96 J. E. DUERDEN. 



by no means so pronounced as in the first and second cycles 

 which are less likely to be influenced by spatial considerations. 

 The sequence given is altogether different from anything which 

 has hitherto been surmised for any coral, and further studies are 

 desirable to determine how far it admits of general application in 

 the group. 



From what has already been revealed it is manifest that the 

 exosepta of corals do not possess any true ordinal sequence 

 comparable with that of the entosepta. Exosepta have been 

 found to be present at each developmental stage, always consti- 

 tuting the outermost cycle, and equalling in number the sum of 

 the inner entosepta ; but until the adult condition is reached they 

 are merely the predecessors of the entosepta. We may con- 

 sider them as the direct continuations of the primary six exosepta 

 which bifurcate at each stage, or, less likely, as arising anew with 

 each cycle of entosepta. Regarded as the persistent representa- 

 tives of the primary exosepta they more nearly conform to the 

 " law of substitution ' in actinian tentacles as established by 

 Lacaze-Duthiers (1872) and Faurot (iSgs). 1 



Studies on other corals, as well as considerations on the tentac- 

 ular development in actinians, suggest that the exosepta may arise 

 in different ways in different species of corals, and that a more 



1 In actinians generally it is found that after the protocnemic stage the tentacles 

 appear two at a time, one entoccelic and one exocoelic, corresponding with the two 

 chambers formed upon the appearance of a new pair of mesenteries ; sometimes the 

 entotentacles appear in advance of the exotentacles, though in Siderastrea radians 

 the reverse is the case. The entotentacles when established are larger than the exo- 

 tentacles, the length of the former being in accordance with the order of appearance 

 of the cycle to which they belong, the largest being the first to appear. The exoten- 

 tacles all attain an equal length and throughout are relegated to the outermost cycle, 

 whatever be the cycle of entotentacles with which they first appeared. They consti- 

 tute a single cycle of which the members are smaller than those of the cycle of ento- 

 tentacles last to appear. The number of exotentacles in the last cycle is always half 

 the total number of tentacles, the number of exocceles being equal to that of the 

 entocodes. 



Being soft polypal structures it is easy to understand how as new entotentacles are 

 added the exotentacles become pushed aside and thus occupy different radii at differ- 

 ent times. The septa, being hard fixed structures, do not admit of rearrangement ; 

 the new growth has to be adapted to the old, as in the fusion of the new entosepta 

 with the old exosepta. 



The tentacles, like the septa, thus arise in such a manner that it is impossible to 

 determine their order of development from their relationships in the mature polyp. 



