84 J- E. DUERDEN. 



Caryophyllia, and some other corals, but hitherto its significance 

 has not been satisfactorily explained. At one time the separate 

 fragments were supposed to be concerned in the formation of the 

 thecal wall, but it will be seen that such is certainly not their fate 

 in Siderastrca. 



About this time, the polyps being two months old, the sec- 

 ond cycle mesenteries began to appear on the column wall, 

 situated in the exoccelic chambers between the primary mesen- 

 teries (Fig. 4). In their development they also presented a con- 

 spicuous dorso-ventrality : the two first pairs appeared within 

 the dorsal exocoeles, the moieties of each pair arising at the same 

 time and remaining equal ; the two next pairs were within the 

 middle exocceles ; and finally appeared the pairs within the ven- 

 tral exocceles. The dorso-ventral succession thus followed by 

 the six pairs remained evident throughout the period under obser- 

 vation, the dorsal pairs being larger than the middle, and the 

 middle larger than the ventral. The comparative development 

 of the first and second cycle mesenteries at this period, and their 

 relationship to the septa, are shown in Fig. 4. A similar bilateral, 

 dorso-ventral succession of the second cycle mesenteries has 

 long been known to occur in the development of actinian polyps, 

 in contrast to the simultaneous origin at one time assumed. 



Clearly the next skeletal stage will be one involving the 

 establishment of septa within the entocceles of the second cycle 

 of mesenteries, and these will constitute the second cycle of 

 septa of the adult corallite. Hitherto it has been generally as- 

 sumed that where a cycle of exosepta is already developed, and 

 then a new cycle of mesenteries appears within the corresponding 

 exocceles, that the exosepta already present become included 

 within the entocceles of the new mesenteries, and thus become 

 entosepta ; an additional outer cycle of exosepta appears later 

 and its members in their turn become entosepta. Thus Delage 

 and Herouard in their " Traite de Zoologie Concrete " (1901, p. 

 558) 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 septe inter- 

 loculaire qui, de ce fait, devient loculaire, et bientot un nouveau 

 cycle de septes se forme dans les nouvelles interloges qui vien- 



