THE MORPHOLOGY OF THE MADREPORARIA, V. SEPTAL SEQUENCE

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Abstract
1. In the coral Siderastrea radians the six members of the first cycle of septa appear simultaneously, shortly after fixation of the larva, situated within the entocoeles of the first cycle of mesenteries. 2. Six members of a second cycle are developed within the primary exocœles shortly after the primary cycle of septa. They are the temporary predecessors of a later permanent cycle, and arise either simultaneously or in bilateral pairs in a dorsoventral order. Later, they become 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 entocœles 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 exoseptum previously developed, these exosepta thereby losing their individuality. 4. Twelve members of a temporary third cycle are situated within the exocœles between the primary and secondary pairs of mesenteries, and represent the bifurcated extensions of the six primary exosepta. The original second cycle exosepta thus become the third exocœlic cycle, their place having been taken by the new second cycle of entosepta. 5. A new third cycle of twelve (or less) septa arises on the appearance of the pairs of third cycle mesenteries, in a similar manner to that followed by the second permanent cycle. New entosepta appear within the entocœles of the third cycle mesenteries, and the bifurcations of the third cycle exosepta then 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 corallum, 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 significance. Until the adult condition is reached the exosepta are the temporary 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 Hexactiniæ, 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 relationships of the septa; the entosepta and exosepta vary in the same degree, so that the true morphological septal formula for a corallite with three cycles and part of another is 6, 6, x, 6 + 6 + x, where x may be any number from 1 to 12. 9. The cycles of septa and mesenteries represent so many distinct recurring phases of growth at intervals all round 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 two-fold.

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