| OCR Text |
Show 1886.] MADREPORARIAN CORAL. 133 appearance, doubtless exaggerated by the action of the alcohol in which the coral is preserved. The tentacles are in four cycles ; the innermost cycle are of the largest size, and correspond in position to the primary and secondary septa, and are therefore twelve in number. The second cycle correspond to the tertiary septa, and are also twelve in number. The third cycle correspond to the quaternaries, and are twenty-four in number. These differences between the three cycles are very slight, consisting only of small differences in size. The fourth cycle, placed considerably nearer the circumference of the coral, are very much shorter and more stumpy than the other three cycles; they correspond to the quinary septa, and, as will be seen below, are evaginations of intermesenterial spaces [exoccelic, Fowler, (4)], not of the intra-mesenterial chambers [entoccelic, Fowler (4)] as are the others ; they are forty-eight in number ; so that there are altogether ninety-six tentacles. The mesenterial chambers are forty-eight in number ; there are two cycles of mesenteries, larger and smaller, forming larger and smaller chambers ; the larger chambers contain septa of the first, second, and third order ; the smaller chambers septa of the fourth order. From these chambers the tentacles of the first three cycles are evaginated ; the fifth cycle of septa, on the other hand, are placed in the spaces between the mesenterial chambers, i. e. exosepta, Fowler ; and the fourth cycle of tentacles are in the same way evaginated from these exoccelic spaces. The whole arrangement will best be understood by reference to the diagram (Plate XII.). In all the chambers, with the exception of those in which are placed the primary septa situated at the long ends of oval mouth-opening, the longitudinal muscles are placed, facing one another; but in the chambers above mentioned, the muscles are placed on the outer faces of the mesenteries; these therefore are the directive mesenteries. So that in the arrangement of the muscles on the mesenteries, Stephanotrochus exactly corresponds to the Hexactinian type, as do all other Madreporarian corals that have been hitherto examined. The tentacles are all of one shape, they are simple conical evaginations of the ccelenteron ; they end in a knob separated from the rest of the tentacle by a narrow neck ; the knob contains no pigment, and is of a white colour ; the main part of the tentacle is madder-coloured like the rest of the polype. A section of the knob and part of the conical part of the tentacle is figured (Plate XIII. fig. 6) ; the knob is packed full of nematocysts, and in fact is a single immense battery of thread-cells. The ectoderm consists of a series of very long and very narrow cells, which are nucleated ; there are also gland-cells present, but the elements are difficult to distinguish ; the endoderm cells of the knob are much longer than those lining the other parts of the ccelomic cavity of the tentacle. I was unable to distinguish any muscular elements in the knob; it is perhaps in consequence of this that the outline of the knob appears straight, since it apparently has not been contracted by the action of the spirit, as has been the rest of the tentacle. |
