104 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 (d-/), 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, c, 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. 



