MORPHOLOGY OF THE MADKEPORARIA. IOI 



proliferation," introduced by van Beneden in 1897, we may say 

 there is only one median band of proliferation in cerianthids, 

 while in hexactinians there are many such bands occurring all 

 round the polyp, the number increasing with age at first six, 

 then twelve, twenty-four, etc. 



In the Zoantheae also mesenterial development is always in the 

 same succession after the protocnemic stage. The increase takes 

 place within only two of the six primary exoccelic chambers, 

 one on each side of the ventral directives ; there are only two 

 bands of proliferation or zones of growth. In this case, how- 

 ever, the order followed by the new mesenteries differs from that 

 in hexactinians and cerianthids ; it proceeds from the ventral 

 (posterior, sulcar) to the dorsal (anterior, asulcar) aspect of the 

 polyp, not from the dorsal to the ventral. 



The bilateral development of the organs, from one border of 

 the polyp to the other, in ordinary actinians and corals would 

 seem to have no phylogenetic significance beyond the group of 

 the ccelenterates ; indeed, even here we appear to have as yet no 

 definite understanding as to what its meaning may be. The 

 approximate radial symmetry of adult ccelenterates is assumed 

 from very diverse developmental conditions (cf., hexactinians, 

 zoanthids, cerianthids, and the tentacles and other cyclic organs 

 in the Hydromedusae and Scyphomedusae). Whatever may be 

 said in favor of the well-known view that the mesenterial arrange- 

 ment in cerianthids suggests the metamerism of higher animals 

 there is clearly no support for such a conception in the develop- 

 ment of the organs in hexactinians. In this latter group we are 

 concerned with a radial cyclic repetition of the organs, even 

 though the members of each cyclic series arise in bilateral suc- 

 cession from one border to the other. 



SUMMARY. 



1. In the coral Siderastrca radians the six members of the first 

 cycle of septa appear simultaneously, shortly after fixation of the 

 larva, situated within the entocceles of the first cycle of mesenteries. 



2. Six members of a second cycle are developed within the 

 primary exocceles shortly after the primary cycle of septa. 

 They are the temporary predecessors of a later permanent cycle, 



