IO2 J. E. DUERDEN. 



and arise either simultaneously or in bilateral pairs in a dorso- 

 ventral order. Later, they become bifurcated peripherally, either 

 by the direct extension of the original septum or by the produc- 

 tion of separate fragments which subsequently fuse. The bifur- 

 cations also appear in a bilateral dorso-ventral order. 



3. The six members of the permanent second cycle of ento- 

 septa 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 exoseptum previously developed, these exosepta thereby 

 losing their individuality. 



4. Twelve members of a temporary third cycle are situ- 

 ated within the exocceles between the primary and secondary 

 pairs of mesenteries, and represent 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 new second cycle of entosepta. 



5. A new third cycle of twelve (or less) septa arises on the 

 appearance of the pairs of third cycle mesenteries, in a similar 

 manner to that followed by the second permanent cycle. New 

 entosepta appear within the entocceles of the third cycle mes- 

 enteries, and the bifurcations of the third cycle exosepta then 

 become the exosepta of the fourth cycle. 



6. The third cycle entosepta, following the mesenteries, are 

 developed in a bilateral dorso-ventral order, but in two series ; 

 first a series within the dorsal moiety of each sextant, and then 

 a second series within the ventral part of each sextant. 



7. Exosepta are present at each cyclic stage in the growth of 

 the corallum, alternating in position and corresponding in num- 

 ber with the sum of the entosepta. They never become ento- 

 septa, but always constitute the outermost cycle of shorter septa ; 

 only the entosepta have any ordinal significance. Until the 

 adult condition is reached the exosepta are the temporary prede- 

 cessors of the entosepta. The developmental relationships be- 

 tween the entosepta and exosepta are closely comparable with 



