82 



SIDERASTREA RADIANS. 



are complete and the fifth and sixth pairs are incomplete ; the six pairs 

 of second-cycle mesenteries are incomplete, and either equal or show by 

 differences in sizes their dorso-ventral succession. The stage may be found 

 among adult Edwardsias, though two or more pairs of second-cycle mesen- 

 teries may here be wanting ; adult members of the Ilyanthidas often present 

 it, and probably all higher actinians in the course of their development. It 

 is repeated by the many pelagic anthozoan larvae described by Van Beneden 

 (1897, p. 189), numbered vni to xv, and which the author regards as belong- 

 ing to the Hexactiniaria. 



Ilia 



Hid 



IIIc 



FIG. 8 (j, A), continued. Series of diagrams illustrating the order of development of the first three cycles of mesenteries. 

 j is a late stage, and k the complete stage in the development of the third cycle. 



A long interval of time, and a fundamental difference in their manner 

 of appearance, are found to separate the second-cycle mesenteries from those 

 of the first cycle. The primary mesenteries appear in bilateral pairs first 

 towards one aspect of the polyp and then towards the other aspect, and so 

 on, and only later constitute unilateral pairs. The secondary mesenteries, 

 on the other hand, are in unilateral, isocnemic pairs from the beginning, 

 and arise within the six primary exocoelic chambers in a regular dorso- 

 ventral succession, alternating with the primary pairs. Though presenting 

 such a difference in development, the two series agree in each forming a 

 cycle or order of six isocnemic pairs when mature. 



It was to emphasize these fundamental differences in origin between the 

 primary mesenteries and the later members that, in a former paper, I termed 



