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THE CORAL SIDERASTREA RADIANS AND 
 ITS POSTLARVAL DEVELOPMENT. 
 
 BY 
 
 J. E. DUBRDBN. 
 
 Washington, U. S. A. 
 
 Published by the Carnegie Institution, 
 
 December, 1904. 
 
THE CORAL SIDERASTREA RADIANS AND 
 ITS POSTLARVAL DEVELOPMENT, 
 
 BY 
 
 J. B. DUBRDBN. 
 
 Washington, U. S. A. 
 
 Published by the Carnegie Institution, 
 
 December, 1904. 
 
Carnegie Institution of Washington, 
 Publication No. 20. 
 
 Press of Gibson Bros., 
 Washington, D. C. 
 
CONTENTS. 
 
 Preface "^ 
 
 Introduction i 
 
 Adult Colony. 
 
 External Characters 7 
 
 Column wall 8 
 
 Tentacles 9 
 
 Disc and mouth 14 
 
 Color 15 
 
 Reproduction 16 
 
 External characters on decalcification 18 
 
 Anatomy and Histology 21 
 
 Column wall and disc 3i 
 
 Tentacles 33 
 
 Stomodaeum 23 
 
 Mesenteries 24 
 
 Mesenterial filaments 29 
 
 Skeletotrophic tissues 30 
 
 Septal invaginations, interseptal loculi, 
 
 and gastro-coelomic cavity 34 
 
 Gonads 37 
 
 Corallum 38 
 
 Histology 40 
 
 Wall or theca 45 
 
 Septa 46 
 
 Synapticula 52 
 
 Columella 53 
 
 Dissepiments 55 
 
 Epitheca and basal plate 56 
 
 POSTLARVAL DErKLOFMKNT. 
 
 Larva 57 
 
 Young polyp 61 
 
 Tentacles 65 
 
 First cycle of exotentacles 65 
 
 First cycle of entotentacles 66 
 
 Secondary exotentacles 69 
 
 Second cycle of entotentacles and third 
 
 cycle of exotentacles 71 
 
 Third cycle of entotentacles and fourth 
 
 cycle of exotentacles 73 
 
 Mesenteries 76 
 
 First cycle of mesenteries (protocnemes)... 76 
 
 Second cycle of mesenteries (metacnemes) 79 
 
 Third cycle of mesenteries 83 
 
 Corallum 86 
 
 First cycle of entosepta and second cycle 
 
 of exosepta 86 
 
 Second cycle of entosepta and third cycle 
 
 of exosepta 93 
 
 Third cycle of entosepta and fourth cycle 
 
 of exosepta 99 
 
 Basal plate II3 
 
 Epitheca iij 
 
 Columella 117 
 
 Anatomy and histology of larva and young 
 
 polyp 119 
 
 Young polyps 122 
 
 References 125 
 
 Explanation of plates 126 
 
 iii 
 
PREFACE. 
 
 The researches of the late Prof. H. de Lacaze-Duthiers (1873, 1897), 
 Prof. G. von Koch (1882, 1897), and Prof. H. V. Wilson (1888) have made 
 us acquainted with many of the early stages in the development of corals. 
 They have served to establish such fundamental facts as the ectodermal 
 origin of the madreporarian skeleton and the sequence of the primary mesen- 
 teries and septa, results which must ever possess an importance to the student 
 of the Anthozoa. But for an understanding of many of the problems of adult 
 coral morpholog}^, especially those associated with the relationships of the 
 mesenteries and septa, it has long been desirable that developmental stages 
 later than those studied by the authors mentioned should be investigated. 
 While resident in the West Indies I have followed day by day the postlarval 
 growth of the coral Siderastrea radians (Pallas) for a third of a year, and 
 secured the development of the tentacles and septa as far as the third cycle, 
 and that of the mesenteries to the completion of the second cycle. The 
 results are herein set forth. 
 
 In many respects the mature polyps of S. radians are of peculiar mor- 
 phological interest, but have never been fully described. An account is 
 therefore first given of the external characters and internal anatomy of the 
 adult colony, and afterwards of the development of the young polyp from 
 the free-swimming larva. The manner of appearance and the relationships 
 of the tentacles, mesenteries, and septa are considered at some length, their 
 establishment being the principal object of the investigation. 
 
 The work was commenced while Curator of the Museum of the Institute 
 of Jamaica, continued as Bruce Fellow at the Johns Hopkins University, 
 Baltimore, and concluded at the American Museum of Natural History, New 
 York. For facilities afforded in carrying out the investigations I am under 
 obligations to the Board of Governors of the Institute of Jamaica, Prof. W. K. 
 Brooks, of the Johns Hopkins University, and Prof. H. C. Bumpus, of the 
 American Museum of Natural History. The research has been assisted by 
 an appropriation from the Carnegie Institution. 
 
 J. E. D. 
 
 University of Michigan, Ann Arbor, Mich., U. S. A., 
 iiTH November, 1904. 
 
THE CORAL SIDERASTREA RADIANS AND ITS POSTLARVAL 
 
 DEVELOPMENT. 
 
 By J. E. DUERDEN. 
 
 INTRODUCTION. 
 
 Tlie following are the more important references to and synonyms of this 
 well-known species of coral : 
 
 Madrefora radians, Pallas, Elench. Zooph., 1766, 322. 
 
 Madrepora astroites, Linnaeus, Sjs. Nat., ed. xii, 1767, 1276. 
 
 Madrefora galaxea, Ellis & Solander, Nat. Hist. Zooph., 1786, 168, pi. 47, fig. 7. 
 
 Astrea galaxea, Lamarck, Syst. Anim. s. Vert., 1801, 371; Le Sueur, M^m. Mus. Hist. Nat. Paris, t. 
 VI, 1820, 285, pi. XVI, fig. 13; Lamouroux, Expos. Mdth., 1821, 60, pi. XLix, fig. 7. 
 
 Astrcea radians, Oken. Lehrb. Naturgesch., 1815, bd. i. 65; Milne-Edwards & Haime, Hist. Nat. Cor., 
 1857, t. II, 506; Gregory, Quart. Journ. Geol. Soc. Lond., vol. Li, 1895, 277. 
 
 Astrea {Sider astrea) g-alaxea, de Blainville, Diet. Sci. Nat., 1830, torn. LX, 335; Man. Actin., 1834, 
 370. 
 
 Astrcea astroites, Ehrenberg, Corall. roth Meer, Abhandl. kongl, Akad. Wiss. Berlin, 1832, 319. 
 
 Siderina galaxea {furs), Dana, Zooph. Wilkes Expl. Exped., 1846, 218, pi. x, figs. 12, 12*, 12c. 
 
 Siderastrcea galaxea, Milne-Edwards & Haime, Ann. Sci. Nat., 1850, torn, xii, 139; Pourtal^s, Deep-Sea 
 Corals, 111. Cat. Mus. Comp. Zool., no. iv, 1871,81; Florida Reefs, Mdm. Mus. Comp. ZooL, 
 1880, vol. VII, pt. I, pi. XI, figs. 14-21, pi. XV, figs. 1-12; Quelch, Reef Corals, Challenger Reports, 
 1886, vol. XVI, 113; Heilprin, Proc. Acad. Nat. Sci. Phila., 1890, 305. 
 
 Siderastral^e'la radia7is, Verrill, Bull. Mus. Comp. Zool., vol. i, 1864, 55 ; Vaughan, Bull. U. S. Fish Com- 
 mission 1900, vol. 2, 1901, 309, pi. XV, pi. XVI, fig. 2; Samm. der Geol. R. — Museums in Leiden, 
 1901, Ser. II, bd. 11, 61 ; Verrill, Trans. Conn. Acad. Science, vol. xi, 1901, 153, pi. xxx, fig. i. 
 
 The generic term has been thoroughly discussed in the recent papers of 
 Gregory (1895, p. 278), Vaughan (1900, p. 154), and Verrill (1901, p. 88). 
 Vaughan, fortunately for coral taxonomy, shows that the name Astrcua^ first 
 used binomially as a coral genus by Lamarck in 1801, can not be retained 
 in madreporarian terminology, having been employed by Bolten in 1798 for 
 a group of gastropod shells. In this conclusion he is supported by Verrill. 
 The next generic name available is the Siderasirea of de Blainville, 1830. 
 The specific term radians of Pallas (1766) has priority of the galaxea of 
 Ellis & Solander (1786), and Vaughan holds that the Madrepora astroites 
 of the twelfth edition of Linnaeus is the same as M. radians of Pallas. 
 
 The species is very common throughout the West Indies, and is recorded 
 
2 SIDERASTREA RADIANS. 
 
 from Vera Cruz and Colon on tlie mainland. It is equally plentiful on the 
 Florida Reefs and flats, and around the more northern Bermudas. Gregory 
 (1895, p. 277) records it as fossil from the Low-level Reefs of Barbados and 
 the Pleistocene Reefs of the Bahamas. 
 
 A glance at the references shows that Siderastrea radians has been fre- 
 quently described and figured, but mainly as regards the corallum ; Le Sueur 
 (1820, p. 285), Pourtales (1871, p. 81), and Verrill (1901, p. 153) have, in addi- 
 tion, contributed brief notes upon the mode of occurrence of the living colonies 
 and the characteristics of the polyps. 
 
 As AstrcBa radians the species is thus systematically described by 
 Milne-Bdwards & Haime in their " Histoire Naturelle des Coralliaires " 
 (vol. II, p. 506): 
 
 Polypary often fixed upon the Valuta turbinellus of Linnaeus, or spherical and free. 
 Budding takes place at the point of union of several calices. Calices subpolygonal, appearing 
 thickened at the borders as a result of the strong development of the septal system, although 
 the walls are indicated only by fine lines. Columella formed by one or two compact tubercles, 
 scarcely visible, more distinct in young individuals. Three cycles of septa complete, and, in 
 general, a variable number of a fourth cycle, unequal. Interseptal chambers extremely 
 narrow. Septa much serrated, strong, very regularly crenulated at the border, nearly equal, 
 the primaries and secondaries a little larger. The teeth serrated, obtuse, and sub-equal. 
 The septa of the second cycle are fused by their internal border to those of the preceding 
 cycle. When the septa are broken from above they are found to be united by strong granules, 
 the spaces between the granules resembling small foramina. A specimen in this condition 
 has been considered by Lamarck as a distinct species under the name Astrcta punctifera. 
 In a vertical section the columella is compact and strong, the septa are perfect lamellae, cov- 
 ered with a radiating series of large granules ; the dissepirhents are rudimentary, horizontal, 
 simple, and 0.5 mm. distant from one another. The species sometimes forms large masses. 
 The large diameter of the calices is from 3 to 4 mm., their depth 2 mm., or a little more. 
 
 An excellent engraving of a colony is given in Bllis & Solander's 
 "Natural History of Zoophytes" (1786, plate 47, fig. 7), under the term 
 Madrepora galaxea. In his report on '* The Stony Corals of Porto Rico," Mr. 
 Vaughan (1901) adds a photographic reproduction of a flat incrusting colony 
 and also one showing the enlarged calices (plates xv, xvi). In the plates 
 accompanying Prof. Louis Agassiz's "Report on the Florida Reefs " (1880) 
 are reproduced (plate xv) a dozen beautifully executed drawings represent- 
 ing the polyps and skeleton in some detail. Many of the actual details with 
 regard to the tentacles and septa are different from those observed on the 
 Jamaica 5. radiajis^ but there seems no doubt that the same species is 
 intended in each case. The drawings of the polyps are interesting as show- 
 ing for the first time the bilobed character of the tentacles and their irregular 
 arrangement at varying distances from the center of the disc. 
 
INTRODUCTION. 3 
 
 The species occurs plentifully in Kingston Harbor, Jamaica, from the 
 shore down to a depth of 5 or 6 feet. Here the colonies are small, and sub- 
 spheroidal or irregular in shape, partly or wholly incrusting some dead 
 shell or stone. Occasionally nearly globular colonies are obtained with the 
 polyps equally developed all over the surface. Such colonies lie exposed on 
 the sea-floor, or partly buried in beds of the aquatic phanerogam Thalassia 
 marina^ and are associated with other free corals, such as Manicina areolata 
 (Linn.), Cladocora arbuscula (Le Sueur), and Porites divaricata (Le Sueur) ; 
 sometimes they are found incrusting shells inhabited by the living mollusc 
 or by hermit crabs. A flat incrusting form, growing upon stones and blocks 
 of coral, occurs on the coral flats outside the harbor in the area of reef 
 formation. Here the species may give rise to large fixed masses, 50 or 
 60 cm. across, while in shallow water the free colonies rarely exceed a 
 diameter of 10 cm. On the reefs S. radians is associated with another 
 common West Indian species of the same genus, viz, S. siderea (Bll. & Sol.). 
 This latter may produce large hemispheroidal blocks, and in situ is readily 
 distinguished from S. radians by the larger size of the polyps and their 
 reddish brown color. 
 
 S. radians seems peculiarly hardy as regards the conditions under which 
 it will thrive. The shore waters within Kingston Harbor are often muddy 
 from the action of the strong day breezes, and at times the living polyps are 
 covered by deposits of sand and mud. Around Jamaica, however, the colo- 
 nies are rarely exposed to the direct rays of the sun, the fall of the tides 
 being very restricted ; but further north, where the difference between high 
 and low tides is greater, both Pourtales and Verrill have found specimens 
 so situated that they are frequently subjected to the direct rays of the sun. 
 With regard to the occurrence of S. radians in Florida, Pourtales, in " Deep- 
 Sea Corals" (1871, p. 81), writes: 
 
 In altitude it ranges higher than any other coral of the Floridian fauna, as small masses are 
 found flourishing in pools left by the tide. I have even seen small clusters left partially dry in 
 a hot sun, keeping up a communication with the water only by a few of the lowest polyps of 
 the group. From their position they must frequently have been thus exposed without incon- 
 venience. According to Prof. Agassiz the polyp has bilobed short tentacles at different 
 distances from the center. 
 
 Prof. A. B. Verrill (1901, p. 154) gives the following notes as to the 
 conditions under which the species is found at the Bermudas : 
 
 This species, which is abundant at the Bermudas, is more hardy than most reef corals, 
 for it can live and grow well in shallow water on mud flats, where it is laid bare by nearly 
 every tide, and where most other corals would be smothered in the mud, though ►S'. siderea 
 
4 SIDERASTRKA RADIANS. 
 
 and some forms of Isophyllia fragilis are usually found with it in such places. It is often 
 partly buried in the white calcareous mud of the flats, and yet seems healthy there. It is also 
 abundant in the small, shallow pools left on the flats by the tide. But it is equally common 
 on the reefs, where it often grows larger. It is also found well grown in Harrington Sound. 
 Exposure to the dry air, or even to the hot sun, for an hour or so, does not kill it, if it be wet 
 beneath. Probably its porosity enables it to absorb sufficient water to prevent drying up. 
 
 The natural occurrence of the living coral under such varying condi- 
 tions as to purity of water and exposure indicates that this species, at any 
 rate, is not so extremely sensitive to its environment as corals generally are 
 found to be. 
 
 The polyps are so small that their external characters can be fully made 
 out only with the aid of a lens or a low-power microscope. Further, they 
 differ so much in appearance, according to the amount of expansion and 
 retraction, that to obtain their complete characteristics it is desirable to keep 
 the colonies under observation for some time and subjected to various condi- 
 tions. As the stocks often lie free on the sea-floor, they can be collected 
 without any injury to the polyps, and are sufficiently small to be kept in the 
 laboratory in ordinary glass vessels. With a little attention no difficulty is 
 experienced in keeping the polyps alive. Indeed, the colonies continued to 
 increase in size during confinement, and while kept under observation new 
 polyps began to arise from the calices of previously dead areas. 
 
 The boring bivalve Lithodomus appendiculatus Phi.* nearly always 
 occurs within the corallum, often several in each colony. The siphon has 
 a dark purple, funnel-shaped incurrent aperture and a small, tube-like 
 excurrent aperture, and protrudes a short distance beyond the surface of the 
 colony, the activity of the mollusc keeping up a strong circulation of water ; 
 larvae and other small floating objects may pass in rapidly at one aperture 
 of the siphon, and after a brief interval may be shot out from the other. 
 Numbers of cirripedes {Pyrgomd) also frequently infest the corallum, and 
 assist in maintaining a current of water over the living colony. It seems 
 not unlikely that these infesting organisms may be of much importance in 
 clearing away the mud and sand which accumulate on the surface of the 
 polyps, and the currents produced by them may also bring food within reach. 
 
 During the greater part of the day the colonies in the laboratory were 
 kept in the shade, or even in darkness, by placing the glass receptacles 
 under some cover. Under these conditions the polyps remained partly 
 expanded, but retracted when exposed for any length of time to the direct 
 rays of the sun. In the early morning and towards evening the vessels were 
 
 • For the determination of this species I am indebted to Prof. L. P. Gratacap, of the American Museum 
 of Natural History. 
 
INTRODUCTION. 5 
 
 removed for a short time into the full rays of the tropical sun, or kept for 
 hours in diffuse light, when all the polyps expanded to their fullest degree, 
 and seemed tingling with vitality ; but in general, like most other corals 
 and actinians, the polyps seem most vigorous in the shade and at night time. 
 Detailed experiments on the phototropism of coral polyps are much needed. 
 
 When the colonies were exposed to strong sunlight for a time, in a limited 
 quantity of water, bubbles of oxygen gas were seen to form within the cavity 
 of the polyps and then to escape through the mouth. The production of 
 oxygen is without doubt dependent upon the metabolic activity of the uni- 
 cellular algae or Zooxanthellse present in great numbers throughout the 
 endodermal tissues (p. 22). The filamentous green algae found perforating 
 the skeleton (p. 38) would doubtless have a similar activity, but the intensity 
 of the light reaching their chloroplasts must be much less than that falling 
 upon the more superficial Zooxanthellae. 
 
 The oxygen produced from the combination of the physiological activities 
 of the plant and animal was ample to keep a limited quantity of water in a 
 proper state of aeration ; and once the colonies were established the water in 
 which they were living remained perfectly clear and sparkling without the 
 addition of other plant life. Generally it was unnecessary to renew the sea- 
 water, but from time to time fresh water was added to replace that lost by 
 evaporation. To make certain that the water contained sufficient calcareous 
 salts in solution to enable the young polyps to build up their skeleton it 
 was partly changed every few days. Like the adult polyps, the developing 
 individuals contained many Zooxanthellae within the endoderm. 
 
 For food the polyps were given small living worms, pieces of molluscs, 
 crabs, or fish. These they took readily by first extending their tentacles 
 and later their mouths over them. Sometimes a hapless polychaete would 
 attempt to creep over a colony, when it was quickly killed and ingested ; other 
 small worms, wriggling with great vigor, were unable to free themselves when 
 dropped upon a colony. The tentacles of the polyps fastened upon the 
 annelid at one or more places, and its movements soon began to weaken, 
 as if the creature were paralyzed. Bach polyp immediately around the 
 worm opened its mouth widely, while those more distant extended their per- 
 istomes towards it, as if all were moved by a common impulse. As many as 
 half a dozen polyps would thus seize hold of a small worm at one time. 
 Naturally a difficulty arose when two polyps commenced to engulf the same 
 worm from opposite ends. The unfortunate annelid soon reached a state 
 of tension between the polyps, and the peristome of each of the latter was 
 drawn outwards to an unusual degree. For over an hour a worm was 
 
6 SIDERASTREA RADIANS. 
 
 seen thus pulled at from each extremity. A large piece of a mollusc might 
 also be seized upon and partly ingested by two polyps at the same time, and 
 in the end their lips would approach and come into actual contact. A larval 
 polyp only four weeks old was able to hold a wriggling fragment of a small 
 worm and attempted to swallow it* 
 
 For examination of their anatomy and histology some of the polyps were 
 preserved in the partly expanded condition in 5 per cent, fomiol and others in 
 a solution of corrosive acetic, after narcotization with menthol or magnesium 
 sulphate. Shortly after preservation they were transferred to alcohol. Where 
 decalcification was necessary this was carried out by means of weak solutions 
 of hydrochloric or nitric acid, a few drops being added from time to time to 
 maintain a slight effervescence. For such a dense corallum as that of Sider- 
 astrea the process of decalcification required three or four days. After the 
 process the space heretofore filled by the skeleton was invariably found to 
 be occupied by a fluffy mass, which on examination under the microscope 
 was found to consist of delicate algal filaments along with fine organic 
 particles. Microscopic sections of the corallum were made by embedding in 
 Canada balsam and then grinding to sufficient thinness, as in the preparation 
 of ordinary rock sections. 
 
 ^ * Experiments conducted since this account was sent to the printer demonstrate that the mucus exuded 
 
 ^^ " bj the ectoderm cells plays an important part in the feeding of coral polyps, particularly where the food 
 particles are small. The latter falling on the polypal surface become embedded in mucus, and if not revers- 
 ing the ordinary direction of movement of the stomodaeal cilia they are slowly wafted awa^' beyond the 
 tentacles ; nutritive particles and solutions bring about a reversal of the action of the stomodseal cilia, and, 
 along with mucus in the form of strands or threads, are gradually drawn into the polypal cavity. 
 
ADULT COLONY. 
 
 EXTERNAL CHARACTERS. 
 
 The polyps are small and subpolygonal in ontline, rarely exceeding 
 4 mm. in diameter when retracted, and 5 or 6 mm. on expansion. Six unequal 
 sides constitute the basal periphery of large polyps, while there are only four 
 or five in smaller examples. In general, the individuals in any colony are 
 irregularly disposed. In some regions an arrangement in parallel rows can 
 be made out, but usually this is interfered with by the intercalation of new 
 polyps among those fully grown. In most branching corals, alcyonarians, 
 and compound actinians, it is usual for the principal oral axis of the polyp 
 to be symmetrically placed with regard to the axis of the colony, but no such 
 regularity of orientation is manifest in the polyps of Siderastrea. 
 
 The superficial polypal tissues are so thin that the white corallum 
 below shows through, and occasionally the internal arrangement of the 
 mesenteries around the stomodseum can be followed. This is especially 
 the case with polyps which have been preserved in formalin. Retracted 
 polyps usually exhibit radiating lighter areas, which correspond with the 
 cycles of septa below, and narrower alternating darker areas, which corre- 
 spond with the interseptal spaces and internal attachment of the mesenteries 
 (plate 6, fig. 32). 
 
 The living polyps making up a colony are separable from one another 
 along a narrow polygonal groove which is always lighter in color than the 
 rest of the polypal wall. Along this groove the column wall is connected 
 with the corallum, not directly, but by the intermediation of the mesenteries, 
 and to whatever degree the polyps become expanded the wall is here never 
 much elevated. The polyps, however, are at no time raised far above the 
 corallum. On full expansion they rarely assume the true cylindrical form 
 with a flattened disc, like most coral polyps, but exhibit merely a dome-like 
 elevation of the walls over the calice, 2 or 3 mm. high (plate 6, fig. 31). 
 
 At no place, except in simple larval polyps and at the margin of a 
 colony, is the boundary of the column wall of the individual polyp directly 
 connected with the skeleton; that is, it is nowhere continuous with the 
 skeletogenic basal wall. The margin of the column of any polyp passes 
 uninterruptedly into that of the surrounding polyps, and, as will be shown 
 later, the mesenteries pass from the column wall to the skeletogenic tissues 
 below, leaving narrow interspaces, which place the digestive cavities of all 
 the polyps of a colony in communication. 
 
 7 
 
8 SIDERASTREA RADIANS. 
 
 On partial retraction the column wall comes to rest upon the corallum,* 
 but the central area of the disc and the tentacles remain a little elevated, the 
 peristome especially so. On fullest retraction these also come to lie closely 
 against the skeleton and follow it all the way, nearly to the bottom of the 
 calice. Unlike the polyps of most corals, the column wall in Siderastrea is 
 incapable of folding over the tentacles and disc upon retraction ; both the 
 column wall and disc are simply drawn down upon the skeletal tissues, 
 leaving the tentacles and mouth exposed. Where, in other coral polyps, an 
 overfolding of the polypal wall occurs, it is brought about by the action of 
 the circular endodermal musculature of the column wall, sometimes in the 
 form of a sphincter ; but histological examination reveals that the columnar 
 musculature in S. radians is of the weakest character. 
 
 The genus Siderastrea^ in many respects, is allied to the mushroom 
 coral Fungia^ both being included within the section Madreporaria Fungida. 
 The genus Fimgia is not found in West Indian shallow waters, but the 
 various figures by Dana (1846), Bourne (1887, 1893), and Saville-Kent (1893)) 
 of different species of Fungia found elsewhere, show that in this genus 
 also the column wall in retraction is not drawn over the disc. The West 
 Indian species of the allied genus Agaricia are also unable to overfold the 
 column wall. 
 
 Apparently the outer surface of the polyps of S. radians is not ciliated, 
 for light particles dropped upon it remain there, and no ciliary motion is 
 recognizable when the living polyps are examined under a moderate magni- 
 fication. Similar particles dropped upon colonies of Manicina areolata are 
 gently swept away and over the sides in such a manner as to plainly indicate 
 ciliary activity, so that probably different coral species vary in this respect. 
 The outer surface is always uniformly ciliated in larvse (plate i, fig. i). 
 
 COLUMN WALL. 
 
 The columnar and discal areas of the polyps are not sharply separated 
 from one another under any condition of expansion or retraction. The outer 
 cycle of tentacles, which in ordinary cases marks the peripheral boundary of 
 the disc, here forms but an irregular circle, and its members are comparatively 
 widely apart (plate 6, figs. 31, 32). Including as column all that part of the 
 polypal wall between the polygonal line of attachment of the polyps to one 
 another and the outermost cycle of tentacles, its extent is very limited, being 
 
 ♦The free superficial regions of the polyp (column and disc) never come into actual contact with the 
 skeleton, as the living parts of the latter are always covered with the basal skeletotrophic tissues. When 
 they are said to rest upon the corallum the tissues lining the corallum always intervene. 
 
ADULT COLONY. 9 
 
 represented by a zone not exceeding 2 mm. across. When the polyps are 
 expanded the proximal region of the column remains somewhat polygonal in 
 outline, but distally it becomes circular in section, rarely assuming the tnie 
 columnar form. 
 
 The column does not present the two topographical subdivisions, calicinal 
 and pericalicinal, like most corals. Instead it extends directly from the edge 
 of the calice, so that it is wholly within it ; in other words, no " Randplatte " 
 or " edge-zone " occurs in Siderastrea. 
 
 Structurally, the column wall is very thin, partly transparent, and 
 smooth ; on expansion vertical lines are clearly visible, indicating the internal 
 attachment of the mesenteries. 
 
 TENTACLES. 
 
 The oral disc is divisible into an outer tentaculiferous region and an 
 inner naked area or peristome, with the mouth in the middle. The tentacles 
 are arranged widely apart, and thus occupy a comparatively broad zone of 
 the disc, leaving only a very restricted smooth central area. As can be seen 
 from plate 6, fig. 32, the tentacular region during retraction includes the 
 greater proportion of the exposed part of the polyp, while the columnar area 
 is very narrow, and little remains of the naked portion of the disc. This 
 exceptional relationship of the columnar, tentacular, and discal areas results 
 from the wide intervals which separate the different cycles of tentacles, a 
 similar condition being apparently characteristic of the Fungidae generally. 
 In most coral and actinian polyps the tentacular cycles are placed close 
 together, the bases of the tentacles of one cycle being partly embraced by the 
 bases of the next cycle, and thus the tentacular zone is very narrow compared 
 with the remainder of the disc. 
 
 The tentacles of 5. radians are remarkable among all West Indian 
 coral polyps on account of their form and arrangement. Under certain con- 
 ditions of partial or even full expansion of the polyps the organs can be 
 discerned with a lens as small sessile tubercles, arranged only approximately 
 in cycles, some as pairs and others singly, and corresponding in position 
 with the septa. In most cases they stand out as small, delicate, simple or 
 bifurcated finger-like outgrowths of the disc. As seen externally each 
 appears to originate over the central termination of the septum with which 
 it corresponds, that is, before the margin of the septum commences to dip 
 downward. 
 
 When the tentacles are fully extended and much enlarged they appear 
 as shown on plate 7, fig. 37. The stem consists of a broad basal part which 
 
lO SIDERASTREA RADIANS. 
 
 in most members of the inner cycles becomes bifurcated a little above midway, 
 each half terminating in a light-colored knob which represents a battery of 
 nematocysts ; the outermost members, however, are simple, like those of the 
 majority of coral polyps.* 
 
 In studying the internal anatomy of the polyp, it is found that the bifur- 
 cated members communicate with the entocoelic chambers, and the simple 
 forms with the exocoelic chambers. Development shows that the double 
 tentacles are at first simple, and such may be the condition of some of the 
 outer entocoelic in nearly mature polyps. The number of tentacles always 
 corresponds with the number of septa, the bilobed tentacles being situated 
 over the entosepta, and the simple tentacles over the exosepta. Their order 
 of appearance is described later in connection with the development of the 
 larval polyps. 
 
 The tentacular walls are smooth, that is, devoid of the nematoblast 
 tubercles which so frequently characterize the tentacles of coral polyps. 
 The aggregation of large nematoblasts is restricted to the apical knobs. 
 The stems are hollow and brown in color, the latter character due to the 
 presence of Zooxanthellae in the endodermal tissues. The knobs, on the 
 other hand, are solid and colorless. The organs are adhesive and able to 
 take particles of food from the tips of forceps. There is little difference in 
 size among the tentacles of the various cycles, but the bifurcated members 
 are larger than the simple. 
 
 During retraction, and even sometimes on full expansion of the polyp, 
 the tentacles appear as unstalked, spheroidal tubercles, sessile on the disc. 
 In the case of the bilobed tentacles, a knob is situated on each side of the 
 septum with which the tentacle corresponds, while in the simple tentacles the 
 knob is directly over the septum (plate 6, fig. 32). No stem can be recog- 
 nized under such circumstances, and sections show that it is not invaginated, 
 but has become part of the discal wall (plate 7, fig. 40). Such a condition 
 of the tentacular stems is often found in corals, but rarely among the larger 
 tentacles of actinians. When the tentacles are fully expanded they stand 
 obliquely to the surface of the disc, and are bent outwardly. The organs 
 have never been found invaginated, a phenomenon which frequently occurs 
 in coral polyps and alcyonarians. 
 
 * Verrill (1901, p. 154) makes the following remarks with regard to the tentacles of 5. radians : " the 
 tentacles are small, short, cylindrical, orclavate; they form several circles, and]appear somewhat scattered 
 those of successive cycles being in different circles and decreasing in size. But they are not bilobed, ?ior 
 trilobed, as Agassiz and Pourtales supposed. This appearance is due to a smaller one standing on one 
 or both sides of a larger one, and close to it." From the part italicized it would appear that the tentacles 
 in Verrill's Bermudan specimens must have been altogether different from those in Jamaica polyps, 
 which, moreover, agree with those observed by Agassiz and Pourtales. 
 
ADULT COLONY. II 
 
 The liexameral cyclic arrangement of tlie tentacles is by no means easy 
 to establish, and would be almost impossible to ascertain without anatomical 
 aid or assistance from the septa below. According to the usually accepted 
 cyclic formula for hexactinian polyps employed in systematic works, S. 
 radians would be said to have three cycles complete and a fourth incomplete, 
 and the cyclic formula would be 6, 6, 12, x, where x may be any number 
 from I to 24. 
 
 On the living polyp six bifurcated tentacles can usually be distinguished 
 as constituting an inner or primary cycle, which is widely separated from 
 the other cycles ; the remaining tentacles are more closely disposed and their 
 arrangement is somewhat obscure, simple and double members being inter- 
 mingled. Among them a second cycle of six bilobed tentacles can sometimes 
 be recognized, alternating with the first and remote from it. Beyond this 
 there is no true hexameral plan determinable. In most polyps a few mem- 
 bers of a third cycle of bilobed tentacles are present, but the full complement of 
 twelve entotentacles, necessary by the laws of actinian symmetry to complete 
 the alternation with the two previous cycles of six each, appears never to be 
 developed. The outer simple exoccelic tentacles correspond in number with 
 the sum of the members of the primary, secondary, and tertiary entotentacles 
 with which they alternate, but some belong to the third cycle and some to 
 the fourth. Only the first and second cycles are completely hexameral ; 
 the third and fourth are incompletely so. 
 
 In transverse sections through the polyps (plate 6, fig. 34) it is found 
 that the first and second cycles of mesenteries consist of six pairs each, but the 
 third cycle rarely or never contains the next number in the hexameral sequence, 
 namely, twelve. The number of entocoelic chambers corresponds with the 
 number of exocoelic chambers, therefore the number of entotentacles will 
 correspond with the number of exotentacles, as a tentacle communicates with 
 each mesenterial chamber. Whatever number of entotentacles be lacking 
 to complete the third cycle of 12, a like number of exotentacles will be 
 wanting to complete the fourth cycle of 24. Thus, considered morpho- 
 logically, the tentacular formula will be 6, 6, x, 6 -f- 6 + x, where x will 
 be the same in the two cases, and may be any number from i to 12 ; the 
 series 6, 6, x will represent the number of entotentacles, and 6 -|- 6 + x the 
 number of exotentacles. A polyp with 38 tentacles will have the morpho- 
 logical formula 6, 6, 7, 19 ; a polyp with 44 tentacles the formula 6, 6, 10, 22. 
 
 Where, in mature polyps, the hexameral sequence is incomplete, it seems 
 preferable to employ the morphological formula as compared with the ordinary 
 cyclic formula. In the first we have the true nature of the tentacles indicated ; 
 
12 
 
 SIDERASTREA RADIANS. 
 
 in the second the penultimate cycle is assumed to be complete, whereas it is 
 made up of two different series of tentacles — entotentacles and exotentacles — 
 and simply represents an arrested stage in the hexameral plan. 
 
 Fig. I is a diagrammatic representation of the arrangement of the tenta- 
 cles in a polyp of Siderastrea with the morphological enumeration added. 
 In the two inner cycles (i, ii) six bilobed tentacles are complete, while only 
 five tentacles of the third cycle (iii) are yet developed ; alternating with the 
 members of the three inner cycles of bifurcated tentacles is a simple tentacle 
 
 IllaX I X Ilia. 
 
 Fig. 1. — Diagram showing the relationships of the septa and tentacles in the same 
 polyp as that from which fig. 34, plate 6, was taken. All the entotentacles are 
 shown with a double apex and the exotentacles with a single apex. The exo- 
 tentacles are all arranged as if forming the fourth or outermost cycle, whereas 
 some will belong to the third and some to the fourth cycle, according as a third 
 cycle entotentacle is present or not. 
 
 (x) belonging to the outermost series, but not forming a true cycle. The septa, 
 indicated by the thick radiating lines, show a corresponding arrangement. 
 For purposes of comparison the same polyp is also shown in transverse section 
 on plate 6, fig. 34, while the septa are diagrammatically represented in fig. 2, 
 p. 13. By comparing figs, i, 2, and plate 6, fig. 34, it is seen that the three 
 inner cycles of tentacles communicate with the entocceles and correspond with 
 the entosepta, while the members of the outermost cycle communicate with 
 the exocoeles and correspond with the exosepta. 
 
 The tentacles of S. siderea are of the same character as those of S. radians^ 
 
ADULT COLONY. 
 
 13 
 
 only somewliat larger ; the entotentacles are bifurcated, and tlie exotentacles 
 simple. In the genus Agaricia^ also belonging to the Madreporaria Fungida, 
 the tentacles are all simple, but in their wide distance apart they resemble 
 those of Siderastrea. 
 
 The fully expanded tentacles of most species of Fungia seem to be small 
 and club-shaped, but in F. crassitentaculata they are much larger than is 
 usual for the organs in corals generally. This is well shown in the figure 
 
 Fig. 2. — Diagram showing the relationship of the septa to one another in the same 
 polyp as that of fig. i. 
 
 oi F. crassitentaculata in Saville-Kent's " Barrier Reef." In all Fungia the 
 organs are widely apart, and, as in Siderastrea and Agaricia^ occupy nearly 
 the whole discal area. 
 
 The wide separation of the tentacles in the genera mentioned usually 
 tends to obscure their hexameral cyclic plan. The two polyps of Siderastrea 
 represented on plate xv in Agassiz's " Florida Reefs " are evidence of this, 
 and also the remarks of Verrill, given in the foot-note on p. 10. Such is also 
 the case with Dana's figure of Fungia lacerta^ but Bourne found the tenta- 
 cles of F. clavata to be arranged in distinct cycles. The positions usually 
 assumed by the tentacles of Siderastrea enable one to understand how such 
 
14 SIDERASTREA RADIANS. 
 
 confusion may arise ; indeed, the polyps were for a long time under observa- 
 tion before any regular arrangement could be established with certainty. 
 It was only after observing the Jamaica polyps under various phases of 
 expansion that the cyclic plan of the first tentacles could be determined,, 
 while, as regards the two outer cycles, such an arrangement is more 
 theoretical than actual. The tentacles on plate 6, fig. 32, from a camera 
 drawing of a preserved polyp, do not readily suggest a hexameral cyclic 
 plan. 
 
 Siderastrea is apparently the only madreporarian genus with true 
 dimorphic tentacles, the peculiarity having been first recognized by Agassiz. 
 The independent origin of the two moieties of the bifurcation, as described 
 later, is also a remarkable feature in the tentacular development of the 
 Zoantharia. Dimorphic tentacles are occasionally met with among the 
 tropical Actiniaria, e. g.^ Rhodactis^ Phymanthus ; but here they are the disc 
 tentacles as compared with the marginal tentacles which show the distinc- 
 tion, and the morphological value of the former is uncertain. At any rate, 
 the disc and marginal tentacles of actinians are not comparable with the 
 entotentacles and exotentacles of Siderastrea. 
 
 DISC AND MOUTH. 
 
 The naked portion of the disc is very limited in extent. It is smooth, 
 circular, and usually partly depressed within the calice. So transparent is 
 the wall that the cyclic arrangement of the internal mesenteries and septa 
 can be easily followed. The central part or peristome is not separable from 
 the rest of the disc, except that it is usually elevated in a conical manner, and, 
 owing to a greater concentration of the endodermal Zooxanthellse, it appears 
 darker than the rest of the polyp. 
 
 The mouth is small and slit-like when closed, oval or circular when 
 open. The lips are not specially marked off from the peristome by any 
 tumidity, such as occurs in many anemones. In living polyps the mouth 
 sometimes closes along the middle by the approximation of the lips, but 
 leaves a small opening at each end. The apertures, however, can not be 
 regarded as representing a siphonoglyph or gonidial groove, for microscopic 
 examination reveals no difference in the character of the stomodasal wall 
 between the sides and extremities. On narcotization the circular muscles of 
 the disc relax, and the oral aperture remains widely open and circular; the 
 stomodseal wall is then seen to be smooth, without ridges and grooves. 
 
ADULT COLONY. 1 5 
 
 COLOR. 
 
 The polyps on the under, unexposed surface of colonies may be color- 
 less, but vary elsewhere from light to dark brown. Owing to the partial 
 transparency of the tissues the white corallum always shows through, and 
 gives rise to an alternation of light and dark radiating areas and to a light 
 polygonal line separating one polyp from the other. Sometimes the disc 
 presents a velvety green color along the mesenterial: radii, and the oral aspect 
 of the tentacles and the angles of the mouth also may be of the same color. 
 The stomodasal walls and knobs of the tentacles are white, while the stems 
 of the latter are usually brown. More rarely the elevated peristome may be 
 rose-colored, and in some cases purple.* 
 
 The different shades of brown exhibited by the polyps are entirely due 
 to the presence of Zooxanthellae or unicellular algse within the endodermal 
 layer. The chromoplasts within these are brown, yellow, or greenish yellow, 
 and when the algse are but few in number they give a light brown color to 
 the walls of the polyps, whereas when present in great numbers, overlying 
 one another, the pol3rps appear a dark brown. For this reason the thin 
 tissues of fully expanded polyps are much lighter in color than the tissues 
 of retracted polyps. 
 
 Examination of the walls of colorless polyps on the under surface of a 
 colony proves that the Zooxanthellae are either absent or occur very sparingly 
 in the endoderm. When, however, the colorless areas are exposed to light, 
 the polypal tissues begin slowly to assume a brownish color, and in a few 
 days yellow cells are to be found in abundance. 
 
 On examining a living tentacle under the microscope the outer ectoder- 
 mal layer is seen to be quite colorless, but the yellow Zooxanthellae are 
 revealed by focusing through to the internal endodermal layer. The white- 
 ness of the spheroidal tips of the tentacles, compared with the brown color of 
 the stems, is also due to the fact that the knobs are wholly formed of ectoderm 
 the endoderm lining the cavity of the stems not being prolonged into them. 
 The superficial green or purple color is somewhat evanescent, and wholly 
 ectodermal in origin. 
 
 Different shades of yellowish brown are the prevailing colors of the coral 
 areas in Jamaica, and whether in the palmate or branching growths of 
 
 ♦ Le Sueur (1820, p. 286) describes the color of the specimens seen by him at Guadeloupe as " d'un 
 rouge m&16 de violet." The color of 5. siderea as given by the same author is also very different from 
 that of the Jamaica specimens. Verrill (1901, p. 154) writes thus of the Bermuda 5. radians: "The 
 general color in life is dull gray, yellowish gray, ocher-yellow, or rusty brown, sometimes tinged with a 
 purplish rosy tint; the polyps are paler, with the lips and tips of the tentacles whitish." 
 
l6 SIDERASTREA RADIANS. 
 
 Acropora {Madrepoj^a) , the hydrozoan Millepora^ or the zoanthid Palythoa^ 
 all owe their origin to the presence of unicellular algae with yellow chromo- 
 plasts within the endodermal cells.* 
 
 Stanley Gardiner and Hickson have both drawn attention to the part 
 which commensal algae probably play in the nutrition of the polyp. The 
 organisms are actually inclosed within the endodermal cells, and there can 
 be scarcely any doubt that the polyp receives from them nutritive carbohy- 
 drates, produced in the course of the metabolic activity of the vegetable cells 
 under the action of sunlight ; further, the Zooxanthellae are constantly 
 increasing in number by fission into two and then four, and perhaps some 
 are utilized as food directly by the polyp. The liberation of free oxygen, 
 already shown to take place by the activity of the chlorophyll within the 
 algal cells, must also be of importance in the vital activities of the polyps. 
 
 In many corals and actinians, however, Zooxanthellae are altogether 
 absent, hence their presence can not be considered as necessary to the life 
 of the polyp ; also, corals grown in the shade are colorless from absence of 
 the algae. Further, both actinians and corals will ingest almost any variety 
 of animal food that is offered them. 
 
 REPRODUCTION. 
 
 New polyps arise asexually as intercalary buds, usually at the point of 
 junction of three or more polyps, so that it is impossible to say whether one 
 polyp more than another is to be regarded as the parent, or as to how far 
 the structures in the bud arise in organic association with those of the older 
 polyps. Over the general surface of any colony bud polyps in different 
 stages of development are usually present, and for some time they remain 
 much lighter in color than the others, owing to a less growth of Zooxan- 
 thellae ; also, around the margin of colonies the addition of new polyps 
 is generally in rapid progress. These latter serve to enlarge the lateral 
 boundaries of the colonies, while the intercalary polyps occupy the spaces 
 produced as the coral grows in height, and, of course, in superficial area. 
 Instances of reproduction by the process known as fissiparous gemmationf 
 
 * In his recent vice-presidential address before the Section of Zoology of the American Association 
 at St. Louis, Prof. C. W. Hargitt comes to the conclusion that the colors in coelenterates and many other 
 groups of invertebrates have probably no adaptive or protective significance, and are in main the result 
 of the metabolic activity of the animal. The rich profusion and beauty of color in coral polyps certainly 
 Beems to have no protective or warning significance, but those due to the presence of commensal alg« 
 have of course an altogether different physiological importance from the other colors. 
 
 t The term is applied to a method of asexual reproduction occasionally found in gemmiferous colonies. 
 It appears as if an enlarged polyp simply divides into two. The method, however, is altogether different 
 from growth in fission colonies generally. ("The Morphology of the Madreporaria — IV. Fissiparous 
 Gemmation," Ann. Mag. Nat. Hist., ser. 7, vol. xi, 1903.) 
 
ADULT COLONY. 1 7 
 
 are very rare, but occasionally much enlarged calices are seen undergoing 
 binary fission. The detailed study of the development of young bud polyps 
 has not been carried out, as the species is unsuited for the purpose. It has 
 been established, however, that the manner of appearance of the tentacles, 
 mesenteries, and septa agrees closely with that of larval polyps to be 
 described later. 
 
 In what seem to be dead parts of a colony, new polyps may arise 
 within the old calices. Outwardly these polyps are at first wholly sepa- 
 rated from one another and from the other living tissues of the colony, and 
 are usually transparent and colorless, due to an absence of Zooxanthellae. 
 Similar renewals of growth have been found in other corals, and give rise to 
 the successive deposits of skeletal matter on dead, corroded surfaces, some- 
 times met with when masses of coral are broken across. The continuity of 
 any one corallite is frequently maintained throughout the vertical extent of 
 the mass, notwithstanding the corroded surfaces here and there. 
 
 The renewal of the polyps within old corroded calices of a coral is a 
 subject for further investigation. So far as could be made out from external 
 obser\ation alone such polyps in Siderastrea are altogether independent of 
 one another, and also unconnected with other living tissues of the colony. 
 It may be that within the deepest parts of the old calices there still remained 
 living remnants of the original polyps, and that from these new polyps were 
 formed as buds ; on reaching the same size as the original polyps the buds 
 would fuse with one another by their peripheral borders and present a 
 continuous covering of soft tissues. New skeletal deposits would cover 
 over the old surface, and the presence of the latter would afterwards be deter- 
 minable only on breaking the colony across. 
 
 Renewals of this character, by the production of independent buds, are 
 to be distinguished from the growth at the living margin of colonies which 
 is frequently seen encroaching over dead areas. The latter is of the same 
 character as ordinary marginal gemmation from the parent stock, even 
 though it extends over old corroded surfaces. Such new growth is very 
 frequent in branching stocks of Acropora^ but may occur in almost any 
 colonial species. 
 
 The death of the polyps over any restricted area of a colony may be 
 brought about by many causes, such as the colony becoming partly covered 
 by or embedded in sand or mud, or by adherence or contact with other 
 foreign bodies, or by exposure. Local death is very frequent in such large 
 hemispheroidal colonies as those of Mceandrina and Orbicella ; the middle 
 portion of the blocks may be dead and disintegrating while the sides are in 
 
1 8 SIDERASTREA RADIANS. 
 
 full vigorous growth. In addition to this, the death of colonies as a whole, 
 without any obvious environmental cause, is not infrequent. Stanley Gardi- 
 ner has recently drawn attention to this subject. During his investiga- 
 tions on the coral reefs in the Maldives and Laccadives the impression 
 was gathered " that practically all the colonies of a species in any one area 
 died, or that there were only the isolated deaths of individual large colonies," 
 apparently without environmental cause. Gardiner is impelled to believe 
 that " The ripening of the generative organs of a large number of polyp 
 colonies of the same species in a single locality or habitat, followed by the 
 subsequent death of all these colonies, is a regular phenomenon." 
 
 In the course of my experience in the coral regions of the West Indies 
 I have met with no instance of the regional death of all the colonies of any 
 species, but frequently individual stocks have been encountered of which all 
 the polyps seemed to be in a state of maceration. This was particularly the 
 case with Forties astrcBoides. Isolated colonies were obtained seemingly alive 
 and normal in color, but upon examination with a lens no distinct polyps or 
 tentacles were recognizable. The whole of the soft tissues seemed to be a 
 gelatinous mass in process of decay, the coloration being due to the persist- 
 ence of the yellow pigment cells characteristic of this species. Such colonies 
 were under exactly similar conditions to others living around them, and 
 without further investigation any suggestion as to the cause of death of 
 the polyps can be only the merest conjecture. 
 
 My own experience leads me to suppose that coral polyps do not die 
 after the ripening of the generative products, but that, from the same indi- 
 vidual, one series of larvae may follow another, for in numerous instances 
 {Favia, Forties) in which polyps charged with larvae, all at the same stage 
 of development, have been examined, there were still many nearly ripe ova 
 within the mesenteries, as if preparing to give rise to another batch of larvae. 
 
 Sexual reproduction in Stderastrea takes place by the formation of 
 planulae from fertilized ova. The planulae undergo partial development, as 
 far as the appearance of the first four pairs of mesenteries, while within the 
 body-cavity of the parent, and are then expelled. After swimming freely for 
 a shorter or longer time they settle and give rise to young polyps. Larvae 
 were extruded toward the end of June and during July. 
 
 EXTERNAL CHARACTERS ON DECALCIFICATION. 
 
 Certain of the external features of the polyps can be studied only after 
 the latter have been freed from the skeletal matrix. The corallum of Sider- 
 asirea is very dense, and the process of decalcification requires several days. 
 
ADULT COLONY. I9 
 
 At the close of the process a delicate mass of algal filaments remains in the 
 spaces previously occupied by the corallum, but is readily removed. The 
 exposed polypal wall then reveals a smooth surface, very complicated in 
 form, but everywhere continuous. 
 
 After removal of the skeleton the individual polyps are seen to be 
 wholly free from one another, except at the surface of the colony. No 
 other connection or communication between one polyp and another exists. 
 Each polyp appears as if made up of a deeply lamellated column, attached 
 above to a flattened layer which represents the column wall and disc. A 
 middle tubular space, formerly occupied by the columella, extends for some 
 distance upwards, when it terminates blindU^ The lamellae represent the 
 tissues which occupied the interseptal spaces. They are now wholly free 
 from one another below, but in the upper region are united centrally. They 
 are easily torn apart along their central and upper lines of attachment. 
 One of the separated lamellae, slightly enlarged, is represented on plate 6, 
 
 fig- 33. 
 
 The whole of the external wall liberated by decalcification is the mor- 
 phological base of the polyp. Though columnar in form it in no way 
 corresponds with the column of the skeletonless Actiniaria, but represents 
 the flattened basal disc which, everywhere continuous, has become much 
 infolded and subdivided in correspondence with the upward growth of the 
 septa and columella. Thus the column of a decalcified retracted polyp is 
 really the vertically elongated, much subdivided basal disc. The true column 
 wall, corresponding with that of an actinian, is here represented by the 
 narrow periphery of the flattened or concave superficial disc which constitutes 
 the upper end of the polyp. To understand the form ultimately assumed 
 by the originally flat basal disc, the arrangement of the septa and columella 
 in the individual corallite must be borne in mind, as every part of these is 
 covered by the basal wall. An early stage in the basal complexity is 
 represented in the section of the young polyp on plate 9, fig. 53. 
 
 The upper part of the liberated tissues appears delicate and somewhat 
 clear and transparent, but as the aboral termination is approached the walls 
 become denser and more opaque white. Such an alteration in the external 
 character of the embedded tissues is met with in nearly all corals, and is 
 usually more marked than in Siderastrea. It is found to be associated with 
 a corresponding thickening and histological modification of the endodermal 
 layer (p. 32). 
 
 The polyps when set free are subpolygonal in outline, and, as already 
 mentioned, are wholly distinct from one another, except at their uppermost 
 
20 SIDERASTREA RADIANS. 
 
 termination, where they are all in communication and joined along the poly- 
 gonal boundary lines of the column wall and skeletogenic tissues immediately 
 below. An interval of only about i mm. separates laterally the columns of 
 contiguous polyps. The length of the polyps liberated from the skeleton 
 varies somewhat, but whatever the thickness of the corallum the polypal 
 tissues are always superficial, never extending downward for more than 
 about 5 mm. The largest polyps vary from 4 mm. to 5 mm. in length after 
 decalcification, while young buds extend much less within the corallum. 
 The diameter also varies from 3 mm. to 5 mm., and in mature polyps is prac- 
 tically the same throughout the column. The soft tissues of young buds, 
 before any columella has appeared, are more obconical, that is, longer centrally 
 and shortening as they pass to the periphery. Mature polyps are more 
 cylindrical and terminate somewhat abruptly, all the lamellae being of prac- 
 tically the same vertical length. Owing to the presence of synapticula, the 
 actual base of the individual lamellae is not always as regular or even as in 
 corals resting upon a smooth dissepiment ; the lower surface of a lamella 
 may, in fact, be partly folded round a sjmapticular bar, as in plate 6, fig. 33. 
 
 The individual interseptal lamellae, like the septa themselves, belong 
 to different cycles and are of different radial lengths. Some are simple, 
 while others are subdivided peripherally into two or three parts. For the 
 greater part of their vertical length, that is, as far as the columella extends, 
 the lamellae are free, either singly or in groups of two or three, but towards 
 the upper extremity they are all joined to one another along their inner 
 margins (plate 6, fig. 34). 
 
 Any simple lamella when seen in surface view appears as a flat, sub- 
 rectangular plate, nearly of the same thickness throughout, and correspond- 
 ing, of course, with an interseptal space (plate 6, fig. 33). It is perforated 
 by rows of circular or oval apertures, arranged roughly in one to four vertical 
 series, the number varying with the cycle to which the lamella belongs. 
 In lamellae extending all the way from the periphery to the columella three 
 or four rows occur, while in narrower lamellae only one or two rows are present. 
 The perforations represent the spaces occupied by the calcareous synapticula 
 which stretch across the interval from one septum to an adjacent septum. 
 Numerous granulations occur on the septal walls (plate 10, fig. 63), but only 
 the large more peripheral ones bridge the interspace between two adjacent 
 septa, and, in doing so, necessarily perforate the soft tissues lining the septal 
 walls and the mesenteries contained within. The smaller granulations merely 
 cause indentations in the lining tissues. 
 
 Transverse sections through the lamellae show that they consist of the 
 
ADULT COLONY. 21 
 
 two lateral walls which line the adjacent faces of two contiguous septa, and 
 inclose between them a narrow chamber, broken up by the synapticular 
 perforations in such a manner as to present a complex canal-like condition 
 (plate 6, fig. 34, and plate 7, figs. 38, 39). The chambers represent the inter. 
 septal loculi less the thickness of the lining wall, but will, however, be spoken 
 of as interseptal spaces. The upper half of each contains a single mesentery, 
 but below they are empty. Moreover, only at the uppermost extremity of 
 the polyp is the mesentery found to extend outwardly as far as the peripheral 
 boundary. In the upper region of the polyp the inner border of each lamella 
 is open so that the chamber, along with all the others, communicates with the 
 central cavity of the polyp, while below all the chambers are closed centrally, 
 or are only in communication toward their centripetal edges (plate 7, figs. 
 38, 39). Where the chambers are wholly cut off from one another the 
 calcareous septa extend throughout the radial length of the polyp, and 
 are centrally united with others or with the columella (plate 7, fig. 39) ; 
 when the chambers are only partly separated, communicating towards the 
 center, then the septa between them do not extend as far as the middle of 
 the calice. 
 
 ANATOMY AND HISTOLOGY. 
 COLUMN WALL AND DISC. 
 
 In radial sections of retracted polyps the column wall and disc appear 
 as a continuous layer, and, histologically, the two are nearly alike through- 
 out. The wall as a whole is very thin, except where the knob of a tentacle 
 is included, when the ectoderm becomes thickened (plate 7, fig. 40, and plate 
 9, fig. 53); to a less degree the layer is also thickened along the line of 
 attachment of the mesenteries. In sections the three coelenterate layers, 
 ectoderm, mesogloea, and endoderm, together, measure only about 0.07 
 mm. in thickness. 
 
 The chief constituents of the ectoderm of the column wall are support- 
 ing cells and clear mucous gland cells, while small nematocysts occur some- 
 what sparsely. The unicellular gland cells are most conspicuous toward the 
 periphery of the layer, where they appear oval, with clear or vesicular con- 
 tents. The nuclei of the various cells are oval, and arranged as a whole 
 within the inner half of the ectoderm, a few, more circular in form, being 
 found near the mesogloea. There is no trace of any ectodermal musculature 
 on the column, but a system of weak fibrils is developed over the disc, their 
 direction being radial. No evidence of external ciliation can be detected 
 even in the best preserved examples. 
 
 The mesogloea in the column, as elsewhere throughout the polyp, is 
 
22 SIDERASTREA RADIANS. 
 
 feebly developed. In the column and disc it appears only as a thin dividing 
 lamella between the ectoderm and endoderm, but in the mesenteries it 
 becomes a little broader. It is homogeneous in structure, except for the 
 presence of minute connective-tissue cells. The mesogloea of corals gener- 
 ally appears perfectly homogeneous, but in several West Indian species the 
 layer is found to contain migrant connective-tissue cells, such as occur in 
 the larger actinians. 
 
 The endoderm of the column ^nd disc is a little broader than the ectoderm, 
 and its cells contain numerous Zooxanthellse, which are altogether absent 
 from the latter. The endodermal cells are much vacuolated, and distinct 
 cell outlines can be made out. The commensal algse are more numerous in 
 some regions of the endoderm than, in others, though there seems to be no 
 regularity in their distribution. Where they are absent, or nearly so, the 
 endoderm is somewhat thinner, its cells are less vacuolated, and the layer as 
 a whole stains more intensely, the nuclei forming a more regular band. 
 
 A delicate circular muscle layer can be discerned over the inner face of 
 the mesogloea of the column and disc, but no concentration, such as can be 
 regarded as constituting a sphincter muscle, occurs at any region of the 
 column. When studying the living characters it was found that the polyps 
 are unable to fold the column over the disc, hence the presence of a special 
 circular or sphincter muscle, as met with in most anemones and some few 
 corals, would be scarcely expected. 
 
 In sections which have been passed through Delafield's haematoxylin 
 the deeper part of the endodermal layer throughout the polyp, but particu- 
 larly the epithelial lining of the mesenteries, contains some substance which 
 stains intensely, and has a very irregular distribution. The appearance, as 
 seen in sections of mesenteries, is represented on plate 8, fig. 50, and in the 
 lining of an interseptal loculus on plate 8, fig, 47. Small, irregular, deeply 
 staining patches lie next the mesogloea on both sides, and prolongations 
 extend for varying distances among the endodermal cells, occasionally 
 reaching almost to the surface. In tangential or oblique sections through 
 the endoderm an appearance of somewhat irregular longitudinal canals is 
 presented. The substance stains and behaves altogether in the same manner 
 as the contents of the mucous cells. No evidence of structure is presented, 
 nor is there any cellular character suggested. I conceive that the appearance 
 is due to intercellular spaces filled in the living condition with mucus or 
 some similar substance, capable of staining strongly in haematoxylin ; per- 
 haps a hint of the lymphatic spaces of the higher animals. 
 
 This is the first time that such a system of apparently intercellular 
 
ADULT COLONY. 23 
 
 Spaces has been observed in the Zoantharia. Its appearance in sections of 
 Siderastrea stained in hsematoxylin is very characteristic, and its presence 
 should be looked for in other forms. 
 
 TENTACLES. 
 
 When the polyps are preserved in the retracted condition, with the 
 tentacles sessile, the columnar and discal walls come to lie upon the skele- 
 totrophic layer of the septal edges, and the two ^nobs of the bifurcated 
 tentacles are far apart, one on each side of a septal ridge (plate 7, fig. 40), 
 while the knob of the simple tentacles lies immediately over the septum with 
 which it corresponds. In this condition the stem of the tentacle has altogether 
 disappeared as such, having become part of the disc. As a result the tenta- 
 cles are recognizable in vertical sections of polyps as mere hemispheroidal 
 thickenings of the ectoderm. In the case of the entoccelic tentacles a thick- 
 ening occurs on each side of a septal elevation of the disc, while a single 
 elevation directly over the apex of the septum represents an exotentacle. 
 In the thickenings long nematocysts extend nearly across the ectoderm, and 
 smaller ones crowd the periphery. The two forms of nematocysts are 
 represented on plate 7, fig. 44. Histologically the portion of the wall join- 
 ing the two tentacular knobs on plate 7, fig. 40, and representing the stem of 
 the tentacle, differs in no way from that of the disc. 
 
 In sections of polyps preserved with the tentacles extended the organs 
 appear as tubular outgrowths of the disc, terminated by knoblike enlarge- 
 ments. The stem seems to differ in no respect histologically from the disc, 
 not even in the greater number of nematocysts ; the apex alone constitutes a 
 battery of stinging cells. The cut ends of a delicate ectodermal muscle 
 layer can be distinguished in transverse sections of the stems, and a feeble 
 circular endodermal layer in longitudinal sections. The tentacula'r muscu- 
 lature is continuous with that of the disc, and a very definite nerve layer 
 occurs at the apex of the tentacles (plate 8, fig. 49). 
 
 STOMOD^UM. 
 
 The stomodaeum is a short, thin-walled, depending tube, strongly ciliated 
 all round, and without any permanent ridges or grooves. Usually it is 
 oval in transverse section, and along its endodermal surface six pairs of 
 mesenteries are attached at equal distances apart. Histologically it is of 
 the same structure all the way round, there being no modification opposite 
 the directive mesenteries such as can be regarded as a siphonoglyph or 
 gonidial groove. This is also the case in all madreporarian polyps yet 
 described. 
 
24 SIDERASTREA RADIANS. 
 
 The absence of a siphonoglyph in Madreporaria affords an interesting con- 
 trast with the one or two grooves usually present in actinian and alcyonarian 
 polyps. A siphonoglyph is generall}^ wanting only in the lowest actinians 
 and in alcyonarians, and its absence in coral polyps would suggest their 
 more primitive nature. This is further borne out by the absence of ciliated 
 bands or Flimmerstreifen from the mesenterial filaments (p. 29). 
 
 The stomodseal ectoderm is thicker than that of the disc, and differs 
 much from it in structure. Large nematocysts are distributed throughout, 
 but the main constituents are long, narrow, ciliated supportiug cells, the 
 nuclei of which are arranged in a broad zone. The ciliation, which is uni- 
 form all round, generally persists in preserved material. Clear mucous 
 gland cells are not numerous, while here and there a granular gland cell 
 stands out very prominently by reason of the deeply staining character of 
 its contents. A distinct nerve layer is recognizable next the mesoglcea, but 
 no muscular fibrils can be detected. The endoderm is similar to that lining 
 the upper part of the polypal cavity. 
 
 At its lower opening into the polypal cavity the ectoderm of the stomo- 
 daeum is reflected up the inner or endodermal surface, and then extends 
 horizontally for a short distance along the free edge and each face of the 
 perfect mesenteries, becoming continuous with their mesenterial filaments. 
 
 MESENTERIES. 
 
 The number and arrangement of the mesenteries in mature polyps are 
 as follows : (a) Six pairs are united with the stomodseum, including two 
 pairs of directives attached at the opposite extremities ; {d) an alternating 
 cycle of six pairs, the members of which are free from the stomodaeum 
 throughout their length ; (c) a variable number of third-cycle mesenteries, 
 alternating with the two previous cycles, but very rarely, if ever, with the 
 whole cycle of twelve pairs developed (plate 6, fig. 34). Thus two cycles 
 of mesenteries are always developed, and a variable number belonging to a 
 third cycle. They may be represented by the formula 6, 6, x, where x 
 represents any number from i to 12. 
 
 Within each intermesenterial space, whether an entocoele or an exocoele, 
 an invagination of the polypal wall occurs which separates adjacent mesen- 
 teries from one another. Except in the uppermost region these septal 
 invaginations extend centrally further than the mesenteries, while in the 
 lower parts of the polyp they extend so far as to meet in the middle and fuse, 
 dividing the gastro-coelomic cavity into distinct chambers, each of which 
 contains a mesentery (plate 7, figs. 38, 39). 
 
ADULT COLONY. 25 
 
 The mesenteries extend from above downward for about two-tbirds of 
 tbe total length of the polyp, the younger outer pairs terminating in advance 
 of the older inner pairs. Below they become very short transversely, and 
 are only slightly folded at their free extremity. Kxcept in the uppermost 
 region of the polyp, their peripheral portion is everywhere perforated by the 
 synapticular bars. Further, they are connected outwardly with the wall of 
 the polyp only in the upper part of their course ; the connection breaks down 
 below, and the organs are seen in transverse sections with both their inner 
 and outer ends free. Thus at an early stage the peripheral portion of the 
 mesentery undergoes degeneration or resorption much in advance of the more 
 central part. The mesenteries, in fact, occupy only the middle and upper 
 part of the gastro-vascular cavity ; the proximal and peripheral regions are 
 practically destitute of them. 
 
 The mesenterial mesoglcea is usually narrow, but there is much varia- 
 tion in the different polyps as to its thickness and hence that of the mesentery 
 as a whole. In the upper region the mesoglcea presents vertical folds for 
 the support of the retractor muscles (plate 7, fig. 41). Generally the fold- 
 ings are diffuse and somewhat complicated, and extend along the entire 
 face of the mesentery ; in other cases they are stronger and more restricted 
 to the middle area. The oblique musculature is very weak throughout, but 
 occasionally the fibrils can be seen cut obliquely in transverse sections. As 
 would be expected, no basilar or parieto-basilar muscles occur. The mesen- 
 terial endoderm is everywhere richly supplied with Zooxanthellse, and also 
 with clear gland cells and others containing large granules. As described 
 in connection with the endoderm of the column wall and disc (p. 22), what 
 seems to be an intercellular system of mucous spaces occurs in the deeper 
 parts of the epithelium on both mesenterial face. 
 
 The arrangement of the mesenteries may be studied in more detail. As 
 represented on plate 6, fig. 34, and also in the diagrammatic figure on page 26, 
 the number of pairs within each of the six primary systems or -sextants 
 varies from one to three. The members of a pair can be easily distinguished 
 by their similarity of size and by the retractor muscles being on the faces 
 turned towards each other. In each of the two ventral systems only one 
 pair of mesenteries (11) is developed, while in the middle and dorsal systems 
 two pairs (11, iii) are present, with the exception of the right middle system, 
 which contains three pairs. Where only one pair occurs, it represents one of 
 the six pairs of first-cycle metacnemes (11), and this cycle is complete in all the 
 mature polyps examined. Where two pairs are present, one pair (iii) is shorter 
 than the other and is a member of the third cycle. It will be observed that 
 
26 SIDERASTREA RADIANS. 
 
 in eacli sextant the third-cycle pair (iii) is situated on tlie dorsal aspect 
 of the second-cycle pair (ii). In the right middle sextant, where two pairs of 
 third-cycle mesenteries occur, a pair is present on both the dorsal and 
 ventral aspects of the longer pair. It is shown later that the mesenteries 
 of the first and second cycles are developed in a very definite sequence, 
 and such would probably be the case for the members of the third cycle were 
 the polyps isolated and free to develop normally all around. The individual 
 polyps in a colonial coral like Siderastrea^ however, are so closely arranged 
 
 Illb 
 
 I 
 
 Fig. 3. — Diagram of mesenteries of fig. 34, plate 6 {cf. figs, i and a, pp. 12, 13). 
 
 that their growth after the first two cycles becomes largely influenced by 
 spatial necessities. Hence, in the particular polyp from which plate 6, fig. 
 34, is taken, one region, the right middle sextant, has progressed farther 
 than the others. Throughout the studies little constancy has been found in 
 the order of development of the members of the third cycle. The amount of 
 variability in the number and disposition of the mesenterial pairs is repeated 
 in the septa (p. 48). In no two polyps, among a dozen or so studied in 
 transverse sections, was the mesenterial plan the same for the third cycle ; 
 also, in none was the full complement of twelve pairs present. The normal 
 sequence of the mesenteries is more fully described in connection with the 
 development of the polyp. 
 
ADULT COLONY. • 27 
 
 The radial length of the mesenteries varies greatly in passing trans- 
 verse sections in review from the oral to the aboral extremity of a polyp. In 
 an expanded polyp the complete mesenteries extend from the colnmn wall, 
 then across the disc, and finally unite with the stomodaeum, down which 
 they pass ; the members of the incomplete cycles, on the other hand, stretch 
 from the column wall only partly across the disc, and terminate at a greater 
 or less distance from the middle of the polyp. When the level of the corallum 
 is reached, all the mesenteries begin to lose their peripheral connection 
 with the polypal wall, and in transverse sections hang freely, unconnected 
 either peripherally or centrally, except where pierced by a synapticulum. 
 Even at the level of the stomodaeum in partly retracted polyps the mesen- 
 teries are greatly shortened transversely, so that the peripheral parts of 
 the septal loculi are empty (plate 6, fig. 34). Where the loculi are inter- 
 rupted transversely by a synapticulum, a mesentery will sometimes stretch 
 from an inner chamber of the loculus to the next chamber, but in no instance, 
 as shown on plate 6, fig. 34, is one found to extend right to the periphery 
 of an interseptal loculus. Instead of this, most of the peripheral chambers 
 are empty, the mesenteries having become resorbed. The organs have a 
 much less radial or peripheral extent on plate 7, fig. 38, resorption having 
 been continued further than at the level represented by plate 6, fig. 34. 
 Mesenteries are wholly absent on plate 7, fig. 39, except in one or two 
 isolated instances. The stages prove conclusively that the mesenteries 
 extend but a very short vertical distance within the region of the synapti- 
 cula, but are longer centrally. The same fact is also well shown on plate 6, 
 fig. 36, representing the upper part of a tangential section of a polyp. 
 
 The descriptions given by Bourne (1887, 1893) of the anatomy of the 
 synapticulate coral Fungia indicate similar degeneration phenomena of the 
 mesenteries within the region of the synapticula in this genus. At first both 
 the primary and secondary mesenteries extend to the very base of the coral- 
 lum, but afterwards they are confined to the upper moiety of the calice, 
 though in Fungia no dissepiments are formed which cut off the polyp 
 from the basal plate. Bourne's figs. 10, 13, and 15 in his first paper show 
 that the mesenteries very rarely extend along the interseptal loculi all the 
 way from one synapticular perforation to another, but in practically all 
 the canals long or short remnants of the mesenteries are adherent to the 
 synapticular wall, and serve for the attachment of separate bundles of the 
 longitudinal muscles. In these cases the middle part of the mesentery within 
 each chamber has been resorbed, leaving only its two extremities. In Sider- 
 astrea there is nothing corresponding to the mesenterial muscle bundles in 
 
28 SIDERASTREA RADIANS. 
 
 the synapticular region whicli Bourne describes for Fungia and Fowler (1888, 
 p. 9) for Stephanophyllia jormosissima ; muscular degeneration proceeds along 
 with that of the mesentery in the West Indian form. 
 
 The actual process of resorption of the mesenteries, from the periphery 
 inwards, can be readily traced. It commences in the peripheral region of the 
 polyp even as high as the level of the retracted stomodseum. The part of 
 the mesentery undergoing degeneration narrows and breaks up into separate 
 fragments ; the mesogloea is often very thin for some distance, and the mus- 
 cular fibrils lining it form only an irregular, discontinuous layer. Generally 
 the portion of the mesentery some little distance from the peripherally fixed 
 part is the first to pass away, thus leaving an interval in the actual continuity 
 of the organ. The endoderm may disappear first, and the naked mesogloea 
 then extends a little beyond and thins out (plate 7, fig. 43), but usually the 
 epithelium persists the longest. Free fragments of the degenerating tissue 
 are often found in the interseptal chambers, and other fragments are occa- 
 sionally seen absorbed by the endoderm lining the chamber. 
 
 Mesenterial resorption must of course take place proximally in all corals 
 which continue to grow upwards as the lower part of the polyp is cut off 
 below by dissepiments. But in Siderastrea the peripheral parts begin to 
 disappear first, and the disappearance as a whole extends upwards propor- 
 tionally much further than in most corals. Perhaps this is in some way 
 connected with the presence of the synapticula in the peripheral region. In 
 other coral polyps the lower edge of the mesentery appears to be resorbed 
 more uniformly. 
 
 The mesenteries are too small and too completely inclosed within the 
 interseptal chambers to permit of their being dissected out and examined as 
 a whole, but by means of serial sections it can easily be seen that the synap- 
 ticula actually perforate the mesenteries. In the first place, the complete 
 loculus shown on plate 6, fig. 33, reveals that the synapticula really per- 
 forate the walls, and necessarily anything inclosed within the two lining 
 walls in the same areas. Then certain of the synapticular perforations 
 included in plate 6, fig. 34, show that a mesentery is continued from each 
 wall bounding the perforation, passing centrally on the one hand and 
 peripherally on the other. On following such a mesentery both upward 
 and downward in serial sections it is found that when the synapticular 
 perforation disappears, having been followed through its vertical extent, the 
 two moities of the mesentery again become continuous. Clearly such condi- 
 tions can result only from the presence of actual fenestrae in the mesentery. 
 Similar relationships can be also followed in the series of sections represented 
 on plate 9. 
 
ADULT COLONY. 29 
 
 Owing to the early disappearance of the mesenteries aborally and periph- 
 erally the organs are, at any one time, necessarily perforated for a very 
 restricted part of their course. Furthermore, as a result of the presence 
 of these synapticular bars, any mesentery must be capable of less retraction 
 than in species where the whole length of the organ is free to respond to the 
 action of the retractor muscle. 
 
 Desmocytes, continuous with the mesogloea of the lining of the inter- 
 septal chambers and the attached mesentery, are usually numerous around 
 the perforations of the synapticula (plate 7, fig. 41, and plate 8, fig. 45). 
 
 MESENTERIAL FILAMENTS. 
 
 Filaments occur on the mesenteries of all three cycles, though usually 
 they are imperfectly developed on the members of the third. On the incom- 
 plete mesenteries the organs commence a short distance above the lower end 
 of the stomodseum, and are continued throughout the vertical extent of the 
 mesentery ; on the complete mesenteries they start from the lower termina- 
 tion of the stomodaeum, the ectoderm of the latter being continuous with the 
 filament. As in all Madreporaria yet described the filaments consist of only 
 a median lobe, supported basally on an expanded mesogloeal axis (plate 7, 
 fig. 42). Trilobed mesenterial filaments having lateral lobes supported on 
 a mesogloeal axis and bearing ciliated bands (Flimmerstreifen), like those 
 found in most Actiniaria, are not known to occur in corals. 
 
 The fully developed filaments of S. radians are sharply separated from 
 the mesenterial endoderm by a well marked constriction on each side. The 
 endoderm is usually slightly swollen immediately behind the filament, but 
 rarely assumes the form of a definite lobe as in many other coral species. In 
 addition, the outline of the filament in section varies somewhat in the case of 
 the first-cycle mesenteries. At first it is cordate, and, histologically, is quite 
 uniform all round; but soon it becomes nearly circular, and the cellular 
 constituents of the middle part differ from those of the lateral {cf, plate 7, 
 figs. 42, 43). 
 
 The cellular constituents of the filaments are mostly ciliated supporting 
 cells, but with these are mingled large, clear, and granular gland cells and 
 nematocyst bearing cells, especially in the middle region of the sections. 
 Towards the sides and posterior borders the cells diminish in height and 
 are nearly all supporting cells (plate 7, fig. 42). At least two kinds of 
 nematocysts occur — a long, narrow, thick-walled form, similar to that in the 
 tentacular knobs, and a large, oval, thin-walled form, with the spiral thread 
 strongly marked (plate 7, fig. 44). 
 
30 SIDERASTREA RADIANS. 
 
 The free end of the mesenteries and, consequently, the filaments also are 
 somewhat exceptional among corals in the small degree to which they are 
 convoluted in the lower regions. In most cases they pass vertically in a 
 straight course from their upper to their lower extremities without any folding. 
 Upon irritation the living polyps of most corals are able to extrude, through 
 temporary apertures in the body-wall, masses of contorted filaments along 
 with the portion of the mesentery to which they are attached, but this 
 phenomenon has been observed on only one occasion in Siderastrea. 
 
 The question of the ectodermal or endodermal origin of the filaments is 
 alluded to in the description of the larva. 
 
 SKELETOTROPHIC TISSUES. 
 
 The skeletotrophic or skeletogenic tissues constitute by far the greatest 
 proportion of the soft parts of the polyps, including as they do the lining of 
 the calicinal wall, septa, and columella throughout. They represent the 
 original flat basal disc of the polyp, which has become greatly infolded in 
 correspondence with the skeletal ingrowths. Both the endoderm and ecto- 
 derm present certain structural peculiarities compared with the same layers 
 in the column wall and disc, and in some respects the calicoblast layer of 
 S. radians differs from that in most other corals. 
 
 The polypal wall lining the uppermost parts of the skeleton is extremely 
 delicate, especially over the edges of the septa. In sections the combined 
 ectoderm and endoderm vary from 0.015 to 0.003 ™^- i^ thickness, both layers 
 being about equal (plate 8, fig. 47). The endoderm is a sjaicytium showing 
 no signs of cellular divisions, and the cytoplasm has at first few or no 
 vacuoles, though below they become somewhat numerous. The contained 
 nuclei are large, round or oval, and closely arranged in a very regular row. 
 Zooxanthellse also occur, their diameter being often equal to the thickness 
 of the layer. Irregular mucous spaces, similar to those described in the 
 endoderm (p. 22), are also seen, except where the layer is at its thinnest. 
 
 The mesoglcea, both here and elsewhere throughout the skeletogenic 
 tissues, is rarely distinguishable as a distinct layer, but constitutes a mere 
 line of division between the ectoderm and endoderm. Where the mesoglcea of 
 the mesenteries is united with the skeletal covering the calicoblast ectoderm 
 is nearly absent, and the mesoglcea may then become swollen and continued 
 into the well-known structures which have been termed desmocytes by 
 Bourne (1899). '^^^ combined mesenterial and skeletotrophic mesoglcea 
 here broadens in a fan-like manner, is striated, and stains more deeply than 
 usual (plate 8, fig. 45). Sometimes the desmocytes appear as more distinct 
 
ADULT COLONY. 3 1 
 
 triangular or wedge-shaped structures, with the narrow apex towards the 
 mesoglcea. 
 
 The function of the desmocytes or desmoidal processes, so closely asso- 
 ciated with the mesoglcea, is considered to be that of attaching the soft 
 polypal tissues to the hard corallum, and they are always most numerous 
 along the line of union of the mesenteries with the basal wall. As the 
 mesenteries in Siderastrea are attached to the skeletogenic tissues only 
 in the uppermost part of the polyp, desmocytes are developed somewhat 
 sparingly ; furthermore, they are rarely found over regions where the 
 mesenteries are not attached. In such areas a broad calicoblast layer 
 usually intervenes between the mesoglcea and the skeleton. Where the 
 skeletogenic tissues, including the mesenteries, are perforated by the synap- 
 ticula, desmocytes are usually present along the line of attachment of the 
 mesentery. 
 
 Bourne (1899) has worked out in a very masterly manner the origin of 
 the desmoidal processes from the single calicoblast cells. These cells, which 
 are strictly the desmocytes, later become connected with the mesoglcea, and 
 the structure takes on a feebly striated character. Although a careful search 
 has been made in sections of Siderastrea prepared in various ways, I have 
 found no certain stages in the formation of the desmoidal processes. Cells 
 of various forms occur within the calicoblast layer in regions where desmoidal 
 processes are already present, or may be expected, but none which could be 
 determined with certainty as desmocytes. Anyone familiar with the details 
 of madreporarian histology will appreciate the credit due to Bourne for work- 
 ing out the development of these structures with such a degree of complete- 
 ness. In their fully formed condition they are undoubtedly continuations of 
 the mesoglcea, and it is remarkable that they should have originated inde- 
 pendently and later come into fusion with the middle layer. Gardiner states 
 (1902, p. 138) that in the polyps of Flabellum rubrum the first appearance of 
 any desmocyte could be seen in a granular mass of protoplasm against the 
 corallum, to which, from the first, it seemed to be attached. Subsequently, 
 by growth inwards, it joins the structureless lamella, which may be thickened 
 so as to meet it. 
 
 The actual calicoblast layer of S. radians^ like the endoderm, is at first 
 very narrow, and in the growing areas of the skeleton shows no evidence of 
 cell limitations. The cytoplasm appears continuous, non-granular, and with 
 or without mucous spaces, the whole staining a bright yellow in picric acid. 
 The nuclei are nearly as numerous as in the endoderm, and are large and 
 finely granular. The margin towards the skeleton is usually irregular in 
 
32 SIDERASTREA RADIANS. 
 
 outline, often with many adhering particles which stain differently from the 
 layer itself (plate 8, fig. 47). 
 
 Away from the actual growing apices of the skeleton, both downward 
 and peripherally, the skeleton-lining tissues at once begin to undergo a 
 marked alteration ; both ectoderm and endoderm become greatly thickened 
 until they are three or four times their former size (plate 8, figs. 45-48). 
 The thickened endoderm is reticular, with many oval nuclei arranged in a 
 regular zone towards the margin ; Zooxanthellse are also numerous, and dis- 
 tinct mucous accumulations appear. 
 
 Vacuoles extend nearly across the layer, and usually appear clear and 
 perfectly transparent, but when stained with iron hsematoxylin they present 
 a grayish reticulum. They are evidently mucous in character, and would 
 seem to represent individual cells. In sections stained with Delafield's 
 haematoxylin and picric acid the vacuolar spaces stand out conspicuously, 
 the boundaries and reticulum being stained a dense blue, as with mucous 
 cells generally, and appearing as if enclosed in a yellow granular matrix. 
 Usually the mucous spaces do not extend as far as the outer margin of the 
 layer, and not always to the mesogloeal boundary. 
 
 The general cytoplasm of the calicoblast ectoderm is finely granular 
 throughout and does not stain readily except in blue-de-Lyon, which colors 
 the particles a bright blue. The nuclei are comparatively small and less 
 numerous than in the upper part of the layer, or in the endoderm. In 
 addition to the granular, matrix-like cytoplasm, cells with coarse granules 
 occur here and there which stain differently from the other granules and 
 appear to be altogether distinct structural elements. In sections stained 
 with carmine and blue-de-Lyon they stand out a conspicuous red against the 
 ordinary blue granules. They rarely if ever extend wholly across the layer 
 and seem comparable with the other coarsely granular cells found elsewhere 
 throughout the ectoderm and in the mesenterial filaments. 
 
 Scattered throughout the calicoblast ectoderm are also a few small oval 
 nematocysts with a close spiral thread. Sometimes several may occur 
 together, but usually they are found singly. Different stages in their devel- 
 opment also can be observed. In the earlier stages they are large, with 
 homogeneous contents, and stain rather deeply, whereas when mature they 
 stain feebly, if at all. 
 
 The presence of nematocysts within the calicoblast layer, where appar- 
 ently they can be of no service to the polyp, is somewhat remarkable. It is 
 to be borne in mind, however, that the calicoblast layer is but a modified part 
 of the ectoderm which, over the column wall and oral disc, always bears 
 
ADULT COLONY. 33 
 
 stinging cells ; and further, the cysts are usually present in numbers at 
 the aboral extremity of the larva which becomes the basal disc when the 
 larva settles. So far as can be made out from sections, without isolation by 
 maceration, the nematocysts in the calicoblast layer are similar to those in 
 the ectoderm of the column wall. Bourne (1899), in his studies of the 
 calicoblast layer of the Madreporaria, also encountered oval bodies which 
 suggested to him degenerate nematocysts. The close spiral thread found 
 in those of Siderastrea^ and their general behavior at different stages towards 
 reagents, leave no doubt as to their true nature. 
 
 The structure of the calicoblast ectoderm, as above given, differs some- 
 what from that usually met with in corals. As described in the papers of 
 Bourne, Fowler, and Gardiner, the layer is thickest in the regions of active 
 growth, and becomes very thin or nearly disappears elsewhere. Such is also 
 its condition in most of the species of corals examined by me (1902, p. 482). 
 It may appear either as a columnar epithelium [Madrepord) or have the 
 character of a syncytium. A well-developed cellular tissue, with abundant 
 protoplasm, would be expected where active formation of the skeleton is in 
 progress. Here, in Siderastrea^ the layer is thickest in those regions where 
 the secretory activity would be considered to be at a minimum. The granular 
 character of the cytoplasm, the small size of its nuclei, and the strong 
 vacuolization would not, however, suggest that the deposition of skeletal 
 matter was actively proceeding in the lower regions, as compared with the 
 large nuclei and more hyaline cytoplasm along the edges of the septa. 
 
 It seems doubtful as to how far the thinness of the endoderm in the upper 
 region is to be associated with a similar condition of the calicoblast endoderm. 
 Over the spines of the columella, which are situated in the lower regions of 
 the polypal cavity, and are probably in a growing state, the two layers pre- 
 sent a great difference in this respect. The ectoderm is as thin as in the 
 uppermost growing parts of the polyp, while the endoderm is greatly thick- 
 ened, measuring as much as 0.05 mm. across, which is nearly equal to the 
 combined thicknesses of both ectoderm and endoderm in the lower regions. 
 
 None of the writers on the anatomy of coral polyps have referred to the 
 marked alteration undergone by the skeletotrophic endoderm in passing from 
 the upper to the lower parts of the polyp. It is, however, characteristic of 
 practically all West Indian corals, the contrast being usually greater than 
 that shown by Siderastrea. 
 
 In his study of the minute anatomy of the polyps of Flabellum rubrum^ 
 Gardiner (1902) found the calicoblastic ectoderm to be in many respects 
 like that of S. radians. The layer is every^vhere complete and persistent, 
 
34 SIDERASTREA RADIANS. 
 
 even in the most roughly decalcified specimens, and no definite cell limita- 
 tions could be distinguished ; but over the greater part of the corallum it is 
 an extremely thin, finely granular layer, and as the edges of the septa are 
 approached the layer thickens, nuclei become more frequent, and tend to 
 exhibit a definite network. Where secretion may be supposed to be going 
 on most actively the layer is more hyaline, elsewhere it is granular. The 
 protoplasm is much vacuolated, as in Siderastrea^ except where secretory 
 activity prevails. 
 
 Where decalcification has been carefully carried out, a layer of the 
 organic matrix within which the skeleton is deposited usually remains 
 behind, and is closely adherent to the calicoblast layer, as shown at the left 
 side of plate 8, fig. 45. The matrix usually appears perfectly homogeneous, 
 very variable in width, and recalls the mesogloea in its behavior towards 
 stains. Bourne (1899) found such a layer in many of the forms examined 
 by him, and regards it as a fine membrane — " limiting membrane" — separat- 
 ing the calicoblasts from the corallum, and comparable with the sheath which 
 incloses the spicules of the Alcyonaria. It is evidently a secretion of the 
 ectodermal layer, and at the apical points of rapidly growing corals, such as 
 Madrepora, I have found the deposit to be continuous throughout the whole 
 thickness of the corallum, and presenting just beyond its border the fibrous 
 scale-like appearance of the early skeleton (1902, p. 484). It is manifest that 
 the mass should be regarded as a homogeneous, mesogloea- like matrix within 
 which the minute calcareous crystals forming the skeleton are laid down, to 
 be compared with the matrix which Bourne has demonstrated for the skeletal 
 spicules of Alcyonaria. As a continuous structure it early disappears, and 
 in the older parts of the skeleton is represented merely by the fine organic 
 particles remaining after decalcification. Only under the most favorable 
 circumstances is it present as a continuous mass in sections of the newest 
 formed parts. Usually all that remains of it after decalcification is the deli- 
 cate membrane bordering the calicoblast layer on its skeletal aspect. When 
 of sufficient thickness to have contained skeletal fibers, now dissolved away, 
 this membrane appears fibrous, but immediately bordering the calicoblast 
 layer it is homogeneous. 
 
 SEPTAL INVAGINATIONS, INTERSEPTAL LOCULI, AND GASTRO-CCELOMIC CAVITY. 
 
 The septal invaginations {refoulements septaux of Delage & H^rouard) 
 are the vertical, somewhat wedge-shaped infoldings or upgrowths of the 
 basal wall which in the living condition cover both sides of the septa, 
 and have been produced pari passu with them. Their presence results in 
 
ADULT COLONY. 35 
 
 mucli subdivision and complexity of the lower part of tlie polypal cavity, as 
 compared with that of an actinian, where the basal disc retains its primitive 
 flatness. In the coral polyp radial parts of the basal disc are pushed vertically 
 upwards into the polypal cavity, concurrently with the deposition of calca- 
 reous matter, so that some portions of the aboral wall are of the height of 
 the septa. The earliest stages in the formation of the basal infoldings over 
 the septa are shown on plate 9, fig. 53. In the later processes of growth 
 the lower edge of the column wall is also carried upwards to about the same 
 height as the septal invagination,* hence it results that the basal wall is 
 represented by so many vertical lamellar infoldings, the column wall and 
 disc appearing as a mere superficial covering to these. 
 
 In transverse sections the invaginations are found to occur between 
 every two mesenteries, that is, they are both entocoelic and exoccelic in 
 position, the two series being equal in number, the exoccelic extending less 
 centrally than the others (plate 6, fig. 34). In the upper region of retracted 
 polyps the invaginations do not extend as far centrally as the primary mesen- 
 teries, though in the stomodseal region they are continued beyond the secondary 
 and tertiary mesenteries. As the lower regions are approached the foldings 
 stretch centrally beyond all the mesenteries until, in the end, the walls of 
 adjacent invaginations unite with one another and thus completely inclose 
 each mesentery in a separate interseptal loculus, the skeletal deposit being 
 also continuous from the periphery to the middle (plate 7, fig. 39). 
 
 As shown in the various figures, the septal invaginations do not remain 
 truly radial. At their inner extremities the exoccelic members turn laterally 
 to unite with an entocoelic invagination, and the entocoelic of the third cycle 
 are united with the entocoelic of the second. In this way continuity of the 
 invaginated walls in transverse section is broken, and the skeletal matter 
 within one invagination is likewise continuous with that in another. On plate 
 7, fig. 39, it is seen that some of the septa are continued radially as far as 
 the central columella, while the others extend centrally only indirectly by 
 fusing with the first. As a result of these many fusions the cyclic plan of 
 
 * In many corals, e. g.^ Cladocorti, Mant'ctna, the proximal margin of the column wall is lower than 
 the upper edge of the septa, and the latter are generally united peripherally in the thecal wall; the septa 
 have grown upwards into the polypal cavity, leaving the lower boundary of the column wall behind. 
 The mesenteries still remain attached to the outer column wall, but internally they have been 
 pushed upwards, as it were, along with the upward growth of the thecal wall. Owing to this, parts of 
 the polypal cavity and mesenteries are peripheral or outside the calice (extracalicular or pericalicular) 
 and the remainder is within the calice (intracalicular). The portion of the polyp outside and around the 
 calice is what is known as the "edge-zone" or " Randplatte," but, as noticed on p. 9, there is no edge- 
 zone in Siderastrea, the polypal cavity and mesenteries being wholly intracalicular when the polyps 
 are retracted. 
 
36 SIDERASTREA RADIANS. 
 
 the invaginations is not readily established ; in fact, it would be almost impos- 
 sible to determine it without the assistance of the mesenteries. In addition, 
 complications are produced by the fact that the invaginations are perforated 
 by the synapticula, and thus, in sections, their walls appear discontinuous. 
 
 The gastro-coelomic cavity in Siderastrea is very complicated in character 
 compared with that of an actinian polyp, or, indeed, with that of many other 
 corals. A double series of divisions, mesenterial and septal, are to be con- 
 sidered, except when the polyp is fully expanded and the column wall and 
 disc are raised wholl}'- beyond the corallum. In this latter condition the 
 polypal cavity is merely divided into entocoelic and exocoelic chambers by 
 the mesenteries, as in an ordinary actinian polyp. Each chamber is 
 prolonged into a tentacle, but there has been found no lateral communica- 
 tion between one mesenterial chamber and another, such as frequently 
 occurs in actinians by means of stomata. 
 
 The ccelomic cavities of the many polyps making up a colony are only 
 partly independent of one another. At the periphery of each polyp aper- 
 tures are found between the united edges of the contiguous column walls 
 and the skeletotrophic tissues, but no apertures or canals elsewhere connect 
 the different pol3'ps. The communicating spaces are intermesenterial in 
 position, the polyps being more or less cut off from one another mesenterially 
 (plate 6, fig. 35). The spaces divide the superficial column wall from the 
 basal disc. Hence in colonial polyps the column wall and base are nowhere 
 in direct continuity with one another, except at the free part of the marginal 
 polyps ; elsewhere they are connected only indirectly by the mesenteries as 
 these pass from column wall to basal disc. 
 
 When the level of the corallum is reached the polypal cavity is encroached 
 upon radially by the septal invaginations, and divided peripherally into inter- 
 septal chambers or loculi, each of which contains a single mesentery. At 
 first all the chambers communicate with one another toward the middle of 
 the polyp, which is still free from any calcareous deposit (plate 7, fig. 38). 
 At the level of the columella the middle region of the polypal cavity is 
 encroached upon, the septal invaginations extend from the periphery to the 
 center, and the entire cavity is broken up into distinct loculi (plate 7, 
 fig- 39)- These are continued as far as the aboral end of the polyp, where 
 each loculus terminates blindly. 
 
 In addition, the interseptal spaces of Siderastrea^ as of other fungids, are 
 encroached upon concentrically by the synapticula which stretch across from 
 septum to septum, and in sections give the discontinuous canal-like character 
 to the polypal cavity seen towards the periphery of figs. 38 and 39, on plate 7. 
 
ADULT COLONY. 37 
 
 In many corals tlie endodenn in tHe lower regions becomes thickened to such 
 a degree as to occupy the entire interseptal loculi, but in S. radians a narrow 
 space remains as far as the aboral termination. 
 
 GONADS. 
 
 Among all the polyps which have been examined anatomically ova only 
 have been found to occur. The polyps studied were taken from colonies 
 other than those which extruded larvae. The gonads are situated below the 
 stomodseal region, only one ripe ovum as a rule being present on each mesentery 
 (plate 7, fig. 43). They occur about the middle of the transverse length of 
 the mesentery, and are found on the members of both first and second orders. 
 The ova are usually longer in diameter along the radial axis, and are indented 
 where the septal granules have intruded upon the narrow interseptal chambers. 
 Along with the single large egg may be two or three developing ova without 
 yolk, but the conditions are not favorable for studying their origin from the 
 ordinary endodermal epithelial cells. 
 
 The absence of male sexual elements from the examples studied can by no 
 means be taken as indicating a dioecious character of the species. Wherever « 
 
 thus far in West Indian corals sexual differentiation is suggested the gonads 
 have been found to be female, but where the ova are best developed sperm aria 
 have been found to accompany them ; spermaria have never been found alone 
 in any polyp. Hence there is much which suggests that coral polyps are pro- 
 togynous. Mr. Stanley Gardiner, however, has recently discussed the subject 
 of protandry in corals (Proc. Camb. Phil. Soc, XI, 1902). From an investi- 
 gation of a large number of developmental stages in Flabellum rubrum he has 
 been able to show that in this form spermaria arise first on the mesenteries and 
 that ova appear later, when the production of sperm acini ceases. " The ova 
 grow enormously, with the final result that the mass becomes entirely female, 
 consisting of usually 2 or 3 large ova, flattened on their sides against one another 
 and occupying the whole area of the former testes." Protandry is thus clearly 
 established in Flabellum rubrum. The claim for protogyny in the West Indian 
 corals hitherto studied rests upon the fact that spermaria have never been 
 found alone, but always in association with large numbers of ova; on the V 
 
 other hand, many polyps have been found with ova alone, often few in number, _ ^ 
 as if sexual maturity were but beginning. 
 
 Were a general protandry to be assumed, we should have to suppose 
 that all the polyps containing only ova had passed beyond their male 
 period and extruded all their spermatozoa, becoming wholly female. Prob- 
 ably, as in other groups of animals and plants, no hard and fast rule is 
 
38 SIDERASTREA RADIANS. 
 
 followed by tlie different species of corals ; some will be protandrous wbile 
 others will be protogynous. 
 
 It is clear from the evidence adduced that a dioecious character can not 
 be assumed from the presence of only ova or sperm aria until the entire 
 developmental history of the species has been followed ; a polyp having at 
 one time only one kind of sexual cells may later show both kinds. 
 
 CORALLUM. 
 
 Freshly macerated coralla are generally white or yellowish in color, but 
 frequently they are somewhat green, either as a whole or in patches. On 
 breaking a green corallum across the coloration is found to extend some 
 little distance below the surface, and, after decalcification, there remains 
 behind a green mass of fluffy texture, which, on examination under the 
 microscope, is found to consist of filamentous algas. Bven where the skeleton 
 presents no superficial green coloration, a filamentous mass remains behind 
 on decalcification, and chlorophyll may be present in small quantities, or the 
 filaments may be dead and altogether devoid of protoplasmic contents. Under 
 the microscope fragments of the skeleton frequently show fibers crossing the 
 interseptal spaces or meandering over the surface, and sections of the corallum 
 are sometimes perforated by similar filaments (plate ii, fig. 66). Thus 
 the algae actually bore their way into the skeleton as well as spread over the 
 surface. They are particularly abundant in decaying masses of coral. 
 
 The coral-boring algae liberated by decalcification are represented by 
 both a non-septate and a septate form, the filaments of both varying greatly 
 in size. Associated with the algal filaments are clusters of a Leptothrix-VCsjt 
 bacterium ; but whether its threads actually perforate the skeletal matter, or 
 merely grow over its surface, living upon the traces of organic matter, seems 
 uncertain. In a former paper I have pointed out the almost universal 
 presence of boring algae in corals, and their importance in disintegration 
 (Bull. Amer. Mus. Nat. Hist, xvi, 1902). 
 
 In addition to the parasitic algae, organisms such as cirripedes, molluscs, 
 or tubiculous worms usually occur, either inclosed within the skeleton or 
 adherent to its surface. These associated organisms are particularly numer- 
 ous on specimens obtained from muddy shores as compared with those from 
 the clearer waters of the reefs, and often interfere with the regularity of form 
 of the corallites of the colony. The cavities resulting from the activity of 
 the boring molluscs weaken the skeleton greatly, so that the colonies can be 
 readily broken into fragments. 
 
 Numbers of the parasitic cirripede Pyrgoma sometimes occur, exhibiting 
 
ADULT COLONY. 39 
 
 all stages in tlie growtli of its protective shell. The older specimens are 
 completely inclosed and fused within the corallum, only the mouth of the 
 shell being exposed at the surface of the colony. Its outer surface is covered 
 by an irregular growth of the coral skeleton, and, being raised a little above 
 the general surface of the corallum, gives a marked irregularity to the colony 
 as a whole. The external ridges on the shell of the parasite have a curious 
 resemblance to the septa of the coral, and in the later growths it is somewhat 
 difficult to distinguish one from the other. 
 
 On certain macerated colonies many early stages were obtained showing 
 the manner of attachment of the cirripede within the living calice. At first 
 the presence of the small shell of the crustacean has produced no modifica- 
 tion of the coral ; it is simply adherent to a few of the septa. But later the 
 lower ridges on the surface of the cirripede, which closely simulate septa, 
 became fused and partly overgrown with the septa, thus causing the oblitera- 
 tion of the middle of the calice. In every case the intruder has fixed itself 
 within a calicinal cavity, never on the ridge connecting two calices. Appar- 
 ently the cirripede larva enters the polypal cavity and then bores through 
 the living tissues to the corallum, and in the process of growth the skeletons 
 of the two become fused. Afterwards the two organisms continue growing 
 together, the coral skeleton almost wholly inclosing the crustacean ; the latter 
 has no boring action. In the later stages the presence of the Pyrgoma 
 results in the production around it of imperfectly developed polyps. 
 
 On breaking through a colony it is frequently found that the corallum 
 is made up of growths of different periods, arranged upon one another in 
 irregular layers, instead of a continuous increase in thickness from the middle 
 to the circumference. An old corallar surface has become dead and partly 
 corroded, and then a later skeletal deposit has been formed upon this, and so 
 on for several successive stages, the vertical axis of the corallites of the two 
 or more periods of growth usually corresponding. The secondary origin of 
 polyps in old calices has been already described (p. 1 7) . The growth of the 
 corallum seems to be more continuous in blocks of the larger S. siderea than 
 in S. radians. 
 
 The actual degree of calcification of the corallum, if one may thus express 
 it, varies much in different colonies, and also in different regions of the same 
 colony. In some examples the septa are comparatively thick, leaving but 
 narrow interseptal spaces, and the calices are shallower than usual, the 
 columella being solid and prominent. In others the septa are thinner, the 
 interspaces correspondingly wider, the calices deep, and the columella either 
 absent as a definite projection or represented by one, two, or three prominent 
 
40 SIDERASTREA RADIANS. 
 
 granules on the floor of the calice. Probably these differenc'es are determined 
 by the varying conditions of growth of the polyps, whether rapid or slow. 
 The growing marginal corallites are always of the less calcified variety. 
 Such possible variations should be borne in mind, for if extreme coralla only 
 were available for study they might be almost regarded as distinct species. 
 
 Examination of the surface of the corallum, and also of sections, shows 
 that the individual calices of a colony are separated from one another only by 
 fusion of the peripheral edges of the septa. There is no thecal formation 
 distinct from that of the septa. The fusion is complete and continuous, how- 
 ever, and results in the entire lateral separation of one calicinal cavity from 
 another. This is clearly shown in the various figures of the corallum on 
 plate lo ; and, as further proof, it may be recalled that on decalcification the 
 tissues of the different polyps so far as they are inclosed within the corallum 
 are wholly cut off from one another. 
 
 The septa of adjacent corallites correspond peripherally, end to end, or 
 they may alternate ; and between these two extremes are all intermediate 
 conditions (plate lo, figs. 62, 64). At the periphery the thickness of the septa is 
 usually greater than the width of the interseptal spaces, so that where the 
 septa of adjacent corallites alternate it is clear that a distinct wall of separation 
 is produced between one calice and another ; but where adjacent septa are end 
 to end a shallow interval may remain on each side of the two continuous 
 septa, and the septa are then feebly exsert. 
 
 The exsert condition of the septa is most marked toward the margin of 
 colonies, that is, in the region of new growth. Elsewhere the intervals are 
 generally filled by calcareous deposits, so as to bring the calicinal edge to 
 the same level as the septa. The usual superficial appearance is as if each 
 septum became bifurcated at its outer edge and then each half united with 
 a similar half of two adjacent septa belonging to a contiguous corallite. 
 The result is a very narrow zig-zag partition wall between adjacent corallites. 
 Usually the calicinal boundary as a whole reaches the same height as 
 the septa, but occasionally it is depressed, and the septal edges are inclined 
 towards it. 
 
 HISTOLOGY. 
 
 The different parts of the corallite will be now described, but first the 
 microscopic structure of the skeleton must be briefly considered. In connec- 
 tion therewith the writer has to express his indebtedness to the. valuable 
 work of Miss Maria M. Ogilvie (Mrs. Ogilvie-Gordon) who, in her paper, 
 Microscopic and Systematic Study of Madreporarian Types of Corals (1897), 
 has done more than any other investigator to further the histological study 
 
ADULT COLONY. 41 
 
 of tlie madreporarian skeleton, following along lines initiated by Pratz (Palse- 
 ontographica, vol. xxix). Mr. T. Wayland Vaughan has given a resume 
 of the subject in the introduction to his paper on The Kocene and Lower 
 Oligocene Coral Faunas of the United States (1900). 
 
 When the larger septa of Siderastrea are examined sideways, under a 
 low magnification, they present the appearance of the three septa shown on 
 plate 10, fig. 63. The peripheral vertical boundary of each septum consists 
 of a narrow, continuous, nearly straight ridge, which is the broken surface 
 of the thecal wall, really a part of the adjacent septum. Next this is the 
 synapticular area, the synapticula being arranged rather closely in several 
 vertical rows. The surface as a whole is striate, becoming distinctly ridged 
 and grooved as the margin is approached. The septum terminates centrally 
 and above in a strongly serrated margin, the teeth being almost conical in 
 shape, continuous with the ridges, and varying but little in size. Minute 
 granules, usually terminating in one or more sharp points, occur over the 
 whole surface, including the synapticular area and even the marginal spines. 
 
 On septa of the second and third orders may also be seen a vertical row 
 of synapticulum-like structures near their inner border. They are shown on 
 the right (central) margin of the left septum on plate 10, fig. 63, and represent 
 the line of fusion or coalescence of the next adjacent septum in the series — 
 the third with the second, and the fourth with the third. They do not occur 
 on septa of the first order, as none of the other septa fuse with these {cf. plate 
 10, fig. 64). The disconnected character of the bodies serves to demonstrate 
 that the central union of one septum with another by the internal margin is 
 not continuous, but interrupted in character. It is best regarded as taking 
 place along the spinous or toothed edge of the smaller septum, in the same 
 way as the complete septa of the first and second orders fuse in an interrupted 
 manner with the columella by their spinous margins. In Siderastrea Miss 
 Ogilvie (1897, p. 179) regards the direct lateral coalescence of the septal 
 surfaces belonging to different septal cycles as homologous with the synap- 
 ticular union of septa, but it may be pointed out that the fusion is altogether 
 on the part of one of the septa, not a simultaneous growth from two adjacent 
 septa, as in synapticula proper. The interrupted coalescence of the septa, 
 both with one another and with the columella, necessarily leads to the pro- 
 duction of foramina near their inner edge, some of which are shown to the 
 left in plate 10, fig. 63. 
 
 The septal striae are directed upward and inward in a half fan-shaped 
 manner, starting from the vertical thecal wall. Those on adjacent septa 
 belonging to contiguous calices radiate in an opposite manner, each towards 
 
42 SIDERASTREA RADIANS. 
 
 the center of its own calice (plate lo, fig. 63). The thecal ridge is thus the 
 area of divergence of the striae of adjacent septa. It is further found that 
 the striae on opposite sides of the same septum correspond with one another, 
 as can be seen by looking edgewise at the inner margin of an individual 
 septum ; hence opposite striae terminate in one serra. The synapticula and 
 granules are set mainly along the ridges of the septal face, but the former 
 may be so broad as to extend over more than one stria. 
 
 In many corals lighter lines run transversely across the striae, parallel 
 to the dentate septal edge, and mark successive septal margins, corresponding 
 to distinct periods of growth in the life of the polyp. The part between two 
 successive curves represents the addition made to the septum during a single 
 period of polypal growth, and is called by Miss Ogilvie (p. no) "the growth- 
 segment of the septum " or " a septal segment." The septa of Siderastrea 
 very rarely exhibit such lighter cross lines ; by Miss Ogilvie the synapticula 
 are considered an essential part of every growth segment in this genus. 
 
 A magnified transverse section through a portion of the corallum shows 
 that the septa, columella, and synapticula are built up of the same structural 
 elements or units (plate 10, fig. 65). These appear in sections as so many 
 distinctly round or polygonal bodies, fused together into a compact continuous 
 whole. Bach unit of structure under low power reveals lighter and darker 
 parts arranged in a concentric manner, and under high power is found to be 
 made up of radiating calcareous fibers (plate 11, fig. 66). The fibers seem 
 to start some little distance from a center, which is either clear or dark, and 
 practically homogeneous in structure ; further, they are seen to be arranged 
 in close concentric lamellae, and the boundary between each two layers is 
 often indicated by dark, finely granular particles, similar to those at the 
 centers of calcification. 
 
 Employing the terminology of Miss Ogilvie, each structural unit is a 
 trabecular here seen in transverse section. The radiating bundles of fibers 
 zx^ fascicles r the light or dark centers from which the fibers radiate are centers 
 of calcification r and the concentric layers qxq. growth lamellce. 
 
 In transverse sections of the septa of Siderastrea the centers of calcifica- 
 tion are some distance from one another, and each bears radiating calcareous 
 fibers all round ; but in the majority of corals the centers are closer, and 
 often appear in sections of the septa as a more or less continuous dark line 
 or band along the septal axis — the dark line of calcification. In this case the 
 separate fascicles or bundles of fibers are more distinct, and arranged in a 
 feather-like manner along each side of the dark line or layer. 
 
 Much discussion has taken place with regard to the nature of the center 
 
ADULT COLONY. 43 
 
 or dark line of calcification. Writers on fossil corals particularly have devoted 
 their attention to the subject. For a complete account the reader is referred 
 to various sections of Miss Ogilvie's work. By some authors the median 
 dark layer has been interpreted as 2i primary septum^ upon the faces of which 
 the lighter layers, then termed stereoplasm^ have been deposited, the two 
 formations making up the septum proper. Miss Ogilvie considers that the 
 dark appearance of the centers results from the presence of the carbonized 
 residue of the originally unchanged parts of the calicoblasts within which 
 she considers the madreporarian skeleton to be formed. It must be stated, 
 however, that the dark appearance is only seen when sections are viewed by 
 transmitted light. With reflected light the middle region appears lighter 
 than the rest of the septum, and thus can scarcely be occupied by black 
 organic matter. 
 
 Undoubtedly the middle line of the septum is often of a different struc- 
 tural nature from the two sides, and varies much in character in different 
 species. In Paleozoic corals I have occasionally found the middle part to be 
 dissolved away, making the individual septum appear as if formed of two 
 distinct lamellae, more or less completely separated. Furthermore, in the 
 very young septa represented on plate ii, fig. 70, the center of each trabecula 
 is clearly seen, under high power, to be devoid of any inorganic matter. In 
 the rather thick section one can focus down each center for some distance, 
 which would certainly not be the case were it occupied by a calcareous 
 deposit. I conceive that the so-called center of calcification is really the 
 organic center or axis around which the skeletal matter is deposited in a 
 radiating or feather-like manner, and that, at an early stage in the living, 
 growing skeleton, the center is occupied by the mesogloea-like matrix within 
 which it has been shown that the calcareous fibro-crystals are deposited (p. 34). 
 The organic matrix of the centers is probably in all cases impregnated 
 later with calcareous matter, differing in character and crystallographic 
 orientation from the true skeleton (stereoplasm), as shown by its different 
 appearance in sections and different solubility. Bourne has remarked (1899, 
 p. ^39) : " I suspect that the dark * centers of calcification ' will be found to be 
 the expression of a core of organic filaments, just as the central dark line in 
 the Alcyonarian spicule is the expression of the central core of threads." 
 Results published since Bourne's paper seem to confirm his conjecture, only 
 the core is probably homogeneous, and later becomes impregnated with inor- 
 ganic matter, probably from solution, not from calicoblastic activity. 
 
 The centers of calcification in 5. radians are distant from one another 
 on an average about 0.125 mm. and the trabeculae show a corresponding 
 
44 SIDERASTREA RADIANS. 
 
 diameter. The latter are very variable in size in the columella (plate lo, 
 fig. 65). Sometimes two centers of calcification, diverging from one another 
 in the median plane, are present in one concentric system, and the trabecula 
 then appears double and is elongated in outline. Fusion between adjacent 
 trabeculse is usually indicated by a narrow, dark line. 
 
 A synapticulum may have either a separate center of calcification (true 
 synapticulum) or be formed from the fascicles of trabeculae in adjacent septa 
 which extend beyond the septa until they meet in the middle of the inter- 
 septal space (false synapticulum, p. 52). Bundles of fibers or fascicles from 
 individual centers also pass into the granulations over each face of the septa. 
 
 The growth lamellae in the individual trabeculae are not always clearly 
 indicated, but sometimes they come out with remarkable distinctness, having 
 a black, finely granular margin (plate 11, figs. 66, 68). Around the centers 
 of calcification they are strictly concentric, but towards the edges of the septa 
 they more nearly follow the septal outline. In transverse sections they are 
 distant from one another about 0.0025 ^™- '^^^ black, finely granular 
 margin is probably organic residue of the same character as that at the 
 centers of the trabeculae. 
 
 A thin vertical section of part of two contiguous septa is represented on 
 plate II, fig. 67, and displays a series of elongated bands. One of these, the 
 continuous median band, represents the thecal wall, and from it the con- 
 stituent parts of each septum diverge. The darker circular or oval areas also 
 seen in the section are the transversely cut surfaces of synapticula, and each 
 originates in connection with a band. 
 
 The broad radiating bands are the septal trabeculae as they appear in 
 longitudinal section. Where the section passes through the middle of a 
 trabecula a row of close centers of calcification is revealed, and fascicles of 
 fibers pass from them obliquely upwards on both sides. Peripherally the 
 fibers of adjacent trabeculae meet all the way in a distinct thin line, so that a 
 compact septum is the result. Fresh trabeculae are seen to be inserted at 
 intervals between the old. The trabeculae are continued upward to the septal 
 margin and there constitute the terminal teeth, each tooth corresponding with 
 a single trabecula, but the preparation from which plate 11, fig. Sy, is taken 
 was not sufficiently wide to show the teeth. They vary but slightly in width, 
 being about 0.125 ^^^- across towards the margin of the septum. From the 
 inclined arrangement of the trabeculae it is obvious that a transverse section 
 of the septum will cut only the more peripheral at right angles ; the others 
 will be represented in varying degrees of obliquity. 
 
 Each separate dark area along the middle of a trabecula represents a 
 
ADULT COLONY. 45 
 
 center of calcification, and from it fascicles of fibers radiate on both sides, the 
 black, finely granular matter extending but a very short distance. Bach 
 center of calcification, therefore, represents a single growth period in the 
 upward progress of the septum. They are distant from one another about 
 0.013 mm. A trabecula is thus a vertical series of fibrous groups, each group 
 of fibers being deposited around a distinct axis of calcification. A trabecular 
 part is the name given to the separate groups of fibers formed by the fasci- 
 cles at one center of calcification. Bach stria seen under low power on the 
 surface of a septum represents a single trabecula, and the tooth in which an 
 opposite pair of striae terminates at the edge of the septum is the growing 
 apex or organic center of the trabecula. Around this organic center new 
 calcareous matter is constantly being added by the activity of the calicoblasts. 
 
 WALL OR THECA. 
 
 The wall or theca of a simple coral or of a corallite is defined by Vaughan 
 (1900, p. 48) as " that part of the skeleton that cuts off more or less com- 
 pletely the interseptal loculus from peripheral communication with the 
 outside." In Siderastrea the thecal wall at first appears indistinct, as if 
 represented only by the united peripheral edges of the septa of adjacent 
 corallites. Some of the septa of contiguous calices are in the same straight 
 line, in which case it is difficult to distinguish just where the calicinal 
 boundary comes ; in other places a septum of one corallite corresponds with 
 an interseptal loculus of the other, when the distinction between the two 
 calices is quite clear. Usually it appears as if the septa were forked 
 peripherally, and the two limbs of one septum unite in a zigzag manner 
 with two adjacent septa in the next calice. 
 
 The nature of the wall is best seen when a corallum is fractured verti- 
 cally, and adjacent corallites can be viewed lengthways. As indicated on 
 plate 10, fig. 63, the theca is then represented by a narrow, vertical, continuous 
 ridge, which completely separates the interseptal loculus of one calice from 
 that of the other. Further, in the section shown on plate 11, fig. 6'j^ this 
 vertical ridge is found to consist of a distinct trabecula, and from it the 
 trabeculae of adjacent septa diverge. 
 
 Miss Ogilvie (1897, p. 180) has given a figure of Siderastrea (sp. ?) 
 somewhat similar to that of plate 10, fig. 63, but the thecal wall (pseudotheca) 
 is there represented as a discontinuous ridge, as if comparable with a row of 
 synapticula, and in such a way as would permit of communication between 
 one calice and another. This, however, is not its condition in S. radians. 
 In all instances I have found the wall to be continuous, and the fact that 
 
46 SIDERASTREA RADIANS. 
 
 when a colony is decalcified adjacent polyps are found altogether cut off from 
 one another would prove that their cavities are not in lateral communication, 
 as would be the case were the boundary wall interrupted. 
 
 The to^rms pseudotheca and eutheca of von Heider (1886) are well known 
 to coral students, the former referring to a theca produced by the septa 
 becoming so much thickened peripherally that they fuse together, while the 
 latter, is applied to a theca formed from separate trabeculas between the 
 peripheral ends of the septa. Vaughan (1900, pp. 48-52) has considered 
 very fully the value to be applied to these distinctions, and comes to the 
 conclusion that " From the great variation not only in the same species, but 
 in a section of a single corallite, no special systematic importance can be 
 attached to the theca being of the so-called true or false variety. True theca 
 marks the Anlagen of new septa, as von Koch and Bourne have shown, or 
 occurs in calices where the septa are distant from one another and their outer 
 ends are not sufficiently thickened to effect peripheral fusion." 
 
 The theca of Siderastrea is described by Miss Ogilvie as a pseudotheca, 
 and, as above shown, it is certainly formed at the peripheral extremities of the 
 septa. On the other hand one, or rarely more, distinct centers of calcification 
 occur laterally in the interval between one septum and another, though not 
 to be distinguished from those of the septa except by being inclined to them at 
 an angle. In her figs. 43<3;, 43<5 (p. 181), Miss Ogilvie represents such centers 
 of calcification, but regards them as synapticular. Although presenting the 
 same appearance in transverse sections, plate 10, fig. 63, and plate 11, fig. 67, 
 show that this is not their true character ; rather, the thecal trabeculse are 
 continuous vertical partitions. According to the accepted definitions it would 
 seem that the theca of Siderastrea should be regarded as a true theca, having 
 a separate trabecula between the ends of adjacent septa from which the tra- 
 beculae of the septa radiate. 
 
 An independent thecal wall, however, is found to be wanting in the 
 corallum of larval polyps, the epitheca being in no way comparable with this. 
 In the simple corallum, as far as reared, the septa remain free and exposed 
 peripherally, except in so far as they may be united by synapticular growths 
 or covered by the epitheca (see plates 4 and 5). Likewise the outer septal 
 edges of the marginal polyps in a colony extend for a considerable vertical 
 height, exposed all the way. 
 
 SEPTA. 
 
 The detailed characteristics of the septa have been already described, so 
 that it only remains to discuss their arrangement and relationships. 
 
ADULT COLONY. 47 
 
 All the septa of a corallite are well developed, both as regards their 
 thickness and radial extent ; even the members of the outermost cycle extend 
 centrally for about half the radius of the calice, and are but little narrower 
 than the others. The individual septa are usually thicker than the width 
 of the interseptal loculi separating one from another, so that, compared with 
 most other corals, the septal system is very compact, and the calicinal cavity 
 as a whole is correspondingly diminished. The septa vary but slightly in 
 thickness according to the cycle in which they belong. 
 
 The outer septa are not strictly radial throughout their transverse 
 length, but incline along their inner edges and fuse at intervals with the 
 lateral faces of the septa of the second and third cycles, thus forming groups 
 of three, five, or rarely seven (fig. 2, p. 13). The grouping, however, is not 
 always obvious at the surface of a colony. Thus Verrill (1901, p. 154) states 
 that the septa of the Bahama representatives are nearly all straight and 
 
 aaAA 
 
 1 X I fx nxi ixmxnxi ,1x111x11x111x1;^ 
 
 Fig. 4. — Grouping of the septa within a sextant (i-l) according as one, three, five, or seven septa are present. 
 The Roman numerals l-iii indicate the order of the septa and x the exosepta. 
 
 seldom fused. It is, however, always a marked characteristic in sections of 
 the corallum from a little below the surface downward (plate 10, fig. 64). 
 
 When examined in detail the grouping is found to be as follows : The 
 twelve largest septa, constituting the primary and secondary orders, extend as 
 far as the middle of the calice and are united directly with the columella. 
 Of these the six primary septa pass uninterruptedly all the way from the 
 periphery to the columella without any connection with the others except, of 
 course, through union by synapticula. The six secondary septa, on the other 
 hand, are also radial, but with them are united the members of the third 
 order, which never reach the columella. The members of the fourth or outer- 
 most cycle fuse one on each side with a tertiary septum. Where only one 
 secondary septum occurs within a primary system the two outermost septa are 
 fused with it, and a grouping of three is produced. Where a tertiary septum 
 and a secondary septum are within a sextant the grouping is in fives, and 
 where two tertiary septa and a secondary occur the grouping is in sevens. 
 The relationships are clearly shown in the series of figures above and also in 
 fig. 2, p. 13. 
 
48 SIDERASTREA RADIANS. 
 
 In the course of development of the septa it is shown that from the 
 beginning the exosepta always constitute the outermost cycle, and are fused 
 with the entosepta immediately preceding them. Thus, when only two cycles 
 of septa are present (plate 4, fig. 23), the outer exosepta fuse with the inner 
 entosepta. Where three cycles are developed (plate 5, fig. 28) the exosepta 
 unite with second-cycle entosepta ; where fotir cycles, they fuse with the third- 
 cycle entosepta. At the early stage of development, represented in plate 5, 
 fig. 28, the second-cycle entosepta are fused with the first-cycle entosepta 
 instead of with the columella as in the adult. The union here takes place 
 in such a manner as to give a decided bilateral symmetry to the corallum. 
 
 The hexameral cyclic plan of the septa is not so manifest in S. radians 
 as in many corals, partly owing to the incompletion of the third and fourth 
 cycles ; but with a little care it can be established by the differences in size 
 of the septa and their other relationships. In mature corallites two inner 
 alternating orders (primary and secondary) of six septa each are always 
 present, while the third and fourth orders are represented by a varying num- 
 ber of septa. Owing to this varying incompleteness of the last two orders a 
 study of the septal arrangement yields some interesting results in connection 
 with the manner in which the later additions of septa take place, and also in 
 the relationships of the entosepta and exosepta. 
 
 The hexameral plan is found to be strictly followed so far as concerns 
 the first and second orders, and for the third and fourth orders so far as these 
 are developed. Were the third and fourth orders to be completed the total 
 number would be 48, arranged in the formula 6, 6, 12, 24; but this number 
 appears to be rarely, if ever, reached in S. radians^ though frequently 
 exceeded in S. siderea. In one of the largest corallites 44 septa were present, 
 but the numbers generally vary from 30 to 36. 
 
 The six members of the primary cycle are readily separable from the 
 others on account of their greater radial extent and thickness ; they pass all 
 the way from the thecal wall to the columella without any of the other septa 
 coalescing with them. They serve to distinguish the six primary systems 
 within which are enclosed varying numbers of smaller septa. The outermost 
 exosepta are also recognizable by their smaller size, one occurring between 
 every two adjacent members of the first, second, and third orders. Less cer- 
 tainty prevails in distinguishing the members of the second and third cycles, 
 though frequently the second-cycle septa are larger and more prominent than 
 those of the third. 
 
 In mature corallites three septa at least are always present within each 
 primary system, while there may be five or seven (fig. 2, p. 13). Where only 
 
ADULT COLONY. 49 
 
 three septa occur in a system the middle member belongs to the second order, 
 and the two lateral ones are exocoelic; where five septa are present, one 
 belongs to the second order, another to the third, and the three alternating 
 with these to the exocoelic cycle ; in the rarer instances in which seven 
 septa are developed the middle member is a second-order septum, the two 
 next in size are tertiary septa, and the four alternating septa belong to the 
 fourth or exocoelic cycle. The members of the outermost cycle fuse in an 
 interrupted manner by their inner ends with the members of the second 
 and third cycles, but not with those of the primary cycle ; the tertiary septa 
 in their turn unite with the secondary. 
 
 The actual ordinal relationships of the septa can be best understood 
 when taken in connection with the mesenteries, as revealed in transverse 
 sections of the polyp. 
 
 Plate 6, fig. 34, represents a transverse section from the lower stomo- 
 daeal region of a partly retracted decalcified polyp. The drawing was made 
 with the help of a camera lucida, but in the process of decalcification and the 
 preparation of the section slight displacements of the lamellae have taken 
 place. Still the general relationships remain as in the living condition, and 
 for all practical purposes the invaginations of the polypal wall correspond 
 with the septa which have been dissolved away. Fig. 2, p. 13, is a diagram- 
 matic representation of the septa of the same polyp, the synapticula and 
 spinous projections being omitted for the sake of clearness. 
 
 In the transverse section (plate 6, fig. 34) a large septal invagination 
 (entosepta, i, 11, iii) occurs within the interspace inclosed by the two 
 members of each pair of mesenteries, and a smaller one in the interspace 
 between every two adjacent pairs (exosepta, x). The entosepta inclosed by 
 the two pairs of directive mesenteries are known as directive septa, but in 
 the corallite itself there is nothing to distinguish these from the other 
 primary entosepta. They indicate the dorso- ventral (postero-anterior, sulculo- 
 sulcar) axis of the corallite, and can only be determined with certainty when 
 in association with the mesenteries. 
 
 The six interspaces between the six primary mesenterial pairs contain 
 a variable number of pairs of mesenteries : the two ventral systems each 
 contain one mesenterial pair (11), the right middle interspace three pairs 
 (hi, II, hi), and the remainder two pairs each (iii, 11). The six pairs of 
 secondary mesenteries (11) are easily distinguished from the tertiary mesen- 
 teries (hi) by their greater size and more central position. Of the latter 
 there are only five pairs in this particular polyp instead of twelve, as 
 considerations of hexameral symmetry would suggest. 
 
50 SIDERASTREA RADIANS. 
 
 The septa are therefore related to the mesenteries in the following 
 manner : 
 
 The six primary septa are inclosed within the entocceles of the primary 
 cycle of six pairs of complete mesenteries, the six secondary septa within the 
 entocceles of the second cycle of mesenteries, the five tertiary septa within the 
 entocceles of the third cycle, while the members of the outermost or quater- 
 nary cycle of septa are all exocoelic, one between each two pairs of mesenteries. 
 The primary, secondary, and tertiary septa are all entosepta ; the members 
 of the fourth cycle are exosepta. Further, the number of exosepta equals 
 the sum of the entosepta, and the number of cycles of septa is one more than 
 the number of cycles of mesenteries. Considering the septa as entosepta and 
 exosepta, the true septal formula is 6, 6, x, 12 + x, where x may be any 
 number from one to twelve. The variations in the number of septa within 
 the different calices are in pairs — an entoseptum and an exoseptum — and affect 
 only the members of the third cycle and their corresponding exosepta. These 
 two cycles always show a corresponding variation which is not explicable on 
 the usual supposition that the cycles of septa are developed in the order of 
 their importance. The order of appearance of the septa and the relationships 
 of the entosepta and exosepta will be discussed more fully in connection with 
 the development of the septa in larval polyps. 
 
 It will be shown that morphologically the exosepta are to be regarded 
 not so much as a separate cycle, appearing after the others are established, 
 but rather as the bifurcated continuations of the original cycle of exosepta, or 
 perhaps as new formations arising along with the entosepta which they 
 inclose ; each entoseptum has an exoseptum corresponding with it, which is 
 formed nearly at the same time. Bxosepta are found to arise along with 
 each cycle of entosepta or even with each individual entoseptum ; but as new 
 cycles of entosepta are formed they are, as it were, shifted outwardly, so as 
 always to constitute the outermost cycle. Until the adult condition is reached 
 the exosepta are but temporary predecessors of the permanent entosepta. 
 The same relationship is found to hold between the entotentacles and the 
 exotentacles. 
 
 The above relationship between the cycles or orders of septa and mesen- 
 teries holds for most species of corals which have been examined. In certain 
 forms the exosepta are absent, e. g.^ Pectinia^ Manicina^ when the cycles of 
 septa correspond in number with the cycles of mesenteries. 
 
 Where the hexameral cyclic sequence of a corallite is not completed it 
 is preferable to speak of the exosepta merely as exosepta or as septa of the 
 outermost cycle, not as a third or a fourth cycle; for ontogenetically som? 
 
ADULT COLONY. 5 1 
 
 belong to the third cycle and some to the fourth. They have no ordinal value 
 comparable with the entosepta. 
 
 In descriptive works on corals, when giving the number of septa 
 characteristic of any calice, it is usual to consider all the inner cycles as 
 hexamerously complete and then to regard all the missing septa as wanting 
 from the last cycle. Thus Milne-Bdwards, in describing the septa of the 
 present species, says : '' Three cycles of septa complete, and, in general, a 
 variable number of a fourth cycle." Also Verrill (1901, p. 153): "They 
 [the septa] form three complete cycles, with part of the fourth cycle devel- 
 oped, so that the number is usually 36 to 40." From the relationships here 
 set forth, and more fully discussed in connection with the development of 
 the septa, it is clear that such cyclic plans do not express the true ordinal 
 or morphological relationships of the septa ; the last and penultimate cycles 
 vary in the same degree, and the latter contains both entosepta and 
 exosepta. 
 
 The septal invaginations on plate 6, fig. 34, indicate how the exosepta 
 in most instances fuse with the third septa, but in the two ventral systems, 
 where no members of the third cycle are developed, they unite instead with 
 the secondary septa. In the serial sections the invaginations also show that 
 the fusion of the exosepta with the entosepta is not continuous throughout 
 their vertical length, but is effected only at somewhat regular intervals. 
 
 The section of a fragment of a corallum represented on plate 10, fig. 64, 
 includes the whole of one corallite and portions of the six surrounding 
 corallites, and shows their relationships to one another. The thecal wall 
 separating one corallite from those adjacent to it is seen to be very limited 
 in thickness. At this level in the complete calice twelve septa, representing 
 the first and second cycles, are united directly with the columella. The six 
 primary septa are distinguished by the fact that they extend all the way 
 from the periphery to the columella without fusion with any of the smaller 
 septa. In three of the primary systems there are only three septa — a second- 
 cycle entoseptum and two complete exosepta fused with it ; in the remaining 
 three sextants are five septa — a second-cycle entoseptum with one exoseptum 
 and a third-cycle entoseptum fused with it, the latter having two exosepta 
 in union with it. 
 
 Comparison may be here made with a somewhat similar horizontal sec- 
 tion of a corallite of S. radians, introduced on plate xv (fig. 12) of Agassiz's 
 " Florida Reefs." Nine distinct, complete septa are represented, and in the 
 interspace between each two occurs a group of three septa, the two lateral 
 united with the middle, which, in its turn, extends to the columella. The 
 
52 SIDERASTREA RADIANS. 
 
 arrangement of the septa in groups of three is continued all the way round, 
 and the hexameral plan is altogether obscured. Such an interpretation of 
 the septal plan is at variance with what is established above for the Jamaica 
 representatives of the same species. 
 
 As seen at the surface of a colony the septal edges extend almost hori- 
 zontally for a short distance from the periphery, and are then inclined down- 
 ward and inward somewhat sharply to meet the columella which forms the 
 floor of the middle of the calice. The actual depth of the calice varies some- 
 what in different colonies, but is usually about 2 mm. 
 
 SYNAPTICULA. 
 
 Viewed with a lens from above, the septa are seen to be joined to one 
 another laterally by thick transverse bars — the synapticula. In any trans- 
 verse section of a calice one to three, rarely four, synapticula are seen crossing 
 the space between every two adjacent septa, but are limited in their distribu- 
 tion to the peripheral half of the calice (plate lo, fig. 64). In a view of the 
 lateral surface of the larger septa they are found to be arranged in two or 
 three, rarely four, somewhat irregular vertical rows, each synapticulum being 
 circular or oval in section, and projecting at right angles from the septal 
 surface (plate 10, fig. 63). The presence of so many connections between the 
 septa gives a porous or reticular character to the more peripheral part of the 
 calice. 
 
 The synapticula are formed by the ultimate fusion across an interseptal 
 loculus of two opposite granulations on the faces of adjacent septa, the calca- 
 reous matter being deposited in a manner similar to that of other parts of the 
 corallite. In Siderastrea it is only the granules in restricted spots on the 
 septal faces which become thus enlarged. The synapticula have a limited 
 distribution, and the other granulations on the septal face remain compara- 
 tively small, there being no intermediate examples. It is clear that in the 
 course of their growth the enlarging granules must first indent the skeleto- 
 trophic tissues which line them, then bring the two opposite layers together, 
 and finally perforate them, as shown on plate 6, fig. 34 ; also, as described on 
 p. 28, the mesentery inclosed within the loculus is perforated at the same time. 
 
 Sections through the synapticula show that they are formed as lateral 
 processes of the septa, either by an extension of the septal trabeculae or from 
 a center of calcification of their own. In the former case a boundary in the 
 middle of the synapticulum indicates where the bundles of fibro-crystals from 
 one septum have met those from the opposite septum. In the other case it 
 
ADULT COLONY. 53 
 
 appears as if the fibers from tlie synapticulum itself were bracing tbe two 
 septa. The first are known as false synapticula and the second as true 
 synapticula, but, as all recent writers have pointed out, there is no morpho- 
 logical distinction between the two. Both kinds appear in any section of a 
 corallite. Whether one or the other form is present depends mainly upon the 
 interval between the two septa. Where sufficiently close, as towards the 
 middle of the calice, the interseptal space can be bridged without the forma- 
 tion of a new center of calcification, while such is necessary when the interval 
 is wide, as towards the periphery. 
 
 Miss Ogilvie (1897, p. 168) considers that in J^ung-ta the synapticula are 
 formed by special interseptal invaginations of the basal wall of the polyp 
 subsequent to the septal upgrowths, without ever perforating the mesenteries, 
 and assumes such to be the case with Siderastrea. Bourne (1886, p. 47) has 
 already shown that in Fungia the mesenteries are really pierced by the 
 synapticula, Fowler (1888, p. 8) has accomplished the same for Stephano- 
 phyllia formosissinia^ and I for Siderastrea siderea (1902, p. 487). The con- 
 ditions revealed by the liberated lamella on plate 6, fig. 33, prove that the 
 synapticula are the products of definite areas of the original skeletogenic layer 
 producing the septa, not of special secondary basal upgrowths. Delage & 
 Herouard (1901) have accepted Miss Ogilvie's interpretation of synapticular 
 formation, giving diagrammatic figures to illustrate how the upgrowth is 
 supposed to take place. 
 
 COLUMELLA. 
 
 The columella is represented in the mature calice by a more or less 
 compact, column-like structure forming the floor of the middle of the calice. 
 In decalcified polyps it is found to have elevated the central part of the 
 skeletogenic tissues in a tubular manner considerably beyond the peripheral 
 parts. Its superficial appearance varies much in different calices ; it may 
 be either papillose or smooth, dependent upon the degree of calcification of 
 the colony. Sometimes only one large, tooth-like papilla occurs, or there 
 may be two close together ; in others, again, there are two or three large or 
 chief tubercles along with several minute projections, or it may be formed 
 wholly of small, scarcely perceptible granules, when, to the naked eye, it 
 appears compact and smooth. This latter condition is found in coralla of 
 which all the parts are strongly calcified. In these the columella is conspic- 
 uous, raised for a short distance above the septa, and its free surface nearly 
 smooth. 
 
 Where several prominent tubercles are present some seem as if con- 
 tinuous with the teeth of the septa, suggesting that the columella is formed 
 
54 SIDERASTREA RADIANS. 
 
 from the united edges of the primary and secondary septa. Others of the 
 projections seem, however, to be quite independent of the septa. Secondary 
 calcareous matter is deposited among the projections, and this gives the more 
 compact character to the structure as a whole. The section represented on 
 plate lo, fig. 65, shows the relationship clearly. The columella is here 
 quite solid and distinct, and is structurally the same as the septa united with 
 it, having its own centers of calcification. In sections a little higher it is 
 spongiform, as the trabeculae are free from one another. 
 
 With such structural details alone available it is practically impossible 
 to say whether the columella of Siderastrea is a true or a false columella 
 (pseudocolumella) as these terms are understood in coral literature. Miss 
 Ogilvie (1897, p. 179) describes it as a ^'paliform pseudocolumella." A true 
 columella is considered to arise as an independent structure from the middle 
 of the basal plate, though the septal edges may secondarily unite with it. A 
 false or pseudocolumella, on the other hand, is an irregular skeletal tissue 
 formed from union of the inner or central ends of the septa, sometimes bound 
 together by a secondary deposit. Recourse to the developing coralla shows 
 the true nature of the columella in S. radians. As described later, and 
 illustrated by the figures on plates 4 and 5, independent skeletal upgrowths 
 are first formed from the basal plate, but later come into intimate relation- 
 ship with similar formations at the edges of the septa, and afterwards the 
 two groups are united into a solid, compact column by a deposit of calcareous 
 matter. 
 
 Developmentally, therefore, the columella of Siderastrea is a true colu- 
 mella, compact all the way, or compact below and spongy above. 
 
 Histologically the columella usually shows two or three large circular 
 trabeculse arranged in a row and surrounded by a number of smaller tra- 
 beculae (plate 10, fig. 65). The larger probably represent the true elements 
 of the columella, which arise directly from the basal plate, while the smaller 
 are the septal teeth which also take part in its constitution. 
 
 Viewed from the surface the columella is usually nearly circular in 
 outline and affords no aid in determining the principal axis of the corallite ; 
 but in sections some distance below the surface it generally presents a longer 
 and a shorter axis, and the former evidently corresponds with the directive 
 or principal axis of the corallite. To each extremity of the longer axis a 
 septum of the first order is attached, and the two are, no doubt, the directive 
 septa, though, owing to the difference in the number of septa on each side, 
 they are not always in the same plane (plate 10, fig. 64). The oval columella 
 may thus afford an important aid in the orientation of the septa and of the 
 corallites in a colony. 
 
ADULT COLONY. 55 
 
 DISSEPIMENTS. 
 
 The dissepiments are extremely thin and delicate transverse partitions 
 which serve to cut ojff the living polyps from the dead part of the skeleton 
 below. In section they are only 0.0 1 mm. thick. Along with the columella 
 they serve for the time being as a support for the basal part of the polyp, the 
 dissepiment being morphologically comparable with the basal plate of the 
 larval polyp. In some cases the partitions are horizontal, but in others they 
 are deeply convex upwardly. They are situated at somewhat different levels 
 in different interseptal loculi, and in the same loculus vary from 0.25 mm. to 
 0.5 mm. apart. The distance apart of two adjacent dissepiments in a loculus 
 represents a period of growth of the polyp in its march upward, the polyps 
 throughout their lifetime retaining approximately the same vertical length 
 and transverse diameter. As a result of the very narrow interseptal loculi in 
 Siderastrea the dissepiments are rather insignificant features of the corallum, 
 but as a basal support they are probably as important to the polyp as in other 
 species of corals in which they are more prominent. Occasionally a dissepi- 
 ment is found intersected by a synapticulum, when both may be considered as 
 constituting the basal support of the particular lamella. Such was probably 
 the case in the interseptal lamella represented in plate 6, fig. 33. Usually- 
 the lamellae present a more nearly straight basal extremity than is here repre- 
 sented, showing that the polyp is cut off below in a regular manner by the 
 formation of dissepiments which alternate with the synapticula. The dissepi- 
 ments may thus correspond or alternate with the synapticula, the latter being 
 formed independently and in advance of them. 
 
 The dissepiments are easily studied in tangential sections of corallites, 
 and it is found that their histological structure is different from that of 
 septa. They possess no centers of calcification, but are thin and laminated, 
 composed of fibro-crystals standing at right angles to the surface. They thus 
 recall the microscopic structure of the basal plate and epitheca, and like these 
 they are lined by the calicoblasts of the polyp only on one side. 
 
 Miss Ogilvie (1897, p. 178) assumes that the synapticula and not 
 the dissepiments constitute the principal basal support of the polyp in 
 Siderastrea. This would follow were the synapticula formed, as she assuines, 
 within special interseptal invaginations of the aboral body wall ; but, as 
 shown on p. 53, such is not their origin. They are produced by special areas 
 of the septal invagination, ultimately resulting in perforation of the polypal 
 wall. As the synapticula actually perforate the polyp they may be conceived 
 as also affording it support ; but only incidentally, as happens to be the case 
 
56 SIDERASTREA RADIANS. 
 
 in the lamella shown in plate 6, fig. 33, is this basal. The synapticula do 
 not take the place nor assume the function of dissepiments. 
 
 EPITHECA AND BASAL PLATE. 
 
 The most careful observation of the margin of adult coralla fails to 
 reveal the presence of an epitheca or covering of the peripheral septal edges. 
 This is somewhat remarkable considering that such a skeletal formation is 
 developed in young polyps reared from the larva, and this whether the 
 polyps are isolated or growing contiguous to others (plates 4, 5). In 
 these larval polyps, however, the extent of the upward growth of the epitheca 
 has been found to vary greatly. In some specimens, apparently stationary 
 as regards growth, it formed a comparatively high external wall to the polyp, 
 wrinkled and diminishing somewhat in diameter from the base upwards, but 
 overtopping all the septa (plate 5, fig. 27), while in others, the largest, 
 actively growing corals, it was either absent or represented only by a low, 
 narrow rim (plate 5, fig. 28). Evidently it is a structure of importance 
 only in the early stages of growth. 
 
 Where, as frequently happens, a part of the growing margin of a colony 
 is free from the incrusted object, a flat, plate-like deposit covers and unites 
 externally the basal edges of the septa. Such a formation by the bud-polyps 
 will correspond with the basal plate of the larval polyp, but its peripheral 
 edge is rarely, if ever, upturned so as to cover the free vertical edges of the 
 septa, as in the case of an epitheca proper. 
 
POSTLARVAL DEVELOPMENT. 
 LARVA. 
 
 Five diflferent colonies of S. radians were collected in Kingston Harbor, 
 Jamaica, on the 6tli of July, many of the polyps of which contained free 
 planulae. Although similar colonies had been obtained from this locality 
 on former occasions, and examined with regard to their fertility, this was the 
 first time that larvae were secured. On a second visit a week later two or 
 three other ripe colonies were collected, and others which on being sectionized 
 were found to contain ova. 
 
 On every colony the fertile polyps were in somewhat restricted patches, 
 not all the individuals coming to maturity at the same time. One, two, or 
 three planulae were plainly visible through the nearly transparent tissues of 
 the parent, and were often observed to pass into the tentacles, which thereby 
 became greatly distended, remaining so even when the polyps were retracted. 
 At times the planulae would glide into the lower regions of the polypal cavity, 
 and were then lost to view. While within the parent cavity it was impossible 
 to determine whether the movement of the larvae was, like that of the food 
 particles, dependent upon the ciliary activity of the lining endoderm or was 
 a result of the larva's own activity. Immediately upon being set free, how- 
 ever, the larvae were able to swim about, the ectoderm being already provided 
 with a layer of cilia. 
 
 The larvae were shot out suddenly, but the actual place of extrusion was 
 not determined, although prolonged observation was made. From their per- 
 sistent entrance into the tentacles it would seem that they made their escape 
 through these organs, and not through the mouth, which was usually closed 
 and depressed. In other instances,.^. ^., Manicina areolata and Favia 
 fragum^ the sexual products and larvae have been seen to be given out 
 through the oral aperture, the proceeding being accompanied by a peculiar 
 jerking motion of the adult polyp. Von Koch (1897), however, found the 
 larvae of Caryophyllia cyathus to be expelled through the tips of the tentacles, 
 and Lacaze-Duthiers (1873, p. 308) occasionally observed the same in Astroides 
 calycularis. Sections show that the knobbed tip of the tentacles in Sideras- 
 trea is without any permanent terminal aperture. 
 
 Larvae were freely extruded at the time of collection of the corals, and 
 continued to be discharged from time to time for about a month. Under ordi- 
 nary conditions one or two would be set free at intervals, but upon disturbance 
 
 57 
 
58 SIDERASTREA RADIANS. 
 
 of the colony a score or so would be sliot out together. Often when the disc 
 had become retracted a larva would remain within the upper peripheral region 
 of the polyp, fixed between the edges of a septum and the wall of a tentacle. 
 
 The larvae were able to swim about immediately upon being discharged, 
 and gyrated through the water first in one direction and then in another. 
 There was evidence of a feeble negative geotropism. During the first day 
 they kept near the surface of the water, or gathered around the sides of the 
 vessels ; afterwards they traversed the water as a whole, though in the main 
 keeping near the surface. 
 
 To the unaided eye some of the planulse appeared as minute, opaque- 
 white, spheroidal bodies ; others were oval ; but the majority were elongated 
 and pear-shaped, as shown in plate i , fig. i . Under the microscope the ciliation 
 was found to be uniform over the whole surface. The broader pole (oral) 
 was deeply pigmented and invariably posterior in swimming ; no oral aperture 
 could be detected at first, but extrusions of Zooxanthellse, yolk, and cell 
 debris took place later, showing that the mouth was already functional. 
 Under the microscope the brownish yellow of the broad end was seen to be 
 due to the presence of numerous Zooxanthellas within the ectodermal la^^er ; 
 otherwise the ectoderm was colorless, while the endoderm was dark and non- 
 transparent. The narrow anterior end (aboral) was perfectly colorless, its 
 ectoderm being free from commensal algae. In some larvae the colorless end 
 (aboral) was larger than the other. The elongated specimens were able to 
 retract and extend themselves, and upon irritation or preservation altered 
 their shape so as to become nearly spherical. The ordinary pear-shaped 
 individuals were about 2 mm. long. 
 
 Throughout their free existence the majority of the larvae remained as 
 wholly opaque objects, and no trace of mesenterial divisions could be seen 
 from the outside. Some specimens, however, were so far developed as to be 
 distended at birth, or became so shortly afterwards. The tissues of these 
 latter were more transparent than the rest, and eight mesenterial divisions 
 were obvious, though not all of the same vertical length (plate i, figs. 3 and 4). 
 The latest extruded larvae, after the colonies had been kept under somewhat 
 unfavorable conditions for two or three weeks, were nearly devoid of external 
 Zooxanthellas and therefore colorless. 
 
 After the first day or two many of the larvae showed positive geotropism 
 and sank to the bottom of the vessels, lying there motionless ; later, some of 
 these resumed their activities. By the evening of the second day a few had 
 fixed themselves by the narrow, anterior, colorless end to the bottom or side 
 of the vessel, or to objects placed within it. At first the larvae would adhere 
 
POSTLARVAL DEVELOPMENT. 59 
 
 by means of the actual tip of the narrow end (plate i, fig. 5) ; this would then 
 flatten out, and the whole body become much shorter, a small round or oval 
 aperture being visible at the free extremity (plate i, fig. 6). 
 
 Whether or not any individual larv^a would settle seemed very uncertain, 
 for out of several hundreds set free comparatively few became permanently 
 fixed. Those which settled appeared to be the larger, better developed 
 specimens. Smaller vessels, glass slides, and cover glasses floated by 
 means of cork were placed within the receptacles so as to afford additional 
 surfaces for adhesion ; in other cases larvas were distributed wdthin vessels 
 coated with paraffin. If fixation were not accomplished within the first few 
 days, it seemed to be impossible afterwards, though the larvae might continue 
 active for several weeks. In one instance about a score of planulse were 
 isolated and kept in a glass dish, and although they appeared healthy and 
 swam freely for a period of twenty days, externally they underwent no change 
 whatever. 
 
 Sometimes a larva would fix itself in a position apart from others ; but in 
 general many would settle close together at one and the same time, their walls 
 in some cases actually pressing one against another. Plate i, fig. 5, represents 
 three larvas in the first stage of fixation, in this case to a small pebble. The 
 narrow extremities nearly touch, and it is obvious that when the larvas shorten 
 and their bases flatten, the latter will exert a mutual pressure as a result 
 of their closeness. In plate i, fig. 6, seven larvae already settled and fully 
 expanded are represented, as seen from above by reflected light. All were 
 closely adherent to a fragment of stone, and formed a miniature colony. To 
 the under surface of a small pebble thirty-eight other specimens were aggre- 
 gated in groups of two, three, or more, the members of any group touching 
 along their margin. One of these groups contained a dozen or more young 
 polyps, all in contact with one another, the mutual pressure producing a 
 distortion of the normally circular base. 
 
 During a single night another group of thirty-two became adherent to the 
 surface of a small glass dish. In this case nearly all the members were touch- 
 ing to a greater or less degree. One of the aggregations, consisting of fifteen 
 polyps, is represented in fig. 5, p. 60, as seen from the under surface of a 
 fragment of glass to which the individuals were attached. The colony when 
 drawn was two or three months old, and the skeleton was already in process 
 of development, being represented by a variable number of septa and by basal 
 and epithecal formations. 
 
 The aggregated larvae, or young polyps, as they may be called after 
 settling, remained closely associated during their subsequent growth, and in 
 
6o SIDERASTREA RADIANS. 
 
 all respects resembled a distinct colony. Siderastrea radians thus exhibits 
 the phenomenon of a coral reaching its colonial stage by direct union or 
 aggregation of a number of independent larvae, the larvae being at first free 
 swimming and wholly unconnected with one another. The close association 
 is probably to be understood as a thigmotactic phenomenon, and calls for a 
 more extended study with abundant material. The subject of " Aggregated 
 Colonies in Madreporarian Corals " has been more fully discussed in " The 
 American Naturalist," June, 1902. It is there shown that similar associa- 
 tions are at times met with in other corals. 
 
 Bxtemally the opaque larvae seemed all alike, but sections reveal slightly 
 different stages of growth as regards the number of mesenteries and their 
 connection with the stomodaeum. The sections further demonstrate that the 
 
 Fig. 5. — Aggregation of fifteen polyps derived from the same number of originally free-swimming larvae. 
 Unly the basal skeletal appearance is represented. 
 
 opacity is due to the thickness of the endoderm and the numerous Zooxan- 
 thellae within its cells. Furthermore, the gastro-coelomic cavity is yet 
 unformed, or very limited in extent, the larvae being a nearly solid mass of 
 cells. The distended larvae, on the other hand, already showed from the 
 outside different stages in the development of the mesenteries, and the 
 internal cavity was more fully established {cf. plate i, figs. 3, 4). 
 
 After fixation all the larvae became distended, the walls more trans- 
 parent, and mesenterial divisions, twelve in number, were rendered conspicuous. 
 From the beginning of fixed life some few were more advanced than others. 
 These were the individuals bom in an already transparent state. On set- 
 tling they were nearly as large again as the other larvae, and their develop- 
 ment throughout was more rapid. 
 
 A few abnormal larvae were extruded and swam about like the rest. 
 The oral extremity of these was double and the aboral single, the bifurcation 
 taking place about the middle of the length (plate i, fig. 2). Lacaze-Duthiers 
 (1873, p. 312) likewise found such double monsters to be somewhat frequent 
 
POSTLARVAL DEVELOPMENT. 6l 
 
 in Astroides. In these the oral end was sometimes bifurcated and sometimes 
 the aboral. 
 
 No trace of any calcareous deposit or skeletal formation was apparent 
 in any of the larvae before fixation, but calcareous matter was laid down soon 
 afterwards, the distinction between basal disc and free polypal wall, or skeleto- 
 genic and non-skeletogenic tissues, being then established for the first time. 
 
 It is convenient to describe the general course of the development and 
 character of the young polyps before proceeding with the detailed growth of 
 the individual organs — tentacles, mesenteries, and septa. 
 
 YOUNG POLYP. 
 
 Some of the larvae became adherent to small pebbles, others to pieces of 
 glass, while most were fixed to the sides and bottom of the glass vessels in 
 which the colonies were placed. In this last case the vessels were sacrificed 
 in order to obtain the young polyps in a condition favorable for examination 
 under the microscope. By carefully breaking the glass suitable fragments 
 were secured with the polyps attached, and these could be transferred from 
 one aquarium to another, or placed in dishes small enough to rest on the 
 stage of the microscope. In this way the growing polyps could be examined 
 at any time from both their upper and under aspects, their partial trans- 
 parency allowing the development of the mesenteries and septa to be followed 
 day by day. Thus the growth of the polyp and the skeleton could be observed 
 together and their various relationships studied. 
 
 Once the larvae were settled they seemed vigorous and hardy, and scarcely 
 any of the young polyps arising therefrom succumbed, although subjected 
 to the somewhat adverse conditions of small aquaria. 
 
 The different organs, polypal and skeletal, began to appear shortly 
 after fixation, the various stages in the development of the tentacles, mesen- 
 teries, and septa being represented by the drawings and photographs on plates 
 1-5. The young polyps were early capable of expansion and retraction. 
 For the most part they remained in the expanded condition, with the tentacles 
 stretching out horizontally, or even overhanging. Sometimes the column 
 would extend vertically upwards and remain equal in diameter throughout ; 
 at other times the lower part of the walls would shrink over the corallum 
 and the remainder appear as a narrow column upon a broad pedestal. Upon 
 retraction the tentacles still remained exposed, but restricted to the central 
 region (plate 2). Even in the adult the tentacles have been found to remain 
 visible upon the fullest retraction, the column being incapable of folding 
 over them. On one occasion only a larval polyp was found with the 
 
62 SIDERASTREA RADIANS. 
 
 column wall completely overfolding the tentacles and covering nearly all 
 the disc (plate 3, fig. 18). 
 
 After the first few days the polyps were able to feed upon small pieces 
 of molluscs or chastopods placed upon the disc by means of forceps or a 
 needle. The tentacles would close upon the fragment and hold it as the 
 forceps were withdrawn. If the pieces offered were small enough they were 
 taken bodily within the gastric cavity, but when too large to be engulfed a 
 portion only would be drawn within the stomodseum, remain there for two or 
 three hours, and then be pushed away over the side of the disc. 
 
 For some time Zooxanthellae were present within the ectoderm of the 
 disc and upper part of the column, and rendered these regions nearly opaque ; 
 but later they wholly disappeared from the outer layer, and, as in the adult, 
 were restricted to the endoderm. As the polyps grew larger their walls 
 became thinner and more transparent. The internal yellow cells were 
 thickly distributed along the two sides of the mesenteries and within the 
 endoderm of the stalk of the tentacles, but were sparse towards the lower 
 part of the column. They formed an important aid in determining the course 
 of the mesenteries within the living polyps. 
 
 The young polyps displayed great power of recovery from injury. In 
 two or three instances the glass to which they were attached was broken in 
 such a way that a polyp was completely divided into two, the halves adherent 
 to different fragments ; yet the two parts continued to live, and each became 
 a distinct, though smaller, polyp, nearly circular on healing, and restoring 
 in every way the normal form. 
 
 On the polyps attaining partial transparency, the internal cavity could 
 be seen to contain granular matter and Zooxanthellae in a regular rapid 
 movement, and, by focussing at different heights, it was possible to make out 
 the complete course of the circulation. The particles passed directly upward 
 along the column, then transversely across the disc, downward along the 
 endodermal surface of the stomodaeum, and thus right into the central cavity 
 of the polyp ; they then continued radially across the base outwards towards 
 its periphery, and once more up the column. The regularity of the course 
 was a little disturbed at the opening of the tentacles into the gastric cavity. 
 Some of the particles would circulate around this region in an eddy-like 
 manner, or pass a little within the tentacular cavity, and afterwards be 
 wafted along in the general current. The course is indicated in fig. 6. 
 
 Occasionally the lips of the stomodaeum would meet in the middle and 
 remain open at each end, but no incurrent or excurrent streams were ever 
 detected through the apertures thus formed. A bolus of organic debris, 
 
POSTLARVAL DEVELOPMENT. 63 
 
 containing Zooxanthellae, would at times be seen whirling round and round 
 within the stomodseum, and would afterwards be jerked out and lost to the 
 polyp. Even after three months such extrusions would still take place. 
 
 The young polyps varied much in their rate of growth, especially in 
 the later stages ; indeed, in some instances, development seemed to be wholly 
 arrested. The isolated polyps progressed most rapidly, but among these 
 many differences were recognizable. The polyps forming colonies made 
 scarcely any progress after the first week or two. In the group represented 
 in plate i, fig. 6, no increase beyond the six tentacles and twelve simple 
 septa occurred up to six weeks, when the colony was preserved. 
 
 The development of the various organs was characterized by certain 
 well-marked intervals of rest and growth. A week or two might pass without 
 any conspicuous change taking place, and then 
 progress would be somewhat rapid. The order of 
 appearance of the organs varied somewhat in dif- 
 ferent individuals ; at first some organs of a system 
 were developed a cycle at a time, and the remainder 
 in successive pairs from one border of the polyp to 
 the other. Perhaps under the artificial conditions 
 the duration of some of the intervals was partly 
 determined by the state of the water and the amount 
 
 . ,~ .-^ , Fig. 6. — Diagram showing the course of the 
 
 and character of the food supplied. Still, there circulation within the imemai cavity of a 
 
 were certain definite resting stages appearing in all ""* ^° ^' 
 
 the polyps and seeming to possess a phylogenetic as well as an ontogenetic 
 
 significance. 
 
 The following were the most conspicuous phases of growth : Of the 
 mesenteries the protocnemes were established at the time of settling of 
 the larvae, and the six pairs are shown to develop in a regular consecutive 
 manner without any marked intervals (p. 76). Four of the six bilateral 
 pairs of protocnemes were connected with the stomodseum from the commence- 
 ment of stationary life, but at the termination of seventeen weeks the fifth 
 and sixth protocnemic pairs had not become complete. No further change 
 in the number of mesenteries took place for nearly four weeks, when, in 
 the larger polyps, the metacnemes began to appear. The different pairs of 
 these followed somewhat quickly upon one another from the dorsal to the 
 ventral aspect of the polyp, until the complete cycle of six pairs was estab- 
 lished (plate 3, fig. 14). The polyps remained at this mesenterial stage 
 for the rest of the time they were under observation — about two months. 
 There was no hint of the appearance of the third cycle. 
 
64 SIDERASTREA RADIANS. 
 
 As regards the tentacles, six primary exotentacles appeared simulta- 
 neously, shortly after fixation (plate i, fig. 6), followed by an interruption 
 of two or three weeks before the entotentacles began to arise. These were 
 at first simple outgrowths of the disc, and a long interval took place before 
 a second moiety appeared in connection with each and thus established the 
 mature form (plate 2, fig. 11). The longest interruption, however, was after 
 the completion of the primary twelve tentacles, for in the most forward polyps 
 about three months elapsed before additional members appeared, when those 
 of another cycle began to arise in a successive manner. 
 
 Of the septa the six entocoelic representatives appeared as a complete 
 cycle at a very early stage (plate i, fig. 7), followed shortly by the six alternat- 
 ing exocoelic members ; these latter were developed in some instances as a 
 complete cycle, but usually in successive pairs (plate 2, figs. 8, 9). Without 
 any marked rest other fragments or nodules were added to each system, but 
 in such a way as to render it doubtful as to which cycle they belonged (plate 
 2, fig. 12). On the whole, the growth of the septa was more continuous 
 than that of either the mesenteries or the tentacles. 
 
 Undoubtedly the most conspicuous interruption for all the systems of 
 organs was that between the protocnemic and metacnemic stages, and the 
 great differences between the manner of appearance of the two series of 
 mesenteries, septa, and tentacles further establishes this as the most impor- 
 tant ontogenetic and phylogenetic interval in madreporarian development. 
 Its significance is discussed later. 
 
 The time of appearance of the organs varied somewhat in different polyps, 
 sometimes even to the extent of several weeks ; but with regard to the actual 
 order followed no important differences were observed in the mesenteries and 
 septa, though less constancy prevailed amongst the tentacles. Where devel- 
 opment was not in complete cycles the succession was bilateral, proceeding 
 from the dorsal to the ventral borders of the polyp. Contrary to what is 
 generally assumed, bilaterality, not radiality, was a conspicuous feature of 
 the growth from beginning to end ; and in most of the systems of organs 
 the bilateral symmetry prevailed for lengthened periods, even where the 
 radial condition was ultimately assumed. 
 
 Many of the young polyps were preserved at different stages — while still 
 adhering to their original pieces of glass — and were prepared for microscopic 
 examination as a whole. The process was as follows : After narcotization 
 by means of magnesium sulphate or menthol the polyps were killed by 
 pouring a solution of formaldehyde over them, and, still adherent, they were 
 transferred to alcohol, afterwards stained, dehydrated, cleared, and mounted 
 
POSTLARVAL DEVKLOPMENT. - 65 
 
 in Canada balsam as permanent preparations. The young coralla from 
 which the photographs on plates 4-5 were taken were prepared by macer- 
 ating the polyps in caustic potash and afterwards washing or carefully 
 brushing away the soft, loosened tissues. After maceration the skeletons 
 could generally be freed from the glass to which they were attached by 
 gently tapping the latter, but in some cases it was necessary to insert the 
 edge of a razor between the basal plate and the surface of the glass. 
 
 The young polyps intended for sectionizing were detached after preser- 
 vation by dissolving away the calcareous deposit with weak hydrochloric 
 acid. In one case a living polyp of fourteen weeks became detached along 
 with the whole skeleton, and, thus free, continued its growth for some time. 
 
 In the course of their growth the polyps increased from i to 2 mm. in 
 diameter, the oldest polyps reared being but 2 mm. across the base. 
 
 TENTACLES. 
 FIRST CYCLE OF KXOTENTACLKS. 
 
 The tentacles began to make their appearance two or three days after 
 fixation of the larvae, as rounded upgrowths over alternate mesenterial 
 chambers. At this stage only the six primary pairs of mesenteries were 
 present, giving rise to six entocoelic and six exoccElic chambers (plate i, 
 fig. 7). Six tentacular prominences appeared simultaneously, equal in size 
 and distance apart. They early showed an opaque white, knob-like apex 
 distinct from a short, more transparent stem. The single cycle which they 
 constituted served to delimit the larva for the first time into oral disc and 
 column wall, no indication of the boundary between the two areas being 
 hitherto determinable. 
 
 The stems of the tentacles were hollow, broad below, narrow distally, 
 and perfectly smooth, that is, without nematoblast tubercles. The endoderm 
 wdthin could be seen to be richly supplied with Zooxanthellae, while the ecto- 
 derm was colorless ; the knob was solid and colorless, without Zooxanthellae, 
 and constituted wholly of ectoderm with numerous radiating nematoblasts. 
 
 At first the actual mesenteric chambers from which the tentacles 
 protruded could not be determined with certainty, owing to the opacity of the 
 polyps. Soon, however, as the polyps grew larger and the walls more trans- 
 parent, the outgrowths were seen to communicate with the exocoelic chambers. 
 As noticed below, this relationship is contrary to the general rule of tentac- 
 ular development hitherto observed among the Zoantharia, and therefore 
 the greatest care was exercised to establish it beyond a doubt. Bxtemally 
 the entocoelic and exoccelic mesenteric chambers could be readily determined 
 
66 SIDERASTREA RADIANS. 
 
 from their relation to the longer oral axis and from the known relation of 
 the complete and incomplete mesenteries at the Edwardsia stage of develop- 
 ment ; the two terminal chambers which include the longer oral axis are 
 always the directive entocoeles, and the two alternating chambers on each side 
 between these are the lateral entocoeles, while all the intermediate divisions 
 are exocceles. Most of the young polyps were so situated as to permit of all 
 these details being clearly observed, and in no instance were the first six 
 tentacles developed as outgrowths of the entocoeles, but were always disposed 
 as in plates i and 2, figs. 7-9. 
 
 As shown in plate i, fig. 7, the six tentacles first to appear alternated 
 with the first septa, the latter being entocoelic in position ; the smaller septa 
 which arise later (plate 2, fig. 9) correspond with the first tentacles, that is, 
 are exosepta. 
 
 FIRST CYCLE OF ENTOTKNTACLES. 
 
 A long interval elapsed between the appearance of the primary cycle of 
 six exotentacles and the development of any others. Toward the close of 
 the fourth week additional small protuberances of the disc began to appear 
 in the interspaces between the primary series and a little nearer the center 
 of the disc. Like the first they soon showed a distinction into apical knob 
 and stem. In most polyps the six new tentacles arose simultaneously, 
 constituting a second inner cycle and alternating with the first ; but in some 
 cases the members developed successively, though no dorso-ventral or other 
 regular sequence could be established. Any intervals which occurred in 
 the appearance of the different members were very brief, except in one 
 instance where a fortnight passed between the appearance of the dorso-lateral 
 and the ventro-lateral pairs. 
 
 There were now twelve tentacles present, arranged in two alternating 
 cycles of six each ; an outer cycle of six large exotentacles, the first to arise, 
 and an inner cycle of six smaller ento tentacles appearing later (plate 2, fig. 11). 
 
 Close examination soon revealed that the individual members of the inner 
 cycle were not disposed symmetrically midway in the interspaces between 
 those of the older cycle, but, as represented in plate 3, fig. 13, were a little to 
 one side. Bven the tentacles arising from the axial entocoeles were some- 
 what to the one or the other side of the principal axis of the polyp. In most 
 cases the four small lateral tentacles were all situated toward the dorsal 
 aspects of their corresponding interspaces, while the tentacles over the direc- 
 tive entocoeles might be to the right or to the left of the directive axis. This 
 peculiar arrangement of the young entotentacles was best observed when 
 the polyps were only partly extended ; during full expansion they appeared 
 
POSTLARVAL DEVELOPMENT. 67 
 
 almost symmetrically disposed with regard to the outer cycle and the ento- 
 coelic chamber belov/ (plate 2, fig. 11). 
 
 The simple character of the entoccelic tentacles on their first appearance 
 is in marked contrast with their bifurcated form in mature polyps. As 
 already mentioned in describing the adult colony, the genus Siderastrea is 
 unique among West Indian corals in that its polyps possess dimorphic 
 tentacles. The entoccelic members consist of a simple stalk, divided above 
 the middle into two smaller halves, each of which is terminated by a white, 
 swollen knob ; the exocoelic tentacles, on the other hand, consist of a simple 
 stem with an apical knob. It was also found that in nearly mature polyps 
 the newly formed entoccelic tentacles of the higher cycles are sometimes 
 simple, while the older members are double (plate 6, fig. 32). 
 
 A few days after the completion of the second (inner) cycle of tentacles 
 in the young polyps, the double character of its members began to assert 
 itself. From certain of the entoccelic interspaces another knob-like pro- 
 tuberance arose beside the first tentacle, which in time became a distinct 
 tentacle with stem and capitulum like the other member (plate 2, fig. 12)- 
 The two moieties thus formed were for some time different in size, but later 
 became equal. The appearance was as if two small tentacles had become 
 intercalated between, but a little central to, the interspaces of the first- 
 formed tentacular cycle. 
 
 The second moiety of the inner tentacles exhibited no constancy in its 
 order of appearance from the different entocoeles. In plate 3, fig. 13, the ven- 
 tral axial member has appeared and also the two dorso-laterals, while over the 
 dorsal and ventro-lateral entocoeles the simple tentacle alone is present. In 
 four other polyps the dorsal axial tentacle was the first to become doubled, 
 and all the others remained single for a few days longer. In another polyp 
 the additional outgrowth first appeared over the two axial entocoeles. 
 
 The new outgrowth was at first wholly independent of the old, and to all 
 appearances the two were to be regarded as distinct tentacles, side by side, 
 each with its own peduncle. Later, however, a single proximal stem was 
 found to arise, which, uniting the two as by a single stalk, raised them some 
 distance above the disc, and the organ thus assumed the form of the ento- 
 tentacles already described in connection with the mature colony (plate 3, 
 figs. 15, 17). 
 
 The adult bifurcated entoccelic tentacle is thus seen to be primarily 
 represented by two distinct tentacles, which only later are united by the 
 growth of a common peduncle. It represents a unique method of tentacular 
 growth among the Anthozoa. The double character appeared on only a few 
 
68 SIDERASTREA RADIANS. 
 
 of the most forward larval polyps, and, for tlie time they were under obser- 
 vation, was in no instance established for all the six members. 
 
 For another period of six weeks or two months no further tentacular 
 increase took place, though in the meantime the second cycle of mesenteries 
 had made its appearance. 
 
 So far, four distinct stages are recognizable in the development of the 
 twelve prototentacles of Siderastrea, each stage separated by marked intervals 
 of rest. They are represented diagrammatically in the figures a-c on p. 72 
 as they would appear when fully completed. 
 
 1. The stage with six exocoelic tentacles, forming a single cycle. The 
 members are equal and appear simultaneously. 
 
 2. The stage with twelve simple tentacles, constituted of an outer cycle 
 of six large exotentacles and an inner alternating cycle of six small ento- 
 tentacles. The latter appear either simultaneously or successively in an 
 irregular sequence. They are not situated midway over the mesenterial 
 chambers. The four lateral members are disposed a little towards the dorsal 
 or ventral half of the interspaces, while the directive members are somewhat 
 to one side or the other of the principal oral axis of the polyp. 
 
 3. Six additional members of the inner entoccelic cycle appear, so that 
 the cycle is constituted of twelve independent tentacles in six pairs. They 
 arise in no definite sequence. 
 
 4. A single outgrowth arises over the middle of each entoccele and forms 
 a common peduncle for each pair of entotentacles. Thus the tentacular 
 outgrowths from each entoccele come to resemble a bilobed adult tentacle. 
 
 The appearance of the exocoelic tentacles in advance of the entoccelic in 
 Siderastrea radians is unique in the tentacular development of Zoantharian 
 polyps according to the researches of Lacaze-Duthiers, Faurot, von Koch, 
 Appellof and others, who have made us acquainted with the post-larval 
 development of many species of anemones and corals. In the corals Caryo- 
 phyllia cyathus^ Manicina areolata^ and Favia fragum the twelve proto- 
 tentacles, constituting an inner and outer cycle, appear simultaneously, or 
 at most there is only a short interval between one and the other ; but 
 wherever such an interval occurs, the inner entoccelic cycle appears in 
 advance of the outer exocoelic, and the entoccelic members are larger than 
 the exocoelic (entacmasous). In general it will be found that the production 
 of other entoccelic structures — tentacles and septa — in corals is in advance 
 of the exocoelic. 
 
 In actinian larvse it is more usual for eight tentacles to appear first, 
 arranged either as a single cycle or as an inner and outer cycle of four each. 
 
POSTLARVAL DEVELOPMENT. 69 
 
 These correspond witli the eight interspaces between the four pairs of 
 Edwardsian mesenteries ; later, as the fifth and sixth pairs of protocnemes 
 increase in size, other two pairs of tentacles arise, and the whole become 
 arranged in two cycles of six each. Of the primary eight tentacles in 
 actinians six are the entocoelic members, and they early assume predomi- 
 nance over the exocoelic series {e. g.^ Lebrunia^ 1899). 
 
 Clearly the number of external tentacles first appearing is determined 
 by the number of internal mesenterial chambers already formed, seeing that 
 the cavities of the former are continuations of the latter. Where, as in the 
 actinian Lebrunia (1899), only four pairs of mesenteries are present when 
 the tentacles make their appearance, it is manifest that only eight tentacles 
 will be developed, and that the other four will be formed only when the 
 growth of the fifth and sixth mesenterial pairs has given rise to four 
 additional mesenterial chambers. In the species of coral larvae so far fully 
 investigated the six pairs of primary mesenteries — four pairs of macrocnemes 
 and two pairs of microcnemes — are fully established at or shortly after the 
 time of fixation and before the tentacles begin to protrude; hence six or 
 twelve tentacles arise simultaneously, one from each mesenterial chamber. 
 
 The long interval which elapses before the more complex entocoelic 
 tentacles are fully formed may perhaps account for the appearance in Side- 
 rastrea of the exotentacles in advance of the entotentacles. The young 
 polyp after fixation has to provide for itself, and for this purpose its weapons 
 of offense and defense, represented by the tentacular nematoblasts, are 
 necessary from the beginning. 
 
 The origin of the bifurcated entotentacles from two independent moieties 
 which appear at different times and are later elevated upon a common 
 peduncle is a second unique feature in tentacular development. Obviously 
 the presence of a double battery of nematocysts on each tentacle, in place of 
 a single battery, will be of advantage to the polyp ; and the growth of one 
 moiety in advance of the other is probably to be taken as indicating an 
 ancestry in which only one was present. 
 
 SECONDARY EXOTENTACLES. 
 
 Much interest attaches to the manner of appearance of the tentacles 
 beyond the prototentacles, as, from the exceptional sequence of the first two 
 cycles, it is impossible to predict from the known development of other forms 
 how or where the next members will arise. One of the earliest stages is 
 represented by the fully expanded disc in plate 3, fig. 15. The second- 
 cycle mesenteries are seen stretching along the disc, decreasing in size 
 
70 SIDERASTREA RADIANS. 
 
 successively from the dorsal to the ventral aspect. The six new pairs of 
 mesenteries have given rise to six additional entocoelic and six additional 
 exocoelic chambers, from which further tentacles may arise ; so that mani- 
 festly the mesenteries with their mesenterial chambers are formed in advance 
 of their tentacular prolongations. At the stage represented in plate 3, fig. 15, 
 all the primary entotentacles are yet simple except the two dorso-lateral 
 members, each of which consists of a broad stem unequally bifurcated at 
 the dorsal extremity. The dorsal and ventral exotentacles are simple, and 
 the broad base of each communicates with the second-cycle entoccele below 
 as well as with the exoccele on each side of it. An additional tentacle is 
 seen on the ventral aspect of the middle primary exotentacle on each side of 
 the polyp, and as yet they are smaller than the exotentacles first formed. 
 The new tentacle has pushed the older a little to one side, and both are 
 obviously exocoelic in position, though in the figure appearing to extend 
 over into the entoccele. There are now eight exocoelic tentacles forming an 
 outermost cycle. 
 
 A somewhat older stage is represented by the next figure (plate 3, fig. 
 16). The two middle exotentacles are equal in size and quite free from one 
 another, and both are now seen to be wholly unconnected with the median 
 second-cycle entocoeles. Another exocoelic member has also appeared on the 
 ventral aspect of the dorso-lateral exotentacle, on both the right and left sides 
 of the polyp, but as yet they are a little smaller than the primary exoten- 
 tacles. The four new exotentacles are situated at the same distance from the 
 center of the disc as the primary cycle of six exotentacles, and therefore, 
 instead of forming a new cycle, they merely add to the number of members 
 in a cycle already established. The outer cycle of exotentacles thus consists 
 of ten practically equal tentacles ; the two wanting to complete the hexameral 
 plan belong to the ventral system, which region is usually found to lag 
 behind the dorsal and middle systems in the extent of its development. 
 
 The two stages (plate 3, figs. 15, 16) demonstrate the important fact 
 that the primary six exotentacles remain exotentacles, and always constitute 
 part of the outermost cycle ; further, any new exotentacles formed become 
 equal in size with the primary exotentacles and are so disposed as to be 
 members of the same cycle. The primary exotentacles do not become 
 entotentacles when new pairs of mesenteries arise in the same radii, as from 
 their original position might have been expected. On the appearance of the 
 second cycle of mesenteries, they seem for a time as if communicating with 
 their entocoeles (plate 3, fig. 15), but this condition is only temporary. When 
 additional exotents^cles arise and the mesenteries are further developed, both 
 
POSTLARVAL DEVELOPMENT. 7 1 
 
 the older and the newer tentacles are seen to communicate only with the 
 exocoeles, and the second-cycle entocceles are for a time without any tentacular 
 outgrowths. The cycle of exotentacles when complete would at this stage 
 consist of twelve members, of which six are primary and six are later forma- 
 tions, the latter appearing after the second cycle of mesenteries (fig. 
 7, d, p. 72). 
 
 The tentacular development of S. radians thus conforms with the law 
 of substitution established by Lacaze-Duthiers (1872, '73) and Faurot (1895) 
 for actinians — exotentacles remain exotentacles, and throughout the growth 
 of the polyp they continually change their relationship to the entotentacles 
 in such a manner as always to constitute the outermost cycle, and all are 
 uniform in size. 
 
 As in the case of the two cycles of primary tentacles the secondary 
 exotentacles appear in advance of the corresponding secondary entotentacles, 
 so that now the cycle of exotentacles contains double the number of members 
 of the cycle of entotentacles. In the several species whose development was 
 carried thus far by Lacaze-Duthiers and by Faurot the exoccelic and entocoelic 
 members appeared together along with a new pair of mesenteries, or the ento- 
 tentacles were in advance of the exotentacles ; later the entotentacles became 
 larger than the exotentacles. 
 
 SECOND CYCLE OF ENTOTENTACLES AND THIRD CYCLE OF EXOTENTACLES. 
 
 Clearly the stage beyond that represented in plate 3, fig. 16, will be one 
 in which the tentacles begin to make their appearance over the second-cycle 
 entocceles. Such a condition is represented in plate 3, fig. 17, taken from the 
 same polyp as plate 3, figs. 15 and 16, when about a fortnight older than the 
 stage of plate 3, fig. 16. It is also the oldest stage reared. From the middle 
 second-cycle entocoele on each side a simple tentacle has arisen, intercalated 
 a little beyond the primary cycle of entotentacles, but yet within the cycle 
 of exotentacles. The two tentacles are the first representatives of the second- 
 cycle tentacles of the adult polyps, and the greatest interest attaches to the 
 fact that they have displaced, as it were, some of the members of the second 
 cycle (exotentacles), so that they now form a third cycle. 
 
 If the development of the polyps had been continued, there is every 
 reason to suppose that a new tentacle would have appeared over each of the 
 dorsal second-cycle entocceles. Also in the further growth of the two ventral 
 systems another exotentacle would have appeared on each side, and then a 
 tentacle over the second-cycle entocoele, just as in the middle systems; then 
 all the simple second-cycle entotentacles would in time have become bifurcated 
 
72 
 
 SIDERASTREA RADIANS. 
 
 like the members of the first-cycle entotentacles. In general, the sequence 
 of the entotentacles would follow that of the mesenteries with which they are 
 associated, although in plate 3, fig. 17, the growth within the middle sextants 
 is in advance of that of the dorsal sextants. 
 
 The completed system of tentacles would consist of: (a) Six bifurcated 
 entotentacles constituting the first cycle ; {d) six alternating bifurcated ento- 
 tentacles forming the second cycle ; (c) twelve alternating exotentacles, which 
 have appeared at two wholly distinct periods, making up the third cycle. 
 
 The developmental relations between the six second-cycle entotentacles 
 and the twelve third-cycle exotentacles are somewhat different in Siderastrea 
 
 a 
 
 d 
 
 Fig. 7. — «r-e, series of diagrams illustrating the order of appearance and relationships 
 of the first three cycles of tentacles. The numerals I and II indicate the orders 
 of entotentacles and X the exotentacles. 
 
 from those hitherto met with in actinians. Lacaze- 
 Duthiers (1872) first found that after the completion 
 of the first two cycles of tentacles (six entotentacles 
 and six exotentacles), the tentacles of the third cycle 
 do not arise peripherally, alternating with the mem- 
 bers of the first and second cycles, as would be 
 naturally supposed, but that six new pairs are insinuated between the two 
 primary cycles at six intervals only. One member of each of the six new 
 pairs increases in size more than the other member, and ultimately consti- 
 tutes the second cycle of tentacles (entotentacles), displacing the primary 
 second cycle of exotentacles ; the other member of each new tentacular pair 
 attains the same size as the displaced exotentacles, and along with them 
 forms the third cycle of twelve tentacles. The third cycle is thus formed of 
 six tentacles from the original second cycle and six tentacles which are later 
 formations ; the members of the permanent second cycle are all new forma- 
 tions which have displaced the older second-cycle members. Lacaze-Duthiers 
 expressed these relationships under the term " law of substitution." 
 
POSTLARVAL DEVELOPMENT. 73 
 
 In Siderastrea the second cycle for a time consists of twelve exotentacles, 
 six of which are primary formations and six are later outgrowths. The latter 
 appear in advance of the corresponding entotentacles, not along with them, as 
 seems to be the case in most actinians. Afterwards, however, a new cycle of 
 six entotentacles is intercalated between the original first and second cycles, 
 displacing the latter to the third cycle, and itself constituting the permanent 
 second cycle. The law of substitution thus holds for Siderastrea as for 
 actinians, though the relative time of appearance^f the entotentacles and 
 exotentacles differs from that usually followed. Faurot (1895) shows that in 
 IlyantJms parthenopens the entotentacle from a new mesenterial pair arises 
 in advance of the exotentacle, exactly the reverse of that in Siderastrea. 
 
 THIRD CYCLE OF ENTOTENTACLES AND FOURTH CYCLE OF EXOTENTACLES. 
 
 As the larval polyps were not reared beyond the beginning of the 
 three-cycle stage, it will be necessary, in order to complete the survey of the 
 tentacular development, to have recourse to bud polyps of a colony. From 
 what has been established above it is manifest that the problem is not to 
 determine the manner according to which the fourth cycle of the adult polyp 
 will arise, but how a third cycle of entotentacles originates and is inter- 
 calated between the previously formed second and third cycles. The exo- 
 tentacles as a cycle are shown to have no ordinal significance in studies of 
 tentacular sequence ; this importance belongs to the entotentacles. Regarded 
 cyclically the exotentacles are but temporary predecessors of the entotentacles 
 of the adult polyp. 
 
 On any colony are numerous polyps in which the tentacles vary from 
 twenty-four to nearly forty-eight. In the polyp represented on plate 6, fig. 
 32, there are three third-cycle entotentacles, each with a corresponding 
 exotentacle. The former are yet at an early stage of development, only one 
 moiety of the tentacle having appeared. In fig. i, p. 12, is represented 
 diagrammatically the tentacular plan of the polyp of which the mesenterial 
 system is given in plate 6, fig. 34. Along with each of the five pairs of 
 third-cycle mesenteries (ill) have appeared two corresponding tentacles, and, 
 comparing with fig. 3, p. 26, one of the tentacles is seen to be entocoelic (iii) 
 and the other exoccelic (x). The new entotentacles, iii, are situated beyond 
 the second cycle of entotentacles, il, but within the outermost exoccelic 
 cycle ; that is, they represent the commencement of a third cycle of ento- 
 tentacles, and the exotentacles are now being relegated to a fourth cycle. 
 At this stage, therefore, some of the exotentacles may be considered as 
 belonging to the third cycle and some to the fourth cycle. Under the circum- 
 
74 SIDERASTREA RADIANS. 
 
 stances it was not possible to say whether the new exotentacles appeared in 
 advance of the entotentacles, but from the order actually observed in the 
 first three cycles it may be assumed that such was the sequence. It will be 
 observed that the new tentacular pairs are situated on the dorsal aspect of 
 the four sextants, along with one pair on the ventral side of the right middle 
 sextant. 
 
 New exotentacles and entotentacles have been found to arise shortly after 
 the appearance of new mesenterial pairs. Therefore, if the third-cycle mesen- 
 teries develop normally with the regularity outlined on p. 83, a single ento- 
 tentacle aud exotentacle would be expected to appear on the dorsal aspect of 
 each sextant, and then the corresponding ventral series would commence, 
 both starting with the dorso-lateral systems and proceeding to the ventro- 
 lateral. In a colony where the polyps are so closely arranged as in Sideras- 
 trea^ however, the later growth rarely proceeds according to such a regular 
 law, so that very little importance can be attached to the order of appearance 
 of the tentacles making up the outer cycles. All the studies on corals seem 
 to indicate that the mesenteries, tentacles, and septa develop conformably to 
 a regular plan for the first and second cycles, but that the regularity is 
 frequently departed from in the later stages. 
 
 If the cyclic hexameral plan of the tentacles in S. radians were com- 
 pleted the formula would be 6, 6, 12, 24. The first three cycles would be 
 entotentacles and the last exotentacles ; the members of any one cycle would 
 be equal in size, but those of the inner cycles somewhat larger than those of 
 the outer cycle. The entotentacles would have appeared in a fairly regular 
 sequence, the inner cycle first, the middle cycle next, and the third cycle 
 last ; but it is altogether otherwise with the fourth cycle, constituted only of 
 exotentacles. Six of its members appeared a little in advance of the first 
 cycle of entotentacles ; other six a little in advance of the second cycle of 
 entotentacles ; and twelve more in advance of the third cycle of entotentacles. 
 The exotentacles at each stage form a complete cycle ; first a cycle of six, 
 then a second cycle of twelve, which afterwards becomes a third cycle, and, 
 lastly, a third cycle of twenty-four, which afterwards becomes a fourth cycle. 
 In this way they always constitute the last or outermost cycle for the time 
 being. At the termination of each stage the number of outer simple exoten- 
 tacles equals the sum of the bifurcated entotentacles making up the inner 
 cycles ; but, until the adult stage is reached, as a cycle they occupy the 
 position which later will be occupied by the entotentacles. 
 
 When, however, the hexameral cyclic plan is not complete in the adult, 
 that is, where the last cycle of entotentacles commenced is not completed 
 
POSTLARVAL DEVELOPMENT. 75 
 
 according to the hexameral sequence, then the formula must be different from 
 that given above if it has to express the true morphological value of the tenta- 
 cles as entotentacles and as exotentacles. Cyclic incompletion has been 
 shown to be practically always the condition in adult polyps of S. radians ; 
 two cycles of entotentacles are hex amerously complete, but the third cycle of 
 entotentacles consists of a variable number, rarely reaching the required num- 
 ber twelve. Regarded cyclically, some of the exotentacles belong to the third 
 and some to the fourth cycles ; but until they form a complete hexameral 
 cycle, it is preferable to consider the exotentacles simply as the outermost 
 tentacles, not as forming a cycle. The morphological formula will thus be 
 6, 6, X, 6 + 6 + X, where x may be any number from one to twelve ; the 
 series 6, 6, x, will represent the number of entotentacles, and 6 -1- 6 + x 
 the number of exotentacles. The number of exotentacles will vary in the 
 same degree as the number of third-cycle entotentacles. Later, a similar 
 formula is established for the septa. 
 
 The different tentacular cycles in the adult polyps of Siderastrea are 
 much more widely apart than in the larval polyps {cf. plate 6, fig. 31 ; 
 plate 3, fig. 17). In the latter the closeness of the cycles recalls the condition 
 characteristic of the majority of anemones and corals. 
 
 The principal facts concerning the development of the tentacles of 6*. 
 radians may be now summarized. 
 
 1. The first tentacles to appear consist of a cycle of six members, one 
 from each of the six primary exocoeles. They arise simultaneously, are 
 equal in size, and remain simple throughout. 
 
 2. Shortly afterwards six smaller tentacles arise, forming a second cycle, 
 situated internally to the first, and communicating with the six primary 
 entocoeles. They may develop either simultaneously or in successive pairs 
 from the dorsal to the ventral aspect. They are not symmetrically situated 
 as concerns the mesenterial chambers. Later, another entocoelic tentacle 
 appears alongside each of the first members, and then a common peduncle is 
 formed which elevates the two moieties so as to form the single bifurcated 
 tentacle of the adult. 
 
 3. The second cycle of mesenteries having appeared, a new exotentacle 
 arises from each additional exocoelic chamber within the six primary systems 
 of the polyp. In general the succession is from the dorsal to the ventral 
 systems, and the new tentacle is on the ventral aspect of the older exoten- 
 tacle belonging to the same system, but many variations occur. The six 
 new exotentacles arrange themselves in the outermost or second cycle, which 
 thus consists of twelve members, equal in size, and all simple. 
 
76 SIDERASTREA RADIANS. 
 
 4. Additional tentacles are now intercalated between the inner and outer 
 cycles already established. They arise, apparently without any regular 
 sequence, from the six entocoeles of the second pairs of mesenteries, and, like 
 the members of the first cycle of entotentacles, are at first simple, but after- 
 wards bifurcated. When the cycle is completed it constitutes the second 
 tentacular cycle of the mature polyp ; the original second cycle of six 
 exotentacles, now consisting of twelve members, becomes at this stage the 
 outermost or third cycle (law of substitution). 
 
 5. As the pairs of third-cycle mesenteries develop, two tentacles, one 
 entocoelic and one exoccelic, arise in connection with each. The entocoelic 
 representatives appear between the second and outermost cycles of the previous 
 stage, and thus constitute a new third cycle ; the exocoelic representatives 
 arrange themselves among the members of the outermost cycle, which now 
 becomes the fourth of the tentacular cycles. Here, as in the two previous 
 cycles, the entotentacles are at first simple, then another moiety appears, and 
 finally a common stalk. 
 
 6. The hexameral cyclic plan of the third-cycle entotentacles in S. 
 radians being usually incomplete, the morphological tentacular formula for 
 the whole polyp with four cycles is 6, 6, x, 6 + 6 + x, where x may be any 
 number from one to twelve. 
 
 MESENTERIES. 
 FIRST CYCLE OF MESENTERIES (PROTOCNEMEs) . 
 
 The earliest larvae reveal, both externally and in sections, only eight 
 mesenteries, arranged in four bilateral pairs, two axial and two lateral. Of 
 these the two lateral pairs are already united with the stomodseum, but the 
 dorsal and ventral axial pairs are free, the ventral pair being a little larger 
 than the dorsal (plate i, fig. 3). In other larvae, only a day or two older, the 
 two ventral mesenteries have become united with the stomodaeum, but the 
 dorsal are still free, and two new pairs, the fifth and sixth, are beginning 
 to make their appearance (plate i, fig. 4, and plate 8, fig. 51). At the stage at 
 which the larva settles four pairs of mesenteries, two lateral pairs and the 
 dorsal and ventral axial pairs, are complete, and the fifth and sixth pairs have 
 become a little larger, but remain free from the stomodseum (plate i, fig. 6). 
 This is the stage represented by all the larvae at or shortly after fixation, and 
 is evidently one of importance in the ontogeny of the polyp, for no further 
 mesenterial development took place for a period of three or four weeks. 
 
 In transverse sections of the larvae the vertical muscle fibers of the 
 mesenteries are already sufficiently well developed to permit of the directive 
 
POSTLARVAL DEVFXOPMENT. 77 
 
 and unilateral pairs being established. On the dorsal and ventral axial 
 pairs the musculature is on the faces of the mesenteries turned away 
 from one another, which, by this means, are recognizable as directives. 
 The musculature on each complete lateral mesentery is on the ventral 
 face turned towards the incomplete mesentery, while this bears its muscle 
 fibers on the dorsally directed face, so that each unilateral pair of mesen- 
 teries at this stage is made up of a complete and an incomplete moiety 
 (anisocnemic). 
 
 On the reasonable assumption that the comparative sizes of the mesen- 
 teries indicate their order of appearance in the larvae, the two bilateral pairs 
 which first unite with the stomodaeum (plate i, fig. 3) are the first and second 
 pairs to arise. At the stage available little difference in size is represented 
 by these two pairs, so that it could not be readily determined which is the 
 first pair and which the second. The results of Von Koch (1897), Wilson 
 (1888), and others who have studied earlier stages of coral larvae, leave no 
 doubt, however, that the ventral of the two pairs is the first, and that the 
 dorsal pair is the second. Frequently in other corals the first pair is for a 
 time much better developed than the others, and bears well-defined filaments 
 (1902, p. 528). From the relationships shown on plate i, fig. 4, and 
 plate 8, fig. 51, there can be no uncertainty as to which are the third 
 and fourth mesenterial pairs. The ventral pair of directives is larger, 
 and unites with the stomodaeum in advance of the dorsal pair ; it is there- 
 fore the third pair in the mesenterial sequence, and the dorsal directives will 
 be the fourth. 
 
 The incomplete mesenteries on plate i, fig. 6, representing the fifth and 
 sixth bilateral pairs, arise practically at the same time, and for the most part 
 remain equally developed. The more dorsal of the two in some instances 
 outstrips the other, and in other corals and actinians is frequently found to 
 unite with the stomodaeum in advance of the more ventral. Whatever differ- 
 ences exist in size or time of union with the stomodaeum support the view 
 now generally held that the incomplete pair of mesenteries between the first 
 and second pairs is to be regarded as the fifth, and hence the pair between 
 the first and third pairs is the sixth. 
 
 The sequence of the protocnemes thus established for Siderastrea 
 radians is in strict agreement with that ascertained by H. V. Wilson (1888) 
 for Manicina areolata^ and by Von Koch (1897) for Caryophyllia cyaihus^ 
 both of whom had a complete series of larval stages for investigation. It is 
 also supported by the various early stages in other corals which I have had 
 under observation (1902, p. 450). The same order is followed by nearly all 
 
78 SIDERASTREA RADIANS. 
 
 actinians,* though the evidence for the Cerian these and Zoanthese is not yet 
 conclusive. 
 
 The length of time the young polyps of Siderastrea remained with 
 only twelve mesenteries — eight macrocnemes and four microcnemes — would 
 indicate that the stage is one of phylogenetic, as well as ontogenetic, 
 significance. No marked resting stage in the mesenterial development 
 occurred until this was reached, while none of the polyps showed further 
 progress until the expiration of three or four weeks, and some of the smaller 
 polyps remained thus for three months. Further, the fact that four pairs 
 united with the stomodaeum, while two pairs remained separate for the whole 
 seventeen weeks, the six pairs of second-cycle mesenteries arising in the 
 meantime, would suggest a different significance for the complete and incom- 
 plete groups (8 + 4). 
 
 The relationship, 8 + 4, is characteristic of the larvae and young polyps 
 of Madreporaria and Actiniaria generally, and persists in some species 
 throughout the life of the polyp. Only the first four pairs of mesenteries 
 are ever united with the stomodaeum in the Bdwardsidae, Gonacttma, and 
 some other Actiniaria, and in the mature polyps of all West Indian species 
 of Madrepora and Porites the same eight protocnemes alone are complete, 
 and the other four remain incomplete. 
 
 The long retention of freedom of the fifth and sixth pairs of protocnemes 
 suggests to my mind an ancestry in which the mesenteries as a whole, includ- 
 ing the metacnemes, were alternately long and short, excluding, of course, 
 the axial directives. Among modem examples this is retained in the mesen- 
 terial system of the zoanthids, Porites^ and Madrepora^ and was perhaps 
 characteristic of the Rugosa. The arrangement of the musculature on the 
 mesenteries in the Cerianthese, always on the face towards one aspect of the 
 polyp, can be also understood if one considers that the incomplete mesenteries 
 present in such forms as the zoanthids are never developed ; the cerianthids 
 retain the simplest protocnemic stage of any of the forms here considered 
 having only four bilateral pairs. 
 
 In the Ceriantheae and Zoantheae the mesenteries beyond the protocnemes 
 arise at only one or two restricted regions of the polyp (bands of proliferation), 
 not all round the circumference, as in most modem anemones and coral 
 polyps ; and four such zones or bands of proliferation would appear to have 
 been characteristic of the mesenterial growth in Palaeozoic coral polyps. The 
 
 * Appellof (1900) has discussed fully the value to be assigned the accounts of Lacaze-Duthiers, Haddon, 
 and others as to various other sequences of the primary mesenteries in actinians. Throughout their work 
 Delage and H^rouard (1902) transpose the order of the fifth and sixth pairs as given above. 
 
POSTLARVAL DEVELOPMENT. 79 
 
 mesenteries and septa in all these are never more than dicyclic. With the 
 later introduction of a radial polycyclic arrangement of the mesenteries and 
 septa, by the addition of exocoelic pairs in all the sextants, the fifth and sixth 
 protocnemic pairs begin to unite with the stomodaeum, and so produce a 
 more approximate radial symmetry in the adult. But that the alternately 
 macrocnemic and microcnemic arrangement is more primitive seems to be 
 still suggested in the long separation from the stomodseum of the fifth and 
 sixth protocnemes, found to be characteristic of ~ recent actiniarian and 
 madreporarian polyps. 
 
 The first twelve mesenteries in Siderastrea have been shown to arise in 
 bilateral pairs, a member on each side of the median axis, and the order is 
 such that each new pair appears alternately in the successively oldest 
 chamber. As the mesenteries become fully established another paired 
 arrangement is introduced. The directives throughout the life of the polyp 
 constitute bilateral pairs ; but now the second and fifth mesenteries on each 
 side form what I have termed unilateral pairs, and likewise the first and 
 sixth mesenteries on each side. Though varying so much in their order of 
 appearance and in their primary relations to one another, the six pairs in 
 the end constitute a regular cycle and appear all of the same value ; at first 
 they are anisocnemic pairs, but they become isocnemic when the fifth and 
 sixth bilateral pairs come into union with the stomodaeum. Thus a trul}^ 
 radial disposition of the parts results from a primary bilateral origin, a result 
 found to be continually recurring in the development of the different systems 
 of organs. 
 
 SKCOND CYCLE OF MESENTERIES (mETACNEMEs) . 
 
 For a period of several weeks after fixation no addition to the twelve 
 protocnemes took place ; the four incomplete members continued to increase 
 in size, though remaining free from the stomodasum. Mesenterial filaments 
 began to show as dense, more opaque tissues within the interior of the polyp, 
 but their relative development could not be followed in the living polyp. 
 
 Towards the end of the fourth week some of the polyps presented 
 rudiments of the second-cycle mesenteries. Their first appearance externally 
 was as two narrow lines along the column wall, towards its aboral termina- 
 tion, and within the dorsal exocoele of the right and left sides. A few days 
 afterwards these were followed by a similar pair of lines within each of the 
 two middle exocceles, and still later by a pair within each ventral exocoele 
 (plate 3, fig. 14). The actual time of appearance of the dorsal pairs, and 
 also the intervals between the three sets, varied somewhat in different polyps ; 
 
8o 
 
 SIDERASTREA RADIANS. 
 
 the periods here given are for the most forward specimens. The six pairs 
 together constituted the second cycle of mesenteries or first cycle of metac- 
 nemes, the different pairs diminishing in size from the dorsal to the ventral 
 aspect of the polyp. Their order of appearance and comparative sizes are 
 diagrammatically shown in fig. 8 {d-f). 
 
 The interval between the appearance of the dorsal and middle, and 
 between the middle and ventral pairs, was usually well marked, so that at 
 
 Fig. 8 (<T, b, c, d). — Series of diagrams illustrating the order of development of the 
 first three cycles of mesenteries, a-c represent the order for the first cycle (protoc- 
 nemes), d, the first stage of the second cycle (metacnemes). 
 
 any time the different phases could be easily ob- 
 served. About four weeks elapsed between the 
 appearance of the first or dorsal pairs and that of 
 the third or ventral pairs. Usually, the pairs on 
 opposite sides of the polyp would arise simulta- 
 neously, but in one instance the left pair in the 
 ventral exocoele was apparent for over a week in 
 advance of the right pair in the corresponding 
 exocoele. In every instance the two members of a 
 unilateral pair appeared simultaneously and developed equally. 
 
 The metacnemes increased slowly in length, and reached the aboral 
 termination of the column long before they extended to the distal extremity. 
 For many weeks the pairs differed in their vertical extent corresponding 
 with their dorso-ventral appearance ; even at the close of the observations the 
 distinctions were strongly indicated, giving to the polyp a bilateral symmetry 
 (plate 3, fig. 17). After the third month they began to extend across the 
 oral disc, the dorsal pairs being first recognizable, then the middle, and later 
 the ventral. They never, however, reached the stomodaeum, this condition 
 being also characteristic of the adult polyp. 
 
POSTLARVAL DEVELOPMENT. 
 
 8l 
 
 A dorso-ventral sequence in the development of the six pairs of second- 
 cycle mesenteries, as compared with their simultaneous appearance, has long 
 been known in actinians ; in fact, from the time of the Dixons' account of 
 
 Illb 
 
 g 
 
 Fig. 8 (*,/, z, h), continued.-Series of diagrams illustrating the order of deyelopment of the first three cycles of mesenteries. 
 * and/- represent the second and third stages for second cycle, and / and h are two early stages of third cycle. 
 
 the relative sizes of the pairs in certain late larvae of Bunodes. Apparently, 
 however, the present is the first occasion on which the actual appearance has 
 been followed stage by stage in the living polyp. 
 
 The developmental stage now reached is one which frequently recurs 
 in zoantharian studies. Of the protocnemes, the eight Edwardsian members 
 
82 
 
 SIDERASTREA RADIANS. 
 
 are complete and the fifth and sixth pairs are incomplete; the six pairs 
 of second-cycle mesenteries are incomplete, and either eqnal or show by 
 differences in sizes their dorso-ventral succession. The stage may be found 
 among adult Edwardsias, though two or more pairs of second-cycle mesen- 
 teries may here be wanting ; adult members of the Ilyanthidae often present 
 it, and probably all higher actinians in the course of their development. It 
 is repeated by the many pelagic anthozoan larvae described by Van Beneden 
 (1897, p. 189), numbered viii to xv, and which the author regards as belong- 
 ing to the Hexactiniaria. 
 
 Fig. 8 (},k), continued. — Series of diagrams illustrating the order of development of the first three cycles of mesenteries. 
 j is a late stage, and k the complete stage in the development of the third cycle. , 
 
 A long interval of time, and a fundamental difference in their manner 
 of appearance, are found to separate the second-cycle mesenteries from those 
 of the first cycle. The primary mesenteries appear in bilateral pairs first 
 towards one aspect of the polyp and then towards the other aspect, and so 
 on, and only later constitute unilateral pairs. The secondary mesenteries, 
 on the other hand, are in unilateral, isocnemic pairs from the beginning, 
 and arise within the six primary exoccelic chambers in a regular dorso- 
 ventral succession, alternating with the primary pairs. Though presenting 
 such a difference in development, the two series agree in each forming a 
 cycle or order of six isocnemic pairs when mature. 
 
 It was to emphasize these fundamental differences in origin between the 
 primary mesenteries and the later members that, in a former paper, I termed 
 
POSTLARVAL DEVELOPMENT. 83 
 
 the first cycle of six pairs of mesenteries " protocnemes," and all subsequent 
 mesenteries " metacnemes," whether becoming arranged in cycles or not. 
 The protocnemic stage is practically the same for all the different groups of 
 Actiniaria and Madreporaria. It is in the metacnemic stage that distinguish- 
 ing characteristics are introduced, development in all cases proceeding in a 
 manner altogether different from that of the protocnemic stage. The interval 
 is thus one of the greatest significance in the phylogeny of the Anthozoa, as 
 well as in the ontogeny of the individual polyp. 
 
 The metacnemic development here established for Siderastrea is that 
 most usual for ordinary anemones and corals ; new isocnemic pairs appear in 
 the six primary exocoeles all round the polypal wall. In the group of the 
 Zoantheae the metacnemes appear at only two restricted regions, one within 
 the exocoele on each side of the ventral directives, and each unilateral pair 
 consists of a large and a small member (anisocnemic). In the Ceriantheae 
 the metacnemes appear as bilateral pairs at only one region, within what 
 seems to be the entocoele of the ventral directives. Furthermore, the metac- 
 nemes in the Zoantheae and Ceriantheae do not form one or more cycles 
 distinct from the protocnemes (monocyclic), as in Siderastrea^ most other 
 modern corals, and ordinary anemones (poly cyclic). 
 
 These differences in the origin of the metacnemes separate in the clearest 
 manner the zoanthids and cerianthids from ordinary actinians and corals. 
 On the other hand, the metacnemic similarity in actinians and madreporar- 
 ians proves that the two groups are much more closely allied to one another 
 than to any other group ; the only important difference between the two 
 consists in the presence or absence of a calcareous skeleton. 
 
 In general, it will be found that in the Actiniaria and Madreporaria the 
 organs as a whole beyond the protocnemic stage develop successively from one 
 border of the polyp to the other. The adult cyclic arrangement is clearly a 
 later modification of a primary dorso-ventral plan ; in some instances, however, 
 the cyclic tendency strikes back, as it were, to the first appearance of certain 
 of the organs, for the prototentacles and protosepta usually arise a cycle at 
 a time. The second-cycle septa in S. radians are also interesting in this 
 respect, for in some pol3'ps the six members of the cycle appeared simulta- 
 neously, but in others in successive pairs (p. 87). 
 
 THIRD CYCLE OF MESENTERIES. 
 
 The polyps reared from the larva were not kept alive beyond the com- 
 pletion of the first and second cycles of mesenteries ; hence for what follows 
 as to the order of appearance of the third cycle recourse will be had to 
 
§4 SIDERASTRKA RADIANS. 
 
 colonial polyps arising asexually as buds. Fortunately, in any colony, 
 individuals of various sizes occur, and thus all the stages in the growth of the 
 third cycle can be secured. As already traced, the mesenteries consist of 
 six equal pairs of protocnemes all united with the stomodaeum, the fifth and 
 sixth pairs having become complete, and six smaller, incomplete pairs of 
 metacnemes, alternating with the former. Following the laws of hexactinian 
 cyclic symmetry the third cycle of mesenteries should consist of twelve equal 
 pairs situated within the exocceles formed by the first and second cycles ; 
 the problem is to determine their order of appearance. 
 
 The earliest stage obtained in the formation of the third cycle of mesen- 
 teries is diagrammatically represented in fig. 8, ^, p. 8i, taken from one 
 of the small bud polyps of a colony. In addition to the primary and second- 
 ary cycles an isocnemic pair of mesenteries has appeared on each side of the 
 median axis, in the exocoele between the dorsal directives and the dorsal pair 
 of second-cycle mesenteries. Such an early stage would be expected on the 
 dorso-ventral succession already established for the second cycle. The 
 chamber within which the next pair of mesenteries will arise is, however, 
 one of much significance. The succeeding exocoelic chamber on each side 
 is between the dorsal pair of second-cycle mesenteries and the dorso-lateral 
 pair of first-cycle mesenteries, and it might be supposed that the new mesen- 
 teries would occupy the exocceles in regular succession, from one border of 
 the polyp to the other. Instead of this it is found that the pairs arise succes- 
 sively within only the dorsal of the two exocceles of each system. This 
 condition is shown in fig. 8, ^, p. 8i, the next stage available, where a third- 
 cycle pair (hi) is found within the dorsal exocoele of each of the six systems. 
 
 A later stage secured in the establishment of the twelve third-cycle 
 mesenteries is given in fig. 8,y, p. 82, where an additional pair (iii, d) has 
 appeared, this time within the ventral exocoele of the two dorsal systems. 
 Clearly, if the succession here indicated were followed with perfect regu- 
 larity, other pairs would appear within the ventral exocceles of the middle 
 and ventral systems, and the cycle would then be completed according to 
 fig. 8, k^ p. 82. No stage exactly corresponding with this figure, however, 
 has been obtained, as the polyps of S. radians very rarely, if ever, complete 
 the third cycle of mesenteries. 
 
 The stages above presented prove that in the establishment of the third 
 cycle of mesenteries the dorso-ventral succession is twofold : First, a series 
 of six pairs within the dorsal exocceles, and then a similar series within the 
 ventral exocceles of each system. 
 
 The regularity in the sequence, represented by fig. 8, g-k^ was secured 
 
POSTLARVAL DEVELOPMENT. 85 
 
 only after examination of a number of polyps. In a colony in which the 
 polyps are so closely arranged as in S. radians it is found that the individuals 
 rarely undergo their later development with perfect regularity all round — 
 some regions will be in advance of the normal sequence and others behind. 
 The polygonal form assumed by the adults is evidence that pressure is 
 exerted upon a form which otherwise would be circular, as in simple polyps 
 reared from larvae. Spatial difficulties may therefore be held sufficient to 
 account in a large degree for the many irregularities obtained in the estab- 
 lishment of the third mesenterial cycle. 
 
 The mesenterial plan of two other polyps is given in figs. 3 (p. 26) and 
 1 1 (p. 100), and illustrates the variability encountered. In fig. 3 the sequence 
 is normal except that a pair of mesenteries (iii^) has appeared within the 
 ventral exoccelic chamber of the right middle system in advance of the pairs 
 in the dorsal exocoeles of the ventral systems. The polyp represented in 
 fig. 1 1 presents many departures from the normal regularity ; two third-cycle 
 pairs occur within three systems, one pair within another, and two systems 
 are without any third-cycle pairs. 
 
 In Astrangia solitaria and Phyllangia americana^ where the polyps 
 are practically separated from one another and retain their cylindrical form 
 throughout, the regularity of development all round is more pronounced, 
 and the general order of appearance of the mesenteries established in S, 
 radians is found to be maintained from beginning to end (1902, p. 459). 
 
 Hitherto, the development of the third-cycle mesenteries has not been 
 actually followed either in actinians or corals. Faurot's studies in 1895 were 
 confined mostly to the tentacles. Carlgren* has described a condition of the 
 mesenteries in the actinian Condylactis cruetttata^ in which the twelve pairs 
 of third-cycle mesenteries as a whole, as well as the exoccelic chambers in 
 which they are situated, show a gradual decrease in size in passing from the 
 dorsal to the ventral border of the polyp. Pairs \\\a and iwd in the enumera- 
 tion of fig. 8, k^ p. 82, were larger than pairs \\\b and iii^, and these than 
 pairs iwc and iii/; hence, if the condition obtained by Carlgren really repre- 
 sents the sequence followed in the growth of the third-cycle mesenteries in 
 Condylactis^ it is altogether different from that of corals, being simple instead 
 of twofold. 
 
 I believe it will be found in corals generally that the sequence of the 
 later mesenteries is by no means so regular as that of the earlier cycles. 
 The order followed by the organs in the first and second cycles is fairly con- 
 stant, but this can not be asserted of the third cycle, and probably the regularity 
 
 * "Zur Mesenterienentwicklung der Aktinien," Ofvers af R. vet.-Akad. Forh., Stockholm, 1897. 
 
86 SIDERASTREA RADIANS. 
 
 will be even less in still higher cycles. As the polyps increase in size the 
 forces of growth are less likely to act with the regularity and uniformity 
 which they do in the earlier stages when the polyps are small ; even though 
 in the end the cycles obtain hexameral completion it will be brought about 
 with much individual variation. 
 
 No polyps of S. radians having mesenteries belonging to a fourth cycle 
 have been found, and nothing is yet known as to the normal sequence 
 according to which the cycle is established in other species. 
 
 The chief facts concerning the mesenterial sequence in S. radians may 
 be now summarized : 
 
 1. The six pairs of first-cycle mesenteries (protocnemes) arise as bilat- 
 eral pairs in a regular alternation from one aspect of the polyp to the other. 
 The first four pairs early unite with the stomodseum, but the two last pairs 
 (v, vi) remain free for a long period. Later, the second and fifth, and the 
 first and sixth mesenteries, on each side, form isocnemic pairs, and the third 
 and fourth pairs constitute the directives. 
 
 2. The six pairs of second-cycle mesenteries arise bilaterally as uni- 
 lateral isocnemic pairs on each side of the polyp, and appear successively 
 in the primary exocoeles from the dorsal to the ventral border ; ultimately 
 they become equal and exhibit perfect radial symmetry. 
 
 3: The twelve pairs of third-cycle mesenteries also arise bilaterally as 
 unilateral isocnemic pairs on each side. Normally six pairs appear in a 
 successive manner from the dorsal to the ventral aspect of the polyp, a pair 
 within the dorsal exoccele of each sextant ; then other six pairs appear in 
 the same succession, a pair within the ventral exocoele of each sextant. 
 Generally some of the pairs of third-cycle mesenteries are wanting in mature 
 polyps. 
 
 CORALLUM. 
 
 FIRST CYCLE OF ENTOSEPTA AND SECOND CYCLE OF EXOSEPTA. 
 
 Three or four days after fixation of the larva the skeleton was first 
 observed through the transparent tissues of the living polyp in the form of 
 six small radiating septal upgrowths, practically equal in size. At the 
 same time a narrow peripheral calcareous ring was seen, its outer surface 
 uncovered by the polypal tissues (plate i, fig. 7). The six septa were per- 
 fectly free from one another and from the outer annulus, and arranged at 
 equal distances apart within the six entoccelic chambers, thus alternating 
 with the cycle of six exocoelic tentacles first to arise. Each septum appeared 
 as a somewhat spindle-shaped bar with the upper edge strongly spinous and 
 the lower edge flat and adherent to the glass to which the polyp was affixed. 
 
POSTLARVAL DEVELOPMENT. S7 
 
 A day or two after the formation of the first cycle of entosepta, the six 
 exoccelic septa began to make their appearance, in some cases simultane- 
 ously, but in others in successive bilateral pairs from the dorsal to the 
 ventral border of the polyp (plate 2, figs. 8, 9). Fig. 8 shows that a pair 
 of septa has appeared within the dorso-lateral exocoeles, mere rudiments of 
 septa are found in the middle exocoeles, and as yet there is no indication of 
 the ventro-lateral exoccelic pair. For a long time, as shown by the coralla 
 on plates 4 and 5, the dorso-lateral pair is better developed than the middle 
 pair, and the middle than the ventro-lateral pair. The ventral pair in nearly 
 all cases remained conspicuously smaller than the other pairs. Where, how- 
 ever, the second cycle is fully developed (fig. 9), its six members are disposed 
 midway in the six interspaces between the members of the primary cycle, 
 and remain shorter than the latter. As in the entosepta the surface is spinous 
 along the edge and over both lateral faces. 
 
 Thus, within the first week two complete cycles of septa (protosepta) 
 were developed — a primary cycle, consisting of six equal entosepta, and a 
 secondary cycle of six smaller exosepta, the latter having appeared later and 
 diminishing in size from the dorsal to the ventral border. A narrow periph- 
 eral calcareous ring, unconnected with the septa, was also formed at the 
 same time. This is shown by later observations to be a marginal continuation 
 or upgrowth of the basal plate (p. 115), and therefore to be regarded as an 
 epitheca. Only the Bdwardsian mesenteries were united with the stomodasum, 
 and of the tentacles the six exoccelic members alone were developed. 
 
 The order of appearance of the twelve protosepta is thus in marked con- 
 trast with that of the twelve protocnemes with which they are associated. 
 The latter have been found to arise in bilateral pairs, first towards one aspect 
 of the polyp and then towards the other, and the six pairs (four macrocnemic 
 and two microcnemic) are fully established before any of the septa arise. 
 The six entosepta, on the other hand, appear simultaneously ; and such is 
 usually the case with the six exosepta in other corals, though not in 
 Siderastrea. 
 
 The symmetrical growth of the skeletal structures, represented on plate 
 2, figs. 9 and 10, generally took place only in completely isolated polyps, 
 free to develop equally all round, and even in these irregularities were some- 
 times introduced. Among the young polyps forming the aggregated minia- 
 ture colony in fig. 5, p. 60, the septal development was scarcely alike in 
 any two. Where, as at the two extremities of the colony, one polyp partly 
 overfolds another, only half the number of septa occurs, while in the others 
 the alternation of large and small septa is inconstant ; further, the epithecal 
 
88 
 
 SIDERASTREA RADIANS. 
 
 wall of two contiguous polyps is common along the lines of adherence, and 
 the outline, instead of being circular, becomes more or less polygonal as a 
 result of the mutual pressure. 
 
 The protoseptal stage was completed in nearly all cases within the first 
 
 Fig. 9 (a-*).— Series of diagrammatic figures illustrating the manner of development of the septa from one to three cycle* 
 {cf. plates 1-5). The epitheca is represented as a white circle distinct from the septa. 
 
 fortnight, and some of the smaller polyps never reached beyond ; indeed, in 
 a few examples, the exocoelic cycle was never completed. Much variation 
 was observable as to the rate at which the corallum was laid down, growth 
 in larger polyps being always in advance of that in smaller individuals. 
 
POSTLARVAL DEVELOPMENT. 
 
 89 
 
 Within the larger, more vigorous polyps, further calcareous upgrowths 
 began to be formed peripherally during the course of the third week, some 
 appearing as angulated extensions of the primary septa and others wholly 
 independent of them (plate 2, fig. 12). Some of the isolated skeletal elements 
 
 Fig. 9 K/-j), continued. — Series of diagrammatic figures illustrating tlie manner of development of the septa from one to three 
 cycles (cf. plates 1-5). The epitheca is shown uniting the peripheral ends of the septa, 
 
 seemed like short additional septa, situated on different radii from the septa 
 first formed. The new members either retained their independence for a 
 long period or early became fused with the original septum in whose inter- 
 space they had appeared. In this latter instance the original septum was 
 
90 SIDERASTREA RADIANS. 
 
 distinctly angulated at its peripheral end, and wliere two additional elements 
 were introduced in each chamber the periphery of the septum was strongly 
 bifurcated. In some few cases no separate fragments w^hatever would arise 
 within an interspace. The septum would then retain its original bar-like 
 form, but become longer and usually more rugged in outline, showing that 
 skeletal matter was being added. The deposition of new matter was usually 
 more forward in the axial chambers than in the lateral, but scarcely any 
 two polyps were alike in the detailed appearance assumed by the skeleton 
 at this period. 
 
 Figs. 12, 19, and 20, on plates 2 and 4, represent the actual polyp or coral- 
 lum during this stage, while fig. 9, d^ p. 88, is an attempt to represent diagram- 
 matically the septal conditions at its close. It is a well-defined phase, and 
 important as showing the diflferent methods by which the septa may increase 
 in length and complexity. Although, as above remarked, no two polyps were 
 exactly the same, a general plan was determinable throughout the many 
 examples studied. The septa in all but the pair of ventral exosepta con- 
 sist of a simple, more central portion and the peripheral additions. The 
 former represents the enlarged primary septa seen in fig. 9, r, p. %^^ while 
 the latter are newer formations. In general the peripheral deposits consist 
 of two or more separate fragments, placed at an angle with the central bar, 
 the angle being greater in the exosepta than in the entosepta ; the entocoelic 
 additions are, in fact, nearly parallel with the primary entoseptum, and thus 
 more strictly radial. The two axial or directive septa .are usually the most 
 strongly developed of the entire series, and remained thus to the end ; in 
 addition, the dorsal and the ventral frequently differ much in form from 
 one another. 
 
 In all the figures it will be seen that the two ventrolateral exosepta 
 remain simple, enlarging but little beyond their primary condition, and 
 the middle exosepta are likewise somewhat less developed than the dorso- 
 lateral. Thus a decided dorso-ventrality is indicated in the growth of the 
 peripheral elements of the skeleton, as was the case with the simple exosepta. 
 
 For many weeks afterwards the only alteration in the septa consisted 
 in increased growth along the plan thus laid down. As the peripheral frag- 
 ments enlarged, the members of any group became fused with one another. 
 After the second month the development of the skeleton within the living 
 polyp could with difficulty be followed, owing to the complexity of the 
 internal tissues. Therefore, from this stage onward the septa will be studied 
 mainly from macerated corallites. 
 
 Fig. 23, plate 4, is from a photograph of the skeleton of a polyp ten 
 
POSTLARVAL DEVELOPMB:nT. 9 1 
 
 weeks old, iu which only six pairs of primary mesenteries and two cycles of 
 tentacles were present, while fig. 9, e^ p. 88, is its diagrammatic representa- 
 tion. Bach septum now bears a closer resemblance to the septa in the 
 mature cor alii te, being narrow centrally and broad peripherally, with spinous 
 projections over the entire surface. The general relationships of the septa 
 to the mesenterial pairs could be made out before maceration ; hence there is 
 no uncertainty as to the orientation of the corallum. The dorsal and ventral 
 directive entosepta present a somewhat bifurcated peripheral extremity, the 
 two limbs being nearly parallel, but the four lateral entosepta are simple 
 bars, with thickened peripheral ends formed from the enlargement and 
 complete ifusion of the originally separate elements. On the other hand, 
 the dorso-lateral and middle pairs of exosepta are strongly bifurcated periph- 
 erally, while the membersof the ventral pair retain their simple character, and 
 are by far the smallest of the six pairs. The peripheral limbs in the bifurcated 
 exosepta may be quite separate from the single radial piece, though usually 
 they are joined. It is readily seen how by the enlargement and fusion of the 
 many detached fragments within each mesenterial space of plate 2, fig. 12, 
 such a septal condition as that of plate 4, fig. 23, has been obtained. 
 
 With the increase in thickness of the septa the interseptal loculi are 
 correspondingly diminished, but no actual synapticular unions are yet formed 
 across the interspaces. The dorso-lateral exosepta bend laterally and nearly 
 fuse centrally with the dorsal directive septum ; the middle exoseptum on 
 each side approaches the corresponding dorso-lateral entoseptum and fuses 
 with it, while the small ventro-lateral exoseptum on each side is fused with 
 the ventro-lateral entoseptum (fig. 9, <?, p. 88). 
 
 Much older corallites, but still at nearly the same stage of development, 
 are shown on plate 4 (fig. 24) and on plate 5 (figs. 25-27), and diagrammati- 
 cally by fig. 9,/, p. 89. The dorso-lateral exosepta are now fused with the 
 dorsal directive septum, the middle exosepta with two dorso-lateral entosepta, 
 and the ventro-lateral exosepta with the ventro-lateral entosepta. The exo- 
 septa, generally, are not so strongly bifurcated as in the corallite on plate 4, 
 fig. 23 (fig. 9, <?, p. 88). Fig. 10, a-d^ on p. 96, shows the diagrammatic rela- 
 tionships of the septa to the mesenteries throughout these early stages. 
 
 The general impression produced by the corallum at this stage, as seen 
 under a simple lens or low power of the microscope, is that of two alternating 
 cycles of septa — a larger and a smaller. The tendency to fusion of adjacent 
 septa, exosepta with entosepta, which was found to be so marked a feature 
 of the adult corallite, is already exhibited by the two cycles, and gives a 
 bilateral symmetry to the calice. 
 
92 SIDERASTREA RADIANS. 
 
 Four distinct stages in the development of the protosepta of Siderastrea 
 radians are thus recognizable : 
 
 1. The simultaneous appearance of six equal entosepta, a few days after 
 the larva settles. A very narrow epitheca, distinct from the septa, appears 
 about the same time. 
 
 2. The appearance several days later, either simultaneously or in a 
 dorso-ventral sequence, of six exosepta which are smaller than the entosepta 
 and alternate with them. The septa of both cycles are simple, spinous 
 wedge-shaped upgrowths from the basal plate. 
 
 3. The appearance towards the periphery of most of the septa of one or 
 more short septum-like bars or skeletal nodules. 
 
 4. The fusion of these detached fragments with the main septa, so as 
 to give rise to a broad peripheral termination which may be either simple 
 or bifurcated ; also the fusion of the exosepta with the entosepta by their 
 inner extremity. 
 
 A distinct dorso-ventrality in the rate of growth is maintained from the 
 second stage onwards, particularly with regard to the exosepta, thereby giving 
 a bilateral symmetry to the calice. 
 
 The protoseptal development of Siderastrea presents a general agree- 
 ment with that of other corals whose early history has been followed. 
 Lacaze-Duthiers (1873, ^^97), however, found that in Astroides calycularis^ 
 Balanophyllia regia^ Leptopsammia^ and Cladopsammia the six exosepta 
 appeared simultaneously with the six entosepta ; but in Caryophyllia cyathus 
 and others there is an interval between the two cycles as in Siderastrea^ 
 where;as in Manicina areolata^ as I have shown (1902, p. 491), it appears 
 to be doubtful whether exosepta ever appear. Lacaze-Duthiers represents a 
 a decided bilateral condition in the early development of the skeleton in 
 Astroides (1873, plate xiv, fig. 29), but in other known cases the septa of 
 each cycle appear simultaneously, and are equal from the beginning. 
 
 The simultaneous appearance of the members of one or both cycles of 
 protosepta and also of the prototentacles may be compared with the succes- 
 sive appearance of the pairs of protocnemes. The protosepta and prototen- 
 tacles resemble one another in that both appear a cycle at a time, and from 
 the beginning exhibit radial symmetry, whereas the protocnemes arise in 
 bilateral pairs according to a well-defined succession, and, for a time, display 
 a strong dorso-ventrality. The simultaneous appearance of all the members 
 of a cycle is maintained for the septa only so far as the first cycle or, at most, 
 the second cycle of the protosepta ; in the later growth of the organs a dorso- 
 ventral sequence is followed, quite as conspicuous as that of the mesenteries. 
 
POSTLARVAL DEVELOPMENT. 93 
 
 When it is recalled that the twelve primary mesenteries and their cham- 
 bers are fully established before the tentacles and septa begin to make their 
 appearance it can be understood how these latter organs may arise a cycle 
 at a time. In their development beyond the protocnemic stage the mesen- 
 teries, tentacles, and septa follow one another very closely, and such would 
 probably be the case were the different pairs of protocnemes closely succeeded 
 by their tentacles and septa.* The organs as a whole in the Zoantharia are 
 unquestionably to be regarded as developing in a bilateral dorso-ventral 
 order, not in a cyclic manner. Hitherto the polyps in Siderastrea are alone 
 in the throwing back of the dorso-ventral ity of the skeleton as far as the 
 exocoelic protosepta. 
 
 The forked or bifurcated continuation of the protosepta, produced either 
 by continued peripheral growth or by the production of independent nodules, 
 has been observed both in Astroides calycularis and in Caryophvllia cyathus^ 
 in addition to the present species. Both Lacaze-Duthiers (1873, 1897) and 
 Von Koch (1882) give figures of the developing corallum which show that 
 the septa are prolonged peripherally much in the same way as in Siderastrea. 
 Von Koch at first considered that the forkings were concerned in the for- 
 mation of the theca by the union of those from adjacent septa, but in 
 Caryophyllia he found (1897) the true theca (Mauer) to arise independently. 
 In Siderastrea the bifurcations are found to be merely peripheral extensions 
 of the septa, becoming fused with the simple septum in the case of the 
 entosepta, but constituting two new septa in the case of the exosepta. They 
 in no way assist in the formation of a theca. 
 
 SECOND CYCLE OF ENTOSEPTA AND THIRD CYCLE OF EXOSEPTA. 
 
 Before describing the further development of the septa it will be helpful 
 to consider what are the septo-mesenterial relationships involved in passing 
 from a polyp with only two cycles of septa to one with three cycles. In the 
 first polyp only six pairs of mesenteries are present, and within the ento- 
 coeles of these are the six primary entosepta ; the alternating six members 
 of the second cycle of septa are within the exocoeles, on radii midway 
 between the entosepta (plate 2, fig. 9). In the second polyp with three cycles 
 of septa two alternating cycles of mesenteries are present ; the first-cycle 
 septa are contained within the entocceles of the first mesenterial cycle, the 
 second-cycle septa are within the entocoeles of the second mesenterial cycle, 
 
 ♦Where in larval actinians only four pairs of mesenteries are present when the tentacles begin to 
 develop. oxAy eight of the latter appear, one from each of the eight mesenterial chambers, and the 
 other four necessary to complete the hexameral plan arise after the establishment of the fifth and sixth 
 pairs of protocnemes. 
 
94 SIDERASTREA RADIANS.- 
 
 while the third-cycle septa are within the twelve alternating exocoeles, and 
 are therefore exosepta. Thus at both stages the exosepta form the outer- 
 most cycle {cf. fig. lo, b^ f, p. 96). The question naturally arises as to 
 whether, on the appearance of a second cycle of mesenteries, the exosepta 
 of the first stage, which there constitute the second cycle, remain as the 
 second cycle of entosepta of the later stage ; were they to do so the twelve 
 exosepta of the latter would be the only new formations. As regards their 
 actual position the six exosepta of the early polyp correspond with the 
 six entosepta of the later polyp, and it would be natural to assume that on 
 the appearance of the second-cycle mesenteries the latter have simply included 
 within their entocoeles the exosepta of a former stage, and then new exo- 
 septa have arisen within the newly formed exocoeles. 
 
 The latter is the view commonly held by writers on coral development. 
 Thus Delage & Herouard, in their "Traite de Zoologie Concrete" (1901, p. 
 558), remark : " Quand, dans les interloges occupees par les septes du dernier 
 cycle, nait un nouveau cycle de couples de cloisons, celles-ci se forment de 
 part et d'autre du septa interloculaire qui, de ce fait, devient loculaire, et 
 bientot un nouveau cycle de septes se forme dans les nouvelles interloges 
 qui viennent d'etre formees. Les cj^cles naissent successivement et jamais 
 un cycle ne commence a se former avant que le precedent soit complet." 
 Similarly, J. Stanley Gardiner (1902), in his account of the anatomy of 
 Flabellum rubrum^ says (p. 133, italicizing added) : "As the growth of any 
 corallite proceeds, more and more septa up to six cycles appear. The former 
 exocoslic order of septa becofne entocoelic by the development of new pairs of 
 mesenteries. The increase of mesenteries takes place pari passu with the 
 formation of new septa." 
 
 Unfortunately, the relationships involved in the above assertions have 
 not been actually followed, though from the known conditions no other 
 arrangement at first sight seems possible. The problem is one of the most 
 important in the developmental histor}^ of corals. 
 
 Without doubt the members of the primar}^ cycle of entosepta in adult 
 polyps are the direct representatives or continuations of the six septa first 
 to arise, just as the six pairs of complete mesenteries throughout the life of 
 the polyp are the representatives of the protocnemes. Throughout their 
 later growth the primary septa retain their individuality, and remain within 
 the entocceles of the primary cycle of mesenteries. Other considerations, 
 however, are involved in the relationships between the septa developing later 
 and those of the adult corallite. 
 
 Most of the polyps of Siderastrea remained for some time at the stage 
 
POSTLARVAL DEVELOPMENT. 95 
 
 already described on p. 92, the second-cycle mesenteries appearing in the 
 meantime and growing towards both the basal and the oral disc. As indi- 
 cated diagrammatically in fig. 10, d^ p. 96, the new mesenterial pairs corre- 
 spond with the space inclosed by the bifurcations of the dorso-lateral and 
 median pairs of exosepta, but seem as if about to embrace the incompletely 
 developed ventro-lateral pair. 
 
 About this time other calcareous upgrowths began to appear peripherally, 
 midway within the exocoelic bifurcations, and necessarily inclosed within 
 the entocceles of the second-C3^cle mesenteries. Plate 3, fig. 15, gives the 
 discal view of such a living polyp in which the six pairs of second-cycle 
 mesenteries had been developed for some time. The new mesenteries, still 
 varying in size in agreement with their order of appearance, have now 
 begun to extend along the periphery of the disc, and the latter is resting 
 upon the upper edges of the septa with the tentacles fully expanded. The 
 septa are clearly seen through the transparent disc. Within the bifurcation 
 of each dorso-lateral and middle exoseptum has appeared an additional free 
 septum, included within the entocoele of the second-cycle mesenteries, and 
 in the same radius as the original exoseptum. Moreover, the exocoelic 
 bifurcations are now seen to be wholly exocoelic in position, situated in the 
 chambers between the pairs of the first and the second-cycle mesenteries ; 
 in the dorso-lateral sextants the forkings are free from the original median 
 exoseptum, but in the middle sextants they are united. The primary ento- 
 septa remain simple straight bars, and such is yet the condition of the 
 ventro-lateral exosepta. Fig. 10, e^ p. 96, is a diagrammatic representation 
 of the polyp and corallum at this stage. 
 
 The new formations within the second-cycle entocoeles suggest an inde- 
 pendent series of septa, and subsequent stages show that they represent the 
 second-cycle entosepta. The polyp of plate 3, fig. 15, in fact, displays the 
 early stages in the development of the second cycle of entosepta and the 
 establishment of a third cycle of exosepta, and in such a manner that the 
 actual relationships between the new septa and the second-cycle mesenteries 
 admit of no misinterpretation. The entosepta of the second cycle appear 
 peripherally as separate formations, but in their later growth centrally, as 
 shown on plate 5, figs. 28, 29, and fig. 10,^ p. 96, they come into union with 
 the primary exosepta, and the two then appear as a single continuous structure. 
 Further, each bifurcation of a primary exoseptum forms a new exoseptum, 
 belonging to a third cycle, and may be either distinct or united by its in turned 
 edge with a second-cycle entoseptum. ^ 
 
96 
 
 SIDERASTREA RADIANS. 
 
 Three important results in the development of the Madreporarian 
 skeleton are thus gained: (i) The primary exosepta do not continue their 
 
 Fig. 10 (j»-f). — Series of diagrammatic figures illustrating the relationships of the mesenteries and septa in the establishment 
 of the first two cycles of mesenteries and three cycles of septa. 
 
 growth peripherall}'^ in a radial manner, and constitute entosepta by becoming 
 included within the entocoeleof the new second-cycle mesenteries. (2) The 
 
POSTLARVAL DEVELOPMENT. 97 
 
 secondary entosepta are new formations, arising within the second-cycle 
 entocoeles independently of other septa ; the second-cycle mesenteries never 
 embrace the exoccelic protosepta as such, but only after their fusion with 
 the new entosepta. (3) The appearance of the second-cycle entosepta is 
 in a dorso-ventral manner, not simultaneous, a cycle at a time. 
 
 Other coralla exhibit different stages, all pointing to the same conclu- 
 sion. That shown diagrammatically in fig. 9, y^, p. 89, is in somewhat the 
 same stage as fig. 10, e. In the upper right sextant the second-cycle 
 entoseptum (11) is yet very distinct from the exocoelic bifurcations, but an 
 irregularity in the rate of growth is exhibited by the corresponding left 
 sextant, in that no entoseptum has yet formed. The growth in the two 
 middle sextants is in advance of it ; each of the sextants contains a primary 
 exoseptum, a secondary entoseptum, and two third-cycle exosepta as distinct 
 formations. As before, the development within the two ventro-lateral sex- 
 tants lags behind that of the middle and dorso-lateral sextants. Fig. 9, g^ 
 from another corallum, also shows important intermediate stages. 
 
 The next stages available are the coralla of the oldest polyps reared, two 
 of which are represented on plate 5, figs. 28, 29, and diagrammatically in fig. 
 9, y, p. 89, and fig. 10,/, p. 96. Bach corallite now consists of three com- 
 plete orders or cycles of septa, the development within the two ventral 
 sextants having reached the same stage as that within the dorsal and middle 
 sextants. The first order contains six large septa radially disposed, the 
 directive septa a little larger than the others. The second order also 
 includes six septa, turned inwardly in such a manner that the two dorsal 
 fuse with the dorsal directive septum, the two middle fuse with the dorso- 
 laterals of the first order, and the two ventral with the ventro-lateral s of the 
 first order. The third cycle contains twelve septa, the two adjacent members 
 fusing with each of the six septa of the second order. The coalescence of 
 the septa in this way imparts a bilateral symmetry to the calice which other- 
 wise would be radial. Further, adjacent septa are now for the first time 
 joined by synapticula. The polyps (plate 3, fig. 17) forming the coralla bore 
 two cycles of mesenteries, so that the first and second orders of septa are 
 entosepta, while the members of the outermost cycle are exosepta. 
 
 Previous stages have demonstrated that the members of the second 
 order of septa, although now continuous bars, have really a twofold origin. 
 The peripheral part of each appeared as a separate septum within an entoccele 
 of the second-cycle mesenteries and, later continuing its growth centrally, 
 fused with an exoseptum which was originally a constituent of the second 
 cycle of septa, so that now the two appear as a single septum. In each 
 
98 SIDERASTREA RADIANS. 
 
 corallum there are indications still remaining of the distinctness of the 
 peripheral and central parts of the entosepta, as in the ventral sextants of 
 fig. 28, plate 5, where the entocoelic moiety is still free from the exoccelic 
 portion. The fusion of the two parts is a mechanical necessity in the 
 process of growth, seeing that both are on the same radii. It is manifest 
 that it is only by securing such intermediate stages that the true character 
 of the septa at the mature stage can be understood. When the secondary 
 entosepta have come into fusion with the primary exosepta there remains 
 no means of distinguishing their compound origin, and the ontogenetic and 
 phylogenetic significance of the secondary entosepta is obscured. 
 
 The twelve exosepta now forming the tertiary cycle undoubtedly arise as 
 continuations of the bifurcations of the six primary exosepta, but in 
 many places, as on plate 5, figs. 27 and 28, they still show a considerable 
 amount of distinctness. Regarding them as continuations of the two forks 
 of each primary exoseptum, it is manifest that they are to be considered 
 as appearing in advance of the entosepta of the second cycle, a relationship 
 which need not be wondered at, considering that in this species the exo- 
 tentacles are also found to arise in advance of the entotentacles. Originally 
 forming the second cycle, the exosepta now constitute the third septal cycle, 
 their place in the sequence being taken by the new second cycle of entosepta. 
 
 It is thus manifest that in the course of development of a coral the 
 exosepta of a former stage do not become the entosepta of a later stage ; 
 the latter are new formations appearing after the establishment of the 
 mesenteries with which they correspond, and consequently the mesenteries 
 and their included entosepta have the same ordinal value. The exosepta, 
 on the other hand, have no ordinal value ; they appear at each cyclic stage, 
 always constituting the outermost cycle. They are, in a measure, temporary 
 structures, predecessors of the entosepta, until the limit of growth of the 
 polyp is reached ; they serve as integral parts of the septal system of the 
 coral during all its intermediate stages, and are then overgrown by later 
 permanent septa.* 
 
 •In some corals the exoseptal predecessors appear to continue their independent growth in situ 
 without losing their individuality as skeletal structures in the central extension of the entosepta. In 
 these cases the entosepta do not grow far enough centrally to fuse completely with the exosepta already 
 there. I believe it will be found that this is the true nature oi pali which are found in some corals as 
 small septum-like plates in front of the larger septa. The fact that pali seem not to occur before the 
 primary cycle of six septa, but only before those of later origin, is what we should expect if this surmise 
 be correct. The primary entosepta have never had exoccelic predecessors, as is the case with the later 
 entosepta. The pali would thus represent the persistent exoccelic predecessors of the entosepta beyond 
 the primary cycle. 
 
POSTLARVAL DEVELOPMENT. 99 
 
 The closest morphological parallel is proved to exist between tlie develop- 
 ment of the septa and the tentacles. As previously shown, exotentacles are 
 present at each cyclic stage, but a new cycle of entotentacles intercalates 
 itself between the last cycle of entotentacles and the exotentacles, hence the 
 latter always remain as the outermost cycle ; only the entotentacles, like the 
 entosepta, have ordinal value. Thus the law of substitution first discovered 
 by Lacaze-Duthiers for the tentacles of hexactinians is found to hold for the 
 septa also. 
 
 Belonging to the soft, fleshy parts of the polyps, it can be easily under- 
 stood how actual displacement of the tentacles may be carried out, but such 
 is not possible with the hard, rigid skeleton. Hence the process of substitu- 
 tion must be conducted in a different manner in the two sets of structures. 
 A new peripheral entoseptum arising independently can not displace an inner 
 exoseptum already occupying the same radius. In its growth centrally the 
 entoseptum simply fuses with the exoseptum, and thenceforward the whole 
 of the septum must be morphologically regarded as an entoseptum. 
 
 The septa, like the tentacles, are thus shown to arise in such a manner 
 that it is impossible to determine their order of development from their rela- 
 tionships in the mature polyp. 
 
 Though the septo-mesenterial relationships will remain the same, I 
 conceive that a like adult condition of the septa may be reached in differ- 
 ent ways in different forms of corals, as is the case with the tentacles. 
 The actual method followed in Siderastrea can by no means be assumed to 
 be that characteristic of corals generally. Probably some of the stages in 
 Siderastrea might be better interpreted were results available from other 
 forms ; such, for instance, as the significance of the forking of the exosepta. 
 The fact that in Siderastrea the septa of one cycle fuse centrally with the septa 
 of the next inner cycle probably obscures the problem somewhat, as com- 
 pared with forms where the septa remain altogether free from one another. 
 
 THIRD CYCLE OF ENTOSEPTA AND FOURTH CYCLE OF EXOSEPTA. 
 
 None of the polyps reared from larvae were kept alive beyond the for- 
 mation of the first three cycles of septa, which consist of two cycles of 
 entosepta and one cycle of exosepta. Therefore the development of the 
 fourth cycle of septa must be studied from the bud polyps of a colony. 
 Fortunately, there are many polyps available for such an investigation, as 
 in any stock most of the individuals are at one stage or another towards the 
 establishment of a complete fourth cycle of 24 septa. 
 
lOO 
 
 SIDERASTREA RADIANS. 
 
 III2 
 
 nil 
 
 Ills 
 
 ni8 
 
 When describing the septo-mesenterial relationships in the mature 
 polyp, it was found that the fourth-cycle septa occur within the exocceles 
 between all the adjacent pairs of mesenteries. On leaving the last section, 
 however, the third cycle was constituted of exosepta ; only the first and 
 second cycles were entosepta ; therefore, as in the case of the secondary and 
 
 tertiary cycles, the problem is first 
 to determine the morphological re- 
 lationship of the tertiary exosepta 
 of the developing polyp to the 
 tertiary entosepta, and also to the 
 quaternary exosepta of the mature 
 polyp. Do the tertiary exosepta 
 at the stage where only second- 
 cycle mesenteries are present be- 
 come the tertiary entosepta when 
 the third-cycle mesenteries appear, 
 or are they continued as the 
 fourth-cycle exosepta, in which case 
 the tertiary entosepta arise de 
 novo ? 
 
 The relationships to be determined have been studied by means of 
 serial sections of nearly mature decalcified polyps. A complete series of 
 stages, representing the development of a pair of third-cycle mesenteries 
 along with the associated septa in their relation with the older mesenteries 
 and septa, is given on plate 9, figs. 54-60. The figures are taken from a 
 series of transverse sections of a retracted polyp, and will be described as 
 seen in passing from the lower stomodaeal region to the uppermost margin 
 of the calice and polyp. On account of the presence of synapticula and the 
 peripheral resorption of the mesenteries, certain complications are introduced 
 which render a clear conception of the stages somewhat more difiicult than 
 would otherwise be the case. The diagrammatic figures on p. 103 will assist 
 in following the description of the sections. 
 
 The polyp from which the drawings were made is that of which the 
 mesenterial plan is diagrammatically represented in fig. 11, p. 100. Within 
 the ventral system on the left side occurs a pair of small third-cycle mesenteries 
 (11I3), situated on the dorsal aspect of a pair of second-cycle mesenteries 
 (11). Plate 9, figs. 54-60, represents the members of this ventral system 
 at different levels, and the mesenteries and septa are indicated by the same 
 
 Fifi. II. — Diagram of mesenteries in the polyp from which figs. 
 54-60, on plate 9, were taken. 
 
POSTLARVAL DEVELOPMENT. lOI 
 
 lettering {a-j) throughout. In the figures mesentery a is the lower moiety 
 of the ventro-lateral pair of first-cycle mesenteries, mesentery/" is the left 
 moiety of the ventral directives, mesenteries d and e are the pair of second- 
 cycle mesenteries, while b and c are the two members of the rudimentary pair 
 of third-cycle mesenteries. The incompletely represented entoseptum to the 
 left of ^ is a member of the primary cycle of septa, as is also the partial 
 septum to the right of f\ the entoseptum d-e belongs to the secondary 
 cycle, while the septa a-d and <?-/, in figs. 54-56 are exosepta. The upper 
 margin of the section in fig. 54, is formed by the stomodaeal wall, while that 
 in all the subsequent figures is either the depressed disc or column wall. 
 This, upon retraction, has come to rest upon the septal edges, and adapts 
 itself to them in outline, being thrown into ridges and furrows. 
 
 Fig. 54 represents a section of the sextant taken from the lower stomodaeal 
 region. The members of the mesenterial pair, b^ c^ of fig. 11, have not yet 
 reached this level, so that the sextant contains only the pair of incomplete 
 second-cycle mesenteries, d^ e. The mesenteries a and y are united with the 
 stomodaeal wall centrally, but their peripheral extremity has undergone 
 resorption and, therefore, is free. At this level the second-cycle mesenteries, 
 d^ e^ are feebly represented, being much reduced peripherally as a result of 
 resorption. The entoseptum d-e is broad, but the exosepta a-d and e-f on 
 each side of it are narrow and partly turned towards it. Exoseptum a-d is 
 bifurcated at its peripheral end, but before dividing is connected by synap- 
 ticula with the entosepta on each side. Within the angle of bifurcation 
 occurs a small, empty loculus. 
 
 Plate 9, fig. 55, is from a section immediately above the level of the 
 depressed stomodaeum, so that it is bounded above by the disc resting upon 
 the septal edges. At this level mesenteries d and e are larger than in the 
 former section and are without any peripheral degeneration; exoseptum e-f\s 
 united by its inner edge with entoseptum d-e^ but the bifurcated exoseptum 
 a-d is free. Within the loculus, at the angle of bifurcation of the exoseptum, 
 the rudiments of the pair of mesenteries, b^ c, have appeared, but at this stage 
 there is no septal formation whatever within their entoccele. Already, there- 
 fore, it is manifest that the new pair of mesenteries does not inclose a pre- 
 viously formed exoseptum ; the two members lie close together within the 
 peripheral bifurcation of an exoseptum, a-d. 
 
 Plate 9, fig. 56, is taken from a section at a somewhat higher level. 
 The second- cycle mesentery d is now united with the depressed disc, but 
 the other moiety {e) of the pair is still free ; in none is there any peripheral 
 resorption. Exoseptum ^-y is once more distinct centrally from entoseptum 
 
I02 SIDERASTREA RADIANS. 
 
 d-e^ as in fig. 54 ; a synapticular growth from the left limb of septum a-d 
 perforates the mesentery b^ and a slight depression of the wall between the 
 mesenteries b and c is the first indication of an entocoelic skeletal ingrowth 
 about to separate the two. 
 
 The section from which fig. 57 was taken reveals important altera- 
 tions taking place. The second-cycle mesenteries d and e are both united 
 with the discal walls, and along with /"are at this level not perforated by 
 any synapticular bars. The two peripheral limbs of septum a-d are now 
 becoming free from one another at their inner angle, and also from the central 
 radial portion ; both may be regarded as distinct exosepta a-b and c-d^ though 
 formerly they appeared only as the bifurcations of the single exoseptum a-d. 
 For the first time an actual entoseptum now occurs in the entocoele of the 
 new pair of mesenteries b and ^, and at its free extremity is on the point of 
 uniting with the synapticular process from the exoseptum a-b. The septum 
 b-c is clearly a new formation, appearing within the now widely bifurcated 
 peripheral extremity of the original septum a-d^ or, better, between the two 
 exosepta a-b^ c-d. 
 
 The septal modifications suggested by fig. 57 are completed in the next 
 stage, fig. 58. The exoseptum c-d is now distinct, like septum a-b, from the 
 central radial part, while exoseptum a-b and entoseptum b-c are united with 
 one another towards their free edge by a synapticulum perforating the 
 mesentery b. 
 
 The section represented by fig. 59 passes through the exsert edges 
 of the septa, so that the column wall forms the lower boundary while 
 the upper boundary passes through the tentacular region of the disc, four 
 tentacular thickenings being represented. The mesenteries extend uninter- 
 ruptedly from one wall to the other, and include the free septal edges within 
 their chambers. The mesentery c is still free at its inner end, while the 
 mesentery b is perforated by the synapticulum still joining septa a-b and b-c. 
 The central portion of the original exoseptum a-d)i2iS now nearly disappeared. 
 
 The next stage, fig. 60, is from a section near the uppermost extremity 
 of the retracted polyp and its calice, so that now each septum and the body 
 wall surrounding it are becoming distinct. The mesenteries b and c both 
 extend across the two portions of the column wall, and the septa a-b, b-c, and 
 c-d are distinct from one another, the entoseptum b-c being as yet smaller 
 than the exosepta on each side of it. 
 
 Omitting the synapticula as merely incidental structures the develop- 
 ment of the three septa a-b, b-c, and c-d C2i.ii be diagrammatically represented 
 as in fig. 12 {a-f) below, which shows also their relations to the mesenteries. 
 
POSTLARVAL DEVELOPMENT. 
 
 103 
 
 The first two figures {a, b) correspond with plate 9, figs. 54-56, of the 
 sections. The exoseptum is here simple, but bifurcated towards its periph- 
 eral extremity. At first no mesenteries are included within the angle of 
 bifurcation, but a pair has appeared in fig. 12, b. In relation to the mesen- 
 teries the two limbs are clearly exocoelic, and may even at this stage be 
 regarded as two individual exosepta. 
 
 In fig. 12, c^ one exoseptum has become distinct, and a short septum has 
 appeared midway between the two older septa {cf. plate 9, fig. 57). Clearly 
 this is a new entoseptum arising a little later than the new pair of mesen- 
 teries. In fig. 12, d, the right exoseptum has also separated from the more 
 central portion which is along the same radius as the entoseptum {cf. plate 
 9, fig. 58). The next figure, e, shows the exsert septa (cf. plate 9, fig. 59); 
 the central part is becoming smaller, while it has disappeared in the last 
 
 rrm-SimfLUi 
 2. £ d. S. 
 
 Fig. 13. — Series of dtasrammatic figures illusirating figs. 54-63 on plate 9. 
 
 figure, / {cf. plate 9, fig. 60), The entoseptum throughout extends the 
 shortest vertical distance of the three, and so far as the development has pro- 
 ceeded remains the smallest radially. Clearly when growth is completed 
 the three septa will be related as shown in the six groups of fused septa in 
 fig. 10,/, p. 96, which is the usual relationship of the exosepta and entoseptum ; 
 the entoseptum in its further growth will have extended more centrally and 
 fused with the central portion of the original exoseptum, and the two exosepta 
 will turn inwardly to unite with it. 
 
 The series of sections plainly demonstrates that a new entoseptum 
 arises shortly after a new pair of mesenteries, not in advance of it, and the 
 two peripheral limbs of a bifurcated exoseptum become distinct, so that each 
 constitutes a new exoseptum. The new entoseptum takes the place of the 
 inner simple part of an exoseptum situated in the same radius. 
 
 The members of the third-cycle entosepta of the older polyp are new 
 formations which take the place of the earlier third-cycle exosepta. The 
 dorsal and ventral moieties of the bifurcated periphery of the third-cycle exo- 
 
I04 SIDERASTREA RADIANS. 
 
 septa become fourth-cycle exosepta ; in other words, the fourth-cycle exosepta 
 in the mature polyp are the peripheral continuations of the third-cycle exo- 
 septa, which in their turn have been shown to be the peripheral bifurcations 
 of the primary exosepta. Exosepta thus remain exosepta to the end, each 
 time constituting a later cycle as new entosepta arise to take their place. 
 
 The results from a study of a series of sections from a nearly mature 
 bud polyp thus agree stage by stage with those obtained in the progressive 
 development of the three cycles of septa in polyps reared from larva. The 
 figures given bear the closest comparison with the corresponding details in 
 fig. lo (d-f), p. 96, which represents three stages in the septal development 
 of a larval polyp. 
 
 Close examination with a lens of the surface of macerated coralla 
 often reveals one or more stages similar to the above. In practically all 
 the corallites of a colony the alternation of large and small (entoccelic and 
 exoccelic) septa is strongly marked ; but amongst the youngest corallites, 
 especially those around the margin of colonies, the regularity of the alter- 
 nating large and small septa is not always so pronounced. Occasionally a 
 group of two or three septa is seen presenting quite different relationships, 
 which can be explained only upon the septal development here described. 
 
 An exoseptum is sometimes seen with its peripheral end conspicuously 
 bifurcated, as in fig. 12, «, p. 103 ; in several instances a stage in which a 
 septum is beginning to appear midway between the bifurcation has been 
 met with, recalling the conditions in fig. 12, ^, d ; while very often in 
 a group of developing septa a marked interval occurs between the central 
 and the peripheral halves of an entoseptum, and the exosepta on each side 
 are wholly free. In these instances it would seem that the new entoseptum 
 has not yet fully united with the central part of the old exoseptum, being 
 in the same stage as fig. 12, d^ e. 
 
 It still remains to be seen what is the order followed in the appearance of 
 the third-cycle entosepta, for these do not appear a cycle at a time any more 
 than the six members of the second cycle of entosepta. The septa alone in 
 the dried corallum are insufiicient to enable their sequence to be made out, as 
 they afford no certain means by which the principal or directive axis can be 
 determined, and from this the dorsal and ventral borders of the calice. It 
 has been shown, however, that in the case of each of the three cycles of ento- 
 septa the mesenteries appear in pairs only a little in advance of the corre- 
 sponding entosepta within them ; therefore, if the sequence of the third-cycle 
 mesenteries be determined, it can be assumed that the third-cycle entosepta 
 follow the same order. 
 
POSTLARVAL DEVELOPMENT. 
 
 105 
 
 The order of appearance of the twelve pairs of third-cycle mesenteries 
 has been fully described on p. 83. It is found that the members arise after 
 the formation of the six second-cycle pairs as unilateral pairs in a twofold 
 succession. Normally a pair appears within the dorsal exocoele of each 
 sextant, successively from the dorsal to the ventral border of the polyp ; then 
 another pair appears within the ventral exocoele of each sextant, following 
 the same succession as the first series. 
 
 tiif X 1 % 
 
 Fig. 13. — Diagram illustrating the order of developmtnt of the first three cycles of septa (I-III). 
 The small letters accompanying the Roman numerals indicate the sequence of the indU 
 vidual septa in the cycle. 
 
 The normal sequence of the third-cycle mesenteries in Siderastrea 
 being established, we are justified in assuming that a like succession will be 
 maintained by the third-cycle entosepta, seeing that they arise shortly after 
 the mesenteries with which they are associated. This is shown in fig. 13, 
 p. 105, by the numerals iii^-iiyC The figure also represents the normal 
 sequence for all the entosepta of the first three cycles. The six primary 
 septa appear together, a cycle at a time ; the six members of the second order 
 arise bilaterally in a simple dorso-ventral succession ; the twelve members of 
 the third also arise bilaterally in a dorso-ventral succession, but in two 
 series — first, a series of six within the dorsal of the two interspaces in each 
 
Io6 SIDERASTREA RADIANS. 
 
 sextant, and then the remaining six in a like order, but within the ventral 
 of the two interspaces. The exosepta constituting the last outermost cycle 
 have no corresponding ordinal significance. 
 
 Studies on the mesenterial sequence of other corals indicate that a similar 
 septal succession will in all probability be followed by forms in which the 
 adult calice shows a regular hexameral cyclic plan. Individual departures 
 from the order may be expected, but are to be looked upon as irregularities ; 
 regularity of growth of the higher cycles is by no means so pronounced as 
 in the first and second cycles, which are less likely to be influenced by 
 spatial considerations. The sequence given is altogether different from any- 
 thing which has hitherto been surmised for any coral, and further studies are 
 desirable to determine how far it admits of general application in the group. 
 
 From what has been revealed it is manifest that the exosepta do not 
 possess any true ordinal sequence comparable with that of the entosepta. 
 Exosepta have been found to be present at each stage, always constituting 
 the outermost cycle, and equaling in number the sum of the inner entosepta. 
 We maj'' consider them as the direct continuations of the six primary exo- 
 septa, or, less likelj'', as arising anew with each cycle of entosepta. Regarded 
 as the persistent representatives of the primary exosepta, they more nearly 
 conform to the law of substitution of the exotentacles in actinians as estab- 
 lished by Lacaze-Duthiers and Faurot. In actinians generally it is found 
 that after the protocnemic stage the tentacles appear two at a time, one ento- 
 coelic and one exocoelic, corresponding with the two chambers formed upon 
 the appearance of a new pair of mesenteries ; sometimes the entotentacles 
 appear in advance of the exotentacles, the reverse of what happens in Side- 
 rastrea radians. The entotentacles are always larger than the exotentacles, 
 the length of the former being in accordance with the order of appearance of 
 the cycle to which they belong, the largest being the first to appear. The 
 exotentacles all attain an equal length and are all relegated to the outermost 
 cycle, whatever be the cycle of entotentacles with which they appeared. They 
 constitute a single cycle of which the members are always smaller than those 
 of the cycle of entotentacles last to appear, and the number of exotentacles 
 in the la^t cycle is always half the total number of tentacles, and, of course, 
 equal to the number of entotentacles. 
 
 As new entotentacles are added the exotentacles become pushed aside so 
 as to occupy different radii at different times. The calcareous septa being 
 hard, fixed structures, do not admit of such rearrangement ; the new septal 
 growth has to be adapted to the old, resulting in the fusion of the new ento- 
 septa with the old exosepta. 
 
POSTLARVAL DEVELOPMENT. I07 
 
 Partial studies on other corals, as well as considerations on the tentacular 
 development in actinians, suggest that the exosepta may arise in different ways 
 in different species, and that a more precise significance as to their relation- 
 ships at different stages ma}'" be forthcoming than is possible in Siderastrea, 
 There are indications that in some forms an entoseptum and an exoseptum 
 arise together, thus more closely recalling the method followed by the tenta- 
 cles. Regarded as arising anew in each cycle, the two exosepta in Siderastrea 
 appear somewhat in advance of the entoseptum which is included between them. 
 
 The relationships proved to exist between entosepta and exosepta involve 
 important considerations when the cyclic hexameral sequence is not completed 
 in the mature corallite, as almost invariably happens in S. radians^ as well 
 as in many other species of corals. As regards both the septa and mesen- 
 teries it is found that the last cycle is rarely a multiple of 6, but some 
 irregular number from i to 32, resulting from the fact that at maturity the 
 polyp does not complete the last cycle begun. Bxosepta have been shown 
 to appear always in close association with entosepta, whatever be the number 
 making up a corallite ; and, as often remarked, the two series are equal in 
 number and the exosepta always outermost in position. 
 
 If we regard a septal cycle as made up only of entosepta, or of exosepta, 
 then in mature corallites of 5. radians the third entoccelic cycle and fourth 
 exoccelic cycle of septa will vary in the same degree. Whatever number of 
 entosepta be lacking from the third cycle to form the complete cycle of 12, 
 a like number of exosepta will be wanting from the fourth cycle. 
 
 When describing the number of septal cycles within a calice, the cyclic 
 hexameral plan of which is incomplete, it is usual in systematic works on 
 corals to regard the hexameral multiples as completed as far as the number 
 of septa will permit, and then to relegate to the last cycle all the remaining 
 septa not included in the hexameral formula. The cycles are all supposed 
 to be hexamerously complete with the exception of the last. Thus, with 
 regard to S. radians^ Milne-Bdwards states : " Three cycles of septa com- 
 plete, and, in general, a variable number of a fourth cycle." Likewise Verrill 
 (1901, p. 153), describing the same species, says: "They [the septa] form 
 three complete cycles, with part of the fourth cycle developed, so that the 
 number is usually 36 to 40." 
 
 The relationships proved to exist between the entosepta and exosepta in- 
 dicate that the above formulae do not express the true morphological charac- 
 ter of the septa. Any hexameral incompletion in the number of septa mak- 
 ing up a corallite affects both the entosepta and the exosepta, that is, both 
 the penultimate and the last cycles. If any septa are wanting to complete 
 
I08 SIDERASTREA RADIANS. 
 
 the hexameral multiple of the last cycle of entosepta, the same number 
 will be lacking from the exosepta. The third complete cycle, as understood 
 by Milne-Bdwards and Verrill, is really made up of both tertiary entosepta 
 and of tertiary exosepta. The two kinds of septa are of very different value 
 in their development and relations to the polyp, and, as a matter of fact, 
 will be scarcely of the same thickness and radial length to justify their 
 being regarded as a cycle. 
 
 The cyclic formula, as above understood, may be written 6, 6, 12, x, 
 where x will represent any number from i to 24. Formulated in this way 
 the number 12 conveys the impression that the third cycle is really com- 
 pleted, and that all the additions made will belong to the next or fourth 
 cycle, whereas they will belong to both the third and fourth cycles. Beyond 
 the two first cycles the septa do not arise a cycle at a time, but the penulti- 
 mate and last cycles are formed concurrently, or almost so. Incomplete 
 cyclic hexamerism, as met with in S. radians, is an intermediate condition 
 in the establishment of two adult hexameral cycles, not of one alone, and 
 attention should be drawn to this in the septal formula. 
 
 According to the relationships now established the morphological septal 
 formula for S. radians should be written 6, 6, x, 6 + 6 + x. In this 
 formula, 6, 6, x will represent the number of septa in the two completed 
 entocycles, x being the number in the last entoseptal or penultimate cycle 
 which does not yet complete the hexameral sequence ; while 6 -|- 6 -|- x will 
 represent the total number of exosepta, x being the same number as before ; 
 some of the exosepta will be tertiaries and some will be quaternaries, the 
 number of the latter being always double the number of tertiary entosepta. 
 The formula for a corallite having 36 septa would, according to the ordinary 
 cyclic formula, be written 6, 6, 12, 12, whereas, considered as entosepta and 
 exosepta, the formula will be 6, 6, 6, 18, the three first numerals indicating 
 the entosepta and the last the exosepta ; the usual cyclic formula of a corallite 
 with 40 septa would be 6, 6, 12, 16, and the morphological formula 6, 6, 8, 20. 
 In the first case 12 of the exosepta will be quaternaries and 6 will be ter- 
 tiaries ; in the second 16 will be quaternaries and 4 tertiaries. 
 
 Where the relationships of the septa to the mesenteries are clearly known 
 the morphological formula will more nearly express the real value of the septa 
 than the ordinary cyclic formula ; the latter has little significance unless the 
 hexameral sequence is fully completed. One can not say that a cycle is 
 really complete unless its constituents all have the same morphological value, 
 which is not the case where some are entosepta and some are exosepta. 
 
 The bilaterality of the polyp during development may be looked upon 
 as associated in turn with each cycle individually. Any cycle tends to attain 
 
POSTLARVAL DEVELOPMENT. I09 
 
 its radial plan before the next cycle commences to form, when the additions 
 take place in such a manner as to again confer bilaterality upon the polj^p 
 as a whole. Thus the first two cycles of septa become perfectly radial 
 before an additional cycle commences, and the growth of this is then con- 
 tinued in a bilateral manner ; likewise, the new second and third cycles 
 assume their radial stage before the members of the fourth cycle make their 
 appearance, proceeding from one border to the other. In like manner the 
 first-cycle mesenteries are nearly radial before those of the second cycle arise 
 and introduce a conspicuous bilateral symmetry ; and on these assuming the 
 radial plan the third-cycle mesenteries begin to appear, again in a bilateral 
 manner. 
 
 The successive dorso-ventral growth followed by the constituent mesen- 
 teries and septa of each cycle may also be regarded as conferring a certain 
 individuality upon the cycle. The difierent cycles, arising independently, seem 
 to represent so many distinct recurring phases of growth in the life of the 
 pol)rp, not a continuous addition from one aspect to the other, as is usual in 
 permanently bilateral animals, particularly segmented forms. The members 
 of a cycle appear in a dorso-ventral sequence, and may retain their differences 
 in size for a long time, but in the end they become equal and thereby confer 
 radial symmetry upon the polyp. Then another cycle commences to form 
 in somewhat the same bilateral dorso-ventral succession, displays for a time 
 its consecutive origin, and afterwards attains radiality. 
 
 The conception of recurring phases of growth in cyclic coral polyps is 
 best realized when comparison is made with the mesenterial increase char- 
 acteristic of the Ceriantheae. Here the mesenteries beyond the protocnemes 
 always develop in a regular bilateral successive manner, from the dorsal 
 (anterior, sulcar) to the ventral (posterior, asulcar) aspect, the oldest being 
 dorsal or anterior and the youngest ventral or posterior, recalling more the 
 method of growth of segmented animals ; in cerianthids there is never a 
 reversal of growth to the anterior end, followed by a successive series to the 
 other, such as occurs in ordinary hexactinians. Employing the term ''band 
 of proliferation," introduced by Van Beneden in 1897, we may say there is 
 only one median band of proliferation in cerianthids, while in hexactinians 
 there are many such bands, the number increasing with age — at first 6, then 
 12, 24, etc. 
 
 In the Zoantheae also mesenterial growth is always in the same succes- 
 sion after the protocnemic stage. The increase takes place within the two 
 exocoelic chambers on each side of the ventral directives ; there are only two 
 bands of proliferation or zones of growth. In this case, however, the order 
 
no SIDERASTREA RADIANS. 
 
 followed by the new mesenteries differs from that in hexactinians and 
 cerianthids ; it proceeds from the ventral (posterior, sulcar) to the dorsal 
 (anterior, asulcar) aspect of the polyp, not from the dorsal to the ventral. 
 
 The bilateral development of the organs, from one border of the polyp 
 to the other, in ordinary actinians and corals, would seem to have no phylo- 
 genetic significance beyond the group of the coelenterates, and as yet we 
 appear to have no definite understanding as to what even this may be. The 
 approximate radial symmetry of adult coelenterates is assumed from very 
 diverse developmental' conditions {cf. hexactinians, zoanthids, cerianthids, 
 and the tentacles and other cyclic organs in the Hydromedusae and Scypho- 
 medusse). Whatever may be said in favor of Sedgwick's well-known view 
 that the mesenterial arrangement found in cerianthids suggests the metamer- 
 ism of higher animals, there is clearly no support for such a conception in 
 the development of the organs in hexactinians. 
 
 SUMMARY. 
 
 1. In Siderastrea radians the six members of the first cycle of septa 
 appear simultaneously, shortly after fixation of the larva, situated within 
 the entocceles of the first cycle of mesenteries. 
 
 2. Six members of a second cycle are developed within the primary 
 exocceles, shortly after the primary cycle of entosepta. They are the tempo- 
 rary predecessors of a later permanent cycle, and arise either simultane- 
 ously or in bilateral pairs in a dorso-ventral order. Later, each becomes 
 bifurcated peripherally, either by the direct extension of the original septum 
 or by the production of separate fragments which subsequently fuse. The 
 bifurcations also appear in a bilateral dorso-ventral order. 
 
 3. The six members of the permanent second cycle of entosepta arise 
 within the entocceles of the second-cycle mesenteries soon after these make 
 their appearance. The two right and left dorsal septa appear first, then the 
 two middle members, and, at a much later period, the two ventral, the series 
 thus exhibiting a decided dorso-ventrality. In the end they become equal, 
 and each fuses with the central part of the corresponding second-cycle exo- 
 septum previously developed, which now lose their individuality. 
 
 4. Twelve members of a temporary third cycle are situated within the 
 exocceles between the primary and secondary pairs of mesenteries, and repre- 
 sent the bifurcated extensions of the six primary exosepta. The original 
 second-cycle exosepta thus become the third exocoelic cycle, their place 
 having been taken by the permanent second cycle of entosepta. 
 
 5. The later development of the septa in buds proves that a new third 
 
POSTLARVAL DEVELOPMENT. Ill 
 
 cycle of 12 or less septa arises on the appearance of the pairs of third-cycle 
 mesenteries, in a similar manner to that followed by the permanent second 
 cycle. New entosepta appear within the entocoeles of the third-cycle mesen- 
 teries, and the bifurcations of the third-cycle exosepta become the exosepta 
 of the fourth cycle. 
 
 6. The third-cycle entosepta, following the mesenteries, are developed 
 in a bilateral dorso-ventral order, but in two series-— first a series within the 
 dorsal moiety of each sextant, and then a second series within the ventral 
 part of each sextant. 
 
 7. Bxosepta are present at each cyclic stage in the growth of the coral- 
 lum, alternating in position and corresponding in number with the sum of 
 the entosepta. They never become entosepta, but always constitute the 
 outermost cycle of shorter septa ; only the entosepta have any ordinal sig- 
 nificance. Until the adult condition is reached the exosepta are the tempo- 
 rary predecessors of the entosepta. The developmental relationships between 
 the entosepta and exosepta are closely comparable with those between the 
 entotentacles and exotentacles. The law of substitution, first discovered by 
 Lacaze-Duthiers for the tentacles of Hexactiniae, is thus found to hold also 
 for the septa. 
 
 8. Where the cyclic hexamerism of a corallite is incomplete the ordinary 
 cyclic formula does not express the true relationship of the septa. The ento- 
 septa and exosepta vary in the same degree, so that the morphological 
 septal formula for a corallite with three entoseptal cycles and one exoseptal 
 cycle is 6, 6, X, 6 + 6 + x, where x may be any number from i to 12. 
 
 9. The cycles of septa and mesenteries represent so many distinct recur- 
 ring phases of growth all around the polyp, not a continuous increase from 
 one extremity to the other as in metameric animals. With the exception of 
 the first the members of each cycle follow a dorso-ventral succession, display 
 a bilateral symmetry for some time, and ultimately assume an approximate 
 radial plan. The succession for the third cycle of entosepta is twofold. 
 
 BASAL PLATE. 
 
 G. von Koch, in the course of his embryological studies of corals, found 
 a deposit of calcareous matter to take place between the ectoderm of the base 
 and the surface of attachment of the polyps. It is the first part of the skele- 
 ton to be formed by the activity of the calicoblasts, and from its position is 
 known in coral literature as the basal plate. It is present in the photographic 
 reproductions on plates 4 and 5, and its relationship to the polyp, at a rather 
 late stage, is shown on plate 9, fig. 53. 
 
112 SIDERASTREA RADIANS. 
 
 In Astroides calyailaris Von Koch (1882) found the basal plate to take 
 the form of a thin circular disc, composed of spheroidal or elliptical crystal- 
 line bodies, from 0.005 t<^ 0.008 mm. in transverse section. At first the 
 calcareous elements were arranged in an interrupted manner, but later the 
 openings were filled by further deposit of skeletal matter, and the plate as a 
 whole became thickened. In Caryophyllia cyathus (1897) the first rudiments 
 of the basal plate consisted of a small central deposit surrounded by six thin, 
 nearly triangular plates, interseptal in position. For a time these were dis- 
 tinct from one another, but later united with the central circular patch, and, 
 by further additions, became joined along their edges, thus constituting a 
 complete flattened disc or plate. The presence of a basal plate has since 
 been recognized in many forms of corals, and it is extremely doubtful 
 whether it is really wanting in any species. 
 
 For a long time the developing polyps of 6*. radians gave no external 
 indication of any skeletal formation which could be regarded as the 
 basal plate. The first evidences of the corallum were the six entoccelic 
 septa, which appeared to rise directly from the surface of attachment. 
 Under transmitted light they stood out as nearly opaque objects, while the 
 interseptal spaces at this and later stages were quite clear and apparently 
 devoid of any calcareous crystals. For nearly two months the laboratory 
 notes contained the assertion that no basal plate was developed in Siderastrea^ 
 for, excepting the presence of the septa and epitheca, there was no interrup- 
 tion in the ordinary light passing through the polyp. When, however, the 
 living or preserved polyps were examined by means of polarized light, the 
 bright colors of the basal region revealed the presence of crystalline matter. 
 This is well seen in the photograph reproduced on plate 4, fig. 22, taken 
 with an exposure of twenty minutes to polarized light. Between crossed 
 nicols the field was black except in the region of the polyp ; the septa also 
 appeared black owing to their thickness and the irregular disposition of their 
 crystalline constituents, while all the interseptal areas were brightly and 
 variously colored. 
 
 It is obvious, therefore, that a basal plate was already developed between 
 the polyp and its surface of attachment, but was too thin to offer any appre- 
 ciable obstruction to the transmission of ordinary light. The plate must have 
 begun to form a day or two after fixation of the larva, for the septal upgrowths 
 were observed on the third and fourth days, and the deposit of basal skeletal 
 matter probably preceded them. As stated below, the plate does not undergo 
 much thickness, even in later stages ; hence, at no time would it offer much 
 interference to the passage of light 
 
POSTLARVAL DEVELOPMENT. II3 
 
 When any of the young polyps, two or three weeks old, were macerated 
 later, it was found that a thin continuous plate remained adherent to the 
 incrusted surface ; but none of the earliest stages in its formation were 
 obtained, showing whether it originated as a continuous layer or in sepa- 
 rate parts. All the poly pal tissues have been macerated from the young 
 coralla represented on plate 4, figs. 19 and 20 ; they now show a well-developed 
 basal plate, slightly upturned at the periphery^^and bearing the septa on the 
 upper surface. 
 
 A portion of the basal plate of a macerated corallum of two weeks, magni- 
 fied about 300 times, is represented in surface view on plate 11, fig. 69. At the 
 upper boundary of the figure the epitheca also is shown in section, and upon 
 the plate are the first thickenings which will form the septa. The entire 
 surface of the plate exhibits a number of very thin, flat scales, roughly polyg- 
 onal in form. Sometimes they appear as if joined edge to edge like the 
 cells of an endothelium, or at other places as if overlapping. The average 
 diameter of the individual scales is about 0.03 mm. In most of them a distinct 
 fibrous structure can be recognized, the fibro-crystals either lying parallel 
 or, less often, presenting a radiating appearance. There is, however, no 
 suggestion of the fibers being arranged around a center of calcification, as in 
 the trabeculae of the septa. 
 
 According to Miss Ogilvie (1897, pp. 114-117) similar scale-like elements 
 are present on the surface of the corallum of corals generally, and she has 
 succeeded in isolating them from the almost transparent dissepiments of 
 Galaxea. Their breadth in this genus varies on an average from r ,01 to 
 0.015 mm., while their height is about 0.003 ^^^- A number of fibers are 
 present in each, sometimes in the form of divergent groups, but often lying 
 loosely side by side. It was from a study of these that Miss Ogilvie came 
 to the conclusion that each isolated skeletal element represented a calcified 
 calicoblast cell, thus returning to the old view of von Heider as to the origin 
 of the calcareous skeleton of corals in contrast with the more accurate results 
 of von Koch, Bourne, and Fowler, which show that the calcareous matter is 
 secreted wholly external to the ectodermal cells. The structure of the calico- 
 blast layer in the polyps of Siderastrea is also very conclusive as to the 
 ectoplastic origin of the calcareous fibers. Everywhere it has been found to 
 be a simple layer, never many cells deep, as would be the case were the 
 calicoblasts themselves calcified and shed from time to time to build up the 
 skeleton. 
 
 From the relationships of the basal plate to the polyp, increase in its thick- 
 ness can obviously take place only on the^upper surface. In addition to the 
 
114 SIDERASTREA RADIANS. 
 
 septal rudiments, small projections are present liere and there on this surface, 
 especially towards tlie middle of the plate, and some continue growing until 
 they become raised much above the general level of the plate and constitute 
 columellar spines. In the basal plate we have the skeleton in its earliest 
 and simplest condition as a flat deposit, and the septal and columellar forma- 
 tions are vertical upgrowths from it. The latter represent areas at which 
 the calcareous matter is laid down with greater rapidity by the activity of the 
 calicoblasts. The basal plate, however, is covered by polypal tissues on only 
 its upper surface, while the upgrowths from it — septa and columella — are 
 covered on both sides ; hence the difference in their microscopic structure 
 shown on plate ii, fig. 70. There are no axial centers of calcification in 
 the elements of the basal plate, while such are very distinct in the septa. 
 
 When the young corallum is detached from a smooth surface, the lower 
 surface of the basal plate is also smooth and even, but when growing over a 
 rough surface it adapts itself to the irregularities. 
 
 The basal plate increased considerably in diameter from the time of its 
 first formation, keeping pace with the general growth of the corallum. In 
 the early stage on plate 4, fig. 19, it is only 1.3 mm. across, while in the 
 corallum of plate 5, fig. 28, it is 2 mm. in diameter. 
 
 As shown in the next section an epitheca begins to form at the edge of 
 the basal plate whenever growth ceases, while when lateral growth is con- 
 tinued there is little or no possibility of any upturned marginal deposit. 
 
 Apparently the original basal plate of iS". radians never becomes much 
 thickened. Some of the oldest coralla reared rather suggest that the central 
 interseptal portions may be resorbed, or, at any rate, remain extremely deli- 
 cate. When coralla of four months were macerated and separated from their 
 surface of attachment it was found that interseptally the basal deposit had 
 disappeared from the central regions, though retained towards the periphery 
 and along the septa. It may be that on account of its thinness interseptally 
 it had broken away in the process of maceration, while it was supported 
 septally and peripherally. 
 
 In colonial corals the basal plate is generally represented only in the 
 corallum of the primary larval polyp, the later bud polyps not admitting of 
 its formation. But wherever in growing colonies of S. radians the marginal 
 corallites extend beyond the incrusted object a thin parchment-like deposit 
 is found basally. It constitutes the basal skeletal support of the young bud 
 polyps, in the same manner as the basal plate of larval polyps, or as the dis- 
 sepiments at a later stage. 
 
POSTLARVAL DEVELOPMENT. II5 
 
 EPITHECA. 
 
 Along with the first formation of the radiating septal upgrowths 
 appeared a narrow, peripheral calcareous ring, somewhat less opaque than 
 the septa, and for a long time wholly unconnected with them (plate 2). 
 The most careful examination of the living polyps proved that the annulus 
 was altogether external to the soft tissues ; while in preserved and cleared 
 specimens the polypal wall was found to pass witliin the inner border of the 
 skeletal deposit, not to be folded over it. The formation is undoubtedly to 
 be regarded as an epitheca, according to the definitions of this structure 
 given below. 
 
 The epitheca increased in height along with the growth of the polyp, 
 at the same time often narrowing a little transversely. It remained 
 throughout uncovered by any polypal tissues on its outer surface, though 
 lined by the polypal wall on its inner surface (plate 9, fig. 53). Where 
 most fully developed its outer surface exhibited distinct incomplete annula- 
 tions, wrinklings, or accretion lines, as if representing separate intervals in 
 the deposition of calcareous matter (plate 4). By the time the polyps were 
 six or seven weeks old the lower region became discolored by the adherence 
 of foreign matter, such as filamentous algae and diatoms, and only the actual 
 margin was fresh and white. Opposite the septa the epithecal margin was 
 sometimes a little indented, but otherwise was of the same height all round, 
 somewhat exceeding that of the septa. Whenever, owing to uncongenial 
 conditions, the polyps shrunk from their former size, the peripheral deposit 
 remained behind, distinct and complete ; the growth of a second annulus 
 then took place at the new margin of the narrowed polyp, in such a way that 
 the new ring was wholly within the old (plate 5, figs. 25-27). 
 
 Later, as the septa increased in radial length, some of them came in 
 contact with or were actually fused with the epithecal deposit, but the micro- 
 scopic structure of the two remained distinct. When the coralla were freed 
 from their surface of attachment the epitheca was seen to be a direct upward 
 continuation of the edge of the basal plate, and, like it, shows no centers of 
 calcification. It is made up of circular lamellae, the fibro-crystals of which 
 are arranged at right angles to the polypal wall (plate 11, figs. 69, 70) ; the 
 epithecal deposit, in fact, corresponds to but one-half of a septum. 
 
 The epitheca was found to vary greatly in the extent of its development 
 in the various polyps. In some of the most forwardly developed specimens 
 it was practically absent or represented only by a thickened marginal ring 
 (plate 4, figs. 19, 20) ; in others it appeared as a very distinct parapet, 
 
Il6 SIDERASTREA RADIANS. 
 
 elevated beyond the level of the septa (plate 5). Where two or more polyps 
 developed contiguous to each other a common epitheca was formed along the 
 line of contact (plate 5, fig. 30), but each became distinct later. In one 
 instance two polyps, along with their septa, were embraced in a common 
 epitheca. 
 
 This surprising variation in the amount of epithecal deposit appears to 
 be determined by the rate of growth of the polyp. As already noticed, the 
 polyps varied greatly in this respect, and the epitheca was best developed in 
 individuals which increased but little in size. The transverse narrowing in 
 the later stages may be taken to indicate that the polyps had become some- 
 what less in their basal diameter. Where a polyp was growing rapidly, 
 enlarging the diameter of its basal disc and extending the septa peripherally, 
 it is manifest that an epitheca could not be formed, or, if formed, would need 
 to be resorbed. In the two coralla represented on plate 5, figs. 28, 29, there 
 is practically no epitheca, but the basal plate is much thicker in its peripheral 
 half than in the middle. Presumably, the polyp did not rest long enough at 
 any one stage to permit of the secretion of an epitheca. Until maturity, 
 therefore, it is possible that individual corallites of the same species of 
 coral may be provided with or be destitute of an epitheca, according to the 
 slow or rapid rate of growth of the polyp ; the morphological value of the 
 structure becomes somewhat lessened when its formation is shown to be 
 dependent to such an extent upon physiological conditions. 
 
 The peripheral skeletal formation in Siderastrea is of interest in con- 
 nection with the much discussed question as to the nature of the thecal and 
 epithecal wall of corals. In some respects the structure recalls that described 
 as theca (Mauer) by von Koch (1897) in his paper on the development of the 
 skeleton of Caryophyllia cyathus^ but a comparison at once establishes their 
 different values. In both species the structure in question arises as an inde- 
 pendent peripheral part of the corallum, and narrows from below upwards. 
 But in the species investigated by von Koch the annulus is inclosed on both 
 its inner and outer sides by an upgrowth of the basal wall, while in Sider- 
 astrea it is external from the beginning, only covered by the poly pal tissues 
 on its inner wall and growing margin (plate 9, fig. 53). Again, in the first 
 the deposit early unites the edges of the septa, but in the other it remains 
 entirely free from septal connection for a long time, and then only joins their 
 edges as an independent external covering. The different character of the 
 theca (pseudotheca and true theca) in various Madreporaria has already been 
 alluded to (p. 45), and the original peripheral structure arising in Caryophyllia 
 would certainly belong to the " true theca " type. Miss Ogilvie (1897, p. 
 
POSTLARVAL DEVELOPMENT. II7 
 
 159) separates tlie theca from the epitheca, as follows : " The essential differ- 
 ence which may be said to distinguish ** theca " from " epitheca " is that the 
 theca, or wall, must be structurally associated with the peripheral ends of 
 septa, whereas the epitheca is in no structural connection with the septa, but 
 is a continuous concentric deposit exterior to all the other skeletal structures 
 of a calyx." And again (p. 248) : " The epitheca is an external basal struc- 
 ture, laid down at the angle of the aboral wall, where it bends towards the oral 
 or peristomal region of the polyp (figs. 22, 36). It is the continuation upwards 
 or outwards of the embryonic * basal plate,' and may be well-developed or 
 remain rudimentary." G. von Koch (1896, p. 254) describes the structure 
 in much the same terms. 
 
 These two definitions serve to distinguish clearly between the periph- 
 eral annulus found in Caryophyllia cyathus and that in S. radians. In 
 the one case it is structurally associated with the outer ends of the septa, 
 while in the other it is quite independent of these ; in one it is developed 
 within a special invagination, " thecal refoulement," of the skeletotrophic 
 wall which lines both its inner and outer surfaces, in the other it is wholly 
 external to the polyp, covered on one side only by the skeletogenic tissues, 
 not by an upfolding. In Caryophyllia von Koch found no trace of an 
 epitheca in addition to the theca, while in Astroides and now in the early 
 corallum of Siderastrea there is found to be an epitheca but no true theca. 
 Whatever calicinal wall is found in the mature corallum of these two genera 
 is a later structure formed by the coalescence of the outer edges of the septa 
 (pseudotheca). 
 
 By Ogilvie (p. 248) and Vaughan (p. 48) the epitheca is regarded as a 
 primitive structure in Madreporaria, while the thecal structures are of 
 secondary origin. The latter writes : " The oldest type is where the ends 
 of the septa did not fuse distally, but simply had their outer ends bound 
 together by an epithecal covering." This clearly describes the early epithe- 
 cate corallum of Siderastrea. In this respect, therefore, the genus must be 
 considered as representing an older type than the truly thecate corallum of 
 such a form as Caryophyllia. Its very rudimentary condition in some of 
 the young polyps and its almost complete absence from the adult colony 
 would seem to prove that in this genus it is an embryonic structure of 
 diminishing importance. 
 
 COLUMELLA. 
 
 For a time the central part of the basal plate was free from any calcare- 
 ous deposit which could be regarded as a columella (plate 4), but, as the septa 
 increased in size, spinous upgrowths began to form near their inner extremity, 
 
Il8 SIDERASTREA RADIANS. 
 
 extending as far as tlie center of the calice (plate 5). Some of these upgrowths 
 appeared as separate projections from the basal plate, as if produced by special 
 invaginations of the basal wall of the polyp — the refoulement columellaire 
 of Delage & Herouard (1901, p. 558) ; others seemed to be direct continua- 
 tions of the septa, not requiring a separate upfolding of the basal disc. The 
 former undoubtedly represent a true independent columella, while the latter 
 by their union might give rise to a so-called pseudocolumella. At no time, 
 however, could any sharp distinction be drawn between the septal and the 
 independent upgrowths, except as regards their position. In the coralla rep- 
 resented on plate 4, figs. 23 and 24, two or three of the middle granules are 
 obviously distinct basal formations, while exactly similar spinous projections 
 occur at the end of some of the septa. The middle of the corallum on plate 5, 
 fig. 28, is also occupied by distinct spinous upgrowths, both basal and septal in 
 character. 
 
 A further stage in the columellar growth is represented in the coralla of 
 figs. 28 and 29. Here the intervals between the spines are becoming partly 
 occupied by the deposition of secondary calcareous matter, so that their act- 
 ual origin is obscured. In the mature corallum it was frequently found 
 (P- 53) that the columella is minutely spinous or papillose as seen from the 
 surface, and that in sections for some distance below it remains spongy, 
 becoming compact in the deeper regions by the later deposition of calcareous 
 matter. Further, where the coralla are very strongly calcified the spinous 
 character of the columella disappears even superficially, the interspaces 
 being altogether occupied by the secondary deposit, which keeps pace with 
 the septal and columellar spinous formations. These differences in the 
 mature calice thus coincide closely with those represented in the larval cor- 
 alla of figs. 23 and 29. 
 
 Both from its origin and mature characters the columella of S. radians 
 is therefore fiDrmed from three independent sources: (i) Separate basal 
 upgrowths; (2) septal spines, continuous with the central ends of the septa ; 
 (3) a secondary deposit filling up the interstices between i and 2. Histolog- 
 ically the trabeculae of all three are alike (plate 10, fig. 65). 
 
 The columella of Siderastrea is thus a " true " columella, to be distin- 
 guished from a " false " or " pseudocolumella " where there is no direct basal 
 upgrowth, but the entire structure is formed from the inner septal edges. In 
 its development it agrees most closely with the account which von Koch 
 (1897, p. 769) gives of the formation of the columella in Caryophyllia. 
 
POSTLARVAL DEVEI^OPMENT. II9 
 
 ANATOMY AND HISTOLOGY OF LARVA AND YOUNG POLYP. 
 
 A number of free-swimming larvae were preserved in corrosive acetic, 
 and later studied by means of transverse and longitudinal sections, when all 
 were found to be at about tlie same stage of development. Only the impor- 
 tant features in which they differ from mature polyps will be here noticed. 
 
 The larval ectoderm is somewhat broader Ihan the same layer in the 
 adult polyp. In sections it measures about 0.04 mm. across while that of the 
 adult is 0.03 mm. The outer surface is strongly and uniformly ciliated, the 
 enlarged base of each cilium being well defined. Numerous clear and gran- 
 ular gland cells occur, and towards the margin a zone of small nematocysts 
 0.015 mm. in length. The layer further differs from that of the adult in 
 containing a few Zooxanthellse, mainly restricted to the oral extremity. 
 These have been already noticed among the external characters as giving a 
 brownish color to the oral pole of the larva, and are apparently liberated 
 from time to time. 
 
 The larval ectoderm is broader at the base, where it measures 0.06 mm. 
 across, and, in addition, has undergone certain histological modifications. 
 The cells as a whole seem more compactly arranged, gland cells are less 
 numerous, and a nerve layer is present ; but nematocysts do not seem more 
 numerous than elsewhere. The whole structure recalls the aboral sense 
 organ which has already been found by McMurrich, Appellof (1900), and 
 myself (1902, p. 524), to occur in certain actinian and coral larvae, and is 
 evidentl}^ widely, though not universally, present in these two groups. The 
 characteristics of the organ, however, are usually more conspicuous than in 
 the present species, especially as regards the degree of development of the 
 nervous elements. 
 
 The stomodaeal communication between the exterior and the larval cavity 
 was already established in all the larvae studied. In most the lumen is circu- 
 lar, while in others it is slightly oval. The surface is more strongly ciliated 
 than that of the outer ectoderm, and fewer gland cells occur; the two agree 
 in the presence of Zooxanthellae, though these are never found in the ecto- 
 derm of the adult stomodaeum. 
 
 The mesenterial filaments on pairs i and 11 on plate 8, fig. 52, stand 
 out conspicuously from the endoderm on account of the number and deeply- 
 staining character of their nuclei, and their histological structure recalls that 
 of the stomodaeal ectoderm. In the larvae, however, the organs are not yet 
 rounded ofif from the mesenterial endoderm. 
 
I20 SIDERASTREA RADIANS. 
 
 In the serial transverse sections from whicli figs. 51 and 52, on plate 8, 
 were taken, the stomodaeal ectoderm appears to be continued uninterruptedly 
 as the mesenterial filaments down the edge of the two lateral pairs of com- 
 plete mesenteries on their becoming free; but the ventral and dorsal direct- 
 ives (hi, iv), which are only attached to the stomodaeal wall for part of their 
 length, are without such modified tissue along their free edge. 
 
 The continuity of the stomodseal ectoderm with the mesenterial fila- 
 ments in the various larvae of Siderastrea^ and the close histological simi- 
 larity of the two, seem at first sight undoubted evidence of the ectodermal 
 origin of the latter, especially when it is found that filaments are not present 
 on mesenteries which do not reach the stomodasum, or do not extend as far 
 as its inner termination. In the larvae of many other Actiniaria and Madre- 
 poraria, however, it has been found that filaments may appear on mesen- 
 teries before they reach the stomodseum, and even in complete mesenteries 
 an interval of undifferentiated endoderm often occurs between the ter- 
 mination of the stomodaeal ectoderm and the filaments. From observa- 
 tions on other corals I consider that the mesenterial filaments arise from the 
 larval endoderm independently of the stomodaeal ectoderm, but that continuity 
 is early established in the case of those mesenteries which unite with the 
 stomodaeum (1902, p. 476). 
 
 The internal cavity of the larvae is Very limited in extent, the endoderm 
 nearly filling the whole chamber. In the youngest examples mere slits 
 represent the lines along which the polypal cavity will be formed later. The 
 cells of the endoderm are much vacuolated, and contain numerous Zooxan- 
 thellae scattered throughout. The lining of the mesenteries and also of the 
 intervening portion of the column wall is not arranged as a simple epithelial 
 layer such as characterizes the adult. Within most of the intermesenterial 
 spaces the endoderm is greatly thickened, and in transverse sections stands 
 out as a very distinct triangular projection into the ccelomic cavity, leaving 
 only a narrow slit between itself and the mesenterial lining. The vertical 
 projections are the " Vorsepten " or prosepta of von Koch (1897) ; they are 
 the persistent parts of the mass of endoderm which at an earlier stage occu- 
 pies the whole interior of the early larva. On plate 8, fig. 52, taken from a 
 rather late larva, the prosepta are still conspicuous, and some are associated 
 with rudiments of the mesenteries ; the mesenterial endoderm is still greatly 
 thickened, and the central cavity is beginning to enlarge. 
 
 In an earlier paper on the larva of the actinian Lebrunia coralligens 
 (1899), ^ tave shown that most anthozoan larvae are for some time nearly 
 solid, owing to the enormous development of the endoderm. There are, how- 
 
POSTLARVAL DEVELOPMENT. 121 
 
 ever, very narrow, canal-like slits, and from these the adult gastro-coelomic 
 cavity is derived by the disintegration of the more central endoderm and the 
 shrinkage of that lining the body-wall and mesenteries. 
 
 Some of the larvae of Siderastrea sectionized reveal a gastro-coelomic 
 cavity further developed than that represented in figs. 51 and 52. Below the 
 stomodseal region the parenchymatous endoderm has broken down, and the 
 middle of the cavity is occupied by organic debris, in which Zooxanthellae, 
 cell walls, and granules of various kinds are recognizable. There is no 
 doubt that this is the organic debris which is extruded from time to time by 
 the larvae soon after their liberation from the parent polyp (p. 58). At the 
 stage in the young polyp at which the septa are well advanced (plate 9, fig. 
 53), the endoderm has become a simple epithelial layer throughout, resem- 
 bling in all respects that of the mature polyps. 
 
 In the earliest larvae sectionized eight mesenteries were already developed, 
 arranged in four bilateral pairs, as shown diagrammatically in fig. 8, <2, p. 80. 
 The two lateral pairs are united with the stomodaeum, while the dorsal and ven- 
 tral axial pairs, representing the directives, are free, and of the two directive 
 pairs the ventral are slightly larger than the dorsal. In larvae a day or so 
 older the ventral directives have united with the stomodaeum, while the dorsal 
 are still free (plate 8, fig. 51) ; also at the stage with three complete mesenteries 
 two other bilateral pairs of mesenteries, the fifth and sixth in the sequence, have 
 made their appearance, arranged as on plate 8, fig. 52. Afterwards the dor- 
 sal directive mesenteries unite with the stomodaeum, and the larva has reached 
 the Bdwardsian stage of mesenterial development presented at the time of 
 fixation (fig. 8, a-c^ p. 80). 
 
 There appears to be no resting stage in the appearance of the mesen- 
 teries between the tetrameral and hexameral condition, such as seems to char- 
 acterize certain actinians {Lebruma), nor in the successive union of the first 
 four pairs with the stomodaeum. As already shown, however, the fifth and 
 sixth pairs remain as microcnemes for a prolonged period. 
 
 As represented on plate 8, figs. 51 and 52, the mesenterial mesogloea is 
 extremely thin. The cut ends of very delicate muscular fibrils can be recog- 
 nized in transverse sections, and, according to their disposition on one side or 
 other of the mesogloea, assist in the determination of the paired arrangement. 
 
 The further development of the mesenteries, after fixation has taken 
 place, can be easily followed through the transparent tissues of the living 
 polyp, and has been already described. 
 
I«2 SIDERASTREA RADIANS. 
 
 YOUNG POLYPS. 
 
 Very few of the young polyps reared were available for anatomical and 
 histological study, owing to the greater importance of the corallum at this 
 stage ; in most cases the soft tissues were removed by maceration in order to 
 secure the skeleton. A vertical section through a decalcified retracted larval 
 polyp, of about two months, is represented on plate 9, fig. 53. The section 
 of the skeleton has also been added, the details being taken from various 
 coralla of this age. The section of the polyps includes the oral aperture, and 
 on the right is truly radial, passing through a mesenterial space and cutting 
 a septal invagination obliquely, while on the left side it passes obliquely 
 through a mesentery and also through a septal invagination. A third invag- 
 ination is included in the middle region of the section and probably represents 
 an early columellar upgrowth. The upper wall on the left has come to rest 
 upon one of the invaginations. 
 
 The septal and columellar invaginations are here of the simplest charac- 
 ter. They are merely continuous upgrowths of the basal wall of the polyp, 
 and agree with it histologically. They line both sides of the calcareous 
 upgrowths from the basal plate, while the latter in its turn is laid down by 
 the flattened part of the disc, increase in thickness taking place only on 
 the upper side. 
 
 Sections of coral polyps at such an early stage are the most favorable for 
 demonstrating conclusively that the madreporarian skeleton is laid down 
 wholly outside the polypal tissues, and that all the skeletal complications are 
 formed within extensions of the basal disc, the invaginations proceeding pari 
 passu with the deposition of calcareous .matter. This truth was first estab- 
 lished by von Koch by means of sections of polyps of Astroides^ somewhat 
 similar to that of plate 9, fig. 53 ; further, von Koch was able to demonstrate 
 the calcareous spheroids of the basal plate and septa in situ. His polyps 
 for this purpose were adherent to pieces of cork, so that the polyp and its 
 attachment could be sectionized together. The polyps of Siderastrea being 
 adherent to glass could be sectionized only after being freed through decal- 
 cification. 
 
 Decalcification of the polyp from which fig. 53 was taken was carried out 
 with great care, but only fragments of a very narrow skeletogenic ectoderm 
 remained. No hints of any mesoglceal processes or desmocytes occurred, 
 but such would scarcely be expected considering their rarity in the adult 
 polyp oi Siderastrea, The mesogloea itself is a very thin lamella. The basal 
 
POSTLARVAL DEVELOPMENT. 1 23 
 
 endoderm also has undergone a great alteration compared with the same 
 layer in the other regions of the polyp. It is much thinner, all traces of cell 
 limitations are lost, and Zooxanthellae are wholly absent, while elsewhere in 
 the endoderm the algal cells occur in some abundance. 
 
 The passage from the calicoblastic ectoderm to the ectoderm of the 
 column wall is gradual, and can be studied on both sides of the section. It 
 is at this point that the epitheca is laid down. Were there any doubt as to 
 the epithecal or thecal nature of the peripheral calcareous ring such sections 
 prove absolutely that the deposit is uncovered by the polypal tissues on its 
 outer sides. Were it otherwise an invagination of the base would occur 
 towards the periphery, but none of the sections show any such folding. 
 
 At the region of the epitheca there is no sharp distinction between the 
 column wall and the basal disc. The ectoderm of the former is a broad 
 columnar epithelium, having many clear unicellular mucous glands distrib- 
 uted among the supporting cells ; the nuclei are restricted mostly to the inner 
 half of the layer. The left half of the disc includes the section of a tentacle, 
 represented only by the knob which is crowded with long, narrow nematocysts. 
 As in adult retracted polyps, the stem of the tentacle is not distinct from 
 the disc. Histologically the walls of the stomodseal invagination closely 
 resemble those of the adult polyps, and the ectoderm is marked off from the 
 rest of the body wall by its numerous ciliated supporting cells. 
 
 On both sides of the section represented in fig. 53 the stomodseal wall 
 terminates freely, but in sections which include a mesentery the ectoderm is 
 seen to be continuous with the mesenterial filaments along the free edge of 
 the mesentery. The two, stomodaeal ectoderm and mesenterial filament, are 
 much alike histologically, and by their brightly-staining character are easily 
 recognizable among the other tissues. 
 
REFERENCES. 
 
 (Only the more frequently recurring references are here given ; the others are inserted in the text.) 
 
 1900. Appellof, A. : " Studien Uber Actinien-Entwicklung." Bcrgens Museums Aarbog. 1900,00. i. 
 1897. VAN Bknedkn, E. : " Les Anthozoaires de la Plankton-Expedition." Rdsultats de la '' Plankton- 
 expedition der Humboldt-Stiftung." Vol. 11, Kiel et Leipsic. 
 
 1887. Bourne, G. C. : "The anatomy of the Madreporarian Coral Fungia." Quart. Journ. Micr, Sci., 
 
 vol. XXVII. 
 
 *893- • " O" the postembryonic development of Fungia." Trans. Roy. Dublin Soc, ser. xi, 
 
 vol. V. 
 1899. : "Studies on the structure and formation of the calcareous skeleton of the Antho- 
 
 zoa." Quart. Journ. Micr. Sci., vol. XLi. 
 
 1901. Delage, Y., and Herouard, E. : "Trait^ de Zoologie Concrete. Les Coelent6rds." Tom. 11, pt. 2. 
 
 1899. DuERDEN, J. E : " The Edwardsia-stage of the actinian Lebrunta, and the formation of the gastro- 
 
 coelomic cavity." Journ. Linn. Soc, Zool., vol. xxvii. 
 
 1902. : " West Indian Madreporarian Polyps." Mem. Nat. Acad. Sciences, vol. viii, 7th Mem. 
 
 1895. Faurot, L. : " ifetudes sur I'anatomie, I'histologie et le d^veloppement des Actinies." Arch, de 
 
 Zool. Exp. et Gdn., ser. 3, tom. iii. 
 
 1888. Fowler, G. H. : "The anatomy of the Madreporaria, IV." Quart. Journ. Micr. Sci., vol. xxviii. 
 
 1902. Gardiner, J. S. : " South African corals of the genus Ftabellttm, with an account of their anatomy 
 
 and development." Marine Investigations in South Africa, vol. 11, Cape Towrn. 
 1882. VON Koch, G. : " Ueber die Entwicklung des Kalkskeletes von Asteroides calyctilaris und dessen 
 
 morphologischer Bedeutung." Mitt. a. d. Zool. Stat, zu Neapel, bd. iii. 
 
 1896. : " Das Skelett der Steinkorallen." Festschrift fiir Carl Gegenbaur. Leipzig. 
 
 1897. : "Entwicklung von Caryophyllia cyathus." Mitt. a. d. Zool. Stat, zu Neapel, bd. xii. 
 
 1872. Lacaze-Duthiers, H. de: "D^veloppement des Coralliaires. Prem. Mem., Actiniaires sans 
 
 Polypier." Arch. <ie Zool. Exp. et Gdn., tom. i. 
 1873. • " D^veloppement des Coralliaires. Deux. Mdm., Actiniaires a Polypier." Arch.de 
 
 Zool. Exp. et G6n., tom. 11. 
 1897. : " Faune du Golfe du Lion. Coralliaires. Zoanthaires Sclerodermas." Arch, de Zool. 
 
 Exp. et Gdn., 3 ser., tom. v. 
 1897. Ogilvie, M. : " Microscopic and systematic study of Madreporarian types of corals." Phil. Trans., 
 
 vol. CLXXXVII. 
 
 1900. Vaughan, T. W. : " The Eocene and Lower Oligocene Coral Faunas of the United States." U. S. 
 
 Geol. Survey, Monogr. xxxix. 
 
 1901. Verrill, a. E. : "Variations and nomenclature of Bermudian, West Indian and Brazilian Reef 
 
 corals, with notes on various Indo-Pacific corals." Trans. Conn. Acad. Science, vol. xi. 
 1888. Wilson, H. V. : "On the development oi Manicina areolata" Journ. Morph., vol. 11 C1889). 
 
EXPLANATION OF PLATES. 
 
 Plate i. 
 
 Fig. I. — Larva immediately on extrusion, viewed by reflected light. The uniform covering of cilia is 
 indicated, and also the distinction between the outer ectoderm and the solid internal endoderm. The 
 darker color of the broader oral pole is due to the presence of Zooxanthellae in the ectoderm. The 
 narrower aboral pole is anterior in swimming; the mouth is not yet functional. 
 
 Fig. 2. — Abnormal larva with two oral poles and one aboral. 
 
 Fig. 3. — Larva shortly after extrusion. The larva is now more swollen and transparent, so that four 
 pairs of mesenterial lines are seen; the mouth is also functional. Two pairs of mesenteries reach the 
 stomodseum, and two other pairs are free. 
 
 Fig. 4. — A second-day larva just before settling. Six pairs of mesenteries are now present, three 
 pairs of which reach the stomodseum. 
 
 Fig. 5. — Three larvee settling close together by the narrow aboral pole. 
 
 Fig. 6. — A group of seven third-day larvae which have settled so close together that their walls partly 
 overlie. Four pairs of mesenteries now reach the stomodeeum, and on three of the individuals the 
 rudiments of the six exocoelic tentacles have appeared. 
 
 FiG. 7. — A living polyp two or three days after settling, viewed by transmitted light. The dark 
 outermost rim represents the epitheca; the next lighter zone is the flat margin of the polypal wall into 
 which the polypal cavity does not extend. The six entosepta are opaque, but in such a view there is no 
 evidence of the basal plate. The six tentacles are exocoelic and alternate with the septa. The internal 
 Zooxanthellae have accumulated mainly along the sides of the mesenteries; the ectodermal Zooxanthella 
 have disappeared except immediately around the mouth. Diameter of original, 1.5 mm. 
 
 Plate 2. 
 
 Fig. 8. — Living polyp with a dorso-lateral pair of exosepta and a rudimentary pair of median exosepta 
 in addition to the six entosepta. 
 
 Fig. 9 — Polyp with six entosepta and six exosepta, the latter still revealing their dorso-ventral order 
 of development by differences in maj^nitude. 
 
 Fig. 10. — A living polyp of about the same stage as fig. 9, fully expanded and viewed from the side 
 as a transparency. Of the mesenteries only the insertions are represented. 
 
 Fig. ii. — Polyp with twelve fully expanded tentacles, six large outer exocoelic and six small inner 
 entocoelic. Fifth week. 
 
 Fig. 12. — Polyp showing the irregular manner in which peripheral additions are made to the primary 
 septa. The ventral directive tentacle is double. Diameter of original, 1.7 mm. 
 
 Plate 3. 
 
 Fig. 13.— Expanded disc showing the doubling of certain of the entotentacles and the asymmetrical 
 position of others. 
 
 Fig. 14— Expanded polyp viewed from the side so as to display the manner of appearance of the 
 second-cycle mesenteries on the column wall. 
 
 Fig. 15. — Expanded disc resting on the septa and exhibiting the relationship of the second-cj'cle mesen- 
 teries to the new entosepta and exotentacles. An additional exotentacle has appeared within the middle 
 sextant on each side, and the dorso-lateral entotentacles have each a single stem bifurcated distally. 
 
 Fig. 16. — Expanded disc showing the manner of increase of the tentacles. An exotentacle protrudes 
 from each of the two exocoeles in the dorso-lateral and median sextants, but as yet there is only one from the 
 ventro-lateral sextants. The tentacle from each dorso-lateral entocoele is already bifurcated as in the adult. 
 
 Fig. 17. — The same polyp as in fig. 16 at a later stage. On each side a second-cycle tentacle (II) has 
 grown out over the entocoele of the median second-cycle mesenteries, and the four lateral first-cycle ento- 
 tentacles (I) are bifurcated. Diameter of original, 2 mm. 
 
 Fig. iS. — A larval polyp of three months in which the disc and tentacles are indrawn and the disc is 
 almost covered by the overfolding column wall. The epitheca is shown around the margin. 
 
 126 
 
EXPLANATION OF PLATES. 1 27 
 
 Plate 4. 
 
 Fig. 19. — Photograph of early corallum with six entosepta and six exosepta. The basal plate (proto- 
 theca) is fully formed, but somewhat irregular in outline; it is a little thicker and upturned towards the 
 margin. The six entosepta are nearly equally developed, while the alternating exosepta vary much in 
 size; the dorso-lateral pair are best developed and peripherally seem double, the middle pair are a little 
 smaller, while the ventro-lateral are yet rudimentary. Diameter, 1.6 mm. 
 
 Fig. 20. — A somewhat later corallum. The directive entosepta are irregular in form and differ 
 from one another, the ventral appearing as if bifurcated peripherally; the two dorso-lateral and right 
 middle exosepta also appear bifwrcated ; the ventro-lateral exosepta are still much smaller than the other 
 exosepta. — - 
 
 Fig. 21. — A young preserved polyp photographed by transmitted light. In such a view the basal 
 plate is not apparent owing to its transparency. The septa are non-transparent and stand out as black 
 objects. Indications of the six pairs of radiating mesenteries are seen and also some of the tentacles, but 
 the details around the mouth are obscured by the mesenterial filaments and stomodaaum. The dotted 
 surface of the polyp is due to the presence of internal Zooxanthellae. The septa are a little further 
 developed than in fig. 20, the exosepta still showing a decided dorso-ventrality. There is a marked 
 difference in form between the dorsal and ventral directive septa. Diameter, 1.7 mm. (In the process of 
 engraving the principal axis of the polyp has been turned counter-clockwise through an angle of about 
 20 degrees from the vertical.) 
 
 Fig. 22. — The same polyp as in fig. 2i"photographed by an exposure of 20 minutes to polarized light 
 between crossed nicols. Only the calcareous crystals transmitted the variously colored light. The 
 mottled surface represents the basal plate which is not indicated in the previous figure. The crystalline 
 matter of the septa is so thick and irregularly arranged that no light was transmitted ; hence the structures 
 appear black. 
 
 Fig. 23. — A typical corallum at a later stage, growth having proceeded upon the septal plan of figs. 19 
 and 20. The polyp remained at this stage for some time, and a well developed wrinkled epitheca has 
 formed, its upper edge higher than the septa ; the latter are fused peripherally with the epitheca, but there 
 is no theca. The directive septa are somewhat bifurcated peripherally and also the dorso-lateral and 
 median exoseptal pairs. The dorso-lateral pair are almost fused centrally with the dorsal directive, the 
 middle exosepta are fused with the dorso-lateral entosepta on each side, and the ventro-lateral exosepta 
 with the ventro-lateral entosepta. The ventro-lateral exosepta are the smallest of the whole series. Sepa- 
 rate columellar nodules are present in the middle. For diagrammatic plan see p. 88. Diameter, 1.8 
 mm. (In the process of engraving the principal axis of the corallite has been turned clockxvise through 
 an angle of about 20 degrees from the vertical.) 
 
 Fig. 24. — Another corallum at about the same stage as fig. 23, but showing variation in detail. 
 
 Plate 5. 
 
 Fig. 25. — A corallum in which the epitheca has gradually receded from the margin towards the 
 middle of the calice as the polyp shrunk in size. The older part of the epithecal deposit covers the 
 peripheral parts of the septa. The newest portion of the epitheca is the inner annulus. The septa are 
 at about the same stage as in fig. 23. 
 
 Fig. 26. — Corallum in which the polyp has twice diminished in size, somewhat suddenly. Three 
 concentric epithecae have been formed, the peripheral ends of the septa being partly exposed between each. 
 
 Fig. 27.— The septa are a little further advanced than in figs. 23-26. In the left dorso-lateral sextant 
 there are three distinct septa, a large median and two lateral; the median septum consists of a larger 
 central (exoseptum) and a small peripheral part (entoseptum). In the right middle sextant are also three 
 septa, the median of which consists of a central and a small peripheral part {cf. fig. 15). 
 
 Fig. 28.— This and the next corallum are the two oldest reared. Each contains three complete cycles 
 of septa, and some of the sextants show important stages in the passage from the second to the permanent 
 third cycle. In the polyps which formed the coralla the first and second cycles of mesenteries were fully 
 developed. The diagrammatic representation of the stage is given on p. 89, and emphasizes the manner 
 of fusion of the various septa with one another. The directive septa are stouter than any of the others. 
 The three septa within the ventro-lateral sextants are not so strongly developed as those within the median 
 and dorso-lateral sextants, and show very distinctly the twofold constitution of the median entoseptum 
 
128 EXPLANATION OF PLATES. 
 
 and the independent character of theexoseptum on each side {cf. fig. 15). Synapticula now connect some 
 of the adjacent septa. The origin of part of the columella from nodular upgrowths, independent of the 
 inner septal edges, is very evident. The basal plate is thicker peripherally than centrally, and the epitheca 
 is only feebly developed at the margin. Diameter of original, 3 mm. 
 
 Fig. 29. — The details presented are practically the same as in the previous figure, but the growth of 
 the three septa within the ventro-lateral sextants has reached the same stage as in the other sextants. 
 The epitheca is better developed than in fig. 28. 
 
 Fig. 30. — Double corallum produced by two ptjlyps the larVBB of which settled close together. Each 
 has an independent epithecal formation, the part common to both being very irregular. The septa have 
 not preserved the regular hexameral plan. 
 
 Plate 6. 
 
 Fig. 31. — Three fully expanded adult polyps, showing the forms assumed by the polyps and the 
 arrangement of the bilobed and simple tentacles. 
 
 Fig. 32. — Nearly mature polyp preserved in formalin in a partly expanded condition; surface view. 
 The tentacles over the third-cycle entosepta (III) are still simple and resemble the exotentacles (X). 
 
 Fig. 33. — An isolated interseptal lamella from a decalcified polyp. The lower margin represents the 
 part of the lamella which rested upon a dissepiment, and about the middle nearly surrounded a synap- 
 ticulum. The apertures (syti) are the spaces formerly occupied by the synapticula. The column wall, 
 disc, a bifurcated tentacle, stomodjeum, and a mesenterial filament are shown. 
 
 Fig. 34. — Transverse section of a mature polj'p, passing through the stomodaeal region. The figure 
 shows the arrangement of the mesenteries, the septal invaginations, and sj'napticula. In this and other 
 figures the spaces occupied by the skeleton are represented by the dotted areas. 
 
 Fig. 35. — Transverse section through a portion of the calicinal ridge of two retracted polyps, showing 
 the continuity between the mesenterial chambers of adjacent polyps, and also the vertical free edge of the 
 mesenteries as they extend from the column wall to the skeletotrophic tissues. In the upper part the 
 septa are exsert, but below they are united with the calicinal wall {ex. sept., exoseptum; ent. sept., 
 entoseptum). 
 
 Fig. 36. — Tangential section towards the periphery of a polyp. The column wall rests upon the septal 
 edges, and the mesenteries have a short vertical extent. 
 
 Plate 7. 
 
 Fig. 37. — a, fully expanded entotentacle ; b, fully expanded exotentacle. 
 
 Fig. 38. — Transverse section of the same polyp as in fig. 34, taken a little below the stomodaeal region. 
 
 Fig. 39. — Transverse section of a polyp towards its aboral region. The mesenterial loculi are alto- 
 gether isolated at this level, and appear further broken up hy the synapticula stretching from one septum 
 to another. The mesenteries have almost disappeared and the middle of the section is occupied by the 
 columella. The skeletal ectoderm and endoderm are both represented. 
 
 Fig. 40. — Tangential section through the tentacular region. A single bilobed entoccelic tentacle is 
 almost resting upon a septal edge. In the septal invagination {sep. inv.) the calicoblast layer is repre- 
 sented. 
 
 Fig. 41. — Transverse section through a mesentery at the level represented in fig. 35. The meso- 
 gloeal plaitings for the support of the retractor muscles are simple in character, the oblique musculture 
 is feebly seen, and also the desmoidal processes where the mesentery is attached to the skeletotrophic 
 layer {sk. I.). 
 
 Fig. 42. — Transverse section through a mesenterial filament immediately below the stomodseal region. 
 
 Fig. 43. — Transverse section of a mesentery some distance below the stomodeeal region, showing the 
 form of the mesenterial filament, and also a single ovum ; the peripheral end of the mesentery is under- 
 going resorption. 
 
 Fig. 44. — a, small nematocysts from the knob of the tentacle and column wall ; b, elongated form of 
 nematocysts from the knob of the tentacle and mesenterial filaments ; c, large oval nematocyst from the 
 lower part of the mesenterial filaments. 
 
EXPLANATION OF PLATES. 1 29 
 
 Plate 8. 
 
 Fig. 45. — Section through part of a mesentery in union with the skeletotrophic tissues where they 
 surround a synapticulum. The mesenterial mesogloea for some distance from its termination is finely 
 striated, forming a desmoidal process. The skeletotrophic ectoderm on the left side shows faintly a 
 narrow portion of the homogeneous skeletal matrix. 
 
 Fig. 46. — Transverse section through an isolated portion of the lining wall of an interseptal loculus. 
 Each lateral extremity was terminated by a synapticulum. The section was stained in iron haematoxylin, 
 and shows very distinctly the mucous spaces in the inner endoderm and calicoblast layer. The latter also 
 contains coarsely granular cells and nematocysts. 
 
 Fig. 47. — Transverse section through the skeletotrophic layers in their uppermost part, showing the 
 very narrow endoderm and calicoblast layer. The layers are syncytial in character. Irregular canal-like 
 spaces are filled with a deeply-staining substance. Small irregular particles are adherent to the free 
 surface of the ectoderm. 
 
 Fig. 48. — Transverse section through a part of the skeletotrophic layers from the lower region. The 
 section gives practically the same details as fig. 46, but is more highly magnified. 
 
 Fig. 49.— Longitudinal section through an expanded tentacle. The stem is simple in character, while 
 the swollen knob has a nerve and a muscle layer at the base and nematocysts and gland cells {gr. g. c.) 
 towards the periphery. 
 
 Fig. 50. — Transverse section through a portion of a mesentery showing the system of irregular 
 spaces in the endoderm. The Zooxantheliae and cut ends of the retractor muscle fibers arranged along 
 mesoglceal plaitings are also represented. 
 
 Fig. 51. — Transverse section through the stomodaeal region of a larva preserved shortly afler extru. 
 sion. The six primary pairs of mesenteries are present, but the dorsal directives (iv, iv) and pairs v and 
 VI are yet free from the stomodaaum. The gastro-ccelomic cavity is beginning to be established in the 
 nearly solid endoderm; Zooxanthellse are present in the thick stomodasal ectoderm. In this and the 
 next figure the histological details of the endoderm are represented, while the ectoderm is convention- 
 ally depicted. 
 
 Fig. 52. — Transverse section through the same larva a little below the stomodoeal region. Mesen- 
 terial filaments are forming at the free end of the first and second pairs of mesenteries. Endodermal 
 thickenings (Prosepten) are present between the larger pairs of mesenteries. 
 
 Plate 9. 
 
 Fig. 53. — Radial vertical section through a young polyp, showing its relationship to the early coral- 
 lum. In the latter are shown the basal plate (A. //.), epitheca («/.), two septa, and acolumellar upgrowth 
 {col.) 
 
 Figs. 54-60. — Serial transverse sections through a portion of a polyp, showing the relationships of 
 the mesenteries and septa in the development of a third-cycle entoseptum (d-c) and two fourth-cycle 
 exosepta (a-6, c-cf). The full description of the series is given on p. 100. 
 
 Plate 10. 
 
 Fig. 61. — A free corallum. Natural size. 
 
 Fig. 62. — A portion of the same corallum magnified about four times, showing various characteristics 
 of the corallites and their relationships to one another. 
 
 Fig. 63. — Upper part of three septa as seen in a vertical surflice view. The left and middle septa extend 
 across a calice, but are separated in the middle by the columella and the broken central edges of other 
 septa in union with it. The right septum belongs to an adjacent calice, and is separated from the middle 
 septum by a vertical ridge which represents the lineof fusion of an adjacent septum not in the same radius. 
 The two or three vertical rows of large elevations on each septum represent the broken surfaces of synap- 
 ticula ; the smaller elevations are granules. The inner central margin of the left septum shows the denti- 
 culations as they unite with the columellar tangle, and near them the fractured surfaces indicating where 
 an adjacent septum was interruptedly united by its inner margin. 
 
 Fig. 64. — Ground surface of a portion of a corallum showing the relationship of a single calice to the 
 six adjacent calices. Surface view. The long axis of the columella represents the principal axis of the 
 calice, the directive septa being at opposite extremities, and not quite in the same plane. 
 
 Fig. 65. — Portion of a thin transverse section of a single corallite showing the microscopic structure. 
 The columella, septa, and synapticula seem as if constituted of spheroids fused together; the spheroid* 
 represent transverse sections through so many individual trabeculaa. 
 
130 EXPLANATION OF PLATES. 
 
 Plate ti. 
 
 Fig. 66. — Transverse section through a single trabecula, penetrated hy filamentous boring algse, and 
 showing the growth lamellae and their fibrous character. 
 
 Fig. 67. — Vertical section through a part of two adjacent septa. The septa are made up of trabeculae 
 arranged in a radiating manner; they diverge from a continuous median trabecula which represents the 
 boundary between the two adjacent calices. The different appearances presented by the various trabec- 
 ulae are dependent upon the part included in the section. Where the section passes along the middle of 
 a trabecula the interrupted dark centers of calcification are seen ; elsewhere only the diverging bundles of 
 fibro-crystals appear. The growth lamellte are not shown (cf. fig. 68). New trabecule are intercalated 
 at intervals. The dark circular and oval patches are sections of synapticula. 
 
 Fig. 68. — Terminal portion of two trabeculaa as seen in longitudinal radial section, that is, parallel to 
 the surface of the septum, more highly magnified than in the previous figure. The section passes along 
 the middle of the trabeculae so that all the dark centers of calcification are seen. In the corallite from 
 which the section was taken the growth lamellae were very distinct, the boundary of each being indicated 
 by a dark granular deposit similar, to that at the centers of calcification. The growth lamellae are arranged 
 parallel with the toothed edge, that is, with the calicoblast layer which secretes them, while the fibro- 
 crystals making up the lamellae are arranged at right angles. The figure should be compared with fig. 
 66, which shows a trabecula in transverse section. 
 
 Fig. 69. — Portion of basal plate and epithecal boundary of a very early corallum. The epitheca is seen 
 in section, and the basal plate is sufficiently thin to allow of the passage of light. The basal plate is made 
 up of more or less distinct granules or scales showing a fibrous structure; the aggregation of darker 
 granules represents the first formation of a septum. 
 
 Fig. 70. — Portion of basal plate, septa, and epitheca of a somewhat older eorallum than that of fig. 69. 
 The septa and epitheca are ground down to nearly the level of the basal plate. Centers of calcification 
 are not present in the basal plate and epitheca but are very prominent in the septa. 
 
SIDERASTREA RADIANS 
 
 PLATE 1 
 
 HELIOTYPE CO., BOSTON. 
 
SIDERASTREA RADIANS 
 
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 HELIOTYPE CO., BOSTON. 
 
SIDERASTREA RADIANS 
 
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 HELIOTYPE CO., BOSTON. 
 
SIDERASTREA RADIANS 
 
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SIDERASTREA RADIANS 
 
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SIDERASTREA RADIANS 
 
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IDERASTREA RADIANS 
 
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SIDERASTREA RADIANS 
 
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3IDERASTREA RADIANS 
 
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THE UNIVERSITY LIBRARY 
 
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