te ee ee a el Bok ka Oe ne, ee Bah = Y Pet Be FD ad | a << (ies \ Ae i ie aah THE CORAL SIDERASTREA RADIANS AND ITS POSTLARVAL DEVELOPMENT. BY J. E. DUERDEN. WasuincTon, U. S. A. PUBLISHED BY THE CARNEGIE INSTITUTION, DECEMBER, 1904. CONTENTS, Preface .ccsces vavees Seupabas ckanentersiccses semekeed Geacoend * Introduction ..scsessecsesseeeeeeee Mea UCRii sad taneasavacioess Aputt Cotony. External Characters...... .....++ at Column wall...cssccseeseeseeeee pivastidtee RUA RRUTERS Fc cox cesses nami sacavtadiunscenessaraicace dan Disc and mouth.,.....seeceeereere éiakenes woeseevece . COOP: .occcvccceacscns: 09 aneseses becesencssncee se ewenves Reproduction. ....sccscccrseeseeecesscesserassensoeees External characters on decalcification....... Anatomy and Histology... ...ccs-secesssseeeceneenesenes Column wall and disc.........s00seeeneee Tentacles...... Stomodzeum.. WASGOT ELI CM esas cee cunshs ccossecnctascvecrsncésvdtease Mesenterial filaments.....,.........s00reesse-soes Skeletotrophic tissues...........+. eussnseacevsness Septal invaginations, interseptal loculi, and gastro-ccelomic cavity......00..++ paenees Gonads......-00++ o tee eeeeeeees ceeceeee sucanesenees .soeer Corelli cave dels cakes ivonsspenvadbacacen.s suevespstseeesesssene HistOlogy.....cccsscssscscceesseasessesescoeses soesenees Wall or theca..ccccccovsessvovcrecseeceees: sensvecesens Septaccsiscccrsecesrssssvesessssosvcccncvessecvansessssess Synapticula........... GOMAITIOUR : cacecusetsonceevedeavenatias Dissepime nts ...c00sccssesooees concen cccees cosscecnee Epitheca and basal plate.........cccssescseeree v 7 PosTLARVAL DEvELOPMENT. ° coe 87 VOUung POlYD< sec pexscecoucsesses esescoesessee OF POREACI OR: cescosescessivshbrenvey eed seedeteesss srsereosvess 65 First cycle of exotentacles........sc0sesseeressee 65 First cycle of entotentacles.......... sercrsceees 66 Secondary exotentacles.........ss00seeeeee dessiee, | 69 Second cycle of entotentacles and third cycle of exotentacles.; icsass ) ey, SE: Fic. 7.—a~e, series of diagrams illustrating the order of app and relationship of the first three cycles of 1 The Is I and II indicate the orders of entotentacles and X the exotentacles. CY\F 50) 8 Ne m1 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.” WH POSTLARVAL DEVELOPMENT. 73 In Szderastrea 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 Szderastrea as for actinians, though the relative time of appearance of the entotentacles and exotentacles differs from that usually followed. Faurot (1895) shows that in Llyanthus parthenopeus the entotentacle from a new mesenterial pair arises in advance of the exotentacle, exactly the reverse of that in Szderastrea. 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. 1, 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 (111) have appeared two corresponding tentacles, and, comparing with fig. 3, p. 26, one of the tentacles is seen to be entoccelic (11) and the other exoccelic (x). The new entotentacles, 111, are situated beyond the second cycle of entotentacles, 1, 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 and 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 Szderas- 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. radzans 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. radzans ; two cycles of entotentacles are hexamerously 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 +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 Szderastrea 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 S. 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 exocceles. ‘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 entocceles. ‘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 entoccelic 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 exoccelic 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 entocceles 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 entoccelic and one exoccelic, arise in connection with each. The entoccelic representatives appear between the second and outermost cycles of the previous stage, and thus constitute a new third cycle; the exoccelic 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 SS. 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 larvee 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 stomodeum, but the dorsal and ventral axial pairs are free, the ventral pair being a little larger than the dorsal (plate 1, fig. 3). In other larve, only a day or two older, the two ventral mesenteries have become united with the stomodzum, but the dorsal are still free, and two new pairs, the fifth and sixth, are beginning to make their appearance (plate 1, 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 stomodzeum (plate 1, fig. 6). This is the stage represented by all the larve 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 larve the vertical muscle fibers of the mesenteries are already sufficiently well developed to permit of the directive POSTLARVAL DEVELOPMENT. 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 larvze, the two bilateral pairs which first unite with the stomodzum (plate 1, 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 larve, 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 1, 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 stomodzeum 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 1, 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 stomodzeum in advance of the more ventral. Whatever differ- ences exist in size or time of union with the stomodzeum 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 Szderastrea radians is in strict agreement with that ascertained by H. V. Wilson (1888) for Manicina areolata, and by Von Koch (1897) for Caryophyllia cyathus, 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 Cerianthez and Zoanthee is not yet conclusive. The length of time the young polyps of Szderastrea 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 stomodzum, 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 larvee 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 stomodeum in the Edwardside, Gomactinia, 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 modern 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 Cerianthez, 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 Cerianthez and Zoantheze 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 modern anemones and coral polyps; and four such zones or bands of proliferation would appear to have been characteristic of the mesenterial growth in Paleozoic coral polyps. ‘The ~® Appelt (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 Hérouard (1902) transpose the order of the fifth and sixth pairs as given above. aah EEE OL ee 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 exoccelic pairs in all the sextants, the fifth and sixth protocnemic pairs begin to unite with the stomodzum, 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 stomodzeum of the fifth and sixth protocnemes, found to be characteristic of recent actiniarian and madreporarian polyps. The first twelve mesenteries in Szderastrea 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 stomodeum. ‘Thus a truly 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. SECOND 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 stomodeum. 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 exoccele 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 exoccele (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; 80 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—/). The interval between the appearance of the dorsal and middle, and between the middle and ventral pairs, was usually well marked, so that at IV IV IV Ill Fic. 8 (a, 5, 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), @, 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 exoccele was apparent for over a week in advance of the right pair in the corresponding exoceele. 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 stomodeum, this condition being also characteristic of the adult polyp. POSTLARVAL DEVELOPMENT. 81 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 I < I IIb Il Fic. 8 (¢,/, g, 4), continued.—Series of diagrams illustrating the order of development of the first three cycles of mesenteries. eand_/ represent the second and third stages for second cycle, and g and & are two early stages of third cycle. the relative sizes of the pairs in certain late larvee 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 equal 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 Ilyanthide often present it, and probably all higher actinians in the course of their development. It is repeated by the many pelagic anthozoan larvye described by Van Beneden (1897, p. 189), numbered vit to xv, and which the author regards as belong- ing to the Hexactiniaria. Illa Fic. 8 (7, &), continued.—Series of diagrams illustrating the order of development of the first three cycles of mesenteries. J is a late stage, and # 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 Szderastrea is that most usual for ordinary anemones and corals; new isocnemic pairs appear in the six primary exocceles all round the polypal wall. In the group of the Zoantheze the metacnemes appear at only two restricted regions, one within the exoccele on each side of the ventral directives, and each unilateral pair consists of a large and a small member (anisocnemic). In the Cerianthee the metacnemes appear as bilateral pairs at only one region, within what seems to be the entoccele of the ventral directives. Furthermore, the metac- nemes in the Zoanthee and Ceriantheze do not form one or more cycles distinct from the protocnemes (monocyclic), as in Szderastrea, most other modern corals, and ordinary anemones (polycyclic). 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 atime. The second-cycle septa in .S. radzans are also interesting in this. respect, for in some polyps 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 84 SIDERASTREA 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 stomodeeum, 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, g, p. 81; 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 exoccele 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 exoccelic 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, 4, p. 81, the next stage available, where a third- cycle pair (111) is found within the dorsal exoccele of each of the six systems. A later stage secured in the establishment of the twelve third-cycle mesenteries is given in fig. 8, 7, p. 82, where an additional pair (m1, 2) has appeared, this time within the ventral exoccele 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, 4, p. 82. No stage exactly corresponding with this figure, however, has been obtained, as the polyps of S. vadzans 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-#, 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. radzans 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 larvee. 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 II (p. 100), and illustrates the variability encountered. In fig. 3 the sequence is normal except that a pair of mesenteries (111¢) has appeared within the ventral exoccelic chamber of the right middle system in advance of the pairs in the dorsal exocceles of the ventral systems. ‘The polyp represented in fig. 11 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 solitarta 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 Condylactts cruentata, 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 111¢ and id in the enumera- tion of fig. 8, &, p. 82, were larger than pairs 114 and mle, and these than pairs 1c and 111/; hence, if the condition obtained by Carlgren really repre- sents the sequence followed in the growth of the third-cycle mesenteries in Condylactzs, 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. Férh., 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. radzans having mesenteries belonging toa 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. radzans 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 stomodeeum, but the two last pairs (v, vt) 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 exocceles 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 exoccele of each sextant. Generally some of the pairs of third-cycle mesenteries are wanting in mature olyps. polyP 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 1, 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 exoccelic 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. 87 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 exocceles, mere rudiments of septa are found in the middle exocceles, 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. Asin 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 sametime. 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 Edwardsian mesenteries were united with the stomodzeum, and of the tentacles the six exoceelic 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 Szderastrea. 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 Fic. 9 (a-e).—Series of diagrammatic figures illustrating the manner of development of the septa from one to three cycles (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 exoccelic 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 Fic. 9 (7-7), continued.—Series of diagrammatic figures illustrating the manner of development of the septa from one to three eycles (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 go SIDERASTREA RADIANS. distinctly angulated at its peripheral end, and where two additional elements were introduced in each chamber the periphery of the septum was strongly bifurcated. In some few cases no separate fragments whatever 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 different methods by which the septa may increase in lengthandcomplexity. Although, as above remarked, no two polyps were exactly the same, a general plan was determinable throughout the many examples studied. The septain 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, ¢, p. 88, 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 entoccelic 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 ventro-lateral 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 DEVELOPMENT. 91 weeks old, in which only six pairs of primary mesenteries and two cycles of tentacles were present, while fig. 9, ¢, p. 88, is its diagrammatic representa- tion. Each septum now bears a closer resemblance to the septa in the mature corallite, 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 fusion of the originally separate elements. On the other hand, the dorso-lateral and middle pairs of exosepta are strongly bifurcated periph- erally, while the members of 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, e, 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, 4, p. 89. The dorsolateral 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 Szderastrea 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 Szderastrea presents a general agree- ment with that of other corals whose early history has been followed. Lacaze-Duthiers (1873, 1897), however, found that in Astrozdes calycularis, Balanophyllia regia, Leptopsammia, and Cladopsammza 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 Szderasérea, whereas in Manzcina 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 toa 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 atatime. 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 Szderastrea are alone in the throwing back of the dorso-ventrality of the skeleton as far as the exoccelic 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 Astrozdes 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 Szderastrea. 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 Caryophyliia 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- cceles of these are the six primary entosepta; the alternating six members of the second cycle of septa are within the exocceles, on radii. midway between the entosepta (plate 2, fig.9). Inthe 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 entocceles of the second mesenterial cycle, *Where in larval actinians only four pairs of mesenteries are present when the tentacles begin to develop, only 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 exocceles, and are therefore exosepta. Thus at both stages the exosepta form the outer- most cycle (cf fig. 10, 6, 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 entocceles the exosepta of a former stage, and then new exo- septa have arisen within the newly formed exocceles. The latter is the view commonly held by writers on coral development. Thus Delage & Hérouard, in their “Traité de Zoologie Concréte” (1901, p. 558), remark: “Quand, dans les interloges occupées 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 bientét un nouveau cycle de septes se forme dans les nouvelles interloges qui viennent d’étre formées. Les cycles naissent successivement et jamais un cycle ne commence a se former avant que le précédent 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 exocelic order of septa become entocelic by the development of new pairs of mesentertes. ‘The increase of mesenteries takes place par 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 history of corals. Without doubt the members of the primary 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 Szderastrea remained for some time at the stage Ee ro" ) 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 exoccelic bifurcations, and necessarily inclosed within the entocceles of the second-cycle 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 entoccele of the second-cycle mesenteries, and in the same radius as the original exoseptum. Moreover, the exoccelic bifurcations are now seen to be wholly exoccelic 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, ¢, p. 96, is a diagrammatic representation of the polyp and corallum at this stage. The new formations within the second-cycle entocceles 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, f, 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 inturned edge with a second-cycle entoseptum. 96 SIDERASTREA RADIANS. Three important results in the development of the Madreporarian skeleton are thus gained: (1) The primary exosepta do not continue their Fic. 10 (a-7).—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 peripherally in a radial manner, and constitute entosepta by becoming included within the entoccele of the new second-cycle mesenteries. (2) The POSTLARVAL DEVELOPMENT. 97 secondary entosepta are new formations, arising within the second-cycle entocceles 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, 4, p. 89, is in somewhat the same stage as fig. 10, ¢. In the upper right sextant the second-cycle entoseptum (II) is yet very distinct from the exoccelic 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, J, p- 89, and fig. to, f, p. 96. Each 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-laterals 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 entocele 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 asingle 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 entoccelic 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.* S20 aotas scokale the exoseptal predecessors appear to continue their independent growth én 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 of ga/¢ 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 the 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 Szderastrea 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 Szderastrea 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 larve 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. 100 SIDERASTREA RADIANS. 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 Fic. 11.—Diagram of mesenteries in the polyp from which figs. the tertiary entosepta arise de 54-60, on plate 9, were taken. novo g 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 stomodzal 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 difficult 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 (111,), 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 = POSTLARVAL DEVELOPMENT. Io1r lettering (a—y) 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 ¢ are the pair of second- cycle mesenteries, while 4 and care the two members of the rudimentary pair of third-cycle mesenteries. The incompletely represented entoseptum to the left of a is a member of the primary cycle of septa, as is also the partial septum to the right of /; the entoseptum d-e belongs to the secondary cycle, while the septa a—d and e-/, in figs. 54-56 are exosepta. ‘The upper margin of the section in fig. 54, is formed by the stomodzeal 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 stomodzeal region. ‘The members of the mesenterial pair, 4, c, of fig. 11, have not yet reached this level, so that the sextant contains only the pair of incomplete second-cycle mesenteries, d,¢. The mesenteries a and / are united with the stomodzal 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-/ 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 stomodzeum, so that it is bounded above by the disc resting upon the septal edges. At this level mesenteries d and ¢ are larger than in the former section and are without any peripheral degeneration; exoseptum e-/ is 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, 4, 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 (¢) of the pair is still free; in none is there any peripheral resorption. Exoseptum e-/is once more distinct centrally from entoseptum 102 SIDERASTREA RADIANS. d-e, as in fig. 54; a synapticular growth from the left limb of septum a—d perforates the mesentery 4, and a slight depression of the wall between the mesenteries 6 and ¢ is the first indication of an entoccelic 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 f 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 inuer angle, and also from the central radial portion; both may be regarded as distinct exosepta a—d 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 entoccele of the new pair of mesenteries 6 and ¢, and at its free extremity is on the point of uniting with the synapticular process from the exoseptum a-d. ‘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—d, 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—d, from the central radial part, while exoseptum a—é and entoseptum é-c are united with one another towards their free edge by a synapticulum perforating the mesentery 4. 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 4 is perforated by the synapticulum still joining septa a—d and d-c. The central portion of the original exoseptum a-d has 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 4 and ¢ both extend across the two portions of the column wall, and the septa a—d, d-c, and c-d are distinct from one another, the entoseptum J-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—d, b-c, and c-d can be diagrammatically represented as in fig. 12 (a—/) below, which shows also their relations to the mesenteries. POSTLARVAL DEVELOPMENT. 103 The first two figures (a, 6) 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, 5. In relation to the mesen- teries the two limbs are clearly exoccelic, and may even at this stage be regarded as two individual exosepta. In fig. 12, ¢, one exoseptum has become distinct, and a short septum has appeared midway between the two older septa (c/. 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 sa \ Y \ \ Y ) \ —+ c' opscecinsrenoore Fig. 12,—Series of diagrammatic figures illustrating figs. 54-60 on plate g. figure, / (f 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, f, 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. 10 (af), 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, a, 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 insufficient 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 exoccele of each sextant, successively from the dorsal to the ventral border of the polyp; then another pair appears within the ventral exoccele of each sextant, following the same succession as the first series. Wa eX iile Wee yx yx | CE Fic. 13.—Diagram illustrating the order of development of the first three cycles of septa (I-III). The small letters accompanying the Roman numerals indicate the sequence of the indi- vidual septa in the cycle, The normal sequence of the third-cycle mesenteries in Szderastrea 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 m1a-11f 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 atime; 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 106 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 may consider them as the direct continuations of the six primary exo- septa, or, less likely, 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- ceelic and one exoccelic, 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 Szde- 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 theyappeared. 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 last 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. ie «. — POSTLARVAL DEVELOPMENT. 107 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 may be forthcoming than is possible in Szderastrea. 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. radzans, 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 1 to 12, resulting from the fact that at maturity the polyp does not complete the last cycle begun. Exosepta 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 S. rvadzans 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. vadzans, Milne-Edwards 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 4o.” The relationships proved to exist between the entosepta and exosepta in- dicate that the above formule 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 108 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-Edwards 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 1 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. radzans, 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 _— ae POSTLARVAL DEVELOPMENT. 109 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 polyp 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 ; aaowlan, 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 different cycles, arising independently, seem to represent so many distinct recurring phases of growth in the life of the polyp, 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 Cerianthee. 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 Zoanthez also mesenterial growth is always in the same succes- sion after the protocnemic stage. ‘The increase takes place within the two exoccelic 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 + @ ce) 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 ccelenterates, and as yet we appear to have no definite understanding as to what even this may be. The » approximate radial symmetry of adult ccelenterates is assumed from very diverse developmental conditions (cf hexactinians, zoanthids, cerianthids, and the tentacles and other cyclic organs in the Hydromedusz and Scypho- meduse). 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 Szderastrea 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 exoccelic 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 in i OS re POSTLARVAL DEVELOPMENT. IIrt 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. Newentosepta appear within the entocceles 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. Exosepta are present at each cyclic stage in the growth of the tical 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 Hexactiniz, 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 1 to 12. g. 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 Astrotdes calycularis 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 to 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 .S. 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 Szderastrea, 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. 113 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 polypal 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. Atthe 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 trabeculze 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 cor to 0.015 mm., while their height is about 0.003 mm. 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 Szderastrea 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 thezupper surface. In addition to the II4 SIDERASTREA RADIANS. septal rudiments, small projections are present here and there on this surface, especially towards the 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 11, 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 S. vadzans 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. vadzans 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. | ; ; iy 4 7 b ; 4 > ~ POSTLARVAL DEVELOPMENT. IIs 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 within 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 alge 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 lamelle, 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, 116 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 Szderastrea 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 Caryophyliia 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 Szder- astrea it is external from the beginning, only covered by the polypal 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. 117 159) separates the 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 Caryophylia von Koch found no trace of an epitheca in addition to the theca, while in Astvozdes and now in the early corallum of Szderastrea 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 alater 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 Szderasirea. In this respect, therefore, the genus must be cotsidered as representing an older type than the truly thecate corallum of such a form as Caryophyliia. 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, 118 SIDERASTREA RADIANS. extending as far as the 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 & Hérouard (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. radzans is therefore formed from three independent sources: (1) Separate basal upgrowths; (2) septal spines, continuous with the central ends of the septa ; (3) a secondary deposit filling up the interstices between rand 2. Histolog- ically the trabeculz of all three are alike (plate 10, fig. 65). The columella of Szderastrea 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 DEVELOPMENT. 11g ANATOMY AND HISTOLOGY OF LARVA AND YOUNG POLYP. A number of free-swimming larve were preserved in corrosive acetic, and later studied by means of transverse and longitudinal sections, when all were found to be at about the 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 than 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 Zooxanthelle, 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 larve, and is evidently 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 stomodzeal communication between the exterior and the larval cavity was already established in all the larvee studied. In most the lumen is cireu- 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 Zooxanthellz, though these are never found in the ecto- derm of the adult stomodzum. . The mesenterial filaments on pairs 1 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 stomodzal ectoderm. In the larve, however, the organs are not yet rounded off from the mesenterial endoderm, 120 SIDERASTREA RADIANS. In the serial transverse sections from which figs. 51 and 52, on plate 8, were taken, the stomodzeal 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 (III, IV), which are onlyattached to the stomodzeal wall for part of their length, are without such modified tissue along their free edge. The continuity of the stomodeal ectoderm with the mesenterial fila- ments in the various larvee of Szderastrea, 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 stomodzeum, or do not extend as far as its inner termination. In the larve of many other Actiniaria and Madre- poraria, however, it has been found that filaments may appear on mesen- teries before they reach the stomodzum, and even in complete mesenteries an interval of undifferentiated endoderm often occurs between the ter- mination of the stomodzal ectoderm and the filaments. From observa- tions on other corals I consider that the mesenterial filaments arise from the larval endoderm independently of the stomodzeal ectoderm, but that continuity is early established in the case of those mesenteries which unite with the stomodzeum (1902, p. 476). The internal cavity of the larve 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- thellz 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 coelomic 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 Lebrunza coralligens (1899), I have shown that most anthozoan larve 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-ccelomic 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 larve of Szderastrea sectionized reveal a gastro-ccelomic cavity further developed than that represented in figs. 51 and 52. Below the stomodeeal region the parenchymatous endoderm has broken down, and the middle of the cavity is occupied by organic débris, in which Zooxanthelle, cell walls, and granules of various kinds are recognizable. ‘There is no doubt that this is the organic débris which is extruded from time to time by the larvee 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 larve sectionized eight mesenteries were already developed, arranged in four bilateral pairs,as shown diagrammatically in fig. 8, a, p. 80. The two lateral pairs are united with the stomodzum, 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 larve a day or so older the ventral directives have united with the stomodzeum, while the dorsal are still free (plate 8, fig. 51) ; alsoat 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 stomodzeum, and the larva has reached the Edwardsian 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 (Ledrunza), nor in the successive union of the first four pairs with the stomodzeum. 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 mesoglcea, 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. 122 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 thisage. ‘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 ard passu with the deposition of calcareous matter. This truth was first estab- lished by von Koch by means of sections of polyps of Astrozdes, 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 zm sztu. 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 Szderastrea 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 of Szderastrea. ‘The mesoglcea itself isa very thin lamella, The basal ee ei) P POSTLARVAL DEVELOPMENT. 123 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 Zooxanthelle 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 stomodeeal 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 stomodzal 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, stomodzal ectoderm and mesenterial filament, are much alike histologically, and by their brightly-staining character are easily recognizable among the other tissues. nu ; sie Bio eh i "ay" i ae yee a | ie . > sd Tt ee hy a a ae ae LE a: srt, ae “i ees deme tigi gk WT al ee Sak es ane . % . | 7 ] "\ = oe : . nat Ae Ley ony ie Pd . ; ls oT Ae 3 f th 1901. REFERENCES. (Only the more frequently recurring references are here given; the others are inserted in the text.) Appe tir, A.: ‘Studien iiber Actinien-Entwicklung.” Bergens Museums Aarbog. 1900, no. tr. VAN BENEDEN, E.: ‘‘ Les Anthozoaires de la Plankton-Expedition.” Résultats de la a expedition der Humboldt-Stiftung.” Vol. m, Kiel et Leipsic. Bourng, G. C.: ‘‘ The anatomy of the Madreporarian Coral Fungia.” Quart. Journ. Micr. Sci., vol, XXVII. — —: ‘‘On the postembryonic development of Fungia.” Trans. Roy. Dublin Soc., ser. tr, vol. v. — —-: “Studies on the structure and formation of the calcareous skeleton of the Antho- zoa.” Quart. Journ. Micr. Sci., vol. xr. Deace, Y.,and Herouarp, E.: “Traité de Zoologie Concréte. Les Celentérés.” Tom. 11, pt. 2. Duerpen, J. E: ‘‘ The Edwardsia-stage of the actinian Ledrunza, and the formation of the gastro- celomic cavity.” Journ. Linn. Soc., Zool., vol. xxvit. —— —: ‘‘ West Indian Madreporarian Polyps. ” Mem. Nat. Acad. Sciences, vol. vi, 7th Mem. Favrort, L.: ‘‘ Etudes sur l’anatomie, ’histologie et le développement des Actinies.” Arch. de Zool. Exp. et Gén., ser. 3, tom. I. Fowter, G. H.: ‘* The anatomy of the Madreporaria, IV.” Quart. Journ. Micr. Sci., vol. xxvut. Garpiner, J. S.: ‘‘ South African corals of the genus F/adel/um, with an account of their anatomy and development.” Marine Investigations in South Africa, vol. u, Cape Town. von Kocn, G.: ‘‘ Ueber die Entwicklung des Kalkskeletes von Asterotdes calycularis und dessen morphologischer Bedeutung.” Mitt. a. d. Zool. Stat. zu Neapel, bd. 11. —— —: ‘Das Skelett der Steinkorallen.” Festschrift fiir Carl Gegenbaur. Leipzig. — —: “Entwicklung von Caryophyliia cyathus.” Mitt. a. d. Zool. Stat. zu Neapel, bd. xu. Lacaze-Durtuiers, H. pe: ‘Développement des Coralliaires. Prem. Mém., Actiniaires sans Polypier.” Arch, de Zool. Exp. et Gén., tom. 1. —— —: ‘Développement des Coralliaires. Deux. Mém., Actiniaires 4 Polypier.” Arch. de Zool. Exp. et Gén., tom. 11. —— ——: “Faune du Golfe du Lion. Coralliaires. Zoanthaires Sclérodermés.” Arch, de Zool. Exp. et Gén., 3 ser., tom. v. Ocirvig, M.: ‘‘ Microscopic and systematic study of Madreporarian types of corals.” Phil. Trans., vol, CLXXXVII. VAuGHAN, T. W.: ‘‘ The Eocene and Lower Oligocene Coral Faunas of the United States.” U.S. Geol. Survey, Monogr. xxxrx. 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. x1. Witson, H. V.: ‘‘On the development of Manicina areolata.” Journ. Morph., vol. 11 (1889). 125 EXPLANATION OF PLATES. PLATE 1. Fic. 1.—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 Zooxanthelle in the ectoderm. The narrower aboral pole is anterior in swimming; the mouth is not yet functional. : Fic. 2.—Abnormal larva with two oral poles and one aboral. Fic. 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 stomodzeum, and two other pairs are free. Fic. 4.—A second-day larva just before settling. Six pairs of mesenteries are now present, three pairs of which reach the stomodeum. Fic. 5.—Three larve settling close together by the narrow aboral pole. Fic. 6.—A group of seven third-day larve which have settled so close together that their walls partly overlie. Four pairs of mesenteries now reach the stomodzum, and on three of the individuals the rudiments of the six exoceelic tentacles have appeared. Fic. 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 exoceelic and alternate with the septa. The internal Zooxanthellw have accumulated mainly along the sides of the mesenteries; the ectodermal Zooxanthellz have disappeared except immediately around the mouth. Diameter of original, 1.5 mm. PLATE 2. Fic. 8.—Living polyp with a dorso-lateral pair of exosepta and a rudimentary pair of median exosepta in addition to the six entosepta. Fic. 9 —Polyp with six entosepta and six exosepta, the latter still revealing their doreo.ventral order of development by differences in magnitude. Fic. 10.—A living polyp of about the same stage as fig. 9, fully expanded and viewed from the side as atransparency. Of the mesenteries only the insertions are represented. Fic. 11.—Polyp with twelve fully expanded tentacles, six large outer exoceelic and six small inner entocelic. Fifth week. Fic. 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. Fic. 13.—Expanded disc showing the doubling of certain of the entotentacles and the asymmetrical position of others. Fic. 14.—Expanded polyp viewed from the side so as to display the manner of appearance of the second-cycle mesenteries on the column wall. Fic. 15.—Expanded disc resting on the septa and exhibiting the relationship of the second-cycle 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. Fic. 16.—Expanded disc showing the manner of increase of the tentacles. An exotentacle protrudes from each of the two exoceeles 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 entoccele is already bifurcated as in the adult. Fic. 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 entoceele of the median second-cycle mesenteries, and the four lateral first-cycle ento- tentacles (I) are bifurcated. Diameter of original, 2 mm. Fic. 18.—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. 127 PLATE 4. Fic. 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. Fic. 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 bifurcated; the ventro-lateral exosepta are still much smaller than the other exosepta, Fic. 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 stomodeum. The dotted surface of the polyp is due to the presence of internal Zooxanthella. 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.) Fic. 22.—The same polyp as in fig. 21: 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. Fic. 23.—A typical corallum ata 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 clockwése through an angle of about 20 degrees from the vertical.) Fic. 24.—Another corallum at about the same stage as fig. 23, but showing variation in detail. PLATE 5. Fic. 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. Fic. 26.—Corallum in which the polyp has twice diminished in size, somewhat suddenly. Three concentric epithece have been formed, the peripheral ends of the septa being partly exposed between each. Fic. 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). Fic. 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 the exoseptum 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, 2 mm. Fic. 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. : Fic. 30.—Double corallum produced by two polyps the larve 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. Fic. 31.—Three fully expanded adult polyps, showing the forms assumed by the polyps and the arrangement of the bilobed and simple tentacles. Fic. 32.—Nearly mature polyp preserved in formalin in a partly expanded condition; surface view. The tentacles over the third-cycle entosepta (IIT) are still simple and resemble the exotentacles (X). Fic. 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 (syw) are the spaces formerly occupied by the synapticula. The column wall, disc, a bifurcated tentacle, stomodzum, and a mesenterial filament are shown. Fic. 34.—Transverse section of a mature polyp, passing through the stomodzal region. The figure shows the arrangement of the mesenteries, the septal invaginations, and synapticula. In this and other figures the spaces occupied by the skeleton are represented by the dotted areas. Fic. 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 (ew. sefz., exoseptum; ent. sept., entoseptum). Fic. 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. Fic. 37.—a, fully expanded entotentacle; 4, fully expanded exotentacle. Fic. 38.—Transverse section of the same polyp as in fig. 34, taken a little below the stomodzal region. Fic. 39.—Transverse section of a polyp towards its aboral region. The mesenteria! loculi are alto- gether isolated at this level, and appear further broken up by 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. Fic. 40.—Tangential section through the tentacular region. A single bilobed entoccelic tentacle is almost resting upon a septal edge. In the septal invagination (sef. zxv.) the calicoblast layer is repre- sented. Fic. 41.—Transverse section through a mesentery at the level represented in fig. 35. The meso- gleeal 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 (sé. 7.). Fig. 42.—Transverse section through a mesenterial filament immediately below the stomodzal region. Fic. 43.—Transverse section of a mesentery some distance below the stomodzal region, showing the form of the mesenterial filament, and also a single ovum; the peripheral end of the mesentery is under- going resorption. Fic. 44.—a, small nematocysts from the knob of the tentacle and column wall; 4, 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. 129 Pirate 8. Fic, 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. Fic. 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 hematoxylin, and shows very distinctly the mucous spaces in the inner endoderm and calicoblast layer. The latter also contains coarsely granular cells and nematocysts. Fic. 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. 3 Fic. 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. Fic. 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. ¢.) towards the periphery. Fic. 50.—Transverse section through a portion of a mesentery showing the system of irregular spaces in the endoderm, The Zooxantheliz and cut ends of the retractor muscle fibers arranged along mesoglceal plaitings are also represented. Fic. 51.—Transverse section through the stomodzal region of a larva preserved shortly after extru. sion. The six primary pairs of mesenteries are present, but the dorsal directives (1v, 1v) and pairs v and vi are yet free from the stomodeum. The gastro-ccelomic cavity is beginning to be established in the nearly solid endoderm; Zooxanthell# are present in the thick stomodwal ectoderm. In this and the next figure the histological details of the endoderm are represented, while the ectoderm is convention- ally depicted. _ Fic. 52.—Transverse section through the same larva a little below the stomodwal 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. Fic. 53.—Radial vertical section through a young polyp, showing its relationship to the early coral- lum. In the latter are shown the basal plate (4. A/.), epitheca (ef.), two septa, and a columellar upgrowth (col.) Fics. 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—4, c-d). The full description of the series is given on p. 100. PLATE 10. Fic. 61.—A free corallum. Natural size. Fic. 62.—A portion of the same corallum magnified about four times, showing various characteristics of the corallites and their relationships to one another. Fic. 63.—Upper part of three septa as seen in a vertical surface 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 line of 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. Fic. 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. Fic. 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 spheroids represent transverse sections through so many individual trabeculew. 130 EXPLANATION OF PLATES. PLATE 11 Fic. 66.—Transverse section through a single trabecula, penetrated by filamentous boring algew, and showing the growth lamellw and their fibrous character. Fic. 67.—Vertical section through a part of two adjacent septa. The septa are made up of trabecule 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- ulw 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 lamellw are not shown (cf. fig. 68). New trabecule are intercalated at intervals. The dark circular and oval patches are sections of synapticula. Fic. 68.—Terminal portion of two trabeculw 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 trabecula so that all the dark centers of calcification are seen. In the corallite from which the section was taken the growth lamell# were very distinct, the boundary of each being indicated by a dark granular deposit similar to that at the centers of calcification. The growth lamell are arranged parallel with the toothed edge, that is, with the calicoblast layer which secretes them, while the fibro- crystals making up the lamellz are arranged at right angles. The figure should be compared with fig. 66, which shows a trabecula in transverse section. Fic. 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 toallow 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. Fic. 7o.—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. LATE 1 P SIDERASTREA RADIANS HELIOTYPE CO., BOSTON. PLATE 2 HELIOTYPE CO., BOSTON. SIDERASTREA RADIANS PLATE 3 SIDERASTREA RADIANS HELIOTYPE CO., BOSTON, wi SIDERASTREA RADIANS SIDERASTREA RADIANS PLATE 6 HELIOTYPE CO., BOSTON. PLATE 7 SIDERASTREA RADIANS HELIOTYPE CO., BOSTON, ‘ SIDERASTREA RADIANS PLATE 8 : _) oT ed Or PAP > ~ cnr PPREDT YER: 73 LE SIDERASTREA RADIANS PLATE 9 HELIOTYPE CO., BOSTON. oe ots + ¥ 4 —— DIANS ERASTREA R D 5 “rei = + IDERASTREA RADIANS seeabte 1 PEMA CS oy Oe aay Sroeand = ea eee as? yee 2 — ad were” s oe bs ¥ ran - wey ES 7 an ee basa “sD