POSTLARVAL DEVELOPMENT. Ill 



cycle of 12 or less septa arises on the appearance of the pairs of third-cycle 

 mesenteries, in a similar manner to that followed by the permanent second 

 cycle. New entosepta appear within the entocoeles of the third-cycle mesen- 

 teries, and the bifurcations of the third-cycle exosepta become the exosepta 

 of the fourth cycle. 



6. The third-cycle entosepta, following the mesenteries, are developed 

 in a bilateral dorso-ventral order, but in two series first a series within the 

 dorsal moiety of each sextant, and then a second series within the ventral 

 part of each sextant. 



7. Exosepta are present at each cyclic stage in the growth of the coral- 

 lurn, 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 Hexactinise, is thus found to hold also 

 for the septa. 



8. Where the cyclic hexamerism of a corallite is incomplete the ordinary 

 cyclic formula does not express the true relationship of the septa. The ento- 

 septa and exosepta vary in the same degree, so that the morphological 

 septal formula for a corallite with three entoseptal cycles and one exoseptal 

 cycle is 6, 6, x, 6 + 6 + x, where x may be any number from i to 12. 



9. The cycles of septa and mesenteries represent so many distinct recur- 

 ring phases of growth all around the polyp, not a continuous increase from 

 one extremity to the other as in metameric animals. With the exception of 

 the first the members of each cycle follow a dorso-ventral succession, display 

 a bilateral symmetry for some time, and ultimately assume an approximate 

 radial plan. The succession for the third cycle of entosepta is twofold. 



BASAL PLATE. 



G. von Koch, in the course of his embryological studies of corals, found 

 a deposit of calcareous matter to take place between the ectoderm of the base 

 and the surface of attachment of the polyps. It is the first part of the skele- 

 ton to be formed by the activity of the calicoblasts, and from its position is 

 known in coral literature as the basal plate. It is present in the photographic 

 reproductions on plates 4 and 5, and its relationship to the polyp, at a rather 

 late stage, is shown on plate 9, fig. 53. 



