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 Siderastrea can by no means be assumed to 

 be that characteristic of corals generally. Probably some of the stages in 

 Siderastrea might be better interpreted were results available from other 

 forms ; such, for iustance, as the significance of the forking of the exosepta. 

 The fact that in Siderastrea the septa of one cycle fuse centrally with the septa 

 of the next inner cycle probably obscures the problem somewhat, as com- 

 pared with forms where the septa remain altogether free from one another. 



THIRD CYCLE OF ENTOSEPTA AND FOURTH CYCLE OF EXOSEPTA. 



None of the polyps reared from larvae were kept alive beyond the for- 

 mation of the first three cycles of septa, which consist of two cycles of 

 entosepta and one cycle of exosepta. Therefore the development of the 

 fourth cycle of septa must be studied from the bud polyps of a colony. 

 Fortunately, there are many polyps available for such an investigation, as 

 in any stock most of the individuals are at one stage or another towards the 

 establishment of a complete fourth cycle of 24 septa. 



