92 



COELENTERATA— ANTHOZOA 



PHYLUM II 



Like the tentacles, the first two cycles of septa (protosepta) may appear 

 simultaneously, or the cycle of six entosepta may arise in advance of the 

 cycle of six exosepta. The order of appearance of the later cycles is not yet 

 thoroughly understood, the relative sizes in the mature corallum by no means 

 indicating the actual order of development. As in the case of the mesenteries, 

 the radial plan of the mature septa is derived from structures which appear 

 bilaterally, in a more or less definite dorso - ventral or antero - posterior 



n. 



IHz 



Fig. 12S. 



Tliree stages in the development of tlie twelve pairs of third-cycle mesenteries. All the six pairs of primary 

 mesenteries are now complete, and the second-cycle pairs are all equal, but free from the stomodaeum. In a a, 

 pair of third-cycle mesenteries (in) has appeared on each side, within the exocoele next the doi\sal directives ; in 

 h a corresponding pair occurs within tlie dorsal of the two exocoeles of all the six systems, the order being from 

 the dorsal to the ventral aspect ; in c another series of six pairs is beginning, situated within the ventral of the 

 two exocoeles in each system. Growth in tlie dorsal region is in advance of that in the ventral (after Duerden). 



succession. Furthermore, as in the case of the tentacles, the exosepta remain 

 exosepta throughout the course of their development, always constituting the 

 outermost cycle. The entosepta beyond the primary six follow the same 

 succession of growth as the mesenteries, so that the t)rder assigned the 

 secondary and tertiary mesenteries in Fig. 129 will also hold for the septa. 

 Reproduction takes place either sexually, when separate individuals are pro- 

 duced ; or asexually, by means of lateral or basal gemmation ; or by fission. 

 In composite coralla, the individual corallites are sometimes united by a 



