. Class Ctenophora. iPeromedusse. {Order (i). Zoantharia. Order (2). Alcyonaria. [Rugosa.] A. Class HYDROZOA. There are two types, polypoid and medusoid, which may be combined in one life history. The mouth leads directly into the gastric cavity. The mesogloea is simple, and without migrant cells. The reproductive cells seem to be usually ectodermic. i. Order Hydromedusse. Simple or colonial forms in which the sexually reproductive persons are either liberated as free swimming medusoids, or are sessile gonophores. (a) Ilydrophora. Two types are included here. The first includes the Tubularians, Hydractinia, and other forms in which the polypes are not enclosed in the protective sheath which often surrounds the colony (gymnoblastic), and in which the free medusoid forms, when present, have their genital organs placed in the wall of the manubrium (Antho- medusDe), and are furnished with ocelli placed at the base of the tentacles. Hydra and its allies may be included here. Examples : Syncoryne sarsii, the free medusoid of which is called Sarsia tubulosa. Bottgainvillea ramosa liberates the medusoid Margelis ramosa. Cordylophora lacustris and Tubularia larynx have sessile gono- phores. The second type includes Campanularians, Sertularians, Plumularians, and others, in which the protective sheath surrounding the colony is continued into little cups enclosing the polypes (calyptoblastic). The free medusoids have their gonads placed in the course of the radial canals (Leptomedusce), and are either "ocellate" or " vesiculate." I 5 6 CCELENTERA. Examples : Plumularia and Serttilaria have sessile gonophores. Campamdaria geniculata liberates the medusoid Obelia geniculata. (b} Hydrocorallinse. Colonial forms which suggest the Hydractinioe in their polymorphism and division of labour, but are distinguished by their power of taking up lime, and so forming "corals." The colonies are complex and divergent, the medusoid persons are probably sessile gonophores, but a simple male medusoid has been described. Millepora, Stylaster. (c) Trachy medusae. These exist only in the medusoid form, and are divided into two groups, Tracho- medusce and Narcomedusoe, accord- ing to the position of the gonads. Gcryonia, Carmarina, Ctinina, Aeginopsis. 2. Order Siphonophoroe. Free swimming colonies of modified medusoid persons (medusomes), with much division of labour. Physalia (Portugese Man-of-War), Diphyes, Velella, Porpita. B. Class SCYPHOZOA. There are two types polypoid and medusoid very rarely occur- ring in one life history. The gastric cavity has partitions with gastric or mesenteric filaments, and there is an ectodermic gullet. The mesoglcea generally contains migrant cells. The reproductive cells are endo- dermic. I. Sub-class ScyphomedusDe, or Acraspeda Jelly fish with gastric fila- ments, sub-genital pits, and no velum (i.) Lucernarke. Sessile forms. Lucernaria. (2. } Discomedusae. Active forms, often with complicated life his- tory. Aurelia, Pe- lagia, Cyanea, Rhizo- stoma. (3.) Conomedusse. Forms with broad pseudo-velum, and other peculiar features. Charybdea. (4.) Peromedusoe. Forms with four tentaculocysts only. Pericolpa. FIG. 49. Diagram of a gymnoblastic HydromedusDe. (After ALLMAN.) , Stem ; , root ; c, gut cavity ; d> endoderm (dark) ; e, ectoderm ; f, horny perisarc ; g~, hydra like "person" (hydranth); g\ the same, contracted ; /;, hypostome bearing mouth ; k, sac like reproductive bud (sporosac) ; ?;z, a modified hydranth (blastostyle) bearing sporosacs ; /, medusoid "person." CLASSIFICATION OF CCELENTERA. 157 II. Sub-class Anthozoa, or Actinozoa Polypoid forms with well developed gullet and septa, and cir- cu moral tentacles. (i.) Zoantharia or Hexacoralla. (a) Actiniaria. Sea anemones. Actinia, Anemonia, Tealia, Cerianthus. (b) Madreporaria. Stone .or reef corals. AstrcBa, Madrepora, Fungia, Mcmndrina. (c) Antipatharia. " Horny " black corals, with an axial skeleton, and occasional dimorphism between nutritive and reproductive "persons," Antipathes. (2.) Alcyonaria, or Octocoralla. Alcyonium (Dead men's fingers), Tubipora (Organ pipe coral), Coral Hum (Red coral), Gorgonia, Pennatula (Sea pen), Monoxenia (non-colonial). The Rugosa, or Tetracoralla, include extinct, or almost entirely extinct, forms, with numerous septa in some multiple of four. C. Class CTENOPHORA. Delicate free swimming organisms, generally globular in form, moving by means of eight meridional rows of ciliated plates, or comb-like com- binations of cilia. The stinging cells are usually modified into "adhesive cells." The mouth is at one pole, and leads into an ectodermic gullet. The gastric cavity is usually much branched. The mesenchyme is very well developed, and includes muscular and connective cells. At the aboral pole there is a sensory organ, including an " otolith," which seems of use in steering. Here, also, there are two excretory apertures. Ex- cept in Beroe and its near relatives, there are two retractile tentacles. All are hermaphrodite. The development is direct. They are pelagic, very active in habit, carnivorous in diet, and often phosphorescent. According to Lang, they have affinities with Planarian "worms," but this is very uncertain. Examples : (a) With tentacles, Cydippe and the ribbon shaped Venus' Girdle (Cesium Veneris}. (b} Without tentacles, Beroe. History. Of corals, as we would expect, the rocks preserve a faithful record, and we know, for instance, that in the older (Palaeozoic) strata, they were represented by a distinct series (Rugosa or Tetracoralla), of which we have at most two or three survivors. We often talk of the imperfection of the geological record, and rightly, for much of the library has been burned,' many of the volumes are torn, whole chapters are wanting, and many pages are blurred. But this imperfect record sometimes surprises us, as in the quite distinct remains of ancient jelly fish, which animals, as we know them now, are apparently little more than animated sea water. We should also grasp the conception, with which Lyell first impressed the world, of the uniformity of natural processes throughout the long history of the earth. Thus in connection with Ccelentera we learn that there were great coral reefs in the incalcul- 158 CCELENTERA. ably distant past, just as there are coral reefs still. So in the Cambrian rocks, which are next to the oldest, there are on sandy slabs markings exactly like those which are now left for a few hours, when a large jelly fish stranded on the flat beach slowly melts away. On the other hand, some forms of life which lived long ago, seem to have been very different from any that now remain, witness, for example, the very abundant Graptolite fossils, which, though probably Coelentera, do not fit well into any of our modern classes. Pedigree. As to the pedigree of the Coelentera, the facts of individual life history, and the scientific imagination of naturalists, help us to construct a genealogical tree a hypothetical statement of the case. Thus it seems very likely that the ancestral many celled animals ancestral to Sponges, Ccelentera, and all the rest were small two layered tubular or oval forms. The many celled animals must have begun as clusters of cells ; the question is, what sort of clusters spheres of one layer of cells, or mouthless ovals, or little discs of cells, or two layered thimble-like sacs? Possibly there were many forms, but Haeckel and other naturalists were led to fix their attention especially on the two layered sac or gastrula, because this form keeps con- tinually cropping up as an embryonic stage in the life history of animals, whether sponge or coral, earthworm or starfish, mollusc or even vertebrate, and also because this is virtually the form which is exhibited by the simplest sponges (Ascones), the simplest Coelentera (Hydra), and even by the simplest " worms " (Turbellarians). If we begin in our survey with such a gastrula-like ancestor, the probabilities are certainly in favour of the supposition that it was a free swimming organism. A gradual perfecting of the locomotor characteristics might yield the two medusoid types of which we have already spoken. But we know that the common jelly fish Aurelia has a prolonged larval stage which is sedentary, vegetative, and prone to bud. If we suppose with W. K. Brooks that many forms, less constitutionally active than others, relapsed into this sedentary state, with postponed sexuality, and with a preponderant tendency to bud, we can understand how polypes arose, and these of two types, one nearer the jelly fish and Lucernarians and leading on to sea anemones and corals, the other nearer the swimming bell type and leading on to a terminus in Hydra. It is certainly suggestive that PEDIGREE OF CCELENTERA. 159 we have jelly fish wholly free (Pelagia), jelly fish with a sedentary larval life (Aurelta), jelly fish predominantly pas- sive (Lucernaria\ and related polypes (Sea anemones, &c.), which only occasionally rise into free activity ; while in the other series we have medusoid types always free (Trachy- medusae), others which are liberated from (Campanularian and Tubularian) sedentary hydroids, other (Sertularian and Plumularian) zoophytes whose buds though often medusoid- like are not set free, and finally. Hydra, which, though it may creep on its side, or walk on its head, is predominantly a sedentary animal, without any youthful free swimming stage. It must be noticed that the most frequent larval form is the planula, so that if we regard the gastrula as the ancestral type, the life history is not here a recapitulation of the race history. GENERAL SCHEME OF CCELENTERA. PREDOMINANTLY PASSIVE. PREDOMINANTLY ACTIVE. C. CTENOPHORA, e.g-., Beroe, Venus' Girdle. , (Active climax.) B. SCYPHO-* ZOA. A f II. Anthozoa or Actinozoa. (Zoantharia) Sea anemones and related corals. (Alcyonaria) Dead Men's Fingers and related corals. The embryos are free swimmers, and a few adults also are locomotor. j / Scyphomedusae or \ Acraspeda. c. Adult Lucernarians usually attached. b. Sedentary larval stage. a. No fixed stage. ^. c. Free embryos. b. Aurelia type of jelly fish. a. Pelagia type of jelly fish. ANCESTRAL GASTR/EA. Y A. HYDRO- , ZOA. i. No fixed stage. 2. No fixed stage. 3. Many Hydroid colonies. (Campanularians and Tubularians.) 4. Many Hydroid colonies, whose reproductive per- sons are not liberated. 5. Coralline Millepores. i. Trachy medusae (always locomotor). 2. Siphonophorae (locomotor colonies of modified medusoids). 3. Liberated reproductive "persons" of these colonies. 4. No free stage, except as embryos. 5. No known free stage. \6. Hydra without any specially locomotor stage. 160 CCELENTERA. Bionomics. The Coelentera are almost all marine. In fresh water we find the common Hydra, the minute Micro- hydra without tentacles, the strange Polypodium, which in early life is parasitic on sturgeons' eggs, the compound Cordylophora, occurring in canals and in brackish water, and the fresh water Medusoid (Limnocodium) found in a tank in the Regent's Park Botanic Gardens, and another similar form recently discovered in Africa. Most of the active swimmers are pelagic, but there are also a few active forms in deep water. Many polypes anchor upon the shells of other animals which they sometimes mask, and there are most interesting constant partnerships between hermit crabs and sea anemones, e.g., Bernhardus prideauxii and Adamsia palliata. The hermit crab is masked by the sea anemone, and may be protected by its stinging powers ; the sea anemone is carried about by the hermit crab and may get crumbs from its abundantly supplied table. This illustrates a mutually beneficial partnership or commensalism, which, however, in some other animals, may degenerate into parasitism. CHAPTER X. UNSEGMENTED "WORMS." Chief Classes. 1. TURBELLARIA \ Plathelminthes 2. TREMATODA or 3. CESTODA J Flat-worms. 4. NEMATODA. 5. NEMERTEA. THE title " worms" is hardly justifiable except as a con- venient name for a shape. For there is no class of worms, the animals to which the name is applied forming a hetero- geneous mob, a collection of classes whose relationships are imperfectly discerned. But the zoological interest of the diverse types, some- times called "worms," is great. For amid the diversity we discern affinities with Coelentera, Echinoderms, Arthro- pods, Molluscs, and Vertebrates. Moreover, it is likely, as has been already noted, that certain " worms " were the first definitely to abandon the more primitive radial symmetry, to begin moving with one part of the body always in front, to acquire head and sides. And if one end of the body constantly experienced the first impressions of external objects, it seems plausible that sensitive and nervous cells would be most developed in that much stimulated, over-educated, region. But a brain arises from the insinking of ectodermic cells, and its be- ginning in the cerebral ganglion of the simplest " worms " is thus in part explained. Again it may be noted that with worm types begins the series of triploblastic coelomate animals, i.e., of those which 11 1 62 UNSEGMENTED " WORMS." have a well-defined mesodenn, and a mesoderm lined in- ternal cavity distinct from the gut. But the appearance of a well-developed coelome is very gradual. It is not at present possible to have much confidence in preferring one arrangement of the many classes of "worms" to another, but it seems useful to separate the segmented Annelids from the unsegmented types. Class TURBELLARIA. Planarians, &c. Turbellarians are unsegmented " worms" living in fresh, brackish, or salt water. They represent the beginning of definite bilateral symmetry. The ectoderm is ciliated, and contains peculiar rod-like bodies (rhabdites), and occasionally stinging cells. A pair of ganglia in the head region give off lateral nerve cords, and there are usually simple sense organs. The food canal has a muscular pharynx, is often branched, and is always blind. In diet the Turbellarians are carnivorous. There are no special respiratory or circulatory organs ; the body cavity is represented at most by small spaces ; the excretory system usually CQnsists of two longitudinal canals whose branches end internally in ciliated (flame) cells. Excepting two genera, the Turbellarians are hermaphrodite, and the repro- ductive organs usually show some division of labour, e.g., in the development of a yolk gland, which seems to have arisen as an over-nourished (hypertrophied) part of the ovary. Classification. A. Rhabdocoelida. Small fresh water and marine forms. The body tends to be cylindrical. The food canal is very slightly branched or quite straight or absent. (1) Acoela. Degenerate forms without intestine, e.. 9 Convoluta, which contains green cells, regarded by some as symbiotic AlgDe. (2) Rhabdoccela. With straight intestine, e.g., Vortex; Microstoma, a unisexual fresh water genus, with stinging cells, forming temporarily united asexual chains, some- times of sixteen individuals, suggesting the origin of a segmented type ; Graffilla and Attoplodium, parasitic (cf. next class). (3) Alloiocoela. With lobed or irregular gut. All marine except one from Swiss lakes \Plagiostoma Lemani}. TURBELLARIA. 163 B. Dendroccelida. Larger, flatter forms with branched intestine. ( i ) Tricladida. Elongated flat " Planarians " ; the mouth and tubular pharynx lie behind the middle of the body ; intestine with three main branches, themselves branched ; two ovaries, numerous yolk glands and testes, a common genital aperture., e.g., Planaria and Dendroccelum (in fresh water), the former sometimes divides transversely; Gunda segmentata (marine) showing hints of internal segmentation ; Geodesmus and Bipalium (in damp earth). c. F . FIG. 50. Diagrammatic figure of a simple Turbellarian. m, Mouth ; ph, pharynx ; f, digestive part of gut ; l.e, longitudinal excre- tory vessels ; e.p, excretory pore ; Ect, ciliated Ectoderm ; Ms, meso- derm ; End, endoderm. FIG. 51. Diagrammatic expres- sion of part of the structure of a simple Turbellarian. Ect, Ciliated ectoderm ; c.g; cerebra ganglion ; /., lateral nerve ; T, testes ; ov, ovary. (2) Polycladida. Large leaf - like marine "Planarians," with numerous intestinal branches diverging from a central stomach ; with numerous ovaries and testes, without yolk glands, mostly with two genital apertures. e.g., Cycloporus (showing beginning of anus), Leptoplana, Thysanozoon. 1 64 UNSEGMENTED " WORMS." Relationships. Two remarkable forms Cceloplana (Kowalewsky) and Ctenoplana (Korotneff) seem in some ways intermediate between Turbellarians and Ctenophora. Thus they have an aboral sense organ, and retractile branched tentacles ; the branching of the food canal is slightly suggestive of that in Ctenophora ; and Ctenoplana has eight dorsal bands of ciliated combs. The resemblance has been made much of by Lang and others, but, apart from direct affinity, there are likely to be resemblances of " convergence " (see p. 33) between forms not far removed from a common stock that of the primitive Metazoa. The occasional presence of a retractile proboscis and of a ciliated groove on each side of the brain is suggestive of two characteristics of Nemerteans. The Turbellaria are also related to the next class the Trematodes. Class TREMATODA. Flukes, &c. The Trematodes are leaf-like or roundish external or in- ternal parasites. With their mode of life we may associate the absence of cilia on the surface of the adults, the well- formed and apparently cellular "cuticle" the presence of attaching suckers (occasionally with hooks), and the rarity of sense organs. It is likely that they have arisen from free Turbellarian-like ancestors, and they resemble the Turbel- larians in being unsegmented, in having anterior nerve centres from which nerves pass backward and forward, in the rudimentary nature of the body cavity, in the ramifying system of fine excretory canals, in the hermaphrodite and usually complex reproductive system. The alimentary canal is usually forked, often much branched, and always ends blindly. In many cases the animals are self-impregnating, but cross fertilisation also occurs. The development of the external parasites is usually direct, of the internal parasites usually indirect, involving alternation of generations. They occur in or on all sorts of Vertebrates, but those which have an indirect development, and require two hosts to complete their life cycle, often pass part of their life in some Invertebrate. Type, The Liver Fluke (Fasciola (Distomd) hepatica). The adult fluke lives in large numbers in the liver and bile duct of the sheep. It sometimes occurs in cattle, horses, and other domestic animals, and rarely in man. TREMATODA. 165 In the sheep it causes the serious disease called liver rot. The animal is flat, oval, and leaf-like, measures about an 8 L e.v. FIG. 52. Structure of Liver Fluke. (After SOMMER.) From ventral surface. The branched gut (gn. ) and the lateral nerve (/.;/.) are shown to the left, the branches of the excretory vessel (e.v. ) to the right. ;;?, Mouth ; ph, pharynx ; g, lateral head ganglion ; v.s, ventral sucker ; c.s, position of cirrus sac ; an arrow indicates the excretory aperture. inch in length by half an inch across the broadest part, varies from reddish brown to grayish yellow in colour. As the word Distoma suggests, there are two suckers, an 1 66 UNSEGMENTED " WORMS." anterior, perforated by the mouth ; a second, imperforate a little further back on the mid ventral line. ut iii. 53. Reproductive Organs of Liver Fluke. (After SOMMER). f. Female aperture. s.v. Seminal vesicle. y.gl. Diffuse yolk glands. sh.g. Shell gland. v.d. Vas deferens. T. Testes (anterior). ov. Ovary (dark). ut. Uterus. c.s. Cirrus sac. p. Penis. m. Mouth. g. Anterior lobes of gut. There is a muscular pharynx and a blind alimentary canal LIFE HISTORY OF LIVER FLUKE. 167 which sends branches throughout the body. The nervous system consists of a ganglionated collar round the pharynx, from which nerves go forward and backward ; of these, the two which run laterally are most important. Although the larva has eye spots to start with, there are no sense organs in the adult. The body cavity is represented only by a few small spaces. Into these there open the ciliated ends of much branched excretory tubes, which unite posteriorly, and communicate with the exterior by a terminal pore. The reproductive system is hermaphrodite and complex. From much branched testes, spermatozoa pass by a pair of ducts (vasa deferentia) into a seminal vesicle lying in front of the ventral sucker. Thence they are expelled by an ejaculatory duct, which passes through a muscular protrusible .penis. The retracted penis and the seminal vesicle lie in a space or " cirrus sac " between the ventral sucker and the external male genital aperture. The ovary is also branched, but less so than the testes. From its tubes ova are collected into an ovarian duct. Nutritive cells are gathered from very diffuse yolk glands, collected in a reservoir, and pass by a duct into the end of the aforesaid ovarian duct. At the junction of the yolk duct and the ovarian duct there is a shell gland, which secretes the " horny " shells of the eggs, and from near the junction, a fine canal (the Laurer-Stieda canal) seems to pass direct to the exterior, opening on the dorsal surface. The meaning of this is still somewhat uncertain. In some cases it is said to be a copulatory duct ; in others it is regarded as a safety valve for over- flowing products. From the junction of the ovarian duct and the duct from the yolk reservoir, the eggs (now furnished with yolk cells, accompanied by spermatozoa, and encased in shells) pass into a wide convoluted median tube, the oviduct or uterus, which opens to the exterior at the base of the penis. Self fertilisation is probably normal, but in some related forms cross fertilisation has been observed. Life History. The fertilised and segmented eggs pass in large numbers from the bile duct of the sheep to the in- testine, and thence to the exterior. A single fluke may produce towards half a million embryos, which illustrates the prolific reproduction often associated with the luxurious i68 UNSEGMENTED " WORMSr 2 3 FIG. 54. Life history of Liver Fluke. (After THOMAS.) i. Developing embryo in egg case; 2. free swimming ciliated embryo ; 3. sporocysl ; 3. Lymncea truncatula ; 4. division of sporocyst ; 5. sporocyst with rediae forming within it ; 6. redia with more rediae forming within it ; 7. tailed cercaria ; 8. young fluke. LIFE HISTORY OF LIVER FLUKE. 169 conditions of parasitism, and almost essential to the con- tinuance of species whose life cycles are full of risks. Out- side of the host, but still within the egg case, the embryo develops for two or three weeks, and eventually escapes at one end of the shell. Those which are not deposited in or beside pools of water must die. The free embryo is conical in form, covered with cilia, provided with two eye spots, and actively locomotor. By means of its cilia it swims actively in the water for some hours, but its sole chance of life depends on its meeting a small amphibious water-snail (Lymncea truncatula), into which it bores its way. In an epidemic among horses and cattle in the Hawaiian Islands, the host was L. cahuensis (Lutz). Within the snail, e.g., in the pulmonary chamber, the embyro becomes passive, loses its cilia, increases in size, and becomes a sporocyst. Sometimes this sporocyst divides transversely (Fig. 54, 4). Within the sporocyst certain cells behave like partheno- genetic ova. Each segments into a ball of cells or morula, which is invaginated into a gastrula, and grows into another form of larva the redia. These rediae burst out of the sporocyst, and migrate into the liver or some other organ, killing the snail if they are very numerous. Indeed the death of the snail is probably necessary for the escape of the final larvae. Each redia is a cylindrical organism with a short alimentary canal (Fig. 54, 6). Like the sporocysts, the redise give rise internally to more embryos, of which some are simply rediae over again, while the last set are quite different, long tailed cercarice, with two suckers and a forked food canal. These emerge from the rediae, wriggle out of the snail, pass into the water, and moor themselves to stems of damp grass. There they lose their tails and become encysted. If the encysted cercaria on the grass stem be eaten by a sheep, it grows, in about six weeks, into the adult sexual fluke. To recapitulate, the developing embryo becomes a free swimming form, which bores into a snail, and changes into a sporocyst. From certain cells of the sporocyst rediae are developed, and these may similarly give rise to other rediae. Eventually, within the rediae the tailed cercariae are formed, 170 UNSEGMENTED " WORMS? and these in favouring circumstances grow into the adult flukes. The above history has been independently worked out by Leuckart and Thomas. It will be noted that the sporocyst is the modified embryo, but that it has the power of giving rise asexually to redke. These develop, how- ever, from special cells of the sporocyst which we may compare to precociously developed parthenogenetic ova. Though the reproduction is asexual, it is not comparable to budding or division. The same power is possessed by the redise, and there are thus several (at least two) asexual generations between the embryo and the adult. Finally, it must be clearly understood that the cercaria is the young fluke. The disease of liver rot in sheep is common and disastrous. It has been known to destroy a million sheep in one year in Britain alone, and in the winter, 1879-80, the mortality attributed to fluke disease was estimated at three millions. It is especially common after wet seasons, and in damp districts. The preventives suggested are drainage of pastures and dressings of lime and salt ; destruction of the eggs, the snails, infected manure, and diseased sheep. It is usual to give the infected sheep some salt and a little dry food. Classification. Trematodes with direct development Monogenetic. e.g., Polystoimim integerrimum. This form with many suckers is often found in the bladder of the frog. It attaches itself in its youth to the gills of tadpoles, passes thence through the food canal to the bladder, where it develops slowly for years. Gyrodactylus, found on the gills and fins of fresh-water fishes. It is viviparous, but the embryo, before it is extruded, itself contains an embryo, and this in turn another, so that three generations of embryos are represented sim- ultaneously. Diplozoon paradoxum, consists of two individuals united. The single embryo (Diporpa) is at first free swimming, but becomes a parasite on the gills of a minnow, and there two individuals unite very closely and permanently. Tristomtim, with three suckers, is not uncommon on the skin of some marine fishes. Trematodes with indirect development Digenetic. e.g., Fasciola or Distonia. Bilharzia, or Gyncecophorus h&matobius, a dangerous parasite of man, widely distributed in Africa. It infests the urinary and visceral blood vessels. The sexes are separate, and the male carries the female inserted in a groove. Monostomum, a form with one sucker. The relationships of the class are on the one hand with the free living Turbellarians, on the other hand with the parasitic Cestodes. CESTODA. 171 Class CESTODA. Tapeworms. The Cestodes are internal parasites, whose life history in- cludes a bladdenvorm -(proscolex) and a tapeworm (strobila) stage, the former in a Vertebrate or Invertebrate host, the latter (with one exception) in the gut of .a Vertebrate. In a few cases the body is unsegmented, e.g., Archigetes and Caryo- phyllaeus, with one set of gonads ; in a few cases, e.g., Ligula, there is a serial repetition of gonads without distinct segmenta- tion of the body ; in most cases, e.g. , Taenia and Bothriocephalus, the body of the tapeivorm forms a chain of numerous joints or proglottides, each with a set of gonads. Thus the class in- cludes transitions from unsegmented to segmented forms, but the latter are imperfectly integrated. The general form of the body is tape-like and bilaterally symmetrical, with hooks, grooves, or suckers ensuring attachment to the gut of the host. The nervous system consists of longitudinal nerve strands and anterior ganglionated commissures ; there are no special sense organs. There is no alimentary system ; the parasite floating in the digested food of its host absorbs soluble material by its general surface. There is no vascular nor respiratory system, and the body cavity is represented merely by irregular spaces. Into some of these spaces there open ciliated funnels, the ends of the fine branches of longitudinal excretory tubes, which are connected transversely at each joint and open terminally by one or more pores. All tapeworms are hermaphrodite, and most, if not all, are probably self-fertilising. The male reproductive organs include diffuse testes, a vas deferens, and a protrusible terminal cirrus. The female organs include a pair of ovaries, yolk glands, a shell gland, a vagina by ivhich spermatozoa enter, a receptacle for storing spermatozoa, and a uterus in which the ova develop. The embryo develops within another host into a proscolex or bladdenvorm stage, which forms a "head" or scolex. When the host of the bladderworm is eaten by the final host, the scolex develops into an adult sexual tapeworm. With the conditions of endoparasitic life, we may associate the occurrence of fixing organs, the absence of sense organs, the low though somewhat complex character of the nervous system, the entire absence of a food canal, and the prolific reproduction. Life History of Tcenia solium. This is one of the most 172 UNSEGMENTED " WORMS." frequent of the tapeworms infesting man. In its adult state it is often many feet in length, and is attached by its " head " to the wall of the intestine. The head bears four suckers and a crown of hooks, and buds off a long chain of joints, which develop complex reproductive organs as they get v.d FIG. 55. Diagram of reproductive organs in Cestode joint. (Constructed from LEUCKART.) ov,, Ovary, with short oviduct ; /., uterus ; t., diffuse testes ; sh.g., shell gland ; y.g., yolk gland ; v.d., vas deferens ; 71. vagina ; r.s., receptaculum seminis ; I.e., longitudinal excretory ducts ; f.e., transverse bridges connecting these. The dotted lines above and below represent the anterior and posterior borders of the proglottis. shunted further and further from the head. The last of the joints or proglottides, is liberated (singly or along with LIFE HISTORY OF T&NIA SOLIUM. 173 others) and passes down the intestine of its host to the exterior. It has some power of muscular contraction, and is distended with little embryos within firm egg shells. When the proglottis ruptures, these egg cases are set free. In certain circumstances, the embryos, within their firmly resistent egg shells, may be swallowed by the omnivorous pig. Within the alimentary canal of this animal the egg shells are dissolved, and the embryos bearing six anterior hooks are liberated. They bore their way from the intestine into the muscles or other structures, and there encyst. They increase in size and become passive, vegetative, asexual " bladderworms." A bud from the wall of the bladder or proscolex grows into the cavity of the same, and forms the future "head" or scolex. It is afterwards everted, and then the bladderworm consists of a small head attached by a short neck to a relatively large bladder. But this remains quiescent, and without power of further development, unless the pig be eaten by some other Vertebrate. When man unwittingly eats " measly " pork, that is pork infested with bladderworms, an opportunity for further de- velopment is afforded. The bladder is lost, and is of no importance, but the " head " or scolex fixes itself to the wall of the intestine. There it is copiously and richly nourished, and buds off asexually a chain of joints. As these joints are pushed by younger interpolated buds further and further from the head, they become sexually mature, developing complex hermaphrodite reproductive organs. The ova produced in these are fertilised, appar- ently by spermatozoa from the same joints ; the proglottis becomes distended with ripe eggs and developing embryos. These ripe joints are liberated, and the vicious circle may recommence. Happily, however, the chances are thirty- five millions to one against the embryo becoming an adult. The above history is true mutatis mutandis for many other tape- worms. It will be observed that the embryo grows into a proscolex or bladder, which buds off a scolex or head, which, in another host, buds off the chain of proglottides, but as it is virtually the same animal through- out, the life history does not include an "alternation of generations." It is doubtful, however, what term should be applied to those cases in which the bladderworm (Cccnurus and Echinococcus] forms not one head 174 UNSEGMENTED " WORMS." only but many, each of which is capable of becoming an adult tapeworm. 'The only known exception to the fact that sexual tapeworms are parasites FIG. 56. Life history of Tccnia solium. (After LEUCKART.) i. Six-hooked embryo in egg case ; 2. proscolex or bladderworm stage with invaginate head ; 3. bladderworm with evaginated head ; 4. enlarged head of adult, showing suckers and hooks ; 5. general view of the tapeworm from small head, and thin neck to the ripe joints ; 6. a ripe joint or proglottis with uterus. of Vertebrates, is Archigetez sieboldii^ a simple cestode which is sexual within the small fresh water worm Tubifex rivulorum. NEMERTEA. 175 Life Histories. ADULT, SEXUAL, OR TAPEWORM STAGE. NON-SEXUAL, PROSCOLEX, OR BLADDER- WORM STAGE. 1. Tcenia so Hum, in man, with four suckers and many hooks. 2. Tcenia saginata or mediocanellata, in man, with four suckers, but no hooks. 3. Bothriocephalus latus, in man, with two lateral suckers, but with no hooks, with less distinct separation of the prog- lottides than in Tcenia. 4. Tcenia echinococcus, in dog. 5. Tcenia ccenurus, in dog. 6. Tcenia serrata, in dog. 7. Tcenia cucumerina, in cat. 8. Tcenia elliptica, in dog. 1. Cysticercus celluloses, in muscles of the pig. 2. Bladderworm in cattle. 3. The ciliated, free swimming embryo becomes a parasite in the pike or burbot, but without a distinct bladder-like stage. 4. Echinococcus veterinorum, in domestic animals, and sometimes in man, producing brood capsules, which give rise to many " heads." 5. C ceriums cerebralis, causing sturdie in sheep, producing numerous " heads." 6. Cysticercus pisiformis, in rabbit. 7. Cysticercus fasciolaris, in mouse. 8. Cysticercus in dog louse or perhaps in flea. The Cestodes are closely connected with Trematodes by such forms as Amphilina, Caryophyllcvus, Archigetes. Zoologically, they are interest- ing, on account of their life histories, the degeneration associated with their parasitism, the prevalence of self-impregnation, and the complexity of the reproductive organs. Practically, they are of importance as para- sites of man and domestic animals. The medical student should consult Leuckart's great work, The Parasites of Man, part of which has been translated by W. E. Hoyle (Edin. 1886). Class NEMERTEA. Nemertines. The Nemertines are worm-like animals, unsegmented, and generally elongate in form. Almost all are marine ; most, if not all, are carnivorous. Among their characteristics, the following are most noteworthy : The skin is ciliated ; there is a remarkable retractile proboscis ; the head bears a pair of ciliated pits ; the nervous system consists of a brain, a com- missure around the proboscis^ and two lateral nerve cords ; there is a ccelomic vascular system, a pair of anterior nephridia, and a simple reproductive system. The sexes are usually separate. In some the development includes a peculiar pelagic larval stage ; in others there is no metamorphosis. External Appearance. Some are ribbon-like, others thread-like, and the cross section is generally a flattened cylinder. They vary greatly 176 UNSEGMENTED " WORMS." in size, from a Lineus, 12 feet long, to the small pelagic Pelagonemertes, which is under an inch. There are no appendages. The colours are often bright. Skin. The ectoderm is covered with numerous short cilia, and many of its cells are also glandular, secreting mucus which often forms a tube around the animal, or is exuded in move- ment. Some of the glandular cells extend into the subjacent cutis, which consists in part of connective tissue. Muscular System. The Nemertines are remarkably contractile, and in some cases the spasms result in the breakage of the body. The muscles are circular and longitudinal, but their arrangement is variable even in individuals. Body Cavity. In the adult there is no distinct coelome, the space between the gut and the body wall being filled up with con- nective tissue. In the larvae, however, a body cavity may be seen, either as an archicoele, i.e., persistent segmentation cavity (Lineus obscurus\ or as a schizocoale, i.e., a space formed by the cleavage of the mesoderm into two layers (Pilidium-laiv^). In the adult, however, only the blood spaces and the cavity of the proboscis sheath are coelomic. Nervous System. In the head there is a brain, generally four lobed, with a commissural ring surrounding the proboscis and its sheath ; from the lower brain lobes two longitudinal nerve stems run along the sides, and are sometimes united posteriorly above the anus (Fig. 57, In). Hubrecht suggests that the nerve stems and the brain may "be looked upon as local accumulations of nervous tissue in what was in more primi- tive ancestors a less highly differentiated nervous plexus, situated in the body wall," as in many Ccelentera. In some cases (Schizonemertea) this nerve plexus persists, and then the longitudinal stems do not give off regular peripheral branches as is the case in another sub-class (Hoplo- nemertea) where there is no definite plexus. It is interesting to find that in Drepanophorus the lateral nerve steins are approximated ventrally, and in Langia^ dorsally ; for these two NEMERTEA. 177 approximations tend towards the two positions most characteristic of the nervous systems of Annelids and Arthropods on the one hand, and of Vertebrates on the other. Lateral Organs. On each side of the head there is a ciliated pit communi- cating with the exterior through an open slit or groove, and communicating internally either with the brain itself, or with adjacent and associated nervous tissue. In those cases in which the development has been studied these so-called P.s P.c d. v. m. FIG. 57. Transverse section of the Nemertean Drepanophorns latus. (After BURGER.) branches ; P., parenchyma ; -., gut ; l.v., lateral blood vessel, beside which excretory vessel ; E.p., excretory pore ; d.v.' , dorsal blood vessel ; Ep., epidermis. lateral organs arise from epiblastic insinkings and cesophageal outgrowths. In the most primitive genus, Carinella, they are absent, except in one species. It has been suggested that they conduce to the respiration of the brain, which is rich in haemoglobin, and they have even been compared with gill slits. In some forms the groove through which they 12 1 78 UNSEGMENTED "WORMS." open to the exterior is rhythmically contractile. It has also been suggested that they are sensory. Sense Organs. Nemertines are very sensitive, and in many this is to be associated with the superficial nerve plexus already men- tioned. Tactile papillae and hairs are also present in some. Eyes and eye spots are of general occurrence, and in some cases otocyst sacs have been observed. Alimentary System. A ventral mouth leads into a plaited oesophagus, which is followed by an intestine with regularly arranged lateral caeca. d.n FIG. 58. Transverse section of a Nemertean Carinella. (After BURGER.) d.n., Dorsal nerve ; fl.c., proboscis cavity ; g., gut ; c.vt., circular muscles ; /.;;/., longitudinal muscles ; d,v.m., dorso-ventral muscles ; /.?/., lateral vessel. Between the caeca run transverse muscle partitions. The anus is terminal. In the adults of the primitive genus, Carinella, the caeca are absent, but they are present in the larva. The Proboscis. In a cavity along the dorsal median line there lies a remarkable organ, known as the proboscis. It is protruded and retracted through an opening above, or, in a few cases, within the mouth, but it has no connection with the ali- NEMERTEA. 179 mentary system. The proboscis is a hollow muscular structure, very richly innervated, and is sometimes protruded with such force that it separates from the body, and then " often retains its vitality for a long time, apparently crawl- ing about as if it were itself a worm " (Hubrecht). It has been compared in its retracted state to a glove finger drawn in by a thread attached to its tip, the thread being the retractor muscle. But in front of the attachment of the retractile muscle there is a non-eversible glandular region which secretes an irritant fluid. In many cases there is a stylet at the tip of the eversible portion, and if this be absent, there are stinging cells or adhesive papillae. There is a hint of a similar structure in some Turbellarians, and the organ is usually interpreted as one which w r as originally tactile, but which has become secondarily aggressive. It is protruded by the muscular contraction of the walls of the proboscis sheath, which forms a closed cavity containing fluid and surrounding the proboscis. (Fig. 57, P.s.) Vascular System. In the majority there are three longitudinal blood vessels or blood spaces, a median and two laterals, which unite anteriorly and posteriorly, and also communicate by numerous transverse vessels. The vessels or spaces are remnants of a coelome. The blood is a colourless fluid, sometimes at least with corpuscles in which haemoglobin may be present. Excretory System. In most, if not all, there are two coiled nephridia, one on each side of the oesophagus opening anteriorly. (Fig. 57, E.p.) Reproductive System. The sexes are usually separate, and the reproductive organs are always simple. They consist of simple sacs, arranged in a series on each side between the intestinal coeca, and communicating with the exterior by fine pores. The ova are often laid in gelatinous tubes, and are probably fertilised shortly before or at the time of extrusion. In three or four forms known to be viviparous the fertilisation must, of course, be internal. i8o UNSEGMENTED " WORMS." Development. (i.) In Cerebratulus, etc., the larva is adapted for pelagic life, and is known as the Pilidium. " In external shape it resembles a helmet with spike and ear lobes, the spike being a strong and long flagellum or a tuft of long cilia, the ear lobes, lateral ciliated appendages" (Hubrecht). (2.) In Linens there is a sedentary larva, which has been interpreted as a reduced Pilidium, and is known as the "larva of Desor." (3.) In Hoplonemertea, the development is direct without metamorphosis. Habits. Most Nemertines are marine, creeping about in the mud, under stones, among seaweed, and the like ; many are able to swim; Pelagonemertes is pelagic; a few live in fresh water; Malacobdella lives in the mantle cavity of marine bivalves, and two others occur on crabs. Most seem to be carni- vorous, eating other "worms." Many break readily into pieces when stimulated, and the Schizonemertea are able to regenerate what they lose in this way. Classification (after Hubrecht) : 1. Palceonemertea : No deep head fissure ; no stylet ; mouth behind brain. e.g., Carinella, Cephalothrix, Carinoma, Polia. 2. Schizoneuierlea : A deep head fissure with a ciliated duct to the brain ; lateral nerves between the longitudinal and inner circular muscles ; mouth behind brain. e.g.) Lineus, Cerebratulus^ Langia. 3. Hoplonemertea : No deep head fissures; lateral nerves inside the muscles ; stylet present ; mouth generally in front of brain. e.g.) Amphiporus, Nemertes, Drepanophorus, Malacobdella^ The last has no head fissures nor spines on the pro- boscis, but bears a posterior sucker. Relationships. Some of the characteristics of the Nemerteans are hinted at among the Turbellarians. Professor Hubrecht has maintained that Nemerteans exhibit affinities with Vertebrates. (See Chapter XX.) Class NEMATODA. Threadworms, Hairworms, c. The Nematodes are unsegmented, more or less thread-like "worms" some of which are free living and others parasitic. The body is covered by a cuticle, often thick. Ciliated epithe- NEMATODES. 181 Hum is altogether absent. From a nerve ring around the pharynx six nerves run forwards and six backwards. An alimentary canal r , consisting of fore, mid, and hind gut, is usually developed. There is no vascular nor respiratory system, but there is usually a body cavity, and there are two excretory tubes opening by an anterior ventral pore. The sexes are usually separate and the reproductive organs simple. The life history is often intricate. Form. The body is usually cylindrical in cross section and tapering at each end. The male is usually smaller than the female, and his tail, concerned in copulation, bears sensory papillae, and usually some spines and a " bursa." Body Wall. (a.) Most externally there is a chitinoid, often wrinkled, cuticle, thick in the larger forms, and perhaps of ser- vice in enabling the animals to resist drought and digestive juices. With its presence may be associated the almost entire absence of cutaneous glands, and the entire absence of cilia, (b.) Beneath this is the sub-cuticula or hypodermis, usually thickened in four longitudinal lines median dorsal, ventral, and lateral. (c.) Beneath the hypodermis is a layer of longitudinal muscles, which sometimes lie in groups defined by the above mentioned lines. Many of the Nematodes are very agile. Nervous System. Around the pharynx there is a nerve ring from which six nerves run forwards and six backwards. One of the latter runs along the median dorsal line a unique position in an Invertebrate. Here and there on the ring and on the nerves there are ganglionic cells, but any aggregation of these into ganglia is rare. Some of the free living forms have eye spots ; and probably all Nematodes have sensory papillae on various parts of the body. Alimentary System. The mouth is terminal or almost terminal ; the anus is ventral and posterior, and occasionally terminal. As 182 UNSEGMENTED " WORMS. the food consists chiefly of juices either from a living host or from putrefying organic matter, it is not surprising to find that the alimentary canal has usually but a narrow cavity. In some forms, e.g., Sphcerularia from the bee, it degenerates altogether. Normally it consists of three parts, a fore gut or oesophagus, lined by the inturned cuticle, a mid gut or mesenteron of endo- dermic origin, and a usually short hind gut or rectum, lined by the cuticle. When the external cuticle is shed, so is that of the fore gut and hind gut (cf. Crayfish). Body Cavity. A ccelome is developed and contains a clear fluid, which probably discharges some of the functions of the absent blood. There are no amoeboid phagocytes. Excretory System. Imbedded in each lateral line there is a long tube containing clear fluid, probably drained from the surrounding tissues. The two longitudinal tubes unite anteriorly, and open in a ventral excretory pore near the head. Reproductive System. The sexes are separate, except in Angio- stomum which is hermaphrodite and self- fertilising. In the male, the testis is usually unpaired, a coiled tube gradually tureofaNematode differentiating into vas deferens, seminal (Oxyuris). (After vesicles, and ejaculatory duct. The genital GALEB.) aperture is close to the anus, and beside it .'"-* Mouth; c.,cu- . .,, j f , . -i ticular ring ; #?., oeso- there are sensory papillae, and often spicules, phagus ; #., bulb con- and peculiar membranous folds of varied form \vhich constitute what is called the copulatory "bursa." The spermatozoa have not the typical form, and are sluggish. In the female, the ovary is a single or paired tube which passes grad- FIG. 59. Illus- trating the struc- . te j, th ; t z ;' st |": ?.<, vas deferens ;#! NEMATODES. 183 ually into an oviduct, a uterus, and a short vagina. The genital aperture is ventral, usually about the middle of the body, but it is occasionally far forward or far back. Development. The ova meet the spermatozoa at the junction of uterus and oviduct. Segmentation is total and may take place before or after laying. Indeed the embyro may be hatched within the uterus. Before the embryo exhibits adult char- acteristics, several, e.g., three, moultings of the cuticle usually occur. LIFE HISTORIES. 1. The embryo grows directly into the adult, and both live in fresh or salt water, damp earth, and rotting plants Enoplidce, e.g., Enoplus. 2. The larvae are free in the earth, the sexual adults are parasitic in plants, or in Vertebrate animals, e.g., Tylenchus scandens, a com- mon parasite on cereals ; Strongylus and Dochmius in man. 3. The sexual adults are free, the larvse are parasitic in insects, e.g., Mermis. The fertilised females of Spharularia bombi pass from the earth into the body cavity of humble-bee and wasp, whence their larvse bore into the intestine and eventually emerge. 4. The larvee are parasitic in one animal, the sexual adults in another which feeds on the first. Thus Ollulanus passes from mouse to cat, Cucullamis from Cyclops to perch. There are other life histories, and many degrees of parasitism. The most remarkable form is Angiostoinum (or Ascaris, or Leptodera] nigrovenosum. In damp earth males and females occur, the progeny of which pass into the lungs of frogs and toads. There they mature into hermaphrodite animals (the only example among Nematodes), which produce first spermatozoa and then ova. They are self-impregnating, and the young pass out into the earth as males or females. Here there is alternation of generations, and a somewhat similar story might be told of Rhabdonema strongyloides from the intestine of man and Leptodera appendiculata from the snail. There are several quaint reproductive abnormalities, thus the female Spharularia bombi, which gets into the body cavity of the humble- bee, has a prolapsed uterus, larger than itself; the male of Trichodes crassicauda passes into the uterus of the female. 1 84 UNSEGMENTED " WORMS." Parasitic in Man. NAME. POSITION. HISTORY. RESULT ON HOST. Ascaris lumbri- coides (common). Small intestine. Probably enter the body as larvae, along with vege- table food or impure water, Julus gut- tulatus perhaps an intermediate host. Rarely danger- ous, but may per- forate intestine, and cause abscesses. OxyuTts vermi- cularis (common). From stomach to rectum, mostly in caecum. From food or water. Rarely more than discomfort. Trichocephalus dis- par (common). Caecum and colon. " Dochmius (A nchy- lostoma) duodcnalis (Europe, Egypt Brazil). R hab done ma strongyloides. Small intestine. Associated with Dochmius. The larvae seem to live freely in the earth. Dangerous an- aemia. Filar la sanguinis kominis (Australia, China, India, Egypt, and Brazil). Mature female in lymphatic glands, embryos in blood. Larvae in a Mos- quito. Elephantiasis, and haematuria. Dracunculus {F*il- aria) medinensis (Guin'eaworm) in Arabia, Egypt, Abyssinia, etc. The female is 1-6 feet long, encysts beneath skin. The male is not known, though his tail is said to have been seen. Larvae in a Cy- clops. Skin abscesses. Trichina spiralis. Becomes sexually mature in the intes- tine ; embryos, pro- duced rapidly and viviparously, bore their way to muscles, and be- come encysted. From " trichi- nosed " pig's muscle to man. Inflammatory pro- cesses, often fatal, are brought about by the migration of the young worms from intestine to muscles. Trichina. The formidable Trichina deserves fuller notice. It is best known as a parasite in man, pig, and rat, but occurs also in hedgehog, fox, marten, dog, cat, rabbit, ox, and horse. The sexual forms live in the intestine, the female about 3 millimetres in length, the male about half as long. After impregnation, the female brings forth numerous embryos viviparously, 60 to So at a time, and altogether about 1500. These bore through the wall of the intestine into the body cavity or blood vessels, and work their way, especially through connective tissue, to the muscle fibres. There they grow, coil themselves spirally, and become encysted within a sheath, at first membranous and afterwards calcareous. In these cysts, which may be sometimes counted in millions, the young Trichinae remain passive, unless the flesh of their host be eaten ACANTHOCEPHALA. 185 by another, pig eating rat, man eating pig. In the alimentary canal of the new host the capsule is dissolved, the embryos are set free, and be- come rapidly reproductive. Among the numerous other parasitic Nematodes the following may be noted : The giant palisade worm {Eustrongyhis gigas] occurs in the renal region of domestic animals, &c. ; the female may be 3 feet long. The armed palisade worm (Strongylus armatus] occurs in the intestine and intestinal arteries of horse, causing aneurisms, colic, &c. The young forms are swallowed from stagnant water, bore from gut into arteries, become adult, return to gut, copulate and multiply. Various other species of Strongylus occur in sheep, cattle, &c. The large Ascaris megalocephala and the much smaller Oxyuris curvula are not uncommon in horses. Syngamus trachealis occurs in the trachea of birds, causing "gapes." Various species of Tylenchus, especially T. devastatrix and T. scandens (or T. tritici], destroy cereal and other crops. Various species of Heterodera (especially H. schachtii and H. radicicola] infest the roots of many cultivated plants e.g., turnip, radish, cabbage. Classification. At present the Nematodes are usually classified in families Ascaridse, Anguillulidce, &c. With these we need not concern ourselves here, but it is important to notice that the Gordiidse, (e.g., Gordius aquaticus the horse hair worm) are very different from all the others. In the adult the mouth is shut and the food canal is partly degenerate. The adults live freely in fresh water ; there are two larval forms, the first in aquatic molluscs, young insects, &c. , the second in adult insects, fish, frog, &c. Class ACANTHOCEPHALA. For a single genus Echinorhynchus , whose larvae live in Arthropods, and the adults in Vertebrates, a special class, ACANTHOCEPHALA, has been established. We may provisionally place this genus, which has about a hundred species, beside Nematodes, but the relationship does not seem to be very close. Mouth and gut are absent. The anterior end bears a protrusible hooked proboscis. Echinorhynchus proteus of Pike, larva in the Amphipod Ganiniarus pulex. , , angustatus of Perch, larva in the Isopod Asellus aqua- ticus. , , gigas of Pig, larva in young Cockchafers. CHAPTER XI. SEGMENTED WORTylS OR ANNELIDA. Chief classes CH^ETOPODA, DISCOPHORA. THE Annelida do not form a well defined phylum, but in- clude segmented worms, in which the segmentation of the body is usually visible externally. There is usually a well developed body cavity, which communicates with the exterior by paired nephridia or segmental organs. The nervous system consists typically of dorsal cerebral ganglia, a com- missural ring round the gullet, and a ventral ganglionated chain. Not infrequently the nephridia function also as genital ducts. The development is either indirect, when it includes a larval Trochosphere stage, or direct. In habit, form, and structure the Annelids exhibit much diversity of type. The Chaetopods, represented on the one hand by the familiar earthworm, and on the other by the marine worms, best exhibit the structure upon which the Annelid type is founded. It seems, however, that with these we may also include the aberrant Echiuridae e.g., Echiurus and Bonellia. A few forms of primitive type (the Archi-Annelida), and the Myzostomata, which are degenerate parasites found on Crinoids, may also be appended to the class Chaetopoda. The divergent leeches (Discophora) are probably Annelids which have be- come modified in consequence of a peculiar habit. Finally, some zoologists provisionally include Sagitta (Chaetognatha) in this series as an Annelid with three segments, and also the Rotifers (Rotatoria), since their adult form somewhat resembles the Trochosphere larvae of many Annelids. According to Lang, the Chaetopods are derived from a CH^TOPODA. 187 leech-like type, this from a Polyclade Turbellarian, and this from a Ctenophore. According to Sedgwick, the Annelids are derived from an Actinozoon-like ancestor. But we cannot here discuss these possibilities, nor the difficult questions concerned with the meaning of segmentation or metamerism. Class CH^ETOPODA. Worms with Bristles. Segmented animals with seta developed in little skin sacs, either on a uniform body wall or on special locomotor pro- trusions known as parapodia. The segments, indicated externally by rings, are often marked internally by parti- tions running across the body cavity, which is usually well developed. The nervous system generally consists of a double ventral chain of ganglia, connected with a pair of dorsal or cerebral centres, by means of a ring round the beginning of the gut. Two excretory tiibes or nephridia are typically present in each segment, and they or their modifications may also function as reproductive ducts. The reproductive elements are formed on the lining membrane of the body cavity, and the development is either direct or with a metamorphosis. The two prominent divisions of this class may be con- trasted as follows : OLIGOCHVETA, e.g., Earthworm. POLYCH/F.TA, e.g., Nereis. With no parapodia, and with few setae. Other external appendages are also want- ing, except that Branchiura has gills. Hermaphrodite. Development direct. Living in fresh water or in the soil. With parapodia and with numerous setae. With antennae, gills, and cirri. Sexes usually separate. A metamorphosis in development. Marine. TYPE OF OLIGOCH^ETA. The Earthworm (Lumbricus}. Earthworms eat their way through the ground, and form definite burrows, which they often make more comfortable by a lining of leaves. The earth swallowed by the bur- rowers is reduced to powder in the gut, and, robbed of some of its decaying vegetable matter, is discharged on the surface as the familiar " worm castings." By the^burrowing the earth is loosened, and ways are opened for plaht roots and rain drops ; the internal bruising reduces mineral matter to more useful form ; while, in burying the surface with earat i88 SEGMENTED WORMS OR ANNELIDA. brought up from beneath, the earthworms have been ploughers before the plough. Darwin calculated that there were on an average over 53,000 earthworms in an acre of arable ground, that ten tons of soil per acre pass annually through their bodies, and that they cover the surface with earth at the rate of three inches in fifteen years. He was therefore led to the conclusion that earthworms have been the great soil makers, or more precisely, that the formation of vegetable mould was mainly to be placed to their credit. According to Gilbert White (1777), "the earth without worms would soon become cold, hard bound, void of fermentation, and consequently sterile;" while Darwin (1881) said that " it may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organised creatures." Though without eyes, earthworms are sensitive to light and persistently avoid it, remaining underground during the day unless rain floods their burrows, and reserving their public life for the night. Then, prompted by " love " and hunger, they roam about on the surface, leaving on the moist roadway the trails which we see in the morning. More cautiously, however, they often remain with their tails fixed in their holes, while with the rest of their body they move slowly round and round. The nocturnal peregrinations, the labour of eating and burrowing, the transport of leaves to their holes, the collection of little stones to protect the entrance to the burrows, include most of the activities of earthworms, except as regards pairing and egg laying, of which something will afterwards be said. When an earth- worm is halved with the spade it does not necessarily die, for the head portion may grow a new tail, while a decapitated worm has even been known to grow a new head and brain. The earthworm is much persecuted by numerous enemies, e.g., centipedes, moles, and birds. The male reproductive organs are always infested by unicellular parasites Gregarines of the genus Monocystis, and little threadworms seem almost always to occur in the excretory tubes. Form and External Characters. The earthworm is often about six inches long, with a pointed head end, and a cylindrical body rather flattened posteriorly. The successive STRUCTURE OF THE EARTHWORM. 189 rings seen on the surface mark true segments. The mouth is over- arched by the most anterior (pre-oral) segment, while the food canal terminates at the blunt posterior end. The skin is covered by a thin transparent cuticle, traversed by two sets of fine lines which break up the light and produce a slight irridescence. On a region extending from the 3 1st to the 38th ring, the skin of mature worms is swollen and glandular, forming the clitellnm or saddle, which helps the worms as they unite in pairs, and perhaps forms the slimy stuff which hardens into cocoons. The middle line of the back is marked by a special red- ness of the skin. On the sides and ventral surface, we feel and see four rows of tiny bristles or setae, which project from little sacs, are worked by muscles, and assist in locomotion. These bristles are fixed like pins into the ground, at times so firmly that even a bird finds it difficult to pull the worm from its hole. As each of the four longitudinal rows is FIG. 60. Anterior region of Earthworm. (After HERING.) Note the eight setae (s) on each segment. R-S f -, Spots between 9-10, IO-TI, indicate openings of recep^acula seminis ; Ovd., openings of oviducts on segment 14 v.ct., openings of yasa deferentia on segment 15. double, there are obviously eight bristles to each ring. On the skin of the ventral surface, there are not a few special apertures, which should be looked for on a full grown worm, but careful examination of several specimens is usually necessary. Almost always plain on the I5th ring are the two swollen lips of the male ducts, less distinct on the 1 4th are the apertures of the oviducts through which the eggs pass, while on each side, between segments 9' and 10, 10 and n, are the openings of two receptacula seminis or spermathecce into which male elements from another earthworm pass, and from which they again pass out to fertilise the eggs of the earthworm when these are laid. Each segment contains a pair of excretory tubes, which have minute ventral-lateral apertures, while on the middle line of the back, between every two adjacent rings, there are minute pores, through which fluid from the body cavity may exude. 190 SEGMENTED WORMS OR ANNELIDA. Skin and Bristles. Outermost lies the thin cuticle, on which intersecting lines produce interference of light and irridescence. Like any other cuticle, it is produced by the cells which lie beneath, and it is perforated by the apertures previously mentioned. The epidermis clothing the worm is a single layer of cells, of which most are simply supporting or covering elements, while many are slightly modified, as glandular or mucous cells, and as nervous cells. As the latter are connected with afferent fibres which enter the nerve cord, the skin is diffusely sensitive. In a few species the skin is slightly phosphorescent. The bristles, which are longest on the genital segments, are much curved, and lie in small sacs of the skin, in which they can be replaced after breakage. Muscular System. The earthworm moves by the contraction of muscle cells, which are arranged in hoops underneath the skin, and in longitudinal bands more internally. The special muscles about the mouth and pharynx have considerable powers of grasping, while less obvious muscular elements occur in the wall of the gut, in the partitions which run internally between the segments, and on the outermost portions of the excretory tubes. The Body Cavity. Unlike the leech, the earthworm has a very distinct body cavity, through the middle of which the gut extends, and across which run the partitions or septa incompletely separating successive segments. In this cavity there is some fluid with cellular elements, of which the most numerous are yellow cells detached from the walls of the gut. Possible communications with the exterior are by the dorsal pores, and also by the excretory tubes which open internally into the cavities of the segments. The Nervous System. Along the middle ventral line lies a chain of nerve centres or ganglia, really double from first to last, but compactly united into what to unaided eyes seems a single STRUCTURE OF THE EARTHWORM. 191 cord. As the segments are very short, the limits of the successive pairs of ganglia are not very evident, especially in the anterior region, but they are plain enough on a small portion of the cord examined with the microscope, when it may also be seen that each ollhe pairs of ganglia gives off nerves to the walls of the body. Anteriorly, just behind the mouth, the halves of the cord diverge and ascend, forming a ring around the pharynx. They unite above in two dorsal or cerebral ganglia. These form the earth- worm's " brain," and give off nerves to the adjacent pre-oral segment or prostomium, on which are numerous sensitive cells. These, coming in contact with many things, doubtless receive impressions, which are transmitted by the associated nerves to the "brain." As Mr. Darwin observed that earthworms seized hold of leaves in the most expeditious fashion, taking the sharp twin leaves of the Scotch fir by their united base, we may credit the earthworms with some power of profiting by experience ; moreover, as they deal deftly with leaves of which they have no previous experience, we may even charitably grant them a modicum of intelligence. From the nerve collar uniting the dorsal ganglia with the first pair on the ventral cord, nerves are given off to the pharynx or gut, forming what is called a " visceral system." The earthworm has no special sense organs, but we have just mentioned sensitive cells, which are particularly abundant on the head end of the worm. By them the animal is made aware of the differences between light and darkness, and of the approaching tread of human feet, not to speak of the hostile advances of a hungry blackbird. The sense of smell is also developed. The afferent or sensory nerve fibres from the nervous cells of the skin enter the nerve cord and bifurcate into longitudinal branches, which end freely in the nearest ganglia. In this the earthworm's nervous system suggests that of Vertebrates. Two facts in regard to minute structure deserve attention. The nerve cells, instead of being confined to special centres or ganglia, as they are in Arthropods, occur diffusely along with the nerve fibres throughout the course of the cord. Along the dorsal surface of the ventral nerve cord there run three peculiar tubular fibres, with firm walls and clear contents. These " giant fibres," which do not seem to be nervous, but are rather supporting elements, have been dignified by the name of neurochord. 192 SEGMENTED WORMS OR ANNELIDA. Alimentary System. Earthworms eat the soil for the sake of the plant debris which it may contain, and also, indeed, because they must swallow as they tunnel. In eating they are greatly helped by the muscular nature of the pharynx, whence the soil passes down the gullet or oesophagus, first into a swollen crop, then into a strong walled grinding gizzard, and finally through a long digestive and absorptive stomach intestine. On the gullet are three pairs of ceso- phageal or calciferous glands the products of which are limy and able to affect the food chemically, probably counteracting the acidity of the decaying vegetable matter. The long intestine has its internal surface increased by a dorsal fold, which projects inwards along the whole length. In this " typhlosole," and over the outer surface of the gut, the yellow cells are crowded. There is no warrant for calling these hepatic or digestive. Structurally they are pigmented cells of the peritoneal epithelium, which here, as in most other animals, lines the body cavity and the outside of the gut. As to their function we know that they absorb particles from the intestine, and go free into the body cavity, whence, as they break, up, 4heir debris may pass out by the excretory tubes. When a worm has been made to eat powdered carmine, the passage of these useless particles from gut to yellow cells, from yellow cells to body cavity, and thence out by the excretory tubes, has been traced. Various ferments have been detected in the gut, a diastatic ferment turning the starchy food into sugars, and others peptic and tryptic not less important. The wall of the stomach intestine from without inwards, as may be traced in sections, is made up of pigmented peritoneum, muscles, capillaries, and an internal ciliated epithelium. In the other parts of the gut the innermost lining is not ciliated, but covered with a cuticle. Vascular System. The fluid of the blood is coloured red with haemoglobin, and contains small corpuscles. Along the median dorsal line of the gut a prominent blood vessel extends, another (supra-neural) runs along the upper surface STRUCTURE OP THE EARTHWORM. 193 of the nerve cord, another (infra-neural) along the under surface, while two small lateral-neurals pass along each side of this same cord. All these longitudinal vessels, of which the first three are most important, are parallel with one another ; the first three meet in an anterior network on the pharynx ; the dorsal and the supra-neural are linked together in the region of the gullet by five or six pairs of contractile vessels or " hearts." The precise path of the blood is not V c.m -l.rti c.ce 1 11. c FIG. 61. Transverse section of Earthworm. (After CLAPAR&DE.) r., Cuticle ; e., epidermis: c.m., circular muscles ; /.;., longi- tudinal muscles ; s., a seta; c.ae., ccelome ; y.c., yellow cells ; T., typhlosole ; z/.e/., ventral blood vessel; s.n.v., sub-neural vessel below nerve cord ; d.v., dorsal vessel. known, but the distribution of vessels to skin, nephridia, and alimentary canal is readily seen. Respiration is effected by the distribution of blood on the general surface of the skin. 13 194 SEGMENTED WORMS OR ANNELIDA. Excretory System* As we have mentioned, small particles may pass from the gut to the body cavity, and thence to the exterior by the excretory tubes. There is a pair of these little kidneys, nephridia or segmental organs, in each segment except the first four. Each opens internally into the segment in front of that on which its other end opens to the exterior. They remove little particles from the body cavity, but probably get finer waste products from the associated blood vessels. Nephridia occur in many animals, in most young Vertebrates as well as among Invertebrates, but they are never seen more clearly than in the earthworm. When a nephridium is carefully removed, along with a part of the segment septum through which it passes, and examined under the microscope, the following three parts are to be seen : (a) an internal ciliated funnel, (b) a trebly coiled ciliated tube, at first transparent then glandular and granular, and (c) a muscular duct opening to the exterior. Minute particles swept into the ciliated funnel pass down the ciliated coils of the tube, and out by the muscular part which opens just outside of the ventral bristles. The coiled tube consists in part at least of a series of intracellular cavities, that is to say, it runs through the middle of the cells which compose it ; the external muscular portion arises from an invagina- tion of skin. Reproductive System The earthworm is hermaphrodite, and its reproductive organs are somewhat difficult to demonstrate with com- pleteness. To see them it will be necessary to dissect several earthworms with special attention to individual parts. (a) The Male Organs consist of two pairs of testes, three pairs of seminal vesicles, and a paired vas deferens. (1) The testes lie near the nerve cord on the septa between segments 10 and n ; each is "a white translucent body of irregular quadrangular form, rarely more than one- tenth of an inch in diameter." (Fig. 62, T.) (2) Mother sperm cells, which give rise by division to young spermatozoa, pass from the testes to the much lobed semina vesicles, where the spermatozoa are matured. STRUCTURE OF THE EARTHWORM. 195 These vesicles (Fig. 62, s.v.) are very prominent, and seem to be outgrowths of the septa between segments nine to twelve. Among the spermatozoa there are parasitic Gregarines (Monocystis) in various stages of development. (3) The spermatozoa pass from the seminal vesicles into two vasa deferentia or male ducts. These open to the exterior on the i5th segment. Each vas deferens bears two ciliated funnels, which collect spermatozoa in segments 10 and n, and soon unite in one duct. (b] The Female Organs consist of two ovaries, and two oviducts each of which has a side receptacle for the eggs. (i) The two ovaries are small bodies situated near the if *VIII fly r Tx^- 5 H-i :> ^\x <* ' ^ :^-x-i- 5 XII :__, T;.O XIII 1 XIV 1 iXV FIG. 62. Reproductive organs of Earthworm. (After HERING.) N. t Nerve cord; J 1 ., anterior testes ; S., sacs of setae ; R.S., recep- tacula seminis ; s., seminal funnels; v.o., vas deferens; ovd., oviduct; ov., ovary; s.v., seminal vesicles; VII1.-XV.) segments. nerve cord on the septum between segments 12-13. Each is pear shaped, the stalk of the pear being a string of ripe ova. They are more likely to be seen than the testes. (2) The two oviducts open internally on the anterior face of the septum between 13-14, and externally on the ventral surface of segment 14. Into the wide ciliated in- ternal mouths, which lie opposite the ovaries, the ripe eggs pass. 196 SEGMENTED WORMS OR ANNELIDA. (3) The egg sac or receptaculum ovorum, near the internal mouth of each oviduct, is a posterior diverticulum of the septum between segments 13-14. Within it a few mature ova are stored. (c) Two pairs of receptacula seminis or spermathecae receive spermatozoa from another earthworm, and liberate them to fertilise the eggs^f this one. They are white globular sacs, opening in the grooves between segments 9-10 and 10- 1 1. According to some, these spermathecae not only receive and store spermatozoa, but make them into packets or spermatophores. Others say that the glands of the clitellum make these packets. At any rate minute thread-like packets of spermatozoa are formed, and a pair of them may often be seen adhering to the skin of the earthworm about the saddle region. When two worms unite sexually they lie apposed in opposite directions, the head of the one towards the tail of the other. What happens is that spermatozoa of the one pass into the receptacula of the other. When the eggs of an earthworm are liberated they are surrounded by a sheath of gelatinous stuff said by some to be secreted by the saddle. As this is peeled off towards the head a spermatophore is also enclosed. Development of the Earthworm. Many cocoons are made about the same time, and each contains numerous ova, and also packets of sperms, so that fertilisation takes place outside the body. These cocoons are buried in the earth a few inches below the surface. They measure about a quarter of an inch in length. The favourite time for egg laying is during the spring and summer, though it may be continued throughout the whole year. The earthworm of the dung heap (L. fatidus) makes this a habit, induced probably by the warmth of its habitat. Of the many ova of the earthworm L. terrestris, only one comes to maturity, while of L. fatidus a few, and of L. communis two may do so. But in the last species the two embryos are often twins formed from one ovum, separa- tion taking place at the gastrula stage. DEVELOPMENT OF THE EARTHWORM. 197 The whole process of growth, until leaving the egg, lasts from two to three weeks, the time varying however with the temperature. The ovum is surrounded by a vitelline membrane, and is laden with yolk granules. It seems that several polar cells are formed, probably by division of the two primary ones separated from the ovum. Segmentation is slightly unequal, and exhibits considerable variation even within the limits of a species. In about twenty-four hours, a nearly spherical, one layered blastosphere or blastula is formed. It consists of only about thirteen cells. During the next twenty-four hours the cells increase in number rapidly, but the blastula remains one layered. Two cells lying together do not take part in this division ; they are rather larger than the rest, and their inner ends project into the cavity and are soon cut off. Gradually these large cells begin to sink in, giving rise to more daughter cells, and at last are quite included in the cavity. Thus there arise two parallel rows of cells within the blastula, and these define the longitudinal axis of the embryo. This is the beginning of the mesoblast which will form all the muscles of the trunk, and which thus takes origin from two primary mesoblasts. After five to six pairs of secondary mesoblasts have been formed, the blastula begins to flatten, and to elongate, becoming an oval disc. The cells of the lower surface become clearer, and the hypoblast is thus defined. The cells of the upper surface are smaller, and become very much flattened ; they compose the epiblast. The mesoblasts lie side by side near one end, forming two rows extending forwards and downwards, but divergent, because of the flattening of the blastula. The hypoblast now becomes concave, and thus the blastopore arises, occupying the whole of the lower surface. The sides close in and the blastopore becomes a slit, which further closes from behind forwards leaving only a small opening, the future mouth. During these processes the cells at the anterior tip of the blastopore, which will give rise to the praeoral lobe, undergo no change, but the mesoblast has been active. As gastrulation proceeds, the mesoblast rows grow forwards and upwards until they come near each other above the I 9 8 SEGMENTED WORMS OR ANNELIDA. r p.c. AI-. fcEn anterior tip of the blastopore, while their middle portions are carried downwards until they lie on the ventral sur- face. Over them the epi blast is thickened in two bands. Two longitudinal rows of epiblast cells near the anterior end, and ending behind in large cells, sink in just as the primary mesoblasts did. The thickening now extends ventrally until the two bands meet, and passing into the blastopore forms the stomatodaeum. Even before this the embryo has begun to swallow the albumen in which it floats. There are now two lateral bands of cells called the germ bands, composed of three layers : outside is the thickened epi- blast, next, the rows of cells which sank in, and inmost the mesoblast rows. The mesoblast rows have met in the middle line by dividing and widening out into a pair of flattened plates, but they still end behind in the two primary mesoblasts. Ccelomic cavities develop in the plates, and the anterior ends meet above the mouth. The epiblastic rows which sank in (there were eight of them, four on each side of the median line and each ending in a large mother cell) go on growing. The mother cells are apparently carried backwards as the em- bryo lengthens, leaving a trail of daughter cells behind them. The cells so formed also divide, the embryo rapidly lengthening and finally becoming FIG. 63. Stages in the development of Earthworm. (After WILSON.) 1. 2-celled stage, p.c, polar bodies. 2. Blastula, m, a primary meso- blast. 3. Gastrula stage, EC, ectoderm or epiblast ; En, endoderm or hypoblast ; in, mesoblast cells. 4. Longitudinal section in late gastrula stage, ec, ectoderm ; en, endoderm ; M, mouth ; st, stoma- todaeum ; m, primary mesoblasts ; nb, neuroblasts ; nc, nerve cord J n, nephridioblasts. DEVELOPMENT OF THE EARTHWORM. 199 vermiform. The two inner rows (neuroblasts) give rise to the nervous system, the next two rows on either side (nephridioblasts) form parts of the nephridia, while of the fourth row nothing definite is known. Each row, ending behind in a single cell, widens out and deepens as it is traced forwards, the neuroblasts are much further forwards than the mesoblasts, with the nephridioblasts just behind them. The neural and mesoblastic rows can be traced round the mouth and help to form the prostomium, the others fade away at the sides of the stomatodaeum. The mesoblast rows grow to meet one another on the median dorsal line. Let us sum up this complex history : Fertilised ovum. Blastosph or blastuh with primitive mesoblasts. Epiblast or Ectoderm '(a) The original outer layer becomes the epidermis. The secondary inner strat- um consists of two rows of neuroblasts which form the nervous system, and of four rows of nephridio- blasts which form parts of the nephridia. Mesoblast " mesoblasts." Lining of General Development of the Organs. Though it will involve a slight repetition, we shall now describe the origin of the various organs. The skin arises from the original outer wall of the gastrula. The "setigerous glands," within which the setts develop, and from which they push their way to the exterior, arise partly from the rows of cells started by the nephridioblasts, and partly in all probability from the outermost of the four cell rows previously mentioned. The double ventral nerve cord arises from the neuroblasts. The two cerebral ganglia originate, according to Kleinenberg, independently of the ventral cord from a median unpaired apical plate of ectoderm, while according to Wilson they arise along with the ventral cord, and have their founda- tions in the thickened anterior end of each of the two neural rows. The history of the excretory system is complex, (a) At the anterior end of young embryos, a group of ectoderm cells, dorsal in position, forms a larval excretory organ, which wholly disappears in later stages. (b] Next appear two ciliated canals in the anterior region, closed inter- 200 SEGMENTED WORMS OR ANNELIDA. nally, opening on the head. These are known as "provisional nephridia " or "head kidneys." They degenerate as the permanent excretory organs develop, (c) The numerous permanent nephridia are for the most part ectodermic, arising from the rows of nephridial cells already described. Two parts of each nephridium, however, have a meso- blastic origin, viz., the innermost part or the ciliated funnel, and the peritoneal investment which ensheaths the whole organ. By the invagination of the blastosphere, a globular gastrula cavity is formed. This forms the archenteron, the future mid gut, and elon- gates with the growth of the embryo. To the completion of the entire alimentary canal, however, two other processes are necessary, an in- tucking of ectoderm from in front the stomatodaum or "fore gut " which pushes the archenteron backwards and forms the future pharynx, and a similar in-tucking of ectoderm from behind the proctod&um or <; hind gut" which meets and fuses with the archenteron, and forms the anus and a small portion of the posterior gut. The mesoderm begins with the two primary mesoblasts already described. These multiply and form mesoderm bands, which, insinua- ting themselves between ectoderm and endoderm, proceed to surround the gut. At the same time, some of the mesoderm cells become migratory, wander on to the head, and also surround the gut, before the final trunk musculature is completed. The migratory mesoblasts of the trunk appear to form a special larval musculature precociously devel- oped, in order to enable the embryo to manage the enormous mass of albumen (absorbed from the capsule) with which its body is distended. The mesoderm bands grow in strength, and form a complete ring encircling the archenteron. Origin of the body cavity. The mesoderm bands, growing in strength, become tw r o layered. These two layers separate, the inner (splanchnic) cleaving to the gut, the outer (somatic) clinging to the body wall. The space between them is the body cavity or cctlome. But as the separa- tion of somatic and splanchnic layers takes place, partitions are also ' formed transversely, to become the septa which partition off the body cavity into a series of segments. The cavity of the pre-oral segment or prostomium differs somewhat from that of the others, being from the first unpaired, instead of including two lateral cavities one on each side of the gut. As to the blood vessels, the ventral or sub-intestinal appears first, as a space between the wall of the archenteron and the underlying meso- derm ; the dorsal vessel has a double origin, arising from the fusion of two lateral vessels which develop like the ventral. The important point is, that the blood vessels are at first long lacunar spaces, which gradually acquire definite walls. By and by the " hearts " and other complications in the vascular system appear. The reprodtictive organs, though probably arising from cells which have kept to some extent apart from the formation of the embryo, certainly appear in association with the mesoderm. The above is the account of the development of Lumbricus given by Wilson ; another investigator, Bergh, differs from Wilson in several important points. First, with regard to the nomenclature of the con- stituents of the germ bands. STRUCTURE OF ARENICOLA. 201 WILSON. Two inner primitive Neuroblasts. cells. Next three cells on either side. Posterior cell of each side. /Two nephridioblasts, \ one lateral cell, Primitive mesoblast. BERGH. Neuroblasts. f Outer or anterior \ myoblasts. Inner or posterior myoblast. According to Bergh the germ band consists originally of three cells on each side, the neuroblast, the primary inner myoblast, the outer myoblast. The primary inner myoblast later gives origin to the three inner myoblasts, nephridioblasts, and lateral cell of Wilson. Further, Bergh states that at an early stage a "nerve plexus" arises on the mid-ventral line, probably from the ectoderm, and that this unites with the neuroblastic rows to produce the nervous system of the adult. Type of POLYCH^TA. The Lob Worm (Arenicola piscatoruni). Habits. On the flat sandy beach uncovered at low tide, the " castings" of the lob worm are very numerous. There the fishermen seek the worms for bait, and have to dig deep, for the burrowers rapidly retreat far into the sand. The burrows of the lob worm are cylindrical tubes, lined by a yellowish green secretion, and the surrounding sand is often discoloured by some change in which the organic juices reduce the iron constituents to lower oxides. The tubes are at first vertical, and afterwards oblique or hori- zontal. The lob worm burrows like the earthworm eating the sand for the sake of the organic particles or small organisms which it contains. The sandy castings, which pass from the end of the food canal, and are got rid of at the mouth of the tube, fall into spiral coils. When getting rid of the castings, the worm lies with its tail upwards and its head downwards, or with its body bent like a bow; when the 202 SEGMENTED WORMS OR ANNELIDA. tide comes in, the mouth may protrude. The animal is able to turn in its burrow. The young are pelagic, and it is said by some that the adults are active and swim about at certain seasons. External Appearance. The lob worm varies in length from eight inches to a foot, and at its thickest part is about half an inch in diameter. There are three regions in the body : (a) the anterior seven segments, of which all but the first have bristles ; (b} the gill bearing region of thirteen segments ; (c) the thinner posterior part of variable length, without either FIG. 64. Arenicola piscatorum. (After CUNNINGHAM and RAM AGE.) Note anterior protrusion of mouth ; setae on anterior region ; setae and gills on median region ; thinner tail region often longer than shown. bristles or gills. The head lobe is very smallj; there are no tentacles or eyes. Anteriorly a soft proboscis is protruded. Skin and Muscles. Each segment is marked by several rings ; there are numerous warts on the posterior region. Most externally lies the cuticle, then the pigmented epidermis, then the circular and the longitudinal muscle fibres. Appendages. Unlike many of the marine Annelids which have on each segment well-developed outgrowths or parapodia, divided into a dorsal notopodium and a ventral neuropodium, Arenicola has very rudimentary appendages. This reduc- tion of appendages must be associated with the animal's mode of life; the same is true of many tube inhabiting worms. The first segment has no trace of appendages, STRUCTURE OF A RE NICOLA. 203 the next nineteen have rudiments. The dorsal part con- sists of a tuft of bristles, whose bases are enclosed in a sac ; the ventral part, separated by a short interval, bears several hooks. Nervous System. The nervous system is in its general features like that of the earthworm, but ganglia are not developed. In the ventral nerve cord, the ring round the gullet, and the slight cerebral enlarge- ment which represents a brain, nerve cells occur diffusely scattered among the nerve fibres. Along the dorsal sur- face of the nerve cord run two "giant fibres " like those in the earthworm. In some species at least, the head lobe is distinctly sensory and there are two ciliated FIG. 65. Anterior "neck organs." Otherwise sense organs are part of nervous system represented only by a pair of otocyst sacs, one in Arenicola. (After on each side of the oesophageal nerve ring. VOGT and YUNG.) These sacs, like those which occur in many other Invertebrates, seem to have to do rather c., Cerebral part on -, , ,. i . r ,_, . ,, dorsal surface ; ee.r. with tne direction of the animals movements oesophageal ring; g.\ than with hearing. Prof. Ehlers notes an in- nerve t; rd'*'f ' 7^! terestin g series : In A. Claparedii, there are nervesT^.', otocyst!* simply two open grooves ; in A. marina, the sacs have open necks and contain foreign particles ; in A. Grubii and A. antillensis, the sacs are closed and con- tain intrinsic otoliths of lime. Food Canal. The mouth is at the end of a protrusible cup-like proboscis; the gullet has smooth walls, and bears an an- terior and a larger posterior pair of glands which secrete a yellowish fluid perhaps digestive; the succeeding part of the gut is covered with yellow cells and many blood vessels, and is divided into rings ; the terminal portion is full of sand from which the nutritive matter has been absorbed ; the anus is at the very end. The Body Cavity. The body cavity is spacious, except in the tail region, and contains a fluid. Anteriorly there are three transverse, 204 SEGMENTED WORMS OR ANNELIDA. partly muscular, partitions or mesenteries which moor the gullet ; in the tail region there are many such ; the median part of the gut swings freely. Posteriorly there are also oblique partitions which divide the segments into a median and two lateral chambers. The Vascular System. The blood has a bright red colour. It flows forward in a dorsal vessel, running along the mid-dorsal line of the b s ,/> FIG. 66. Dissection of anterior region of Arenicola. (After COSMOVICI.) M.2., m.3. Second and third mesenteric septa ; #., ventral nerve- cord ; b.s., bristle sac ; z/.z/., ventral vessel ; n.6., sixth nephridium ; z., intestine; l.v., lateral vessel; d.v.^ dorsal vessel; ^., heart; /., one of the two larger oesophageal glands ; a*., oesophagus. gut, backward in a ventral vessel below the gut. Two sub-intestinal vessels lie between the ventral vessel and the gut, and receive tributaries from the anterior gills. (Some deny that the sub-intestinal vessel is double.) On each side of the digestive part of the gut there is a lateral vessel. Just behind the posterior pair of cesophageal glands lies a very contractile heart. It consists of two lateral chambers STRUCTURE OF ARENICOLA. 205 or ventricles, each of which receives blood from the dorsal vessel, from a sub-intestinal vessel, and from a lateral vessel, and drives blood into the ventral vessel. Each of the lateral vessels before entering the heart expands into a kind of auricle. The longitudinal vessels are all connected by transverse branches. From the ventral vessel arise afferent branchial vessels. From the seven posterior gills efferent branches enter the dorsal vessel ; while those from the six anterior gills join the sub-intestinals. Each efferent vessel gives off FIG. 67. Cross section of Arenicola. (After COSMOVICI.) E. Epidermis; c.m., circular muscles ; /.?., longitudinal muscles; b.c., body cavity ; gl., gill; s., setae; ./., nephridial pore; a.br., afferent branchial ; e.lr., efferent branchial; ., ventral nerve-cord with blood vessels above; d.v., dorsal vessel; /.#., lateral vessel; s.z'.v., sub-intestinal vessel ; v.v., ventral vessel ; -., gut. a branch to the skin, while the dorsal and sub-intestinal vessels give off numerous branches to the walls of the gut. It seems that the flow of the blood is not always quite the same. Respiratory Sy stern. There are thirteen pairs of gills. Each is a tuft of thread-like branches, through the thin walls of which the 206 SEGMENTED WORMS OR ANNELIDA. red blood shines. As the papillae on the proboscis are hollow and contain vessels, they are doubtless of respira- tory significance. Indeed, the gills may be regarded as exaggerated papillae. Excretory System. In the anterior region, from the fifth to the tenth seg- ments, there are six pairs of nephridia. Each consists of three parts a funnel opening into the body cavity, a glandular portion, and a bladder communicating with the exterior. Reproductive System. The sexes are separate and similar. The reproductive organs are very simple, and lie on the peritoneal mem- brane of the body cavity. They are developed in close association with the nephridia. The reproductive cells are liberated into the body cavity, and there matured. In August and September they pass out by the nephridia. Little is known in regard to the development, beyond the fact that the young are for a time free swimming pelagic forms. Development of Polychceta. As an example of the development of the marine Chsetopods, we may take Eupomatus, which has been investigated by Hatschek. Here segmentation is complete but somewhat unequal, and results in the formation of a blastula, with its upper hemisphere composed of small (ectodermic) cells, and the lower of large (endodermic) cells. Among these latter are two spherical cells the primitive mesoblasts. Invagina- tion takes place in the usual way to form a gastrula, the primitive meso- blasts divide and form mesoblastic bands. During these processes the external form has altered considerably. The apical (aboral) region of the gastrula becomes tilted forward, an ectodermic invagination arises posteriorly, and uniting with the archenteron produces hind-gut and anus, while a similar insinking anteriorly, in the region of the blasto- pore, forms fore-gut and mouth. The larval gut so formed has a distinct ventral curve. Cilia appear on the surface at an early stage, and now form a distinct pre-oral ring, and also a less constant post-oral ring. At the apex of the pre-oral region an ectodermic thickening takes place, this gives rise to an apical ganglion with which sensory structures are often associated. The mesodermic bands give rise to muscle cells used in swimming, and also to the " head kidneys " a pair of larval ex- cretory tubes. The larva so formed is a typical Trochosphere, such as occurs in the great majority of Polychseta, in a more or less modified It FIG. 68. Development of Polygordius. (After FRAIPONT.) #., Mother sperm cell ; 3., c., sperm morulae ; d., spermatozoa. i. Ovum with large nucleus ; 2. 2-cell stage ; 3. 4-cell stage ; 4. blastosphere ; 5. gastrula ; ac., archenteron ; 6. closure of gastrula mouth or blastopore ; 7. for- mation of stomatodaeum^z 1 .), and proctodaeum (/r.), invaginated to meet archenteron (#.) j 8. complete gut formed; 9. elongation of larva; ap. sfi., apical spot; "/., ciliated ring; neph., primitive nephridia ; 10. formation of posterior segments; ii. Form of adult Polygordius. 208 SEGMENTED WORMS OR ANNELIDA. guise in many other worm-types, and also in Molluscs. We may summarise the chief characters of the Trochosphere thus : (1) There is a prominent pre-oral region with an apical ganglion and a ring of cilia. (2) The gut has a distinct ventral curve and a threefold origin. (3) The larval body cavity is simply the persistent segmentation cavity, and in it posteriorly lie the primitive mesoblasts. The Trochosphere is a free swimming pelagic larva which, among worms, corresponds largely to the future head region of the adult. Its metamorphosis into the adult probably takes place in the most primitive fashion in the little worm Polygordius. We shall, therefore, follow it there (Fig. 68). In the larva, which is a typical Trochosphere, the first sign of segmentation appears in the bands of mesoblast. These become divided into successive segments, while at the same time the posterior region of the larva elongates greatly, carrying the larval gut backwards with it. Meanwhile, a cavity appears in each of the mesoblastic segments. These cavities, taken together, form the adult body cavity ; the outer and inner walls form the somatic and splanchnic layers ; the posterior and anterior walls of adjacent segments fuse to form the septa of the adult worm ; the inner (splanchnic) walls of the primitive segments on each side fuse above and below the gut to form the dorsal and ventral sup- porting mesenteries of the gut. The head region is at first dispropor- tionately large, but later by an independent process of growth becomes reduced. The larva abandons its pelagic life and becomes adult. Comparing the development of Polychseta with this, we find that the Trochosphere is often modified, and that segmentation tends constantly to appear at an earlier stage. As a further step in the same direction, we may note that in some Polychreta the Trochosphere stage is no longer recognisable as such. A general contrast of the modes of Development in different Annelids. A. B. " Larval " Types " Fcetal " Types as in as in marine ChiX3topods, Earthworm, Leech, c. Polygordius, &c. Development indirect. Development direct, within egg A free swimming Trochosphere capsule ; Trochosphere stage almost stage, with trunk almost or wholly or wholly suppressed. suppressed, with head region greatly developed, with adapta- ^ A N tions to free marine life. Lumbrtcus type Clepsine type with little nutri- with much nutri- tive material in tive material in ovum, with gas- ovum, with gas- trula formed by trula therefore invagination (em- formed by over- bolic). growth (epibolic.) CLASSIFICATION OF CH&TOPODA. 209 General Survey of Chcetopoda. I. Oligochccta. Of Lumbricus there are many species, e.g., the common earthworm L. terrestris, the dunghill worm L. fcetidus, and L. com munis or trapezoides, whose ova usually form twins. We may conveniently include under the title " earthworms " a great array of animals more or less like Lumbricus, and usually described as terricolous Oligocketa. These are arranged by Beddard in four main groups LUMBRICINI, GEOSCOLEC.INI, ACANTHODRILINI, and EUDRILINI, with a divergent group MONILIGASTRES. It is enough for us to notice here that the modern classification is chiefly based on the modifications of the excretory system. The largest " earthworm " is a Tasmanian species Megascolides gippslandicus measuring about six feet in length, said to make a gurgling noise as it retreats underground. To these must then be added a number of families, Tubificida, Enchytraidce, &c., which live in mud and water, and are often called limicolous Oligochoeta. Of these a very common representative is the little river worm Tubifex rivulorum, often found in the mud of brooks, and well suited in its transparency and small size for microscopic examination. Also notable is the fresh water Nats, with remarkable powers of asexual budding. Another interesting ally of Tubifex is Branchiura, which has paired contractile gills on each of the posterior segments of its body, thus resembling a Polychaete. The leech-like Branchiobdella, which is parasitic on the crayfish, is apparently an abnormal Oligochaete. II. Polychata. Living in surroundings usually very different from those of the more or less subterranean earth- and mud-worms, the marine Polychoeta have a richer development of external structures, and a more complex life history. From the sides of the body rings distinct outgrowths form the first genuine legs. These, known as parapodia, bear bundles of firm bristles, and are typically divisible into a ventral neuropodium and a dorsal notopodium. Each of these is usually furnished with a probably tactile process, the two being known respectively as the notopodial cirrus and the neuropodial cirrus. With the notopodium the first true gills, which contain prolongations of the body cavity, are often associated, but the respiratory plates which occur in the sea mouse, &c. , are probably metamorphosed dorsal cirri. Parapodia are absent from the anterior region, but this is frequently well furnished with tactile cirri, as well as with eyes, " ear sacs," and other sensitive structures. The eyes show an interesting series of gradations from simple pigment spots to very com- plicated structures (e.g., Alciope], exhibiting cornea, crystalline lens, retina, &c. In many cases in these marine worms the blood is red as it is in most Oligochsetes, but it may be colourless (Aphrodite], green (Sabella), or yellow. The pigment is usually dissolved in the plasma, and its variations in character and amount among nearly allied forms are of great interest to the comparative physiologist. The gut is frequently branched and of large calibre. In some cases (CapitellidDe) it possesses an accessory communicating tube (Nebendarm) which is of interest, U SEGMENTED WORMS OR ANNELIDA. having been compared to the notochord of Ve-rtebrates. The sexes are usually separate, the reproductive organs simple and devoid of accessory structures. The nephridia function as genital ducts. There is a metamorphosis in development. (a) Some of these marine Polychcetes lead a free and more or less active life, crawling between tidemarks or on the sea bottom, burrowing in the sand, or swimming in the open water. These Errantia have well- developed appendages, and a large pre-oral segment, and are generally furnished with eyes and well-developed antennre. Gills are usually associated with the dorsal parts of the parapodia. Most of them feed on other animals, and have sharp "horny jaws," while the anterior part of the gut is protrusible as a proboscis. Nereis and Nephthys are two common genera, species of which may be unearthed by digging in the sand close to rocks, though at times these or other species are seen swimming freely. The sea mouse, Aphrodite, has irridescent bristles, a feltwork of matted hair covering large gill plates which lie along its back, a very large muscular pharynx, and a gut with numerous irregular branches extending throughout the body. A very common shore form, a little like a small Aphrodite, is Polynoe. As an actively errant worm, with well developed eyes, Alciope may be noted, and the family of Syllicke is re- markable for the unusually prolific asexual budding, which sometimes results in a chain or even an irregular branched aggregate of individuals. As the cuticle is often irridescent, and as the red blood may shine through the skin, these marine worms are frequently beautiful. The list of nymphs and goddesses has been the source of such titles as Nereis, Aphrodite, Eunice, and Hermione, and one can almost believe the legend, according to which a specialist on Errantia christened his daughters after his seven favourites. (b] Other marine Polycheeta, however, lead a more sluggish life within various kinds of tubes, limy, sandy, papery, or gelatinous. As one would expect, their parapodia are minute, apt to degenerate, and often used solely for clambering within the tube. The pre-oral region is small, but the anterior rings usually bear gills, cirri, and tentacles, often in rich profusion. These Sedentaria rarely have a protrusible pharynx, and FIG. 69. Parapodium of a Marine Polychrete, Heteronereis. (From QUATREFAGES. ) A, Notopodium ; B, neuropodium ; a, notopodial cirrus ; f, neuropodial cirrus ; b, c, g; gill plates ; e, i, tufts of bristles. CLASSIFICATION OF CHALTOPODA. 211 never " jaws." Most of them feed on minute Algse swept in by the cilia on the tentacles and other structures about the mouth. The fisherman's lob-worm (Arenicola) burrows on the sandy shore like Lumbricus in the fields. Common also on the shore within a tube of glued sand particles is Terebella or Lanice conchilega^ where the ex- cretory tubes are partly united by a longitudinal tube in a manner sug- gestive of the segmental duct which connects the nephridia of a young Vertebrate. The twisted limy tubes of Serpula are common outside shells and all sorts of marine objects, and the animal bears a stopper or operculum, with which it closes the mouth of its tube, but through which it probably at the same time breathes. In deep water, within a yellow parchment -like tube, Chatopterus may be dredged, perhaps the strangest form of all. III. Echiuridce. In holes in the rocks on some of the warmer European coasts lives a curious "worm" Bonellia viridis, of a beautiful green colour, with a globular body and a long, grooved, anteriorly forked, pre-oral protru- sion. Such, at least, is the female, but the male is microscopic in size, hopelessly degenerate, living parasitically in or on his mate. The male resembles in some ways a Turbellarian, is mouthless and gutless, and little else than a migratory spermatophore. By means of cilia, it moves from one part of the female to another, and fertilises the eggs in a modified excretory tube, which serves the female Bonellia as a uterus. Here illustrated in extreme, we see the usual inequality (in size) between the sexes. Less abnormal than Bonellia are the genera Echiurus and Thal- assenia. In this small sub-order the adults have, at most, indistinct traces of the segments which the young forms exhibit. Nor are there parapodia, cirri, or gills, but setae are always represented (except in the male Bonellia} by two anterior bristles, and in Echiurus by posterior spines as well. The nerve cord is unsegmented, and there is but a slight anterior ring without a brain. The anterior part of the body forms a muscular, well-innervated, ciliated proboscis, with the mouth deeply situated at its base ; the gut is much coiled, bears a curious adjacent tube known as the "collateral intestine," and a pair of excretory "anal glands " opening into the body cavity by ciliated funnels. There is a terminal anus. There are dorsal and ventral blood vessels, and two or three pairs of nephridia, one or more of which function as reproductive ducts. The sexes are separate, and the reproductive elements are formed on the walls of the body cavity, into which they are liberated. There is a metamorphosis in development, the larvse differing from the adults in many ways, e.g., in being segmented. Appendix (i) to Chcetopoda. PRIMITIVE CHyETOPODS AND ANNELIDS (Archi-Choetopoda and Archi-Annelida). An aberrant Chsetopod type is represented by Saccocirrus, a small marine "worm" with many primitive characteristics. The body is 212 SEGMENTED WORMS OR ANNELIDA. segmented, and very uniform throughout ; the pre-oral region is small, but the mouth segment is large ; there are bundles of setae on the rings ; the nervous system remains embedded in the epidermis. More primitive, however, are the Archi-Annelida represented by Polygordius, Protodrilus, and Histriodrihis all marine. The small body is segmented and uniform ; there are no setre, parapodia, cirri, or gills, but the head bears a few tentacles ; as in Saccocirrus, the pre-oral region is small, and the segment around the mouth is large ; the very simple nervous system is retained in the epidermis. Polygordius is a thin worm, an inch or more in length, living at slight depths in sand or fine gravel, often along with the lancelet. It has a few external cilia about the mouth in a pair of head-pits, and sometimes on the body ; it moves like a worm, but has no bristles. It feeds like an earthworm, or sometimes more discriminatingly on uni- cellular organisms. The females are usually larger than the males, and in some species break up at sexual maturity. The development includes a metamorphosis, and the larvae seem to throw some light on the nature of the ancestral Annelids. They are ciliated, free swimming, light loving, surface animals, feeding on minute pelagic organisms, seeking the depths as age advances. According to some, the larva represents a primitive unsegmented ancestral Annelid with medusoid affinities ; according to others, the larval characteristics are adaptive to the mode of life, and without historic importance. Protodrilus is even smaller than Polygordius, with more cilia, mobile tentacles, and two fixing lobes on the posterior extremity ; the move- ments are Turbellarian-like, the reproductive organs hermaphrodite, the development direct. Histriodrihis is parasitic on the eggs of the lobster. Appendix (2) to Chatopoda. PARASITIC AND DEGENERATE CH^TOPODS. MYZOSTOMATA. The remarkable forms (Myzostoma) included in this small class, live parasitically on feather stars, on which they form galls. They are regarded as divergent offshoots from primitive Annelids, the larval form showing some distinctly Chaetopod characters. The minute disc-like body is unsegmented, and bears five pairs of parapodia, each with a grappling hook, with which five pairs of suckers usually alternate. There are also abundant cirri. The skin is thick, the body muscular, the nervous system is concentrated in a ganglionic mass, which encircles the gullet, and gives off abundant branches. There is a protrusible proboscis and a branched gut ; the mouth and anus are ventral. The ova arise in the reduced body cavity, and pass by three meandering oviducts to the anal aperture. The testes are paired, branched, and ventral, with associated ducts, which open anteriorly on the side of the body. The sexual relations are interesting, for one species is herma- phrodite and another unisexual, between which there is an intermediate species with ovaries and rudimentary testes. The hermaphrodite form may bear on its body dwarfish males, analogous to the complemental pigmies on some hermaphrodite barnacles. HABITS OF LEECHES. 213 Class DISCOPHORA or HIRUDINEA. Leeches. This class includes forms in which the body is elongated or flattened, devoid of appendages or bristles, and marked externally by rings, which are much more numerous than the segments as displayed in the internal structure. The body cavity is much reduced and communicates with the well- developed vascular system. The nephridia are numerous and segmentally arranged. There is always a posterior sucker, and the mouth is frequently suctorial also. Almost all are herma- phrodite, the male organs are numerous and usually segmentally arranged. The nephridia do not function as ducts for the reproductive organs. Leeches show several very distinct Annelid characters, but on the other hand differ from ringed worms and agree with flat worms in having suckers, in the absence of bristles, in the frequently flattened form and other features. It is impossible to say how far these resemblances with flat worms are due to the adoption of a peculiar mode of nutrition, but the evidence on the whole seems to be in favour of Annelid affinities. Most leeches are worm-like aquatic animals, with blood sucking propensities, but some live in moist soil, and others keep to the open surface, while the parasitic "vampire" habit, familiarly illustrated by the apothecary's ancient panacea, is in many cases replaced by carnivorous habits and predatory life. The medicinal leech (Hirudo) is typical of the majority, for it lives in ponds and marshes, and sucks the blood of snails, fishes, frogs, or of larger available victims. The giant leech (Macrobdella valdiviana\ said to measure 2\ feet in length, is subterranean and carnivorous, while the wiry land leeches (Hcemadipsa, &c.), of Ceylon and other parts of the East move in rapid somer- saults along the ground, fasten on to the legs of man or beast, and gorge themselves with blood. By attaching the head end by the mouth and loosening the tail sucker, then fixing the tail and extending the anterior region, many leeches move very quickly and deftly, while at other times, or in other forms, the mode of locomotion is by graceful serpent- like swimming, or by gentle gliding after the manner of snails. The hungry horse leeches, "whose daughters cry 214 SEGMENTED WORMS OR ANNELIDA. Give, Give," are species of Hcemopis, greedily suctorial though their teeth are too small to be useful in blood- letting ; but the popular name is also applied to species of the common genus Aulastoma, whose members are car- nivorous. Other common leeches are species of Nephelis, predacious forms with indiscriminating appetites, and the little Clepsine, also common in our ponds, notable for its habit of carrying its young about on its belly. Numerous marine forms prey upon fishes and other animals, e.g., the " skate sucker " Pontobdella, with a leathery skin rough with knobs, and Branchellion on the Torpedo, remarkable for numerous leaf-like respiratory plates on the sides of its body. Perhaps the strangest habitat is that of Lophobdella, which lives on the lips and jaws of the crocodile. Type. The Medicinal Leech (Hirudo medirinalis). This is the commonest and most familiar of leeches, once so constantly used in the practice of medicine that leech became synonymous with medical practitioner. It lives in ponds and sluggish streams, and though not common in Britain, is very abundant in many regions of the Continent, where leech farms, formerly of great importance, are still to be seen. Leeches feed on the blood of fishes, frogs, and the like, and are still caught in the old fashion on the bare legs of the callous collector. As animals are naturally averse to bloodletting and hard to catch, leeches make the most of their opportunity, and feed very greedily. They gorge themselves with blood and keep on slowly digesting it for many months, it may be indeed for a year. Watched in a glass jar, the leech will be seen to move by alternately fixing and loosening its oral and posterior suckers, while some slight provocation, such as some drops of chloroform or alcohol, will induce the animal to swim about both actively and gracefully. At times it may also be seen to cast off from its skin thin transparent shreds of cuticle, a process which, in natural conditions, usually occurs after a heavy meal, when the animal as if in indigestion spasmodi- cally contracts its body, or rubs itself on the stems of water plants. Numerous eggs are laid together in cocoons in the damp earth near the edge of the pool. Thence after a STRUCTURE OF THE LEECH. 215 direct development, young leeches emerge and make for the water. External Features. The leech usually measures from two to six inches in length, and appears cylindrical or strap-like, according to its state of contraction. The slimy body shows over a hundred skin rings, its dorsal surface is beautifully marked with longitudinal pigmented bands, while the ventral surface is mottled irregularly ; the suctorial mouth is readily distinguished from the unperforated hind sucker, above which on the dorsal surface the alimentary canal may be seen to end. It is, however, necessary to consider the external characters in greater detail. As already noted, the rings of the body are merely superficial wrinkles ; it is therefore not difficult to realise that there may be doubt as to their exact limits, and that the apparent number may differ accord- ing as they are counted from the dorsal or ventral surface. According to Whitman's precise investigations, 102 skin rings in all are represented, and these correspond to 26 somites or true segments. These segments may be recognised externally by conspicuous pigment spots ("segmental papillae"), which in the middle region of the body occur on every fifth ring. In type, therefore, five rings correspond to a segment, but at either end of the body the number of rings is abbreviated. In the head region a pair of "eyes" occurs on each of the 1st, 2nd, 3rd, 5th, and 8th rings ; these are homologous with " segmental papillae," and there- fore in this region eight rings correspond to five segments. Careful examination of the surface of the body will show further, the swollen protrusion of the male organs on the middle ventral line between rings 30 and 31, the aperture of the female organs five rings further back, and also on the ventral surface seventeen pairs of small lateral apertures, through which a whitish fluid may be squeezed the apertures of the excretory organs. The skin of segments 9-11 is especially glandular, and forms the so-called clitellum or saddle, the secretion of which forms the cocoon for the eggs. The Skin. The skin is so closely connected with the connective and muscular tissue lying beneath that little can be seen of its structure except in sections. Most externally lies the cuticle a product of the epidermis periodically shed as we have already noticed. In this shedding some of the genuine epidermis cells are also thrown off. These are somewhat hammer-like units with the heads turned outwards, while the spaces between the thick handles contain pigment and the fine branches of blood vessels. As the latter come very near the surface a respiratory absorption of oxygen and out- ward passage of carbonic acid is readily effected. Opening between the epidermal elements, but really situated much deeper, are numerous long necked, flask shaped glandular 216 SEGMENTED WORMS OR ANNELIDA. cells, the contents of which form the mucus so abundant on the skin. Underneath the epidermis there is much con- nective tissue, and not a little pigment, yellow and green, brown and black in colour. The Muscular System. The muscular system consists of spindle shaped cells arranged externally in circular bands like the hoops of a barrel, internally in longitudinal strands like staves. Besides these there are numerous muscle bundles running diagonally through the body, or from dorsal to ventral surface, and there are other muscles associated with the lips, tooth plates, and pharynx. The Body Cavity. The body cavity is almost quite obliterated in the adult leech, where the predominant connective tissue has filled up nearly every chink and crevice. It is to be seen in the embryo, and its remnants may be detected here and there in the adult. The virtual absence of the body cavity, and the spongy compactness of the whole animal, make the leech a tedious subject to dissect. Nervous System. The nervous system mainly consists of a pair of dorsal ganglia lying above the pharynx, and of a double nerve cord with twenty-three ganglia lying along the middle ventral line. The dorsal (or supra cesophageal) ganglia are connected with the most anterior (or sub-cesophageal) pair on the ventral chain, by a narrow nerve ring surrounding the beginning of the gut. From the dorsal centres nerves proceed to the " eyes " and anterior sense spots, from the ventral centres the general body is innervated, and from the beginning of the ventral chain special nerves supply the alimentary canal, forming what is called a visceral system. The Sense Organs. The sense organs of the leech are ten so-called "eyes," besides numerous sense spots usually occurring on every fifth skin ring. The eyes are arranged round the edge of the mouth, and look like little black spots. Microscopic examination shows them to be definite cups, surrounded by connective tissue with black pigment, and containing clear strongly refracting cells, each in connection with a fibre of the optic nerve. STRUCTURE OF 7^HE LEECH. 217 It has been shown (Whitman) that the eyes of leeches are serially homologous with the segmental sense organs. At the one extreme there are purely tactile organs, at the other extreme there are purely visual organs, and between these there are compound sense organs, in part tactile and in part visual, a series which is full of suggestiveness in regard to the evolution of sense organs (cf. of the series sensitive setae in the crayfish. The visual organs of the leech are not able to form FIG. 70. Transverse section of Leech. (Simplified from A. G. BOURNE.) c., Cuticle; e., epidermis; C.M., dermis and outer muscles (cir- cular and oblique) ; /.;/*., longitudinal muscles (the peculiar connec- tive tissue is hardly indicated) ; r.fn.^ radial muscles ; /.#., lateral blood vessel ; d.s., dorsal blood sinus ; v.s., ventral sinus enclosing nerve cord (n) ; g~., median part of crop, with lateral pockets (/) ; f., testes ; f., nephridial funnel ; v.d., vas deferens. images of external objects, but the animals are exquisitely sensitive to alterations of light. The Alimentary System. When the leech has firmly fastened itself to its prey by the hind sucker, it brings its muscular mouth into action, pressing the lips tightly on the skin, and protruding three chitinous tooth plates which lie within. Each of these 218 SEGMENTED WORMS OR ANNELIDA. tooth plates is worked by muscles, and is like a semi- circular saw, for the edge bears from 60 to 100 small teeth. Rapidly these saws cut a triangular wound, whence the flowing blood is sucked into the mouth by the muscular pharynx. The process may be observed and felt by allowing a hungry leech to fasten on your arm. As the blood passes down the pharynx, it is influenced by the secre- tion of salivary cells which lie among the muscles, and exude a ferment which prevents the usual clotting. The blood greedily sucked in gradually fills the next region of the gut the crop which bears on each side eleven storing pockets. These become wider and more capacious towards the hind end, the largest terminal pair forming two great sacs on each side of the comparatively narrow posterior part of the gut. As all the pockets point more or less backwards, it is evident why a leech to be emptied of the blood which it has sucked must be pressed from behind forwards. The pockets filled, the leech drops off its victim, seeks to retire into more private life, and digests at leisure. The digestion does not take place in the pockets, but in a small area just above the beginning of the terminal part or rectum. This rectum, running between the two last pockets, is separable from the true stomach just mentioned by a closing or sphincter muscle. It ends in a dorsal anus above the hind sucker. The Vascular System. The vascular system consists of four main vessels running longitudinally, one above the gut, one round about and obscuring the nerve cord, and one on each side of the body. These are all connected with one another by looping vessels, and all give off numerous branches which riddle the spongy body. The main side vessels are most distinct, are con- tractile throughout, and give off to the skin, gut, and excre- tory organs, a rich supply of branches. The dorsal and ventral vessels, though quite distinct, are less definite, being rather blood spaces than well-formed vessels. That the lateral vessels do most of the work of circulation is certain, but the precise course of the blood is not satis- factorily known. The blood itself is a red fluid with floating colourless cells diverse in form. STRUCTURE OF THE LEECH. 219 Excretory System. The excretory system includes seventeen pairs of excretory tubules or nephridia, opening laterally on the ventral surface, ending blindly within the body, but extracting waste products from the blood vessels which cover their walls. Each consists of two parts, a twisted horse shoe shaped glandular region where the actual ex- cretory function is discharged, and a spherical, internally ciliated bladder opening to the exterior. Within the latter there is a whitish fluid in which microscopic examination shows numer- ous waste crystals. The nephridia secrete a clear fluid which helps to keep the skin moist, and thus makes respiratory diffusion easier. The Reproductive System. The leech, like many other Inverte- brates, is hermaphrodite, containing both male and female reproductive organs. The essential male organs or testes are diffuse, being represented by nine pairs, lying on each side of the nerve cord in the middle region of the body. Each is a firm globular body, within which mother sperm cells divide into balls of sperms. The spermatozoa pass from each testis by a short canal leading into a wavy longitudinal vas deferens. This duct followed towards the head forms a coil (so-called seminal vesicle) as it approaches the ejaculatory organ or penis. From the coil on each side the sperms pass into a swollen sac at the base of the penis, where by the viscid secretion of special (" prostate ") glands, they are glued together into packets or spermatophores. These pass up FIG. 71. Dissection of Leech. rg., Nerve ring around oesophagus, here incom- plete ; /., penis; s.v., seminal vesicle ; 07'., ovary ; uf., uterus ; v.d., vas deferens; t., one of the testes; ., nephridium with bladder (bl.) ; -., a ganglion on ventral nerve cord ; s., posterior sucker. 220 SEGMENTED WORMS OR ANNELIDA. the narrow canal of the muscular penis, pass out on the middle ventral line between rings thirty and thirty-one, and are transferred in copulation to the female duct of another leech. The female organs are more compact. The two small tubular and coiled ovaries are enclosed in a spherical vesicle, the walls of which are continued as two oviducts which unite together in a convoluted common duct. This is surrounded by a mass of glandular cells, which exude a glairy fluid into the duct. Finally, the duct opens into a relatively large muscular sac the "uterus," which opens through a sphincter muscle on the middle ventral line between rings thirty-five and thirty-six. The favourite breeding time is in spring Two leeches fertilise one another, uniting in reverse positions so that the penis of each enters the uterus of the other. Spermatophores are passed from one to the other, and the contained sperms may remain for a long time within the uterus, or, liberated from their packets, may work their way up the female duct, meeting the eggs at some point, or reaching them even in the ovaries. The development has been most carefully worked out for the little leech Clepsine, and we shall follow it there. Development of Clepsine. The eggs are laid in water, and surrounded by a cocoon ; they are large, and contain much food yolk. Cleavage is complete but unequal. At the four cell stage, there are three sub-equal smaller cells and one larger posterior cell, which marks the future hind end of the body. From each of these cells a small cell is cut off, and in this way four macromeres and four micromeres are produced. The number of micromeres is continually increased by the splitting off of cells from the macromeres, so that a disc of small cells is formed. Except for this continued splitting off of small cells, three of the four macromeres remain passive for a considerable period ; they contain most of the food yolk, and serve as reservoirs of nutriment. The other, or posterior, macromere divides into two cells of unequal size, the larger speedily again divides into two primitive mesoblasts, the smaller divides into eight symmetrically arranged cells, the neuro-nephroblasts. At this time free nuclei appear in the other three macromeres (ento- blasts) without any corresponding process of cell division, these surround themselves with protoplasm, and form the endoderm cells lining the gut. The disc of small cells is now spreading over the surface of the entoblasts, over the neuro-nephroblasts, and over the primitive mesoblasts, which DEVELOPMENT OF. CLEPSINE. 221 have sunk slightly inwards. The small cells are ectodermic, they con- tribute to the formation of the epidermis, and apparently form also the ectoderm of the head region. The ectodermic structures of the body, on the other hand, are formed by the eight neuro-nephroblasts. These, together with the two mesoblasts underlying them, undergo continuous division in a forward direction, and so produce long rows of cells the germ bands. The two germ bands are widely separated posteriorly, but commence to unite anteriorly, the union travelling backward. As the neuro-nephroblasts must be regarded as ectodermic in origin, we see that the spreading of the micromeres over the surface of the egg, and the union of the germ bands, constitute together the delayed epibolic gastrulation. Each germ band consists of three layers, first a thin epidermic stratum, then the layer of the neuro-nephroblasts, and finally the mesodermic layer. Of the neuro-nephroblasts, the inner two form the ventral nerve chain, the next two on either side the nephridia, while the fate of the outer on each side is unknown. The mesoblast rows give rise to the mesoderm,the gut is formed by the entoblasts, and an anterior ectodermic invagination forms the pharynx. At this stage the embryos leave the cocoon and attach themselves to the ventral surface of the mother. A little later the form of the body becomes approximated to that of the adult, and an anus is formed by the fusion of ectoderm and endoderm. The most interesting point about this development is that, although the method of gastrulation differs widely from that of Lumbricus, the history of the germ bands shows very marked resemblances in the two forms. It can hardly be that these resemblances are due to adaptation, so that we must consider that development confirms the view which is otherwise probable, that the leeches are true Annelida. In Clepsine the eight neuro-nephroblasts are not, as in Lumbricus^ obviously ectodermic in origin, but are early covered over by the ectodermic micromeres. Both analogy and the future course of development, however, prove that they do, nevertheless, belong to the outer layer, and that their position is due to a hastening of events. Classification. 1. Rhychobdellidae, in which the fore part of the pharynx can be protruded as a proboscis. There is an anterior as well as a posterior sucker. The blood plasma is colourless. The ova are large and rich in yolk ; the embryos are hatched at an advanced stage, and soon leave the cocoon, which contains no albuminous fluid. e.g., Clepsine, Pontobdella, Branchellion. 2. Gnathobdellidne, in which there is no proboscis, but the pharynx usually bears three tooth plates. The mouth is suctorial. The blood plasma is red. The ova are small and without much yolk ; the embryos are hatched at an early stage, and swim about in the nutritive albuminous fluid of the cocoon. e.g., Hirudo, Hietnopis, Hcemadipsa, Aulastoinci) Nephelis. 222 SEGMENTED WORMS OR ANNELIDA. Appendix (i) to Annelid Series. Class CH^TOGNATHA. Arrow Worms. There are two little marine " worms," Sagitta and Spadella, which are so different from all others, that they have been placed in a class by themselves. It is possible to regard them as Annelids with three segments. The translucent body, which is about an inch long, has three distinct regions, a head bearing a ventral mouth with spines and bristles (whence the name Chsetognatha), a median region with lateral fins, and a trowel-like tail. The nervous system consists of a supra-cesophageal ganglion in the head, a sub-cesophageal about the middle of the body, long commissures between them, and numerous nerves from both. There are two eyes and various patches of sensitive cells. The food canal is complete and simple ; it lies in a spacious ciliated body cavity, which arises in the embryo as two pockets (coelome pouches) from the primitive gut cavity or archenteron. Corresponding to the external divisions, the cavities of head, body and tail are distinct. P^IG. 72. Development of Sagilta (after O. HERTWIG), illustrating formation of a body cavity by pockets from the archenteron, and early separation of reproductive cells (R.}. EC., Ectoderm ; En., endoderm ; ac., archenteron ; ft., reproduc- tive cells; bl., blastopore ; cp., coelome pouches; ;;/., mouth; i. section of gastrula ; 2 and 3. origin of coelome pouches. There is no vascular system, nor are there any certain nephridia. It is possible that the latter may be represented by the genital ducts. The animals are hermaphrodite, and the simple reproductive organs lie near one another posteriorly. The two ovaries project into the body cavity, and their ducts open laterally where body and tail meet. The two testes project into the cavity of the tail ; and their ducts have internal ciliated funnels, and open on the tail. It is interesting to know that two reproductive cells are set apart at a very early stage, and that each divides into the rudiment of an ovary and of a testis. The development is very regular. The eggs undergo complete seg- mentation ; a gastrula is formed by the invagination of a hollow ball of cells ; the body cavity arises in the form of two pockets from the gastrula cavity or archenteron. ROTATORIA. 223 Appendix (2} to Annelid Series. Class ROTATORIA. Rotifers. Rotifers are beautiful minute animals, abundant in fresh water, also found in damp moss, and in the sea. They owe their name and the old-fashioned title of wheel animalcules to the fact that the rapid movements of cilia on their anterior end produce the appearance of a rotating wheel. The food seems to consist of small organisms and particles caught in the whirlpool made by the lashing cilia. The little animals are tenacious of life, and can survive prolonged drought. If they are left dry for long, however, they die, though the ova may survive and subsequently develop. The body is usually microscopic, and is sometimes (e.g., in Melicerta and Floscularid] sheltered within an external tube. There is no internal segmentation, but there are sometimes external rings, and a ventral out- growth or " foot " is sometimes segmented. The anterior end bears, on a retractile ridge, the ciliated ring or "trochal apparatus." / The nervous system is a single dorsal ganglion with a few nerves. An unpaired eye and some tufts of sensory hairs are usually present. <\ The food canal extends along the body in a well-developed ccelomg, and the fore gut contains a mill in which two complex hammers beat upon an anvil. The canal ends posteriorly on the dorsal surface between the body and the foot, and as the terminal portion also receives the excretory canals and the oviduct, it is called a cloaca. There is no vascular system, but a nephridial tube of a primitive type lies on each side of the body, and opens posteriorly into the cloaca. The sexes are separate ; the reproductive organs are simple. Except in the marine parasite Seison and two other forms, the males are dwarfed and degenerate, destitute even of a true food canal. In many cases at least, sexual union (effected by a penis) seems to be ineffective, and there is no doubt that many, if not most, Rotifers are parthenogenetic. The females lay three different kinds of eggs, according to their conditions and constitution either small ova, which become males, or thin shelled "summer ova," or thick shelled " resting or winter ova," the two last developing into females. Many species, however, are viviparous. We include the Rotifers beside the Annelids proper, because it seems possible to regard them as derived from ancestors somewhat like Annelid larvae. Rotifers living in fixed tubes or envelopes, Melicerta, Floscularia, Stephanoceros. Free Rotifers, Notommata, Hydatina, Brachiomis. Parasitic on the marine Crustacean Nebalia, Seison. Pedalion occupies a unique position ; it has hints of appendages and a peculiar jumping motion. Equally incertce sedis, but plausibly regarded as a specialised Trocho- sphere, is the genus Dinophilus, with the nature of which advanced students should make themselves acquainted. At this stage I may also mention that there are several sets of small worm-like animals of whichjwe know very little. It is quite possible 224 SEGMENTED WORMS OR ANNELIDA. that some of them may become of great interest to the systematic zoologist, but we do not yet understand what places in the system they should occupy. Moreover, as they are small, unfamiliar, and unknown to myself, I shall simply refer the curious to what more complete works say about the Gasterotricha, Echinoderidce, Demoscolecidce, and Chcietosomidne. Appendix (3) to Annelid Series. Class SIPUNCULOIDEA, e.g., Sipmmilus. Marine worms usually living in the sand. The body is elongated and apparently unsegmented. The oral or anterior region can be invaginated by special muscles. There are no setae. They are sometimes, but perhaps erroneously, placed beside Echiuridce as Gephyrea Achaeta. The nervous system consists of an oesophageal ring, and a median ventral nerve cord, which shows slight hints of segmentation. There is a spacious body cavity. SlPUNCULID.4 PKIAPULID^:. The anus is dorsal and anterior, and the food canal is usually in a spiral : the mouth is surrounded by tentacles. There is a closed vascular system, with branches to the tentacles. An anterior pair of nephridia serve also as genital ducts, removing the repro- ductive cells from the body cavity. The sexes are separate. Examples Sipunculus. Phascolosoma. The alimentary canal is straight or slightly looped, and the anus is dorsal and posterior. There are no tentacles. There is no vascular system. No anterior nephridia, but a pair of tubes open beside the anus, and are said to be excretory in the young, genital in the adult. The sexes are separate. Examples Priapulus. Halicryptus. Appendix (4) to Annelid Series. Under the old term Molluscoidea are sometimes included the three classes Phoronoidea, Polyzoa or Bryozoa, and Brachiopoda. Prof. Lang includes them along with Sipunculoids in the provisional group Prosopygii. The Molluscoidea are characterised by the presence of a true ccelome, formed in development by the folding off of pouches from the archenteron, and by the shortening of the dorsal region of the body, which results in the close approximation of mouth and anus. The mouth is typically furnished with ciliated tentacles, and is often over- hung by an epistome ; both tentacles and epistome, when present, contain spaces which are part of the body cavity. Except in Polyzoa, two or four nephridia are present, and serve also as genital ducts. There is always a metamorphosis in development, and the larvae are peculiar. Class PHORONOIDEA. The crown of tentacles is shaped like a horse shoe, each tentacle is supported by an internal skeleton. The nervous system lies in the POL YZOABRA CHIOPODA. 225 ectoderm, and consists of a ring round the mouth, and of a cord down the left side of the body. There is a closed vascular system with nucleated red cells. The body cavity is well-developed. The sexes are united. The larva, known as an Actinotrocha, is a much modified trochosphere. Phoronis, the only genus, is a worm-like marine animal, always found enclosed in a fixed leathery tube, and social in habit. Class POLYZOA. As usually defined the class includes two sub-classes, the Ectoprocta and the Entoprocta, but it seems doubtful whether the Entoprocta should not be raised to the dignity of a distinct class. The Ectoprocta include fresh water and marine forms in which the anus is outside the basis of the tentacles. The nervous system is represented by a ganglion placed between the mouth and anus. There is no vascular system. In Cristatella, at least, there are two nephridia. All are colonial and bud very freely ; the marine forms show con- siderable division of labour among the members of the colony. (a) Tentacles in a crescent Fresh water, Cristatella, Lophopus, etc. [b] Tentacles in a circle Marine, except Paludicella ; Fhistra, the common sea-mat ; Membranipora, encrusting seaweed, etc. ; Cellepora^ very calcareous ; Alcyonidtum, gelatinous. The Entoprocta include the colonial Pedicel Una, with a few allied genera, and Urnatella, also the non-colonial Loxosoma, in which the buds separate as soon as they are formed. All are stalked and minute. The anus is included within the tentacular circle. In the metamorphosis of Pedicellina, there is an elongation of the dorsal region of the body, and a consequent approximation of the mouth and anus on the shortened ventral surface. There is no apparent body cavity in the adult, and the mesoderm arises from two primitive mesoblasts. The nephridia are anterior, minute, and do not serve as genital ducts, but resemble the " head kidneys " of Annelid trochospheres. In all these three respects the Entoprocta differ from the Ectoprocta, and from the Molluscoidea generally, but the significance of this is uncertain, more especially as it is possible that the differences may in part arise from defective observation. Class BRACHIOPODA. The Brachiopods or Lampshells are quaint marine animals, once very numerous, but now decadent. The body is enveloped dorsally and ventrally by two folds of skin or mantle, these secrete a shell, usually of lime, but sometimes organic. The development of this shell has appar- ently modified both the position and the relations of the organs. There is no real resemblance between a Brachiopod shell and that of a bivalve Mollusc, except that both consist of two valves. In Brachiopods these lie dorsally and ventrally, in Lamellibranchs they are lateral ; moreover, in Brachiopods the ventral valve is usually the larger. It is hardly necessary to say that the Brachiopod organism is not the least like a Mollusc. 15 226 BRACHIOPODA. A considerable part of the space between the valves of the shell is rilled up by two long " arms," which are coiled in a spiral, and often supported by a calcareous skeleton. These arise in development from the specialisation of a horse-shoe shaped " lophophore," such as is characteristic of the Polyzoa. The mouth is placed between the arms, and opens into the ciliated food canal. This may end blindly, or may be furnished with an anus placed near the mouth ; in Crania the anus is dorsal and posterior. The muscular system is well-developed, the shell is both opened and closed by means of muscles. There is a nerve- ring round the gullet, with a slight brain and an inferior ganglion. Sen- sory structures in many cases perforate the valves. Above the gut lies the heart, which is connected with blood vessels. Two (or more rarely four) nephridia open near the mouth, and serve also as genital ducts. The pos- terior region of the body often forms a stalk by which the shell is moored, but in many this stalk is absent, and the animal is directly attached to the substratum. The sexes are sometimes separate, but perhaps some are hermaphrodite. There is a metamorphosis in the development, and the larvae resemble those of Polyzoa. Of the details little is yet known. FIG. 73. Interior of Bra- chiopod Shell, showing cal- careous support for the "arms." (After DAVIDSON.) TESTICARDINES. The valves are hinged. There is no anus. Terebratula. IValdheimia. ECARDINES. There is no hinge. There is an anus. Crania. Lingiila, persistent since Palaeozoic ages. CHAPTER XII ECHINODERMA. Class I. HOLOTHUROIDEA (Scytoderma). Sea Cucumbers. 2. ECHINOIDEA. Sea Urchins 3. ASTEROIDEA. Starfishes 4. OPHIUROIDEA. Brittle stars 5. CRINOIDEA. Feather stars "I 6. BLASTOIDEA. Extinct VPELMATOZOA. 7. CYSTOIDEA. Extinct J IN contrast to the "worms," the Echinoderms form a well-defined series. They may be described as sluggish marine animals, generally of radiate symmetry, with a tendency to form limy skeletons. The radial symmetry led the older zoologists to place the Echinoderma near Ccelentera, but the larval Echinoderm is more specialised than most of the larval "worms," and is bilateral in its symmetry. It seems likely that the adult radial symmetry is an adaptation to sedentary life, and that the Echinoderms represent an offshoot of some "worm" stock. Yet it is interesting to notice that in both Ccelentera and Echino- derma the nervous system shows a marked absence of centralisation, which may be connected with the absence of a definite head region, and this again with the relatively sedentary habit. GENERAL CHARACTERS. The Echinoderms include forms in which the bilateral symmetry of the larva is replaced in the adult by radial symmetry. In addition to the dominant radial symmetry, the adults show to a varying extent a tendency towards a bilateral form, but this is never the same as that of the larva, nor is it equivalent in the different types. Lime is always deposited in the 228 ECHINODERMA. mesodermic tissues (mesenchyme), and in consequence there is frequently a very complete skeleton. From the primi- tive gut of the larva, pouches grow out to form the usually spacious ccelome and the characteristic water vas- cular system. The branches of this system, together with the nerves, exhibit in most cases a typical five-rayed arrangement. In development there is a marked distinc- tion between mesoblast derived from gut pouches, and mesen- chyme produced by immigrant amoeboid cells. There is usually a very striking circuitousness or indirectness in development. The Echinoderms are all marine. By reason of their FIG. 74. Pluteus larva with rudiment of adult. (After JOHANNES MULLER.) durable skeletons, they are extremely well represented as fossils, yet this does not alter the fact that the group is well-defined, and shoivs no close relation to any other, whether in its living or extinct representatives. The average habit is sluggish, and this may be correlated with the constant development of lime in the tissues. This power of Jorming skeletal substance is indeed so deep-seated that lime may appear in almost any of the organs of the body. ihe diet is vegetarian (most sea urchins), carnivorous^ ( starfishes ),~or 'consists of the organic particles found in sand STARFISH. 229 and mud, the Holothurians in particular practising this worm-like mode of nutrition. Most Echinoderms have to a remarkable extent the power of /V7C//W off ftfttf rp.Pfine.ratit$g fftrtin? 1 ^ of tfajr hqsfo. This power is frequently reckoned as one of their means of defence, but they often mutilatej^M^dv^ vpp.rtfy a* a Mnseque.^ qf unfavourable conditions of life. The self- mutilation, or autotomy, as it is called, seems to be entirely a reflex action, nof~ voluntary . The peculiar water vascular system attains great develop- ment, and has usually respiratory or locomotor functions. It is possible that in some cases it may also~Junctwn as an organ of excretion. Well-defined excretory organs are conspicuously rare. Soluble waste products seem generally to diffuse out into the water, ivhile the insoluble are here, as in sea squirts, stored up in the tissues in the form of granular masses. The Holothurians are in form nearest to the supposed worm-like ancestor, and are perhaps primitive forms, which do not lead up to any of the other classes. From primitive unspecialised Cystoids, the Echinozoa, and Pelmatozoa have perhaps taken origin. Of the Echinozoa the Asteroidea and Ophiuroidea are very closely related, and seem to be connected by fossil forms. In our survey of the group it is more convenient to begin with the familiar starfishes than with the more primitive forms. The general characters of each class may be read from the synoptic table at the end. Class ASTEROIDEA. Starfish. The description applies especially to the common five- rayed starfish (Asterias or Asteracanthion rubens}. It is often seen in shore pools exposed at low water, but its haunts are on the floor of the sea at greater depths. There it moves about sluggishly in any direction by means of its tube feet. Form. Each of the five arms bears a deep ventral groove in which the tube feet are lodged. The mouth is in the middle of the ventral surface, the food canal ends about the centre of the dorsal disc. With this flat, five-rayed form, the 11-13 rayed sun star (Solaster), the pincushion- like Goniaster, and the flat pentagonal Palmipes, should be contrasted. 230 ECHINODERMA. Integument. (a) The body is covered by a ciliated ectoderm. This includes supporting, glandular, and sensory cells, and beneath it there is a network of nerve fibrils with ganglionic cells. (b) The middle layer of the integument consists of a double stratum of ground substance, the outer part of which contains the chief limy structures except the ambulacra! ossicles which are formed more internally. There is also a thin muscular layer. The whole of this middle layer is formed in development from the mesenchyme tissue. (c) Internally the body wall is lined by a ciliated epithelium, derived in development from the wall of the ccelomic pouches. (See Development.) Between two of the arms lies the perforated madreporic plate, the entrance to the water vascular system, thus defining the bivium, while the other three arms constitute the trivium. The Calcareous Skeleton. In association with the inner mesodermic layer of the integument, there is developed on the ventral surface of each arm a double series of sloping plates. These two series meet dorsally, like rafters, in the middle line of the arm, forming an elongated shed. The rafter-like plates are called ambulacral ossicles ; the groove which they bound lodges the nerve cord, the blood vessel, the water vessel, and the tube feet of each arm. In association with the outer mesodermic layer of the integument, numerous smaller plates are developed, e.g., the adambulacrals, which articulate with the outer lower ends of ambulacrals. The dorsal surface bears a network of little ossicles, and many of these bear spines. Peculiarly modified spines, known as pedicellarice, look like snapping scissor blades mounted on a single soft handle. They have been seen gripping Algae and the like, and probably keep the sur- face of the starfish clean. Muscular System. A starfish is not very muscular, but it often bends its arms upwards by means of the muscular layer noted above, and may sometimes be seen tightly embracing an oyster. Other muscles affect the size of the ventral grooves, and muscular elements also occur on the protrusible part of the stomach, and in connection with the water vascular system. SENSE ORGjANS. 231 Nervous System. Underneath the ciliated ectoderm lies a network of nerve fibrils, with some ganglionic cells. But besides these diffuse elements there is a pentagon around the mouth, and a nerve along each arm. The system is not separable from the skin. Sense Organs. A red eye spot, sensitive to light, lies on the terminal ossicle at the tip of each arm, and is usually upturned. It FIG. 75. Alimentary system of Starfish. (After MULLER and TROSCHEL.) The dorsal surface has been removed ; the digestive caeca, the stomach, &c., are shown. is a modified tentacle, bearing numerous little cups, lined by sensitive and pigmented cells, containing clear fluid, and covered by cuticle. The skin is diffusely sensitive. The terminal tube foot of each ray seems to be olfactory. 232 ECHINODERMA. Alimentary System. The starfish is fond of young oysters and other bivalves, and may be found with part of its stomach extruded over them. This protrusible or cardiac portion of the stomach is glandular and sacculated, and bulges slightly towards the arms ; it is followed by an upper or pyloric portion, giving off five branches, each of which divides into two large diges- tive caeca, a pair in each arm (Fig 75.) These glands con- tain a yellowish pigment (enterochlorophyll) and secrete tryptic, peptic, and diastatic ferments. From the short tubular intestine between the stomach and the almost central dorsal anus two little outgrowths are given off, perhaps homo- logous with the " respiratory trees " of Holothuroids. Some parts of the food canal are ciliated. Body Cavity. The coelome is distinct, though not much of it is left unoccupied either in the disc or in the arms. It is lined by ciliated epithelium, and contains a fluid with amoeboid cells. A few of these have a pigment which probably aids in respiration ; others are phagocytes, which get rid of injurious particles through the " skin gills ; " others continue the work of digestion. Water Vascular System. When we watch a starfish crawling up the side of a rock we see that scores of tube feet are protruded from the ventral groove of each arm, that these become long and tense, and that their sucker-like terminal discs are pressed against the hard surface. There they are fixed, and towards them the starfish is gently lifted. The protrusion is effected by the internal injection of fluid into the tube feet, the fixing is due to the subsequent withdrawal of the water producing a vacuum between the ends of the tube feet and the rock. As to the course of the fluid, it is convenient to begin with the madre- poric plate, which lies between the bases of two of the arms (the bivium}. This plate is a complex calcareous sieve, with numerous perforating canals and external pores. It may be compared to the rose of a water- ing pan, but the holes are much more numerous, and lead into small canals which converge into a main ciliated canal. The latter runs down through the body, and is like a complex calcareous filter. It is called the stone canal. WATER VASCULAR SYSTEM. 233 The stone canal leads into a water ring round about the mouth. From this circumoral ring are given off nine glandular bodies (Tiedemann's bodies), and five radial tubes, one for each of the arms. Considerations of symmetry suggest that there should be ten glandular bodies, but the stone canal has taken the place of one. In many starfishes there are five or ten little reservoirs (Polian vesicles) opening into the circumoral ring, but in Asterias rtibens these are hardly distinguishable from the first ampullae of the radial vessels. Along each arm, then, there runs a radial vessel. It lies in the ambulacral groove beneath the shelter of the rafter-like ossicles. From it branches are given off to the bases of the tube feet, but from each of these bases a canal ascends between each pair of ambulacral ossicles, and expands into an ampulla or reservoir on the dorsal or more internal side. The fluid in the system may pass from the radial vessels into the tube feet, and from the tube feet it can flow back, not into the radial vessel, FIG. 76. Diagrammatic cross section of starfish arm. (After LUDWIG.) n., radial nerve ; b.v., radial blood vessel according to Ludwig, sep- tum in blood vessel according to others iv.v., radial water vessel ; am., ampulla ; tf., tube foot ; p.c., a pyloric caecum cut across ; s.p., a calcareous spine ; g. , a skin gill ; lac., spaces in the skin ; go.^ ova in ovary; a.o., ambulacral ossicle. but into the ampullae. There are muscles on the walls of the tube feet, ampullae, and vessels. At the end of each arm, there is a long unpaired tube foot, which seems to act as a tactile tentacle, and has also olfactory significance. To recapitulate, the madreporic plate leads into the stone canal, this passes into the ring round the mouth with its nine vesicles, from the ring radial vessels run along the arms, they give off branches to the tube feet, and the base of each tube foot communicates with an ampulla. 234 ECHINODERMA. Vascular System. We have not yet reached certainty in regard to this system. German authorities, e.g.* Ludwig, describe (i) a radial blood vessel above the nerve in each arm ; (2) a circumoral vessel around the mouth ; (3) a heart lying beside the stone canal and leading into (4) an aboral ring which gives off vessels to the genital organs. But others say that the so-called " heart " is a solid glandular organ, that the aboral ring is merely the connecting strand or rhachis of the genital organs, and that the radial and circumoral vessels described are really thickened septa within the true vessels. French authorities describe (a) a radial perihremal space or blood vessel divided by a median mesentery, and (b] the union of these in a circumoral ring. But the latter encloses (c) another annular vessel with which a sinus (d) surrounding the stone canal communicates. Finally, an aboral pentagon (e) gives off five pairs of genital blood vessels. Respiratory System. From the dorsal surface and sides of a starfish in a pool, numerous transparent processes may be seen hanging out into the water. They are the simplest possible respiratory structures, contractile outgrowths of the skin, with cavities continuous with the ccelome, and are called " skin gills." It is likely that pigmented cells of the body cavity fluid act like rudimentary red blood corpuscles ; the water vascular system may help in aeration ; and the whole body is of course continually washed with water. Excretory System. The u skin gills" are said to have an excretory function ; for phagocytes, bearing waste, seem to traverse their walls. It may also be that excretion is somehow concerned in forming the carbonate of lime skeleton, but facts are wanting. Reproductive System. The sexes are separate, and they are like one another, both externally and internally. The organs develop periodi- cally, and lie in pairs in each arm. Each is branched like an elongated bunch of grapes, and is surrounded by a blood sinus. Each has a separate duct, which opens on a porous plate, between the bases of the arms on the dorsal surface. In Asterina gibbosa, however, the eggs are extruded ven- trally. The eggs are fertilised in the water, and the free swimming larva, which will be described along with those of of the other classes, is known as a Bipinnaria or as a Brachiolaria. OPHIUROIDEA OR BRITTLE STARS. 235 Other Starfishes. Astropecten and most forms related to it have blind food canals ; Brisinga has 9-12 long arms, arising abruptly from a small disc as in Brittle stars, and has no ampullae, eye spots, or skin gills ; Luidia has three-bladed pedicellariie ; in most forms the genital ducts end on plates with a single aperture, and so on. The commonest European forms are species of Asterias or Aster- acanthion, Astropecten, Cribrella, Solaster, Goniaster. The largest are such as Asterias gigantea (from the Pacific coast of N. America), measuring 2 feet in diameter, or Pycnopodia helianthoides, about a yard in diameter, and with over twenty arms. There are many deep sea forms, such as the ophiuroid-like Brisinga, the widely distributed Hymenaster, and the blue Porcellenaster ccertileus, but the majority occur in water of no great depth. Parental care is incipient among Asteroids, for a large Asterias has been seen sheltering its young within its arms : there is a definite brood pouch in the form of a sort of tent on the dorsal surface of Pleraster. Many Asteroids break very readily, or throw off their arms when these are seized. Professor Forbes describes how a fine specimen of Luidia thus escaping him gave a " wink of derision " as it passed over the side of the boat. The lost parts are slowly regenerated, and strange forms are often found in process of regrowth. Thus the " comet form " of starfish occurs when a separated arm proceeds to grow the other four. Asteroidea first occur in Silurian strata. Class OPHIUROIDEA. Brittle stars, e.g., the common Ophiopholis bellis. The body of a brittle star differs from that of a starfish in the abruptness with which the arms spring from the central disc (cf. Brisinga). These arms are muscular, and useful in wriggling and clambering ; they do not contain outgrowths of the gut, nor reproductive organs. Moreover there is no ambulacral groove, and the tube feet which project on the sides are too small to be of locomotor service. The madre- poric plate is situated on the ventral surface, usually on one of the plates around the mouth. The food canal ends blindly. The reproductive organs lie in pairs between the arms, and open into pockets or bursse formed from inturnings of the skin, which communicate with the exterior by slits opening at the bases of the arms. Water currents pass in and out of these pockets, which probably have both respiratory and excretory functions. 236 ECHINODERMA. The free swimming larva is a Pluteus^ very like that of Echinoids. Ophiuroids are first found in Silurian strata. 1. Euryalicla. Skin without plates, arms simple or branched and capable of being rolled up. A s trophy ton. Gorgonocephalus. 2. Ophiurida. Skin with plates, arms simple. OphiopholtS) Ophiocoma, Ophiothrix, are common genera. Amphiura squamata is hermaphrodite. Class ECHINOIDEA. Sea Urchins, e.g., the common Echinus edulis, Strongylocentrotus lividus. Most sea urchins live off rocky coasts, and not a few shelter themselves sluggishly in holes. They move by means of their tube feet and spines, and seem to feed on seaweeds, and on the organic matter found in mud and other deposits. After the perils of youth are past, the larger forms have few formidable enemies. "' , kin, and Skeleton. The hard and pficluy body is more or less spherical. The food canal begms~in the middle of the lower^surface ; it ends at the opposite pole in the middle of an aj/ical disc formed of a central plate surrounded by five " ocular " and five " genital " plates. The ocular or radial plates bear eye specks ; the genital or basal plates bear the apertures of x the genital ducts, but one of the five is modified as the madreporic plate. From pole to pole run ten meridians of calcareous plates which fit one another firmly ; five of these (in a line with the ocular plates) are known as ambulacral areas, for through their plates the locomotor tube feet are extruded ; the five others (in a line with the genital plates) are called inter-ambulacral areas, and bear spines, not tube feet. Altogether, therefore, there are ten meridians, and each meridian area has a double row of plates. On the dry shell from which the spines have been scraped, the ambu- lacral plates are seen to be perforated by small pores, four pairs or so to each plate. Through each pair of pores a tube foot is connected with an internal ampulla. In the ( starfish the ambulacral areas are wholly ventral, and the THE NERVOUS SYSTEM. 237 apical area seen on the dorsal surface of the young forms is not demonstrable in the adult. The " posterior " ambulacra, those between which the modified basal or madreporic plate lies, are often distin- guished as the " bivium," the other three form the " trivium," and the middle one of the three is " anterior." On the shell there are obviously many spines, most abundant on the inter-ambulacral areas. Their bases fit over ball-like knobs, and are moved upon these by muscles. But besides these, there are two modified forms of spines, (a) the minute pedicellariae, with three snapping blades on a soft stalk, and sometimes with apical glands ; and (b} small globular sphaeridia, which show some structural resemblances to otocysts. It is said that like true otocysts they are con- cerned with the perception of direction of motion. In front of the mouth project the tips of five teeth, which move against one another, grasping and grinding small particles. They are fixed in five large sockets, and along with fifteen other pieces form " Aristotle's lantern," a complex masticating apparatus, of whose history we- know little. It surrounds the pharynx, and is swayed about and otherwise moved by muscles, many of which are attached to five beams which project inward from the margin of the shell round about the mouth. As in other Echinoderms, the skeleton of lime is meso- dermic. The shell is covered externally by a delicate ciliated ectoderm, beneath which, in a thin layer of con- nective tissue, there is a network of nerve fibres, and some ganglion cells. Internally, there is another thin layer of connective tissue, and a ciliated epithelium lining the body cavity. The skeleton grows by the formation of new plates around the apical disc, and also by. the individual increase of each. In a few forms the shell retains some plasticity. Nervous System. The nervous system consists of a ring around the mouth, of radial branches run, ing up each ambulacral area, and of the superficial network. Tube feet, sphaeridia, pedicellariae, and spines are all under nervous control, and each radial nerve ends in the " eye speci's " of the apical " ocular plates." It is probable that all the tube feet are sensory, and 238 ECHINODERMA. this is certainly the main function of ten which lie near the mouth. Alimentary Canal. The alimentary canal passes through Aristotle's lantern, and the intestinal portion lies in two and a half coils around the inside of the shell to which it is moored by mesenteries. It contains fine gravel, sand, and some organic debris. It ends near the centre of the apical disc, whence the pedi- cellariae have been seen removing the faeces. Accompanying the first coil of the gut is a canal or " siphon," which opens into the gut at both ends. Accord- ing to Cuenot, a current of water traverses this tube, which thus, by reason of its thin walls, carries oxygen to the cor- FIG. 77. Ventral half of Sea Urchin. (From CARUS, after TIEDEMANN.) to the sessile Cystoids, Blastoids, and Crinoids (Pelmatozoa), the other to the free Echinoidea, Asteroidea, and Ophiuroidea. Of these the existing Asteroidea and Ophiuroidea are late divergences from a common stock. [TABLE. CONTRASTS BETWEEN ECHINODERMS. 251 bS"o"rt J'3 * ft rt <* 4) - t/) 4) g ja< 53.a w JJ. G c3 B .H o > c SG <" G c 18 a'sjs-a tf. c (fl It! I *|i g. iljj >J o,^ j3 . KOIDE = E - -|i|^ M ^ & ^ > w A i > . s ^ .s *P1 S CL UPHIU liiif Pi H G G J3 WHG rt H J-S o rt rt >-S^ s Js.S'o -o j8- I'g . O c i|i " g g.S all G g || o S "5 ^ J 1 w a (U 13 >rt "2 " >-, w t* ASTEROIDEA The body is fla pentagonal or s The arms have ; ventral ambulacral | The skin bears mar plates, tubercles, et pedicellariae are pre s Sj . s ^ UJ .W C n > C'^3 s*S ^^=""3 G 'c "O -^ ^ati < w t| g i| S-g l.d S ll? o||.S oJiU I Q - 2 .S ^*I *a . S is S; t/3 O ^ i*a '0*2 ^s 3 ..5 d JH c i|| 2 c* ~ g D S*o G S*"* P 2 ^S G rf ! 9) *" t/; C K* lili g|j| g - H ^ i Ja 3 -S & a %. c^- *62 o HH : 3 HOLOTHUROIDEA. The body is elongated and worm-like, with a tough muscular skin, in which limy plates are embedded. They move partly by mus- cular writhings, partly by means of the tube feet. There is a circumoral nerve ring with radial branches. Sometimes there are "ear sacs." The mouth surrounded by tentacles is at or near one pole, the anus at or near the other. The circumoral water ring communicates with the tentacles ; the madre- poric plate usually opens into the body cavity ; the tube feet are often restricted and often mere papillae without terminal discs. The reproductive organs are branched tubes in the body cavity ; they open near the base of the wreath of tentacles, and do not exhibit a five rayed ar- rangement. Larva an A uricularia. 254 CRUSTACEA. and there in England and Ireland, and is common on the Continent. It is absent from districts where the water contains little lime. The food is very varied from roots to water rats ; cannibalism also occurs. The animals swim backwards by powerful tail strokes, or creep forwards on their "walking legs." Their life is tolerably secure, but frequent moultings occur which are expensive and hazardous. When hatched the young are like miniature adults ; for a time they cling beneath the tail of the mother. External Appearance. The head and thorax are covered by a continuous (cephalo- thoracic) shield ; the abdomen shows obviously distinct segments movable upon one another. As indicated by the appendages, there are three groups of segments or metameres five in the head, eight in the thorax, six in the abdomen, as well as an unpaired piece or telson on which the food canal ends. (According to some authorities there are twelve segments in the cephalothorax, and seven in the abdomen.) Each of the nineteen segments bears a pair of append- ages. Among other external characters may be noticed the stalked movable eyes, the two pairs of feelers, the mouth with six pairs of appendages crowded round it, the gills under the side flaps of the thorax, and the varied post-oral appendages. '(i) The external shell or cuticle, composed -of various strata of chitin, coloured with pig- ments, hardened with lime salts ; The BODY WALL I (2) The ectoderm, epidermis, or hypodermis, consists of: 1 which makes and remakes the cuticle ; (3) An internal connective tissue layer or dermis, with pigment, blood vessels, and nerves. Internal to this lie the muscles. Between the rings and at the joints the cuticle contains no lime, and is therefore pliable. As a sacrificed product of epidermic cells, it is dead and cannot expand. Hence, as long as the animal continues to grow periodic moulting is necessary. The old husk becomes thinner, a new one is formed beneath it, a split occurs across the back just behind the shield, the animal withdraws its cephalothorax and then its abdomen, and an empty but complete shell is left behind. APPENDAGES. 255 The moulting is preceded by an accumulation of glycogen in the tissues, and this is probably utilised in the rapid growth which intervenes between the casting of the old and the hardening of the new shell. How thorough the ecdysis or cuticle casting is, will be appreciated when we notice that the covering of the eyes, the hairs of the ears, the lining of the fore gut and hind gut, the gastric mill, and the tendinous inward prolongations of the cuticle to which some of the muscles are attached are all got rid of and renewed. The moults occur in the warm months, eight times in the first year, five times in the second, thrice in the third, after which the male moults twice, the female once a year, till the uncertain limit of growth is reached. It is not clearly known in what form the animals procure the carbonate of lime which is deposited in the chitinous cuticle, but experiments made by Mr. Irvine at Granton Marine Station proved that a carbonate of lime shell could be formed by crabs even when the slight quantity of carbonate of lime in sea water was replaced by the chloride. Moulting is an expensive and exhausting process, and great mortality is associated with the process itself or with the defenceless state which follows. The process is a disadvantage attendant on the advantage of armature. Inequalities in the legs are usually due to losses sustained in combat, but these are gradually repaired by new growth. The surface of the body bears hairs or bristles of various kinds. These have their roots in the epidermis, and are made anew at each moult. There are simple glands beneath the gill flaps, and on the abdomen of the female there are cement glands, the viscid secretion of which serves to attach the eggs. Appendages. The limbs of a Crustacean usually exhibit considerable diversity ; in different regions of the body they are adapted for different work ; yet all have the same typical structure, and begin to develop in the same way. In other words, they are serially homologous organs, illustrating division of labour. Typically each consists of a two-jointed basal piece (protopodite), and two jointed branches rising from this an internal endopodite and an external eocopodite ; but in many the outer branch disappears. The protopodite has usually two joints, a basal or proximal coxopodite, and a distal basipodite; the five joints which the endopodite frequently exhibits are named from below upwards ischio-, mero-, carpo-, pro-, dactylo-podites details of some use in the comparison and identification of species. 256 CRUSTACEA. THE APPENDAGES OF THE CRAYFISH. No. NAME. FUNCTION. STRUCTURE. i Antennules (pre- Tactile, olfactory, Typical. oral ?) with ear sac at base. 2 Antennae (pre- Tactile, opening of Small exopodite. oral?) kidney at base. 3 Mandibles. Masticatory. Four joints, of which three form the palp (endopodite and upper joint of protopo- "rfx-N dite). ffi^ 4 ist Maxillae. 9 Thin single jointed protopo- dite, small endopodite, no exopodite. 5 2nd Maxillae. Produces respira- Thin protopodite, filamen- tory current. tous endopodite, the "baler" is formed from the epipodite, probably along with the exopodite. 6 ist Maxillipedes (foot-jaws). 9 Thin protopodite, small en- dopodite, large exopodite. 7 2nd Maxillipedes 9 Two-jointed protopodite, five- jointed endopodite, long exopodite. 8 3rd Maxillipedes Masticatory. Two -jointed protopodite, large five - jointed endo- rt . podite, slender exopodite. O oo" H" 9 Forceps (clawed). Fighting, seizing. No exopodite. In the claw the last joint bites against a 10 Walking Legs prolongation of the second (clawed). last. ii j> Genital opening in female. 12 Without claws. 13 j? Genital opening in male. .4 Modified swim- merets in male, ( Serve in the male Xas canals for the Protopodite and endopodite form a canal ; no exopodite. in female rudi- seminal fluid. mentary. a 15 Modified swim- All the three parts. merets in male, 0(0" normal in female. ,> 16 Swimmerets. C Move slightly like n ^1 T 7 ? } oars, and carry j, 18 I the eggs in the MUSCULAR SYSTEM. 257 We can fancy how the Crustacean form of limb might arise from the biramose parapodium of a Polychsete. The hard chitinous cuticle of the Arthropod makes joints possible and necessary. In regard to the foregoing list it should be noted that the eye stalks are no longer included in the series since their development is not like that of the limbs, and, moreover, that though the two pairs of antennae lie far in front of the mouth, it is possible that they were originally post-oral. With many of the thoracic appendages, gills, plate-like epipodites, and setae are associated. It is interesting to connect the structure of the appendages with their functions. Thus it may be seen that the great paddles are fully spread when the crayfish drives itself backwards with a stroke of its tail, while in straightening again the paddles are drawn inwards, and the outer joint of the exopodite bends in such a way that the friction is reduced. It is likely that some of the crowded mouth parts, e.g., the first maxilke, are almost functionless. The hard toothed knob which forms the greater part of the mandible is obviously well adapted to its crushing work. In connection with the skeleton, the student should also notice the beak (rostrum} projecting between the eyes ; the triangular area (epistoma) in front of the mouth, and the slight upper and lower lips ; how the gills are protected by lateral flaps of the body wall ; that each posterior segment consists of a dorsal arch (tergum\ side flaps (pleura), a ventral bar (sternum], while the little piece between the pleura and the socket of the limb is dignified by the name of epimeron. The hindmost piece (telson) on which the food canal ends ventrally is regarded by some as a distinct segment, by others as an unpaired appendage. The most difficult fact to understand clearly, is that the cuticle of certain mouth parts (e.g., the mandibles), and of the ventral region of the thorax, is folded inwards, forming chitinous " tendons " or insertions for muscles, protecting the ventral nerve cord and venous blood sinus, and above all, con- stituting the complex, apparently, but not really, internal, " endophragmal " skeleton of the thorax. Muscular System. The muscles are white bundles of fibres. On minute examination these show clearly that transverse striping which is always well-marked in rapidly contracting ele- ments. They are inserted on the inner surface of the cuticle, or on its internal foldings (apodemata). The most important sets are (i) the dorsal extensors or straighteners 17 FIG. 82. Appendages of Norway Lobster. Ex, Exopodite ; En, endopodite ; protopodite dark throughput ; Ep, epipodite. i. Antennule, E, position of ear ; 2. antenna, K, opening of kidney ; 3. mandible, P, palp ; 4. first maxilla ; 5. second maxilla, B, baler ; 6. first maxillipede ; 7. second maxillipede ; 8. third maxillipede the basal joint of protopodite is called coxopodite, the next basipodite ; the five joints of the endopodite are called, ischiopodite (z) ; meropodite (;;z) ; carpopodite (c) ; propodite (/) ; dactyloppdite (ct) ; 9. forceps ; (7) coxopodite, (6) basipodite ; 10-13. walking legs ; 14. modified male appendage ; 15-18. small swimmerets ; 19. large paddles. NERVOUS AND SENSORY SYSTEMS. 259 of the tail ; (2) the twisted ventral muscles, most of which are flexors or benders of the tail, which have harder work, and are much larger than their opponents ; (3) those mov- ing the appendages ; (4) the bands which work the gastric mill. Nervous System. The supra-oesophageal nerve centres or ganglia, forming the brain, have been shunted far forward by the growth of the pre-oral region. We thus understand how the nerve ring round the gullet, connecting the brain with the ventral chain of twelve paired ganglia, is so wide. The dorsal or supra-oesophageal ganglia are three lobed, and give off nerves to eyes, antennules, antennae, and food canal, besides the commissures to the sub-cesophageal centres. The sub-cesophageal ganglia, the first and largest of the ventral dozen, innervate the six pairs of appendages about the mouth. There are other five ganglia in the thorax, and six more in the abdomen. Though the ganglia of ^ach pair are in contact, the ventral chain is double, and at one place, between the 4th and 5th ganglia, an artery (sternal) passes between the two halves of the cord. From each pair of ganglia nerves are given off to appendages and muscles, and apart from the brain, these minor centres are able to control the individual move- ments of the limbs. In the thoracic region the cord is well protected by the cuticular archway already referred to. From the brain, and from the commissure between it and the sub-cesophageal ganglia, nerves are given off to the food canal, forming a complex visceral or stomato-gastric system: Similarly from the last ganglia of the ventral chain, nerves go to the hind gut. If the brain be regarded as the fusion of two pairs of ganglia, as the development suggests, and the sub-cesophageal as composed of six fused pairs, then these, along with the eleven other pairs of the ventral chain, give a total of nineteen nerve centres, a pair for each pair of appendages. Sensory System. A skin clothed with chitin is not likely to be in itself very sensitive, but some of the setae are. These are 260 CRUSTACEA. not mere outgrowths of the cuticle, but are continuous with the living epidermis beneath, and though some are only fringes, both experiments and histological examination show that others are tactile. On the under surface of the outer fork of the antennules, there are special innervated setae which have been credited with a smelling function. Other likewise specialised hairs have sunk into a sac at the base of the antennules, and are spoken of as auditory. The sac opens by a bristle- guarded slit on the inner upper corner of the expanded basal joint, and con- tains a gelatinous fluid and small "otoliths" which seem to be foreign particles. This "ear" is somehow connected with directing the animal's movements. In some other Crus- taceans, the auditory hairs are lodged in an open depression ; this has be- come an open sac in the Crayfish, a closed bag in the Crab. Small hairs on the upper lip of the mouth have been said to have a tasting function, but imagination is apt to help conclusions as to the precise nature of the sensitiveness of such simple structures. p.r- R N FIG. 83. A single eye element or omma- The stalked eyes, which used to be tidium of the Lobster - regarded as appendages, arise in de- (After _ G ' H ; PARKER ' } velopment from what are called " pro- cephalic lobes " on the head. They are compound eyes, that is, they con- sist of a multitude of elements, each of which is structurally complete in itself. On the outside there is a cuticular cornea, divided into square facets, one for each of the optic elements. Then follows a focussing layer, corresponding to the epidermis, consisting of many crystalline c, Cornea ; c.h, corneal hypodermis ; cp, cap of crystalline cone ; co, crys- talline cone ; d.r, distal retinula elements ; p.r, proximal retinula ele- ments ; R, rhabdome ; N, nerve fibre. ALIMENTARY SYSTEM. 261 cones. Each crystalline cone is composed of four crystalline cells, which taper internally. Internal to each crystalline cone lie a number of retinula cells. The innermost of these surround four little red rods, united closely into what is called a rhabdome. At its base, a nerve fibre enters from the adjacent optic ganglion at the end of the optic nerve. Thus each element consists of corneal facet, crystal- line cone, and retinula, and the retinula consists of internal rhabdome, and external retinula cells. Between the in- dividual optic elements, lie some pigment cells. Opinions differ as to the visual powers of Crustaceans, but their eyes are able to form images of external objects, and these images are erect, not inverted as in the eyes of Vertebrates. Alimentary System. The food canal consists of three distinct parts, a fore gut or stomatodaeum developed by an intucking from the anterior end of the embryo, a hind gut or proctodasum similarly invaginated from the posterior end, and a mid gut or mesenteron which represents the original cavity of the gastrula. The mouth has been shunted backwards from the anterior end of the body, so that the antennules and antennae lie far in front of it. The fore gut, which is lined by a chitinous cuticle, includes a short gullet, on the walls of which there are small glands hypothetically called " salivary," and a capacious gizzard, or " stomach," which is distinctly divided into two regions. In the anterior (cardiac) region there is a complex mill ; in the posterior (pyloric) region there is a sieve of numerous hairs. The mill is very complex, but there is no difficulty in dissecting it carefully, nor in seeing at once that there are supporting " ossicles " on the walls with external muscles attached to them, and internally projecting teeth which clash together and grind the food. Three of the teeth are conspicuous ; a median dorsal tooth is brought into contact with two large laterals. On each side of the anterior part of the gizzard, there are two limy discs or gastroliths, which are broken up before moulting, and though quite inadequate to supply sufficient carbonate of lime for the new skeleton, seem to have some relation to this process. The occurrence of chitinous cuticle, hairs, 262 CRUSTACEA. teeth, and gastroliths in the " stomach," is intelligible when the origin of the fore gut is remembered, and so is the dis- mantled state of this region when moulting occurs. The mid gut is very short, but it is the digestive and absorptive region. From it, there grows out on each side a large digestive gland with two ducts. This gland is more than a "liver," more even than a " hepatopancreas." It absorbs peptones and sugar, and makes glycogen like the Vertebrate liver, its digestive juices are comparable to those of the pancreas and the stomach of higher animals. The hind gut is long and straight. It is lined by a chitinous FIG. 84. Longitudinal Section of Lobster, showing some of the organs. H, Heart ; A.O, ophthalmic artery ; a.a, antennary artery ; a.h, hepatic artery ; St, sternal artery ; S.A, superior abdominal artery ; M.G, mid gut : D.G, digestive gland ; H.G, hind gut ; Ex, exten- sor muscles of the tail ; Fl, flexor muscles of the tail ; I. A, inferior abdominal artery ; G. gizzard ; C, cerebral ganglia. cuticle, as its origin suggests. There are a few minute glands on its walls. Body Cam'fy. The space between the gut and the body wall is for the most part filled up by the muscles and the organs, but there are interspaces left which contain a fluid with amoeboid cells. These interspaces seem to represent enlarged blood sinuses (a haemoccele) rather than a true body cavity or VASCULAR AND RESPIRATORY SYSTEMS. 263 ccelome. One of the spaces forms the pericardium, or chamber in which the heart lies. Vascular System. Within this non-muscular pericardium, and moored to it by thin muscular strands, lies the six-sided heart, which receives pure blood from the gills (via the pericardium) and drives it to the body. The arterial system is well developed. Anteriorly, the heart gives off a median artery to the eyes and antennules, a pair of arteries to the antennae, and a pair to the digestive gland. Posteriorly, there issues a single vessel, which at once divides into a superior abdominal, running along the dorsal surface, anoT a sternal which goes vertically through the body. This sternal passes between the connectives joining the 4th and 5th ventral ganglia, and then divides into an anterior and posterior abdominal branch. All these arteries are continued into capillaries. From the tissues the venous blood is gathered up in channels, which are not sufficiently defined to be called veins. It is collected in a ventral venous sinus, and passes into the gills. Thence purified by exposure on the water-washed sur- faces, it returns by six vessels on each side to the pericardium. From this it enters the heart by six large and several smaller apertures, which admit of entrance but not of exit. The blood contains amoeboid cells, and the fluid or plasma includes a respiratory pigment, haemocyanin (bluish when oxidised, colourless when deoxidised), and a lipochrome pigment, called tetronerythrin. Both of these are common in other Crustaceans. Respiratory System. Twenty gills vascular outgrowths of the body wall lie on each side of the thorax, sheltered by the flaps of the shield. A current of water from behind forwards is kept up by the activity of the baling portion, or scaphognathite, of the second maxilla. Venous blood enters the gills from the ventral sinus, and purified blood leaves them by the six channels leading to the pericardium. Observed superficially, the gills look somewhat like feathers with plump barbs, but their structure is much more 264 CRUSTACEA. complex. The most important fact is that they present a large surface to the purifying water, while both the stem and the filaments which spring from it contain an outer canal continuous with the venous sinus, and an inner canal com- municating with the channels which lead back to the pericardium and heart. Three sets of gills are distinguishable. To the basal joints of the six appendages from the second maxillipede to the fourth large limb inclusive, the podobranchs are attached. They come off with the appendages when these are pulled carefully away, and each of them bears in addition to the feathery portion a simple lamina or epipodite. The membranes between the basal joints of the appendages and the body, from the second maxillipede to the fourth large limb inclusive, bear a second set, the arthrobranchs , which have no epipodites. In connection with the second maxillipede there is a single arthrobranch, in connection with each of the five following appendages there are two, so that there are eleven arthrobranchs altogether. There remain three pleurobranckSy one on the epimeron of the fifth large limb, and two others quite rudimentary on the two preceding segments. The bases of the podobranchs bear long setae. In Nephrops and the common lobster the number and arrangement of the gills is slightly different. Excretory System. A kidney or " green gland " lies behind the base of each antenna, and its opening is marked by a conspicuous knob on the basal joint of that appendage. Each kidney consists of a dorsal sac communicating with the exterior, and of a ventral coiled tube which forms the proper renal organ. The latter is supplied with blood from the antennary and abdominal arteries, and forms as waste products uric acid and greenish guanin. Each kidney may be regarded as homologous with a nephridium. In Palcemon, the kidney is connected by a glandular duct with a delicate dorsal "nephro-peritoneal sac," possibly coelo- mic. more probably an enlargement of the nephridial system. The crayfish has also, near the gills, small branchial glands which excrete carcinuric acid from the blood, and also help in phagocytosis, that important process in which wandering amoeboid cells resist infection and help to repair injuries (cf. possible function of thymus in Fishes). Reproductive Organs. The male crayfish is distinguished from the female by his slightly slimmer build, and by the copulatory modification REPRODUCTIVE ORGANS. 265 of the first two pairs of abdominal appendages. In both sexes the gonads are three lobed, and communicate with the exterior by paired ducts. The testes consist of two anterior lobes lying beneath and in front of the heart, and of a median lobe extending back- wards. Each lobe consists of many tubules within which the spermatozoa develop. From the junction of each of the anterior lobes with the median lobe, a genital duct or vas deferens is given off. This has a long coiled course, is in part glandular, and ends in a short muscular portion opening on the last thoracic limb. The spermatozoa are at first disc- like cells, they give off on all sides long pointed processes like those of a Heliozoon, and remain very sluggish. The FIG. 85. Female reproductive organs of Crayfish. (After SUCKOW.) ov, Ovaries ; ov' , fused posterior part ; od, oviduct : vu, female aperture on the second walking leg. seminal fluid is milky in appearance, and becomes thicker in its passage through the genital ducts. It is possible that the genital ducts represent modified nephridia, and that the cavities of the gonads are coelomic. The ovaries are like the testes, but more compact. The eggs are liberated into the cavity of the organ, and pass out by short thick oviducts opening on the second pair of walking legs. As they are laid they seem to be coated with the secretion of the cement glands of the abdomen, and the 266 CRUSTACEA. mother keeps her tail bent till the eggs are glued to the small swimmerets. Before this, however, sexual union has occurred. The male seizes the female with his great claws, throws her on her back, and deposits the seminal fluid on the ventral surface of the abdomen. The fluid flows down the canal formed by his first abdominal appendages, and these seem to be kept clear by the movements of the next pair, which FIG. 86. Section through the egg of Astacus after the com- pletion of segmentation. (After REICHENBACH.) st. Stalk of the egg ; ch, chorion envelope ; bl, peripheral blastoderm, within which are the yolk pyramids (dark). are also modified. On the abdomen of the female the agglutinated spermatozoa doubtless remain until the eggs are laid, when fertilisation in the strict sense is achieved. The Development has been very fully worked out, and is of interest in being direct, without the metamorphosis so common among the DEVELOPMENT. 267 Arthropoda. The spherical ovum is surrounded by a cuticular vitelline membrane, and contains a considerable quantity of yolk. After ferti- lisation the segmentation nucleus divides in the usual way into 2, 4, 8, and so on, but this nuclear division is not followed by division of the plasma. Eventually the nuclei, each surrounded by a small amount of protoplasm, approach the surface of the egg and arrange themselves regularly round it. The peripheral protoplasm then segments round these nuclei, and thus we have a central core of unsegmented yolk enveloped by a peripheral ring of rapidly dividing cells. In the central yolk free nuclei may be frequently found, these are the so-called yolk pd S.S FIG. 87. Longitudinal section of later embryo of Astacus. (After REICHENBACH.) ec, Kctoderm ; 7/2, mesoderm cells ; cg> cerebral ganglia ; st> stoma- todaeum ; A, anus : T, telson ; g, ventral ganglia ; ss, sternal sinus ; 4d t proctodffium ; /t, heart ; 7/z^", mid gut ; yolk pyramids (dark). nuclei. Such a type of segmentation is called peripheral or centro- lecithal, and is very characteristic of Arthropod eggs. Over a particular region of the segmented egg, known as the " ventral plate," the cells begin to thicken ; at this region an invagination occurs, which represents the gastrula. At the anterior lip of the blastopore the mesoderm appears, being many celled from the first. Soon the blasto- pore closes ; the cavity of the gastrula thus becomes a closed sac the future mid gut. The cells of this archenteron take up the core of yolk FIG. 88. Embryo of Crayfish, flattened out, with removal of yolk, magnified about 40 times. (After REICHENBACH.) Note rudiments of eyes and appendages, and in the middle line the nervous system. SYSTEMATIC SURVEY OF CRUSTACEA. 269 into themselves in a way which early suggests their future digestive function. On the surface of the egg there have already appeared ectodermic thickenings the so-called eye folds, rudiments of the appendages, and of the thoracic and abdominal regions. In the later stages invaginations of the ectoderm form the fore and hind gut, which grow inward from opposite ends to meet the endoder- mic mid gut, also the ear sac and the green glands. The gills are formed in great part from ectodermic outgrowths or evaginations. From the mid gut the digestive gland is budded out. The heart, the blood vessels, blood, and muscles are due to the mesoderm. As usual, the nervous system arises from an ectodermic thickening. The eye arises partly from the optic ganglia of the "brain," partly from the "eye folds," and partly from the epidermis. When the young crayfish are hatched from the egg shells, they still cling to these, and thereby to the swimmerets of the mother. In most respects they are miniature adults, but the cephalothorax is convex and relatively large, the rostrum is bent down between the eyes, the tips of the claws are incurved and serve for firm attachment, and there are other slight differences. The noteworthy fact is that the development is com- pleted wilhin the egg case, and that it is continuous without metamor- phosis. (The shortened life history of the crayfish is interesting in relation to its fresh water habitat, where the risks of being swept away by currents are obviously great ; but it must also be remembered that the tendency to abbreviate development is a general one.N There is some maternal care in the crayfish, for the young are said sometimes to return to the mother after a short exploration on their own account. SYSTEMATIC SURVEY OF THE CLASS CRUSTACEA. (i) Entomostraca, lower forms. They are usually small and simple. The number of segments and ap- pendages is very variable. The larva is generally hatched as a simple unsegmented Nauplius. There is no gastric mill. (2) Malacostraca, higher forms. They are usually larger and more complex. The head consists of 5, the thorax of 8, the abdomen of 6 (7 in Leptostraca) segments. The larva is usually higher than a Nauplius. There is a gastric mill. {A pus, Bronchi- pus, and Artemia (brine-shrimps), Daph nia^Moina, Polyphemus. 2. Ostracoda, Cypris, Cypridina. 3. Copepoda, Cyclops, Arguhts, many parasites. 4. Cirripedia, acorn shells and bar- nacles, e.g., Balamts and Lepas. Leptostraca, e.g. Arthrostraca, Thoracostraca, Nebalia. Amphipods (sand hoppers, &c.). Isopods (wood lice, &c.). Cuma. Squilla. Mysis. Shrimp, lobster, crayfish, crab. 270 CRUSTACEA. FIRST SUB-CLASS. ENTOMOSTRACA. These are the more primitive Crustaceans, often small and simple, with a variable number of segments and append- ages. The newly hatched larva is usually a Nauplius. The adult may retain the unpaired simple frontal eye, which is always found in the Nauplius, and has no gastric mill. Order i. Phyllopoda. Order 2. Ostracoda. Order 3. Copepoda. Order 4. Cirripedia. Order I. Phyllopoda. In these at least four pairs of swimming feet bear respiratory plates. The body is generally well segmented, and is protected by a shield-like or bivalve shell. The mandibles are without palps, and the maxillse are rudimentary. (a) Branchiopoda. The body has numerous segments and (10-20 or more) appendages with respiratory plates. The shell is rarely absent, usually shield-like or bivalved. The heart is a long dorsal vessel with numerous openings. The eggs are able to survive prolonged desiccation in the mud. Branchipus, a beautifully coloured fresh water form, with hardly any shell. Artemia. Brine shrimps. Periodically parthenogenetic. By gradually changing the salinity of the water, Schmankewitsch was able, in the course of several generations, to modify A. salina into A. milhlenhausii^ and vice versa. Artemia fertilis is one of the four animals known to occur in the dense waters of Salt Lake. Limnadia^ with bivalve shell. Periodically parthenogenetic. A mollusc-like bivalve shell is still more marked in Estheria. Apus, a fresh water form with a large dorsal shield. Periodically parthenogenetic. One species herma- phrodite. Of these Apus is certainly the most interesting. It is over an inch in length, and therefore a giant among Entomostraca. It has an almost world wide distribution. "It possesses peculiarities of organisation which mark it out as an archaic form, probably standing nearer to the extinct ancestors of the Crustacea than almost any other living member of the group." The appendages are very numerous and mostly leaf-like. They may be regarded as representing a primitive type of Crustacean limb. Professor Ray Lankester enumerates them as follows : fi. Antenna. Pre-oral. -! 2. Second antenna. (This is sometimes absent, and apparently always in certain species. ) ^3. Mandible. Oral. -[ 4. Maxilla. ( 5. Maxillipede. CLASSIFICATION OF CRUSTACEA. 271 (6. First thoracic foot (leg-like). Thoracic | 7-16. Other ten thoracic feet (swimmers). (Pre-genital).i The i6th in the female carries an egg sac or brood t chamber. There are eleven thoracic rings on the body. Abdominal f 17-68. Fifty-two abdominal feet, to which there corres- ( Post-genital). \ pond only seventeen rings on the body. The large dorsal shield is not attached to the segments behind the one bearing the maxillipedes. Many of the thin limbs doubtless function as gills. The genital apertures are on the i6th append- ages. The anus is on the last segment of the body. There is a pair of ventral ganglia to each pair of limbs ; the ventral nerve cords are widely apart ; and the cephalic ganglion is remarkably isolated. Professor Ray Lankester called this cephalic ganglion an "archi-cerebrum," to emphasise its pre- oral position and its distinctness from the posterior ganglia. Subsequent research has shown, however, that in Apus, as in other Crustaceans, the cephalic ganglion is a " syn-cerebrum, i.e., it is composed of pre-oral ganglia fused with post-oral ganglia which have been shunted forwards. (b) Cladocera. Small laterally compressed "water fleas," with few and somewhat indistinct segments. The shell is usually bivalved. The head often projects freely. The second pair of antennae are large, two-branched, swimming appendages, and there are 4-6 pairs of other swimming organs. The heart is a little sac with one pair of openings. An excretory organ (the shell or maxillary gland) opens in the region of the second maxillae. It is the Entomostracan equivalent of the antennary green gland of Malacostraca. The males are usually smaller and much rarer than the females. The latter have a brood chamber between the shell and the back. Within this many broods are hatched throughout the summer. Periodic parthenogenesis (of the " summer ova") is very common. "Winter eggs," which require fertilisa- tion, are set adrift in a part of the shell modified to form a protective cradle or ephippium. Daphnia, Moina, Sida, Polyphemus, Leptodora, and many other " water fleas " are extraordinarily abundant in fresh water, and form part of the food of many fishes. A few occur in brackish and salt water. Order 2. Ostracoda. Small Crustaceans, usually laterally compressed, with an indistinctly segmented or unsegmented body, rudimentary abdomen, and bivalve shell. There are only seven pairs of appendages. _, . Cypris (fresh water), Cypridina (marine). Order 3. Copepoda. Elongated Crustaceans, usually with distinct seg- ments. There is no dorsal shell. There are five pairs of biramose thoracic appendages, but the last may be rudimentary or absent. The abdomen is without limbs, and of its five segments the first two are sometimes united. The females carry the eggs in external ovisacs. Many are ecto-parasitic, especially on fishes ("fish lice ") 272 CRUSTACEA. and are often very degenerate. The free living Copepods form an important part of the food supply of fishes. ->_ Cyclops, free and exceedingly prolific in fresh water. Cetochilus free and abundant in the sea. Sapphirina, a broad flat marine form about quarter of an inch long, occasionally parasitic. The male surpasses all animals in the brilliancy of its "phosphorescent" colour. Chondr acanthus. As in many other cases, the parasitic females carry the pigmy males attached to their body. Caligus, a very common genus of " fish lice." Lerncea, Penella, etc. The adult females are parasitic, and almost worm-like. The males and the young are free. That the males are often free and not degenerate, while their mates are parasitic and retrogressive, may be understood by considering (i) the greater vigour and activity associated with maleness ; (2) the fact that parasitism affords safety and abundance of nutrition to the females during the reproduc- tive period. Arguhis, a divergent form temporarily parasitic on carp, &c. It has a shield-like cephalothorax and a small cleft abdomen. A protrusible spine projects in front of the blood sucking mouth; the mandibles and first maxillae are adapted for piercing ; the second maxilke or maxillipedes for adhesion. There are four pairs of two-branched swimming appendages. There are two large compound eyes. The female has no ovisacs ; the eggs are laid on foreign objects. Order 4. Cirripedia. Barnacles and acorn shells, and some allied degenerate parasites. Marine Crustaceans, which in adult life are fixed head down- wards. The body is indistinctly segmented, and is enveloped in a fold of skin, usually with calcareous plates. The anterior antennae are involved in the attachment, the posterior pair are rudimentary. The oral appendages are small, and in part atrophied. In most there are six (or less frequently four) pairs of two-branched thoracic feet, which sweep food particles into the depressed mouth. The abdomen is rudi- mentary. There is no heart. The sexes are usually com- bined, out dimorphic unisexual forms also occur. The herma- phrodite individuals occasionally carry pigmy or " comple- mental " males. The spermatozoa are mobile, which is unusual among Crustacea. Lepas, the ship barnacle, is as an adult attached to floating logs and ship bottoms. The anterior end by which the animal fixes itself is drawn out into a long flexible stalk, containing a cement gland, the ovaries, &c., and involving in its formation the first pair of antennae and the front lobe of the head. The second antennae are lost in larval life. The mouth region bears a pair of small mandibles and two pairs of small maxillae, the last pair united into a lower lip. The thorax has six pairs of two-branched appendages, and from the end of the rudimentary CLASSIFICATION OF CRUSTACEA. 273 abdomen a long penis projects. At the base of this lies the anus. Around the body there is a fold of skin, and from this arise fiye calcare- ous plates, an unpaired dorsal carina, two scuta right and left anteriorly, two terga at the free posterior end. The nervous system consists of a brain, an cesophageal ring, and a ventral chain of five or more ganglia. There is a fused pair of rudimentary eyes. No special circulatory or respiratory organs are known. Two excretory (?) tubes lead from (ccelomic) cavities to the base of the second maxillee, and are probably comparable with shell glands and with nephridia. There is a complete food canal and a large digestive gland. Beside the latter lie the branched testes, whose vasa deferentia unite in an ejaculatory duct in the penis. From the much branched ovaries in the stalk, the oviducts pass to the first thoracic legs, where they pass into a cement making sac, opening to the exterior. The eggs are found in flat cakes between the external fold of skin and the body. The life history is most interesting. Nauplius larvae escape from the FIG. 89. Acorn shell (Balanus tintinnabulum). (After DARWIN.) ^, Tergum ; s, scutum ; d, opening of oviduct ; _/J mantle cavity ; x, depressor muscle of tergum; g, depressor muscle of scutum; h, ovi- duct ; r, outer shell in section ; a, adductor muscle of scuta. egg cases, and after moulting several times become like little Cyprid water fleas. The first pair of appendages become suctorial, and after a period of free swimming, the young barnacle settles down on some floating object, mooring itself by means of the antennary suckers, and becoming firmly glued by the secretion of the cement glands. During the settling and the associated metamorphosis, the young barnacle fasts, living on a store of fat previously accumulated. Many important changes occur, the valved shell is developed, and the adult form is gradually assumed. While the early naturalists, such as Gerard (1597), regarded the barnacle as somehow connected with the barnacle goose, 18 274 CRUSTACEA. and zoologists, before J. Vaughan Thompson's researches (1829), were satisfied with calling Cirripedes divergent Molluscs, we now know clearly that they are somewhat degene- rate Crustaceans. We do not know, however, by what con- stitutional vice, by what fatigue after the exertions of adoles- cence, they are forced to settle down to sedentary life. The food consists of small animals, which are swept to the mouth by the waving of the curled legs. Growth is some- what rapid, but the usual skin casting is much restricted ex- cept in one genus. Neither the valves, nor the uniting mem- branes, nor the envelope of the stalk, are moulted, though dis- integrated portions may be re- moved in flakes and renewed by fresh formations. In the allied genus Scalpellum, some are like Lepas^ hermaphrodites, without complementary males (Sc. balanoides] ; others are hermaphrodite, with comple- mentary males (Sc. villosum\ &c. ; and others are unisexual, but the males are minute and parasitic (Sc. regium). Balanus, the acorn shell, en- crusts the rocks in great num- bers between high and low water marks. It may be de- scribed, in Huxley's graphic words, as a crustacean fixed by its head, and kicking the food into its mouth with its legs. The body is surrounded, as in Lepas, by a fold of skin, which forms a rampart of six or more calcareous plates, and a four- fold lid, consisting of two scuta ,-, and two terga. When covered FlG ' 9- -Development of Sacculma. by the tide, the animal B (After DELAGE.) (Not drawn to scale.) tide, the animal pro- trudes and retracts between the valves of the shell six pairs of curl-like thoracic legs. The structure of the acorn shell is in the main like that of the barnacle, but there is no stalk. A. Free swimming Nauplius, with three pairs of appendages ; B. Pupa stage ; C. Adult protruding from the tail of a crab. CLASSIFICATION OF CRUSTACEA. 275 The life history also is similar. A Nauplius is hatched. It has the usual three pairs of legs, an impaired eye, and a delicate dorsal shield. It moults several times, grows larger, and acquires a firmer shield, a longer spined tail, and stronger legs. Then it passes into a Cypris stage, with two side eyes, six pairs of swimming legs, a bivalve shell, and other organs. As it exerts itself much but does not feed, it is not unnatural that it should sink down as if in fatigue. It fixes itself by its head and antennae, and is glued by the secretion of the cement gland. Some of the structures, e.g., the bivalve shell, are lost ; new structures appear, e.g., the characteristic Cirriped legs and the shell. Throughout this period, which Darwin called the "pupa stage," there is external quiescence, and the young creature continues to fast. The skin of the pupa moults off ; the adult structures and habits are gradually assumed. At frequent periods of continued growth, the lining of the shell and the cuticle of the legs are shed. In spring these glassy cast coats are exceedingly common in the sea. Acorn shells feed on small marine animals. They fix themselves not to rocks only, but also to shells, floating objects, and even to whales and other animals. Alcippe and Cryptophialus (with only three or four pairs of feet) live in the shells of other Cirripedes or of Molluscs ; Proteolepas is parasitic in the mantle of other Cirripedes, and like a grub. On the ventral surface of the abdomen of crabs, Sacculina, the most degenerate of all parasites, is often found. Its complete history has been beautifully worked out by Professor Delage. It is in shape an ovoid sac, and is attached about the middle of a segment. On the lower surface of the sac there is a cloacal aperture, opening into a large brood chamber, usually distended with eggs contained in chitinous tubes. The brood chamber surrounds the central "visceral mass," consisting of a nerve ganglion, a cement gland which secretes the egg cases, and the hermaphrodite reproductive organs ; of digestive or vascular systems there is no trace. The parasite is attached by a peduncle, dividing up, within the body of the crab, into numerous "roots," which have been compared to the placenta of a mammalian fcetus. The "roots" ramify within the body of the crab, and by them the Sacculina obtains nutrition and gets rid of its waste products ; it is therefore practically, even at this stage, an endoparasite. The larvae leave the brood chamber as Nauplii, they moult rapidly and become Cyprid larvae. These fix themselves by their antennae to young crabs, at the uncalcified membrane surrounding the base of the large bristles of the back or appendages. The thorax and abdomen are cast off entirely ; the structures within the head region contract ; eyes, tendons, pigment, the remaining yolk, and the carapace are all lost ; and a little sac remains, which passes into the interior of the crab. Eventually it reaches the abdomen, and, as it approaches maturity, the integuments of the crab are dissolved beneath it, and the sac-like body protrudes ; essentially, however, Sacculina is always endoparasitic. It appears to live for three years, during which time the growth of its host is arrested, and no moult occurs. 276 CRUSTACEA. SECOND SUB-CLASS. MALACOSTRACA. These are higher Crustaceans in which the body consists of three regions with a constant number of segments, five to the head, eight to the thorax, and six to the abdomen (except in forms like Nebalia, which have seven). The terminal piece or telson of the abdomen is regarded by some zoologists as a distinct segment. Apart from this telson, and also the segment next to it in Nebalia, all the segments bear paired appendages. More or less of the thorax is fused to the head region, and the anterior thoracic limbs are usually auxiliary to mastication. Two compound lateral eyes and a gastric mill are always present. There is an antennary excretory gland, probably comparable with the Entomostracan maxillary gland. (The female genital apertures are on the third last pair of thoracic legs, the male apertures on the last pair. Very few are hatched in the Nauplius stage, many, however, at the Zoaea level, while others have no metamorphosis at all. Legion i. Leptostraca. Nebalia. Legion 2. Arthrostraca, with three orders, Anisopoda, Isopoda, Amphipoda. Legion 3. Thoracostraca, with four orders, Cumacea, Stomatopoda, Schizopoda, Decapoda. Legion I. Leptostraca. Marine Crustaceans of great systematic interest, because they retain in many ways the simplicity of ancestral forms, and link Malacostraca to Phyllopods. The most important genus is Nebalia. A bivalve shell covers the whole of the lank body, except the last four abdominal segments ; the head is free from the thorax ; the eight segments of the thorax are free from one another, and the plate-like appendages resemble those of Phyllopods ; the abdomen has seven segments and a telson with two forks ; the elongated heart extends into the abdomen, and has seven pairs of lateral apertures or ostia. Nebalia and its congeners are probably related to certain ancient fossil forms from Palaeozoic strata Hymenocaris, Ceratiocaris, &c. Legion 2. Arthrostraca. (Edriophthalmata, sessile eyed.) There is no shell fold or shield, except in the order Anisopoda. The first thoracic segment (rarely with the addition of the second) is fused to the head, the corresponding appendages serve as maxillipedes, the other thoracic segments (seven or six) are free. The eyes are sessile. The heart is elongated. CLASSIFICATION OF CRUSTACEA. 277 Order I. Anisopoda. The fusion of the first two thoracic segments to the head, the presence of a cephalothoracic shield, and other divergent features distinguish Tanais, Apsetides, 6<:., from the Isopoda. Order 2. Isopoda. The body is flattened from above downwards. The first thoracic segment is fused to the head, while the other six or seven are free, and there is no cephalothoracic shield. The abdomen is usually short, and its appendages, usually overlapped by the first pair, are plate-like, and function in part as respiratory organs. ^=*The "wood lice" (Oniscus, Porcellio) are familiar animals which lurk in damp places under stones and bark, and devour vegetable refuse. Some related forms (e.g., Armadillo), which roll themselves up, are called " pill bugs." In the terrestrial forms there is obviously a departure from the ordinarily aquatic habit of Crustaceans, and the exopodites of some of the abdominal appendages have tubular air passages. Asellus is a very common form, living in both fresh and salt water. Idotea is not uncommon among the shore rocks. The "gribble" ( Linmoria lignorum) is a destructive marine Isopod which eats into wood. Among the marine Cymothoidse which are often parasitic on fishes, some, e.g., Cymothoe, are remarkable in their sexual condition, for they are hermaphrodites in which the male organs mature and become functional when the oviducts are still closed, while at a later period in life the male organs are lost and the animals become functionally female. The Bopyridre infest the gill chambers of other Crustaceans, e.g., prawns. The pigmy males are usually carried about by their mates. Among the parasitic Cryptoniscidce, we again find herma- phrodites with associated pigmy males. In not a few cases they seriously affect the reproductive organs of their male hosts. Many of these Isopods, like not a few other Crustaceans, are extremely interesting to those who care to think about the problem of sex. Thus, to cite one other instance, the males and females in the genus Gnathia are so unlike, that they have been mistakenly referred to different sub-families. Order 3. Amphipoda. The body is laterally compressed. In most it is only the first thoracic segment which is fused to the head, in the " no-body-crabs " (Caprellida), and "whale lice" (Cyamida}, two segments are involved. The thoracic limbs bear respiratory appendages. Of the six pairs of legs w 7 hich the abdomen usually bears, the anterior three are usually more strongly developed as swimmers, while the posterior three directed backwards are used in jumping. 278 CRUSTACEA. Ganunarus pulex is very common in fresh water. Other species occur on the sea-shore. There also the " Beach fleas " (Talitrus and Orchestia] are exceedingly abundant. On solid ground they move on their sides in a strange fashion, but they swim very swiftly. Hyperia, Phronima, and many marine Amphipods, have a habit of living as commensals with other animals. Caprella, a common marine gymnast on Hydroids, &c., has the trunk of the body reduced to the quaintest possible minimum. Legion 3. Thoracostraca. (Podophthalmata, with stalked eyes.) Several or all of the thoracic segments are fused to the head, and there is a cephalothoracic shield overlapping the gills. The two eyes are stalked except in Cumacea. Order I. Cumacea. The cephalothoracic shield is small, and four or five thoracic segments are left uncovered and free. The eyes are sessile and adjacent or fused. There are two pairs of maxillipedes. The females have no abdominal appendages except on the last segment. The genera are marine, e.g., Cuma or Diastylis. Order 2. Stomatopoda. The shield is still small and does not cover the three posterior thoracic segments. The body is somewhat flattened, the abdomen is very strong. Five anterior thoracic appendages are directed towards the mouth, and serve to catch food, and to clamber. The five anterior abdominal legs carry feathery gills, the sixth pair forming swimming paddles. The elongated heart extends into the abdomen, which also contains the reproductive organs. The genera are marine, e.g., Squilla. Order 3. Schizopoda. A delicate shield covers the whole of the thorax, but there is still some freedom as to one or more of the posterior thoracic segments. The eight thoracic appendages are uniformly biramose, but the first two may serve as maxillipedes. The abdominal appendages of the male are strongly developed, those of the female are weak except the last, which in both sexes form paddles. They are marine forms, e.g., Mysis (without gills on the thoracic legs), Lophogaster and Euphausia (with gills on the thoracic legs). The last named starts in life as a Nauplius. As an adult it has luminous organs on the eye-stalks, thoracic legs, and abdominal segments. Order 4. Decapoda. The shield is large and firm, and is fixed to the dorsal surface of all the thoracic segments. Of the thoracic appendages, the first three pairs are maxillipedes, the five other pairs are jointed walking legs (whence the term Decapod). Sub-order I. Macrura. Abdomen long. Homarus (lobster) ; Nephrops (Norway lobster, sea crayfish) ; Astactis (fresh water crayfish) ; Palinurus (rock lobster), whose larva was long known as the glass crab (Phyllosoma) ; Pencsus, a shrimp which passes through Nauplius, Zoeea, and Mysis stages ; Lucifer and Sergestes are also hatched at a stage antecedent to the Zooea ; Crangon vulgaris (the British GENERAL NOTES ON CRUSTACEANS. 279 shrimp) ; PahEtnon, Pandalus, Hippolyte (prawns) ; Galathea (with the abdomen bent inwards) ; Pagurtis, Etipagurus (hermit crabs) ; Birgus latro (the terrestrial robber or palm crab), in which the upper part of the gill cavity is shut off to form a " lung," the walls having numerous vascular plaits. Opinion seems to incline against recognising a separate sub- order (Anomura) for the soft-tailed hermit crabs. Sub-order 2. Brachyura. Abdomen short, and bent under the thorax. It is narrow in the male, and does not usually bear more than two pairs of appendages ; it is broader in the female, and bears four paired appendages. The ventral ganglia have fused into an oval mass. Cancer (edible crab) ; Carcinus m&nas (shore crab) ; Portunus (swimming crab) ; Maia (spider crab) ; Lithodes (stone crab) ; Porcellana ; Dromia (often covered by a sponge) ; Pinnotheres (living inside bivalves) ; Gelasimus (fiddler crab, a very adept burrower) ; Telphusa (a fresh water crab) ; Gecarcinus (land crabs, only visiting the sea at the breeding season). * History. Fossil Crustaceans are found in Cambrian strata, but the highest forms (Decapoda) were not firmly established till the Tertiary period. Some of the genera, e.g., the Branchiopod Estheria, living from Devonian ages till now, are remarkably persistent and successful. How the class arose, we do not know : it is probable that types like Nebalia give us trustworthy hints as to the ancestors of the higher Crustaceans ; it is likely that the Phyllopods, e.g., Apus, bear a similar relation to the whole series ; the Copepods also retain some primitive characteristics ; but it is difficult apart from mere guessing to say any- thing definite as to the more remote ancestry. We naturally think of a segmented worm type as a plaus- ible starting-point for Crustaceans, and it is not difficult to understand how a development of cuticular chitin would tend to produce a flexibly jointed limb out of an unjointed parapodium, how the mouth might be shunted a little back- wards, and two appendages and ganglia a little forwards, and how division of labour would result in the differentia- tion of distinct regions. GENERAL NOTES ON CRUSTACEANS. Of a class that includes animals so diverse as crabs, lobsters, shrimps, "beach fleas," "wood lice," barnacles, acorn shells, and " water fleas," it is difficult to state general characteristics, other than those facts of structure which we have already summarised. Admitting the parasitism of many Crustaceans, and the sedentary life of barnacles and acorn shells, we must still 280 CRUSTACEA. allow that great activity characterises the class. With this may be connected the brilliant colouring, the power of colour change, and the phosphorescence of many forms. Except some primitive and degenerate forms, all are seg- mented. The typical appendage consists of a basal piece with two jointed branches. The cuticle is always chitinous, and often very much calcified. The abundance of chitin may, to some extent, explain the absence of cilia in Crus- taceans and other Arthropods. The rigidity of the cuticle partially explains the necessity of frequent moults. As the muscles contract very rapidly, they illustrate the striated condition with great clearness. In crabs and some others the ventral ganglia are concentrated. Sensory organs are generally well developed; both "eyes" and "ears" may occur away from the head. Much of the alimentary canal, which is almost always simple, consists of fore gut and hind gut. These are anterior and posterior invaginations of skin which meet the mid gut or archenteron the original gastrula cavity and are especially large in the higher Crustaceans or Malacostraca. The frequent presence of a gastric mill is quite intelligible, for it occurs in the fore gut. The body cavity is never very large, being mainly filled up with muscles and organs, and of a true coelome there is little trace. In the blood hsemocyanin is the commonest respira- tory pigment. In the body or skin lipochrome pigments, such as those which change from bluish green to red as the lobster is boiled, frequently occur. Of modes of respiration, there are many grades, by the general surface, by currents of water in and out of the posterior part of the food canal, by thin plates on the legs, by well-formed gills. We miss the numerous excretory nephridia of Annelids ; the green- glands of lobsters, &c., probably represent a pair ; the shell- glands of Phyllopods and Copepods and some other struc- tures seem to be in part at least excretory. It is possible that shell making is an organised method of getting rid of some waste products. There are many peculiarities con- nected with reproduction ; thus parthenogenesis for pro- longed periods is common among " water fleas " ; herma- phroditism occurs in barnacles, acorn shells, &c. ; the hermaphrodites are sometimes accompanied by pigmy " complemental " males ; the two sexes are often very GENERAL NOTES ON CRUSTACEANS. 281 diverse. The spermatozoa are usually exceptional in being very slightly motile. Some appendages are often modified for copulation or for carrying the eggs. Development. The ova of most Crustacea show con- siderable similarity to those of Astacus, and the segmen- tation is typically of the kind already described. But while this is the most typical case for Crustacean, and, indeed, for Arthropod development, it is pos- sible, within the limits of the class Crustacea, to trace out a complete series, in which the first term is a segmentation of the complete and equal type, like that of a worm, and the last the purely peripheral. In the same way, though gastru- lation is usually much disguised, we find all cases from an invagina- tion of the simplest em- bolic type (Lucifer), and through the condition described for Astacus^ to the formation of endo- derm by the ingrowth of a solid plug of cells (Arthrostraca, &c.). FIG. 91Zcxea of common Shore Crab Compared with As- (Carcimts mcenas}. (After FAXON.) tacus, however, the most The appendages are numbered. important point we have to notice is the frequent occurrence of a very striking metamorphosis in the life history. In other words, the larva hatched from the egg is rarely like the parent, and only acquires the adult char- acters after a series of profound changes. In some cases (Nebalia, Mysis) a metamorphosis takes place within the egg-cases, and in the few forms in which development seems to be direct, slight traces of metamorphosis are found. 282 CRUSTACEA. Almost all the lower Crustaceans and the higher forms Euphausia and Penceus are hatched in a Nauplius stage. In the remaining cases the Nauplius stage is indicated within the egg by the moulting of a larval cuticle (so in Astacus}. The Nauplius is characterised by a typically rounded body, and by the presence of three pairs of append- ages, which are the only obvious indications of segmenta- tion. The first pair of appendages are unbranched and bear larval sense organs, the next two are biramose swim- ming organs. There is an unpaired median eye, but no heart and frequently no hind gut. The three pairs of appendages become the first and second pairs of antennae and the mandibles of the adult. The head region of the Nauplius becomes the head region of the adult, the posterior region also persists, the new growth of segments and append- ages takes place (with numerous moultings) in the region between these. The second important form of larva is the Zoaea, which has all the appendages on to the last maxillipedes inclusive, an unsegmented abdomen, and two lateral compound eyes in addition to the unpaired one of the Nauplius stage. Most Decapoda are hatched in the Zoaea stage. - (a) The crayfish (Astacus] is hatched almost as a miniature adult. The development is therefore very direct in this case. (b) The lobster (Homarus} is hatched in a Mysis stage, in which the thoracic limbs are two-branched and used for swimming. After some moults it acquires adult characters. (c) Crabs are hatched in the Zocea form, and pass with moults through a Megalopa stage, in which they resemble certain Hermit Crabs. The abdomen is subsequently tucked in under the thorax. (d) Penceus (a kind of shrimp) is hatched as a Nauplius, becomes a Zocea, then a Mysis, then an adult. Its relative Lucifer starts as a Meta-Nauplius with rudiments of three more appendages than the Nauplius. Another related form, Sergestes, is hatched as a Protozocea> with a cephalothoracic shield and an unseg- mented abdomen. Thus there are two grades between Nauplius and Zocea. Three facts must be borne in mind in thinking over the life histories of crayfish, lobster, crab, and Penceus: (l) there is a general tendency to abbreviate development, and this is of more importance when meta- morphosis is expensive and full of risks ; (2) there is no doubt that larvae exhibit characters which are related to their own life rather than to that of the adult ; (3) it is a general truth, that in its individual development the organism has to recapitulate to some extent the evolution of the race, that ontogeny recapitulates phylogeny. But while there can be no BIONOMICS. 283 doubt that the metamorphoses of these Crustaceans is to some extent interpretable as a recapitulation of the racial history, for there were unsegmented animals before segmented forms arose, and the Zocza stage is antecedent to the Mysis, &c., yet it does not follow that ancestral Crustaceans were like Nauplii. On the contrary, the Nauplius must be regarded as a larval reversion to a type much simpler than the ancestral Crustacean. Moreover, this idea of recapitulation offers a philosophical rather than a material explanation of the facts. Bionomics. Most Crustaceans are carnivorous and predatory ; others feed on dead creatures and organic debris in the water ; a minority depend upon plants. Parasitism occurs in over 700 species, in various degrees, and of course with varied results. Most of the parasites keep to the outside of the host (e.g.. Fish lice), and suck nourishment by their mouths ; the Rhizocephala (e.g., Sacculimi), send ramifying absorptive roots through the body of the host. Sometimes the parasitism is temporary (Ar- gulus] ; sometimes only the females are parasitic (e.g., in Lernced). The parasites tend to lose appendages, segmen- tation, sense organs, &c., but the reproductive organs become more fertile. The hosts, e.g., crabs infested by Rhizo- cephala, are sometimes materially affected, and even ren- dered incapable of reproducing. Some Crustaceans live not as parasites but as commensals with other animals, doing them no harm, though sharing their food. Thus there is a constant partnership between some hermit crabs and sea anemones. The hermit crab is concealed and protected by the sea anemone ; the latter is carried about by the Crustacean and gets fragments of food. Masking is also common, especially among crabs. Some will cut the tunic off a sea squirt and throw it over their own shoulders. Many attain a mask more passively, for they are covered with hydroids and sponges, which settle on the shell. There is no doubt, however, that some actively mask themselves, for besides those known to use the Tunicate cloak, others have been seen planting seaweeds on their backs. The protective advantage of masking both in offence and defence is very obvious. The intelligence of crabs and some of the higher Crus- taceans is well developed. Maternal care is frequent. 284 CRUSTACEA. Fighting is very common, but the loss of limbs is readily repaired. Deep sea Crustaceans are very abundant, and often remarkable " for their colossal size, their bizarre forms, and brilliant red colourings ; " some are blind, others are brilliantly phosphorescent. Yet more abundant are the pelagic Crustaceans (especially Entomostraca and Schizo- pods) ; they are often transparent except the eyes, often brightly coloured or phosphorescent. Many Crustaceans live on the shore, and play a notable part in the struggle for existence which is so keen in that densely crowded region. The lower Crustaceans are abundantly represented in fresh water, in pools, streams, and lakes. A few, such as wood lice and land crabs, are terrestrial, and some blind forms occur in caves. CHAPTER XIV. 'PERIPATUS, MYRIOPODS, AND INSECTS. Series ARTHROPODA. Sub-division TRACHEATA ANTENNATA. . Classes PROTOTRACHEATA. Peripatus. MYRIOPODA. Centipedes and Millipedes. INSECTA. Insects. THESE three classes form a series of which winged insects are the climax. The type Peripatus is archaic, and links the series to the Annelids ; the Myriopods lead on to the primitive wingless insects. We may speak of the series as Tracheata Antennata, for all breathe by tracheae tubes which carry air to the recesses of the body and all have antennae. First Class of Tracheata Antennata PROTOTRACHEATA, including one genus, Peripatus. GENERAL CHARACTERS. The body is worm-like inform, soft skinned^ and without external segmentation. There is a pair of prominent pre-oral antennce. The true appendages are a pair of jaws in the mouth, a pair of slime secreting oral papilla, numerous pairs of short, imperfectly jointed legs, each with two claws, and a pair of anal p apt lice. The legs contain peculiar (coxal) glands. Respiration is effected by numerous trachece, ivhose openings are somewhat scattered on the surface of the body. The heart is simply an elongated dorsal vessel with valvular openings. There is a series of excretory tubes or nephridia. The halves of the ventral nerve cord are ividely separate. The single genus Peripatus is represented by numerous 286 PERIPATUS, MYRIOPODS, AND INSECTS. (tivelve) species, widely distributed; in its possession of trachea and nephridia it is an interesting connecting link ; in many ways it seems to be an old fashioned survivor of an archaic type. The species of Peripatus are beautiful animals. Professor Sedgwick says "The exquisite sensitiveness and continu- ally changing form of the antennae, the well- rounded plump body, the eyes set like small diamonds on the side of the head, the deli- cate feet, and, above all, the rich colouring and velvety texture of the skin, all combine to give these animals an aspect of quite exceptional beauty." As to their habits, Mr. Hatchett Jackson says " They live under stones, in rotting wood, &c., in moist places, are nocturnal in habit, and feed on insects, &c., which they ensnare by the ejectioji of slime from the oral papillae." To their shy habits, their persistence is possibly in part due. They are able to move quickly, some- what after the fashion of Millipedes, especi- ally like Scolopendrella. Young forms roll up when touched, and have been seen to climb up vertical glass plates. FIG. 92. External form of Peripatus. (After BAL- FOUR.) Note antennae and simple feet. The species acknowledged by Sedgwick are : Four from South Africa P. capensis^ P. balfouri, and P. brevis from Table Mountain, and P. moseleyi from near Williamstown ; two from Australasia P. nova zealandice from New Zealand, and P. leuckarti from Queensland ; seven from neotropical regions P. edwardsii from Cara- cas, P. imthurmi or demeraranus from Demerara, P. trinidadensis and P. torqiiatus from Trinidad, P. iuliformis from St. Vincent, P. chilensis from Chili, P. quitensis from Ecuador, besides which there are some doubtful forms. The list shows how widely this remarkable genus is distributed. As the different species have similar habits, and live in very similar conditions, the differences between them perhaps illustrate purely con- stitutional variations. A more Detailed Account of Peripatus. Form. The body suggests an Annelid or a caterpillar, but, apart from the appendages, there is no external segmentation. Over the soft skin are numerous minute warts with small bristles. The mouth is ventral and anterior ; the anus terminal and posterior. DETAILED ACCOUNT OF PERIPATUS. 287 Appendages. The two large, ringed antennae do not seem to be homologous with limbs. The first pair of appendages double sickle- like jaws lie in the mouth cavity. A little further back are two oral papillae from which slime is exuded. Then there are the 14-42 stump- like legs, each with two terminal chitinous claws. In the young P. capensis the leg is said to be five -jointed, but in the adults there is no trace of this. In respect to its legs, therefore, Peripatus is hardly an Arthropod. Skin. The chitinous cuticle, ordinarily thick in Arthropods, is delicate. The ectoderm [hypodermis, or epidermis] is a single layer of cells. The Muscular System is very well developed. ( I ) Externally there is a layer of circular muscles ; (2) within this lies a double layer of diagonal fibres ; (3) internally there are strong longitudinal bundles. Finally, in connection with this internal layer, there are fibres which divide the body cavity into a median and two lateral compartments. The median includes heart, gut, slime glands, reproductive organs ; the laterals include the nerve cords, the salivary glands ; the legs con- tain nephridia and coxal or crural glands. Striped, rapidly contracting muscles are characteristic of Arthropods, but m Peripatus the muscles are unstriped, excepting those which work the jaws and are perhaps the most active. The Nervous System consists of a dorsal brain and two widely separate lateral ventralnerve cords. These are connected transversely by numer- ous commissures, are slightly swollen opposite each pair of legs to which they give off nerves, and are united posteriorly over the anus. There are only hints of ganglia, but there is a continuous layer of ganglionic cells. The brain is very homogeneous, simpler than that of most Insects. From the brain nerves pass to the antennae, &c. , and two viscerals or sympathetics, soon uniting, innervate the anterior part of the gut. Sense organs are represented by two simple eyes on the top of the head. These are most like the eyes of some marine Annelids. Behind each there lies a special optic lobe connected with the brain, but the eye itself arises as a dimple in the skin. Alimentary Canal. Round about the mouth, papillae seem to have fused to form a " mouth cavity," which includes the mandibles, a median pad or tongue, and the opening of the mouth proper. The mouth leads into a muscular pharynx, into which opens the common duct of two large salivary glands, which extend far back along the body. Mouth, pharynx, and short oesophagus are lined by a chitinous cuticle, like that of the exterior. The long digestive region or mid gut extends from the second leg nearly to the end of the body. Its walls are plaited. Finally, there is a short rectum, lined by a chitinous cuticle. Circulatory System. The dorsal blood vessel forms a long contractile heart. It lies within a pericardial space, and receives blood by seg- mentally arranged apertures with valves. The circulation is mostly in ill-defined spaces in the apparent body cavity or " haemoccele." The Respiratory System consists of very long and very fine unbranched tracheae, which are widely distributed in the body ; a number open together to the exterior in flask-like depressions. These openings or stigmata are diffuse and irregular in Peripatus edwardsii, but in P< capensis 288 PERIPATUS, MYRIOPODS, AND INSECTS. there is a dorsal and ventral row on each side. In P. novce zealandice the tracheae are said to be branched. The Excretory System. A pair of nephridia lie in each segment. Each consists of an internal terminal funnel, a looped canal, and a wide vesicle which opens near the base of each leg. They are not very differ- ent from those of many Annelids, but their occurrence in a Tracheate is remarkable. The salivary glands and the genital ducts are probably modified nephridia. It may be noted, too, that the same is perhaps true of the " coxal glands " of Limulus and of the antennary glands of Crustaceans. Crural or Coxal Glands lie in the legs and open to the exterior. Their meaning is uncertain, their occurrence is variable. Thus in P. edwardsii they occur in the males only, in P. capensis they are present in both sexes. In the male of P. capensis the last pair are very long (a.g., Fig. 93). The large mucus glands, which pour forth slime from the oral papillae, are regarded as modified coxal glands. Reproductive System. (a) Female (of P. edwardsii]. From the two bvaries, which are surrounded by one connective tissue sheath, the eggs F.I F.2 FIG. 93. Dissection of Peripatus capensis. (After BALFOUR.) at., Antennae; or,p., oral papillae; e.g., cerebral ganglia ; sl.d., duct of slime gland (sl.g.) ; s.o.8, segmental organ or nephridium eighth ; v.c., ventral nerve connected by transverse commissures (co.) with its fellow; v^.g., last crural gland; s.o.i?, seventeenth neph- ridium; -.0., genital aperture; A, anus; p.d.c., posterior com- missure ; F.if, seventeenth appendage : a.g., last crural gland; F.I, F.2, first and second legs ; oe.co., oesophageal nerve commis- sure. pass by two long ducts leading to a common terminal vagina opening between the second last legs. These ducts are for the most part uteri, but on what may be called the oviduct portions adjoining the ovaries, there are two pairs of pouches (a) a pair of receptacula seminis (for storing the spermatozoa received during copulation), and a pair of receptacula ovorum for storing fertilised eggs. The eggs are hatched in the uteri, and all stages are there to be found in regular order. The young embryos seem to be connected to the wall DETAILED ACCOUNT OF PERIPATUS. of the uterus by what has been called a " placenta," so suggestive is it of mammalian gestation. The older embryos lose this " placenta," but each lies constricted off from its neighbours. When born the young resemble the parents except in size and colour. In P. novce zealandice, the ova pass from the ovary into the uterus in December, and the young are born in July a long period of gestation. (d) Male (of P. edwardsii]. The male elements are produced in small testes, pass thence into two seminal vesicles, and onwards by two vasa deferentia into a long single ejaculatory duct, which opens in front of the anus. In the ejaculatory duct the spermatozoa are made into a long packet or spermatophore, which is attached to the female during copulation. [While it is characteristic of Arthropods, in which the de- velopment of chitin is so pre- dominant, that ciliated epi- thelium is absent, it seems that in Peripatus, which is much less chitinous than the others, ciliated cells occur in some parts of the reproductive ducts, and perhaps also at the internal funnels of the nephridia. This is indeed what one would expect.] Development of Peripatus. There is a strange variety of development in different species of this genus. Thus there is much yolk in the ovum of P. novce zealandice, extremely little in that of P. capensis. In the former species the yolk has a manifold origin ; it is said to arise in the protoplasm or the ovum itself from the breaking up of the germinal vesicle, from surrounding follicle cells, and from yolk present within the ovary. In P. capensis and P. a, Anus ; bl, blastopore ; m, mouth ; balfouri spermatozoa reach the &.i?e SeSmentS; "" 2 ne f vary, anS there probably the ova are fertilised, but in P. novce zealandice the spermatozoa are confined to the receptaculum seminis, near which fertilisation seems to occur. In the maturation of the ova of P. capensis and P. balfouri two polar bodies are extruded as usual, but none have been observed in the case of P. novce zealandice. In P. capensis the " segmentation " is remarkable, for true cleavage of cells does not occur. The fully "segmented" ovum does not exhibit the usual cell limits. It is a protoplasmic mass or syncytium with many nuclei. Even when the body is formed, the continuity of cells 19 7 FlG. 94. Embryos of Peripatus capensis, showing closure of blas- topore and curvature of embryo. (After KORSCHELT and HEIDER.) 290 PERIPATUS, MYRIOPODS, AND INSECTS. persists, nor does the adult lack traces of it. To Professor Sedgwick, this singular fact suggested the theory that the Metazoa may have begun as multinucleate Infusorian-like animals. The gut appears as a large vacuole within the rnultinucleated mass, and a gastrula stage is thus established. In the ova of P. nova zealandicz, which have much yolk, a superficial multiplication of nuclei forms a sort of blastoderm, which spreads over almost the entire ovum. The segmentation in this case has been called centrolecithal (the type characteristic of Arthropods), but it is again true that for a long time the cells do not exist as well defined units. It has been said, indeed, that " the embryo is formed by a process of crystallis- ing out in situ from a mass of yolk, among which is a protoplasmic reticulum containing nuclei." From these examples the student will perceive how difficult it is to give a succinct account of the development of Peripatus. Development of Organs. The hypodermis is ectodermic, the cuticle an external product thereof. The muscles are as usual derived from the mesoderm, which arises from two ventral mesodermic strands. These are subsequently divided into hollow segments. The true body cavity or coelome is represented by the original cavities of the mesoderm segments. In the adult this series of truly ccelomic cavities is hardly represented except by the inner- most portions of the nephridia. The apparent body cavity is a secondary cavity, consisting, for the most part, of blood carrying or vascular spaces, subsequently established in the mesoderm. It is divided into five regions, the central space, the two lateral cavities, and the cavities of the legs. The appendages are outgrowths of the body wall. They, and all the segmentally arranged parts, develop progressively from in front back- wards. The nervous system is derived from ectodermic thickenings which sink inwards. It develops from in front of the mouth backwards. The food canal consists of the long endodermic mid gut or mesenteron (the gastrula cavity), of an anterior ectodermic invagination form- ing pharynx and gullet (fore gut or stomatodseum), and of a short posterior ectodermic invagination forming the rectum (hind gut or proctodseum). The nephridia have a twofold origin. The internal funnel is derived directly from part of a mesodermic segment or vesicle. The rest of the nephridium is invaginated from the ectoderm. The reproductive organs arise on the epithelium of a persistent portion of the true coelome or primitive body cavity. Zoological Position of Peripatus. Professor Lang, in his work on Comparative Anatomy, summarises the synthetic characters of Peripatus as follows : MYRIOPODA. 291 ANNELID CHARACTERISTICS. TRACHEATE CHARACTERISTICS. The presence of tracheae. The nature of the heart and the lacunar circulation. The modification of appendages as mouth organs. The form of the salivary glands. The smallness of the genuine body cavity or ccelome. Segmentally arranged nephridia as in Chaetopods. Segmentally arranged coxal glands, like similar glands in some Chaetopods. The muscular ensheathing of the body. Less important are the stump- like legs and the simple eyes. The ladder like character of the ventral nervous system (cf. primitive Molluscs, Phyllopod Crustaceans, and Nemerteans) is probably primitive. That salivary glands and genital ducts are homologous with nephridia is a fact of much morphological interest. It is possible that the slime glands are modifications of coxal or crural glands, and that the latter are homologous with the parapodial glands of some Annelids. It is not certain that the antennae, jaws, and oral papillae of Peripatus precisely correspond to the antennae, mandibles, and first maxillae of Insects. Our general conclusion is that Peripatus is an archaic type, a survivor of forms which were ancestral to. Tracheata and closely related to Annelids. Second Class of Tracheata Antennata. MYRIOPODA. Centipedes and Millipedes. These animals retain a worm-like shape ; the numerous rings of the body and the appendages they bear are very uniform ; there is little division of labour. It would be rash to assert that any of the modern Myriopods are stages in the pedigree of Insects, but it is likely that the two classes are branches from one base. Simple wingless insects, known as Collembola and Thysanura, are closely approached by such Myriopods as Scolopendrella. Both centipedes and millipedes live on land, but two or three of the latter (e.g., a species of Geophilus) occur on the seashore. Most are very shy animals, lurking in dark places and avoiding the light. The head bears a pair of antennae, and two pairs of appendages mandibles and maxillae. The limbs are six- or seven-jointed, clawed, and very uniform. They have many more legs than insects, but they make less of them. The nervous system, heart, excretory tubules, &c., are like those of Insects. 292 PERIPATUS, MYRIOPODS, AND INSECTS. The development in many ways suggests and leads up to that of Insects. MYRIOPODA. CENTIPEDES. CHILOPODA. MILLIPEDES. DIPLOPODA (or CHILOGNATHA). Carnivorous. Poisonous. Body usually flat. A pair of appendages to each segment. Many-jointed antennae. Toothed cutting mandibles. Each maxilla consists of an ex- ternal palp, and a bilobed median portion. The next appendage is leg-like. Then follows a large basilar plate, beside which are the two poison claws. A single posterior genital aper- ture. Examples Scolopendra. Lithobius. Vegetarian. Harmless, Body cylindrical. By the imperfect separation of the segments all but the most anterior seem to have two pairs of append- ages each, and also two paired ganglia, and two pairs of stigmata. Seven-jointed antennae. Broad masticating mandibles. Maxillae are represented by a four-lobed plate. No basilar plate. Genital apertures open on the second or third pair of limbs. Examples -Julus. Geophilus. In reference to habitat, it is interesting to note that at least two myriopods Geophilus submarinus and Linotcenia maritima, occur on British coasts. As distinct from the two chief sub-classes, it is perhaps necessary to recognise other two Pauropoda, e.g., Pauropus, and Symphyla, e.g., Scolopendrella. The last-named approaches closely to the most primi- tive insects (Collembola and Thysanura). Third Class of Tracheata Antennata. INSECTA. Insects occupy a position among the backboneless animals like that of birds among the Vertebrates. The typical members of both classes have wings and the power of true flight, richly aerated bodies, and highly developed nervous and sensory organs. Both are very active and brightly THE COCKROACH. 293 coloured. They show parallel differences between the sexes, and great wealth of species within a narrow range. One expects to find that insects, like birds, have a high body temperature. GENERAL CHARACTERS. Like other Arthropods , Insects have segmented bodies, jointed legs, chitinous armature, and a ventral chain of ganglia linked to a dorsal brain. Compared with Peripatus and Myriopods, adult insects show concentra- tion of the body segments, decrease in the number and increase in the quality of the appendages, and wings withal. Insects are terrestrial and aerial, and rarely aquatic animals ; usually winged as adults, breathing by means of trachea, and often with a metamorphosis in the course of their growth. The body is divided into three distinct regions, head, thorax, and abdomen. The head bears three pairs of mouth appendages (= legs), and a pair of pre-oral out-growths the antenna ; the thorax bears a pair of legs on each of its three segments, and, typically, a pair of wings on each of the posterior two ; the abdomen has no appendages, unless rudimentary modifications of these be represented by stings, ovipositors, &c. First Type of Insects, Periplaneta (or Blatta)* The COCKROACH. The cockroaches found in Britain are immigrants, either from the East (P. orientalis], or from America (P. americand) ; the two species closely resemble one another. They are omnivorous in their diet, and active in their habits, but they hide during the day and feed at night. They are ancient insects, for related forms occurred in Silurian ages ; they are average types, for they are neither very simple nor very highly specialised. Their position is among the Orthoptera, in the same order as locusts and grasshoppers. The young are hatched as miniature adults, except that wings are absent ; in other words, there is no metamorphosis in development. The skin consists of an external chitinous cuticle and a subjacent cellular layer the epidermis or hypodermis from which the cuticle is formed. The newly hatched cockroaches are white, the adults are dark brown. 294 PERIPATUS, MYRIOPODS, AND INSECTS. External Characters. THE HEAD. APPENDAGES OF THE HEAD. OTHER STRUCTURES ON THE HEAD. It is vertically i. A pair of stout toothed mand- The antennae (probably not elongated and separated from ibles working sideways. 2. The first maxillae, each con- homologous with append- ages), long, slender, many the thorax by a neck. sisting (a) of a basal piece or protopodite with two joints jointed, tactile. The large black compound a basal cardo, a distal stipes ; eyes. (b) of a double endopodite The "upper lip" or labrum, borne by the basal piece, in front of the mouth. and consisting of an inner The white oval patches near lacinia and a softer outer the bases of the antennae, Jalea ; an exopodite or maxillary palp also borne by the basal possibly sensory. piece, and consisting of five joints. 3. The second pair of maxillae, fused together as the " labi- um,'' consisting (a) of a fused basal piece or protopodite with two joints a basal sub- mentum, a smaller distal mentum ; on each side this protopodite bears (b) a double endopodite (ligula) consisting of an inner lacinia, and an outer para- glossa ; (c) an exopodite or labial palp, consisting of three joints. THE THORAX. THE APPENDAGES OF THE OTHER STRUCTURES ON THE It consists of three THORAX. THORAX. segments : (a) prothorax, (b) mesothorax, (a) First pair of legs. (b) Second pair of legs. () A pair of elytra or wing- covers (modified wings) rudimentary in female of P. orientalis. (c) metathorax. (Each segment is bounded by a (c) Third pair of legs. Each leg consists of many joints a basal " coxa " with a small (c) A pair of membranous wings, sometimes used in flight, folded when not in dorsal tergum, " trochanter " at its distal use, absent in female of and ventral end, a " femur," a " tibia," P. orientalis. sternum.) a six-jointed tarsus or foot Between the segments of the ending in a pair of claws. thorax are two pairs of respiratory apertures or stigmata. THE ABDOMEN. APPENDAGES (?) OF THE OTHER STRUCTURES ON THE ABDOMEN. ABDOMEN. It consists of 10 (or Two cigar-shaped tactile anal A pair of stigmata occur be- 1 1) distinct seg- cerci, attached under the tween the edges of the ments, with edges of the last tergum, terga and sterna in the terga and ster- are possibly relics of the first eight abdominal seg- na as in the last abdominal appendages. ments. There are there- thorax. The ninth sternum of the male fore twenty stigmata in all. bears a pair of styles, pos- The anus is terminal, beneath sibly relics of appendages. Both sexes have complex hard the tenth tergum of the abdomen ; a pair of "podi- structures (gonapophyses) cal plates " lie beside it. beside the genital apertures. The genital aperture is ter- They are possibly relics of minal, ventral to the anus. appendages. The opening of the sper- matheca the female's receptacle for sperma- tozoa lies on the ninth sternum of the abdomen NERVOUS SYSTEM. 295 Moulting, which involves a casting of the cuticle, of the internal lining of the tracheae, &c., occurs some seven times before the cockroach attains in its fifth year to maturity. The muscles, which move the appendages, and produce abdominal movements essential to respiration, are markedly cross striped. Nervous System. A pair of supra-oesophageal or cerebral ganglia lie united in the head. As a brain, they receive mx.p ~S.ni FIG. 95. Mouth appendages of Cockroach. DUFOUR.) (After I. Mn, Mandibles ; II. First Maxillae ; c, cardo ; st, stipes ; L, lacinia ; G, galea; mx.p, maxillary palp; III. Second Maxillae or Labium ; s.m, submentum ; m, mentum ; L, laciniae ; pg, para- glossa ; l.p, labial palp. impressions by antennary and optic nerves. By means of a paired commissure surrounding the gullet, they are connected with a double ventral chain of ten ganglia. Of these, the first or sub-cesophageal pair are large, and give off nerves to the mouth parts, &c. ; from each of the ganglia of the thorax and the abdomen nerves are given off to adjacent parts. There are three pairs of ganglia in the thorax, and six 296 PERIPATUS, MYRIOPODS, AND INSECTS. in the abdomen, of which the last is the largest. From the cesophageal commissures two visceral nerves are given off, which form in a somewhat complex manner the innervation, of gullet, crop, and gizzard. Besides the large compound eyes, there are other sensory structures some of the hairs on the skin, the maxillae (to some extent organs of taste), the antennae (tactile and olfactory), the anal cerci (tactile), and possibly the oval white patches on the head. Alimentary System. (i) The fore gut (stomatodaeum) is lined by a chitinous cuticle continuous with that of the outer surface of the body. It includes (a) the buccal or mouth cavity, in which there is a tongue-like ridge, and into which there opens the duct of the salivary glands ; (b) the narrow gullet or oesophagus ; (c) the swollen crop ; (d} the gizzard FIG. 96. Transverse section of Insect. (After PACKARD.) k, Heart ; g, gut ; , nerve cord ; st, stigma ; tr, trachea ; tv, wing ; f) femur of leg. with muscular walls, six hard cuticular teeth, and some bristly pads. There is a pair of diffuse salivary glands on each side of the crop, and between each pair of glands a salivary recep- tacle. The ducts of the two salivary glands on each side unite, the two ducts thus formed combine in a median duct, and this unites with another median duct formed from the union of the ducts of the receptacles. The common duct opens into the mouth. (2) The mid gut (mesenteron) is lined by endoderm. It is short and narrow, and with its anterior end seven or eight REPRODUCTIVE SYSTEM. 297 club-shaped digestive outgrowths are connected. These seem to have a pancreatic function. (3) The hind gut (proctodaeum) is lined by a chitinous cuticle. It is convoluted and divided into narrow ileum, wider colon, and dilated rectum with six internal ridges. From the beginning of the ileum, the excretory Malpighian tubules are given off. Respiratory System. The tracheal tubes, which have ten pairs of lateral apertures or stigmata, ramify throughout the body. Circulatory System. The chambered heart lies along the mid dorsal line of abdomen and thorax. It receives blood by lateral valvular apertures from the surrounding pericardial space, and drives it forwards by a slender aorta. The blood circulates, however, within ill-defined spaces in the body. The Excretory System consists of sixty or so fine (Mal- pighian) tubules, which rise in six bundles from the beginning of the ileum, and twine through the " fatty body " and in the abdominal cavity. Reproductive System OF THE MALE. OF THE FEMALE. The testes are paired organs, sur- rounded by the fatty body below the 5th and 6th abdominal terga. They atrophy in the adult. From the testes, two narrow ducts or vasa deferentia lead to two seminal vesicles. These seminal vesicles (the ' ' mush- room-shaped gland ") open into the top of the ejaculatory duct. This duct opens on the loth sternum. Beside the aperture there are copulatory structures (gona- pophyses). With the ejaculatory duct a gland is associated. The ovaries are paired organs, in the posterior abdominal region, each consisting of eight ovarian tubes. These are bead-like strings of ova at various stages of ripeness. From the ovarian tubes of each side, eight eggs pass at a time into a short wide oviduct. The two oviducts unite and open in a median aperture between the 8th and Qth abdominal sterna. Beside the aperture are hard structures (gonapophyses) which help in the egg laying. Here also a pair of " collet erial " glands pour out their cementing secretion by two apertures. The spermatheca is a paired sac with a single aperture on the Qth abdominal sternum. 298 PERIPATUS, MYRIOPODS, AND INSECTS. Sixteen ova, one from each ovarian tube, are usually enclosed within each egg capsule. The latter is formed from the secretion of the colleterial glands. Each egg is enclosed in an oval shell, on which there are several little holes (micropyles), through one of which a spermatozoon enters. Spermatozoa, from the store within the spermatheca, are included in the egg capsule. The development is similar to that of other insects, and it has already been mentioned that there is ho metamorphosis. At an early stage in development, some cells associated with the mesoderm are set apart as reproductive cells, and originally these have a segmental arrangement as in Annelids ; at a later stage other meso- derm cells join these, some forming ova, others epithelial cells around the latter. The distinction between truly reproductive cells and associated epithelial cells, which is said to be late of appearing in some of the higher insects, is established at a very early stage in the cockroach. Second Type of Insects. The BRITISH HIVE BEE (Apis mellifica.) This is a much more highly specialised type than the cockroach. It belongs to the order Hymenoptera. The Hive Bee (Apis mellifica) is a native of this country, and is the species most commonly found domesticated. It is the only British representative of the genus Apis, and exhibits, in its most fully developed form, the social life which is foreshadowed among the Humble Bees. As a consequence of this social life, there is much division of labour, which expresses itself alike in habit and in structure. The males (drones) take no part in the work of the colony, and have solely a reproductive function ; the females are divided into two groups the queen bees and the workers. In the workers, which do, in fact, perform all the work of the hive, the vegetative organs attain their highest degree of development, but the reproductive organs are normally abortive and functionless. In the queens, of which there is but one adult to each hive, the enormous development of the reproductive organs seems to act as a check upon the vegetative organs, which are of less advanced type than those of the workers. The workers are further divisible into nurses, which are young and do not leave the hive, being THE BRITISH HIVE BEE. 299 occupied with the care of the larvae, and the foraging bees, which are older workers, and gather the food to supply the whole colony. In considering the relation between the life of the Hive Bee and that of many allied forms (Bombus, &c.), it is important to notice that the habit of laying up stores of food material for the winter, enables the colony, and not merely an individual, to survive, and must thus have greatly assisted in the evolution of sociality. The body shows the usual division into head, thorax, and abdomen, and varies considerably in the three different types, being smallest in the workers. It is entirely covered with hairs, some of which are sensitive, while others are used in pollen gathering, &c. The head bears antennae, which are composed of a long basal and numerous smaller joints. They are marvellously sensitive, serving to communicate impressions, and also con- taining organs of special sense. A pair of compound eyes, largest in the drones, and three median ocelli are also present in the head region. Of the true appendages of the head, the mandibles are in the workers very powerful and used for many purposes connected with comb building. In the first maxillae, the maxillary palps are aborted, but internal lacinia, external galea, and basal stipes and cardo are present as usual. The second pair of maxillae are much modified to form the labium or so-called lower lip. The united basal joints form the mentum and sub-mentum. From the mentum at either side springs the long labial palp, which represents the outer fork of the typical appendage. The inner fork is divided into two parts at each side, of these the inner (laciniae) are united and much elongated, the two outer or paraglossae are free and closely apposed to the laciniae; the whole structure is known as the ligula. When the bee is engaged in sucking honey from a flower, the maxillae and labial palps are closely apposed to the ligula, and thus an air-tight tube is formed. When not in use, the whole structure is folded back upon itself. In the queen and in the drone the mouth parts are shorter, and are not used in honey gathering. The thoracic appendages consist as usual of three pairs of legs, which have the usual parts. On the first leg, at 300 PERIPATUS, MYRIOPODS, AND INSECTS. the junction of the tibia and the first tarsal joint, there is a complicated mechanism which is employed in cleaning the antennae ; this is present in all three forms, and varies with the size of the antennae. In the workers the third leg is remarkably modified for pollen gathering purposes. The first tarsal joint bears regular rows of stiff straight hairs on which the pollen grains are collected ; they are borne to the hive in the pollen basket, placed at the back of the tibia, and furnished with numer- ous hairs. In queen and drone, these special ar- rangements of hairs are absent. The second and third thoracic segments bear each a pair of wings. These are largest in the drones and relatively smallest in the queen, who flies but seldom. At the base of each wing there is a respiratory spiracle. In the adult queen and worker, the abdomen is divided into six segments ; in the drone, into seven. There are no abdominal appendages. On the ven- tral surface in the worker, but not in the queen or drone, there are four pairs of wax pockets or glands, which secrete the wax which, after mastication with saliva, is employed in building the combs. The abdomen also bears in queen and worker five pairs of spiracles, but in the drone, on account of the additional segment, there are six pairs. The total number of spiracles is thus fourteen for queen and worker, and sixteen for the drone. The posterior region of the abdomen bears FIG. 97. Head and mouth parts of Bee. (After CHESHIRE.) a, Antenna ; in, mandible ; g; gum flap or epipharynx ; mx.p, maxillary palp ; pg and mx, galea and lacinia ; /./, labial palp ; /, ligula ; b, bouton at end. NERVOUS AND ALIMENTARY SYSTEMS. 301 the complicated sting. In the worker, this consists of a hard incomplete sheath, which envelops two barbed darts. The poison flows down a channel lying between the darts and the sheath. Ramifying through the abdomen are found the two slender coiled tubes which constitute the poison gland. At the posterior end of the body these unite and open into a large poison sac. When a bee uses its sting, the chitinous sheath first pierces the skin, and then the wound is deepened by the barbed and pointed darts, while at the same time poison is steadily pumped down the channel mentioned above, and pours out by minute openings at the bases of the darts. The poison contains formic acid, and is fatal to the bee if directly introduced into its blood. Associated with the sting there are a pair of delicate tactile palps. In the queen, the sting is curved and more powerful, but it is apparently only used in combat with a rival. In the worker, the sting, and with it a portion of the gut, is usually lost after use, and, in consequence, death ensues ; the queen, on the other hand, can withdraw her sting from the wound with considerable ease. There is no trace of sting in the drone, as is natural when we consider that it is merely a modifica- tion of an ovipositor. Nervous System. In the adult this exhibits considerable fusion of parts. The supra-cesophageal ganglia are very large, and send large lateral extensions to the compound eyes. This " brain " is best developed in the active workers. The sub-cesopha- geal mass is formed by the fusion of three pairs of ganglia. In the thorax there are two pairs of ganglia, of which the second supplies the wings and the two last pairs of legs. In the worker there are five pairs of abdominal ganglia, but in the queen and drone only four. The sense organs are the simple and compound eyes, and the antennae, which are furnished with numerous sensitive structures. Alimentary System. The oesophagus is a narrow tube which runs down the thoracic region. In the abdominal region it expands into the crop or honey sac. The crop opens by a complicated 302 PERIPATUS, MYRIOPODS, AND INSECTS. orifice, with a remarkable stopper arrangement, into the digestive region or chyle stomach, which is separated by a pylorus from the coiled small intestine. The inner wall of the small intestine bears numerous rows of chitinous teeth set in longitudinal ridges, and is perforated by the apertures of the excretory tubules. At the junction of the small with the large intestine, there are six brownish plates, perhaps functioning as valves. In connection with the anterior region of the gut, there is a very complicated series of glands. First, we have in the workers only, on B FIG. 98. Nervous system of Bee. (After CHESHIRE.) A, Of larva ; B, of adult ; , antenna ; mx, maxilla ; *, mandible ; w, origin of wing ; 1-5, abdominal ganglia. either side of the head, a long coiled gland which is intracellular in type. ^ It is largest in the so-called " nurses " which feed the young, and diminishes in size kter. According to Mr. Cheshire, this gland secretes a nitrogenous fluid which is furnished to all the larvae in their early stages, but is supplied to the future queen during the whole of the OTHER SYSTEMS. 303 feeding period, and also during the period of egg laying ; this secretion was formerly termed ** royal jelly." In addition to this pair of glands, there are in the worker three other gland systems. Of these, the second and third pairs have a common central outlet on the mentum, and secrete the saliva which is plen- tifully mixed with the nectar dur- ing suction. The fourth pair is small, and the ducts open just within the mandible. The last three pairs of glands are found also in drone and queen. The method of feeding in the bee differs considerably in the three types. In the worker, the honey sucked up from flowers is mixed with saliva, passes down the gul- let into the crop, thence by the opening of the "stomach mouth " it may reach the true stomach and so be di- gested, or may be carried in the crop to the hive and there emptied into the cells by regurgitation. The pol- len, which is frequently mixed with the honey, is separated from the latter by means of the stomach mouth, and is digested. Before impregna- tion, the queen, like the FIG. 99. Food canal of Bee. part after CHESHIRE.) (In mx> Maxilla ; a, antenna ; e, eye ; s.g; salivary glands ; a?, oesophagus ; h.s, honey sac; s, stopper; c.s, chylific stomach ; m.t, malpighian tubules : j.z", small intestine ; /./, large intestine ; st, sting. worker, feeds on pollen and honey; after it, she is always fed by the attendant workers. The drones, like the young workers, avail themselves of the general food supply of the colony, and do not themselves collect honey. Other Systems. The respiratory system is represented by the ramifying tracheal tubes. They open to the exterior by the lateral spiracles, which can be completely closed. In connection 304 PERIPATUS, MYRIOPODS, AND INSECTS. with the tracheae there are large air sacs which aid greatly in flight. The circulatory system is in essentials the same as that of the cockroach. The blood contains a few nucleated amoeboid corpuscles. The excretory system consists of numerous fine Mal- pighian tubules which open into the small intestine. Reproductive System. In the drone the reproductive organs consist of a pair of testes, each furnished with a narrow vas deferens, expanding at its distal end into a seminal vesicle. The seminal vesicles open into the ejaculatory duct, and at their junction a large paired mucus gland opens. When maturity is reached the testes diminish in size, while the spermatozoa accumulate in the terminal expanded part of the ejaculatory duct, and there become aggregated into a compact spermatophore. With the terminal portion of the male duct copulatory organs are associated. Mating takes place only once in the life of the queen, and is followed by the death of the drone. In the queen the large ovaries occupy considerable space in the abdo- minal region. As usual, each consists of numerous (100-150) ovarian tubes containing ova in various stages of development. The ovarian tubes open into the right and left oviducts, which again unite to form the common oviduct. With the anterior portion of the common duct the globular spermatheca is associated. In connection with it there is a gland corresponding to the mucus gland of the male. The oviduct terminates in a copulatory pouch. Previous to laying, the eggs are fertilised by sperms set free from the spermatheca. In the case of drone eggs this liberation of spermatozoa does not take place, and the eggs in consequence are parthenogenetic. Queens which have never mated, or which have exhausted their stock of male elements, habitually lay drone eggs, but those which are laying abundant fertilised eggs at times also lay unfertilised eggs. This with- holding of spermatozoa is said to be "voluntary," and related to the needs of the colony, but the physiological reason is unknown. The workers possess female organs similar in type to those of the queen, but of an extremely rudimentary nature. The eggs are laid singly in the cells of the comb, at the rate of about two per minute, for weeks together. They are of the usual insect type. According to the size of the cell in which it is deposited, and the food with which it is furnished, the fertilised ovum develops into a worker or into a queen. The development takes place within the cell, and includes a complete metamorphosis. CLASSIFICATION OF INSECTS. 305 a D * S id T3 J3 (U jtf W G | B - 5 - & 3 c .h ^ 'o, V 8 sis g.S- '5, S^ d ^ a. SI || d> 5 to +e i_ OJ is c 3 8. > <^ S " o J rt J * o aS c i : bjo-| 0*1*3 j *"" wT ^ g c rj J "i ^ P D Sc | d<3 o JD > W3? * J So 0^ b/) oo D-c . > (8 to S "inl 3 "I Is 'i 2 * B C cS n w H B T3 Si a >^ 'So c en 03 cj W ^ Hymenoptera. Ants, bees, wasps, gall flies, saw : Menogn. or Metagn. , or a sort of compromise 1 with four transparent wings. Larvas are footle* Lepidoptera. Butterflies and moths. Metagn. Two pairs of uniform, scaly wings. Diptera. Two winged flies. House fly, gad fly, Metagn., but sometimes with power of biting, folded wings, and posterior "balancers" or ' footless maggot, without a distinct head. Siphonaptera or Aphaniptera. Fleas. Metagn. , but also with power of piercing. Nc Ectoparasitic. Larva a footless maggot. Coleoptera. Beetles. Menogn. , rarely Metagn. Fore wings modifie folded when not in use. Larvas very diverse, bee parasites Strepsiptera are probably allied. Trichoptera. Caddis flies. Menogn. Hind wings usually larger than fore The body is hairy, rarely scaly. The larvae usually live in water within special cases, and c Panorpata. Scorpion flies. Menogn. Two pairs of narrow membranoi Larva like a caterpillar. Neuroptera. Ant lions and lace winged flies. Menogn. Two pairs of glassy wings with man live in water, and have tracheal gills. .^ vd 10 t CO ci M M H ON s CD s QJ cu cD V 'd 'H T3 73 6 CD 6 6 o o o O 5 C/3 "t/T JB bo c o 1 -y> o M a, rt H ""' bjo & 5OTA, Winge( some degenerf C. METABOLA Complete metar either MENOGNATHA ways with bitin or METAGNATHA jaws replaced apparatus). 11 | "rt be . W a i 2 20 306 PERIPATUS, MYRIOPODS, AND INSECTS. - 0) 1/3 3s 8 - 2 v "g r^ CU i/) M '3 > bi) fe t: o < 11 * i J3 J?s |i B _0 'C Ig la a i 1 | ill g g to}.; 1* tn M "^8 ^ b/) o > n 'So 5 cu d ' cu Rhynchota or Hemiptera, e.g., Phylloxera, Aphide bugs, water scorpions, lice. (The male cocci metamorphosis. ) The mouth parts are adapted for sucking and for of wings or none. The parasitic forms have no several respects degenerate. Thysanoptera, e.g., Thrips. Ametab. Suctorial mouth organs. Wings very r absent. Only three or four pairs of stigmata. O Corrodentia, e.g., Bird lice, termites. Ametab. Mouth parts adapted for biting. Win lice have no compound eyes. Orthoptera, e.g., Cockroach, locust, cricket, mole " walking leaf." Ametab. Mouth parts adapted for biting. Anteri firmer than those behind, or modified into wing cc times absent. Plecoptera, e.g., Perla. Hemimetab. Mouth parts adapted for biting. Tw< The larvae live in water, and breathe by tracheal the adult. Odonata, Dragon flies. Hemimetab. Mouth parts adapted for biting. wings. The larvae live in water, and breathe by Ephemerida, May flies. Hemimetab. Mouth parts of adult somewhat c Fore wings large, hind wings small or absent. L by tracheal gills, and have biting mouth organs. Dermaptera, Earwigs. Ametab. Mouth parts adapted for biting. Ant< large, but folded both longitudinally and crosswii Collembola, Springtails, e.g., Podura, Smynthurus. Thysanura, e.g. Campodea, Lepisma. 00 t^ vd 10 4 CO ri H d M b QJ 0) o3 OJ O % < >H W w S O-hi, M $ ffi H r^ ^ cu > c || 1 S5.a w c S o <^ c S'D, w g 11 ^ . w ^ x: S P- ^^ - s ^ ^'S T3 G 5 <:-s u ,q * c '1 i " '^ ^ "^