94 



SIDERASTREA RADIANS. 



while the third-cycle septa are within the twelve alternating exocoeles, and 

 are therefore exosepta. Thus at both stages the exosepta form the outer- 

 most cycle [cf. fig. lo, b^ /, p. 96). The questiou naturally arises as to 

 whether, on the appearance of a second cycle of mesenteries, the exosepta 

 of the first stage, which there constitute the second cycle, remain as the 

 second cycle of entosepta of the later stage ; were they to do so the twelve 

 exosepta of the latter would be the only new formations. As regards their 

 actual position the six exosepta of the early polyp correspond with the 

 six entosepta of the later polyp, and it would be natural to assume that on 

 the appearance of the second-cycle mesenteries the latter have simply included 

 within their entocoeles the exosepta of a former stage, and then new exo- 

 septa have arisen within the newly formed exocoeles. 



The latter is the view commonly held by writers on coral development. 

 Thus Delage & Herouard, in their "Traite de Zoologie Concrete" (1901, p. 

 55S), remark : " Quand, dans les interloges occupees par les septes du dernier 

 cycle, nait un nouveau cycle de couples de cloisons, celles-ci se forment de 

 part et d'autre du septa interloculaire qui, de ce fait, devient loculaire, et 

 bientot un nouveau C3'cle de septes se forme dans les nouvelles interloges 

 qui viennent d'etre formees. Les cj'^cles naissent successivement et jamais 

 un cycle ne commence a se former avant que le precedent soit complet." 

 Similarly, J. Stanley Gardiner (1902), in his account of the anatomy of 

 Flabellum rubrum, says (p. 133, italicizing added): "As the growth of any 

 corallite proceeds, more and more septa up to six cycles appear. The former 

 exoccelic order of septa become entoccelic by the development of new pairs of 

 mesenteries. The increase of mesenteries takes place pari passu with the 

 formation of new septa." 



Unfortunately, the relationships involved in the above assertions have 

 not been actually followed, though from the known conditions no other 

 arrangement at first sight seems possible. The problem is one of the most 

 important in the developmental history of corals. 



Without doubt the members of the primary cycle of entosepta in adult 

 polyps are the direct representatives or continuations of the six septa first 

 to arise, just as the six pairs of complete mesenteries throughout the life of 

 the polyp are the representatives of the protocnemes. Throughout their 

 later growth the primary septa retain their individuality, and remain within 

 the entocoeles of the primary C3'cle of mesenteries. Other considerations, 

 however, are involved in the relationships between the septa developing later 

 and those of the adult corallite. 



Most of the polyps of Siderastrea remained for some time at the stage 



