110 SIDERASTREA RADIANS. 



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 phylo- 

 genetic significance beyond the group of the ccelenterates, and as yet we 

 appear to have no definite understanding as to what even this 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 Scypho- 

 medusae). Whatever may be said in favor of Sedgwick's well-known view 

 that the mesenterial arrangement found in cerianthids suggests the metamer- 

 ism of higher animals, there is clearly no support for such a conception in 

 the development of the organs in hexactinians. 



SUMMARY. 



1. In Siderastrea 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 entosepta. They are the tempo- 

 rary predecessors of a later permanent cycle, and arise either simultane- 

 ously or in bilateral pairs in a dorso-ventral order. Later, each becomes 

 bifurcated peripherally, either by the direct extension of the original septum 

 or by the production of separate fragments which subsequently fuse. The 

 bifurcations also appear in a bilateral dorso-ventral order. 



3. The six members of the permanent second cycle of entosepta arise 

 within the entocceles of the second-cycle mesenteries soon after these make 

 their appearance. The two right and left dorsal septa appear first, then the 

 two middle members, and, at a much later period, the two ventral, the series 

 thus exhibiting a decided dorso-ventrality. In the end they become equal, 

 and each fuses with the central part of the corresponding second-cycle exo- 

 septum previously developed, which now lose their individuality. 



4. Twelve members of a temporary third cycle are situated within the 

 exocceles between the primary and secondary pairs of mesenteries, and repre- 

 sent the bifurcated extensions of the six primary exosepta. The original 

 second-cycle exosepta thus become the third exoccelic cycle, their place 

 having been taken by the permanent second cycle of entosepta. 



5. The later development of the septa in buds proves that a new third 



