88 J. E. DUERDEN. 



The significance of the twelve exosepta which constitute the 

 outermost third cycle remains to be considered. They undoubt- 

 edly represent the direct continuations of the bifurcations of the 

 six primary second cycle exosepta, but their relationships have 

 now changed ; instead of appearing as extensions of an older 

 cycle of septa they themselves constitute a cycle. The details 

 exhibited by the particular species studied seem inconclusive 

 towards determining whether the exosepta of the later stage are 

 to be regarded as but continuations of the exosepta of the pre- 

 vious stage or as entirely new formations. The former would 

 certainly seem to be suggested ; probably the point could be 

 definitely established in some other coral species in which the 

 exosepta in the adult are not fused at their inner extremity with 

 the entosepta. Whichever view is accepted the exosepta of the 

 third cycle are obviously developed in advance of the entosepta 

 of the second cycle. 1 Thus the relative sizes and positions which 

 the cycles will ultimately assume in the mature calice do not 

 represent their actual order of appearance. 



A third important morphological relationship is definitely 

 established, namely, that the later septa do not arise a cycle at a 

 time as is usually assumed, any more than do the mesenteries. 

 Beyond the primary cycle of six entosepta, the members of which 

 always appear simultaneously, there is a decided dorso-ventrality 

 in the sequence of the septa of each cycle, which for the time 

 being gives a marked bilateral symmetry to the corallum. It 

 is only later, when the development of the septa within each sex- 

 tant has reached the same stage, that an approximate equality 

 and radial symmetry is attained. Thus the primary symmetry 

 in corals is bilateral, and the arrangement is retained for a long 

 period in the ontogeny of both mesenteries and septa. There- 

 fore in the individual septa making up a cycle, as has been proved 

 for the cycle itself, the adult size does not represent the actual 

 order of appearance. 



To return to the further development of the septa in Sidcras- 



1 It is worthy of note in this connection that the exotentacles in Siderastrea radians 

 have been found to appear thoughout in advance of the entotentacles, being the only 

 zoantharian in which this relationship is known to occur. Hence there is nothing 

 contrary to the laws of hexactinian development in the above conception that the 

 exosepta beyond the first series appear in advance of the corresponding entosepta. 



