106 SIDERASTREA RADIANS. 



sextant, and then the remaining six in a like order, but within the ventral 

 of the two interspaces. The exosepta constituting the last outermost cycle 

 have no corresponding ordinal significance. 



Studies on the mesenterial sequence of other corals indicate that a similar 

 septal succession will in all probability be followed by forms in which the 

 adult calice shows a regular hexauieral cyclic plan. Individual departures 

 from the order may be expected, but are to be looked upon as irregularities ; 

 regularity of growth of the higher cycles is 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 any- 

 thing 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 been revealed it is manifest that the exosepta do not 

 possess any true ordinal sequence comparable with that of the entosepta. 

 Exosepta have been found to be present at each stage, always constituting 

 the outermost cycle, and equaling in number the sum of the inner entosepta. 

 We may consider them as the direct continuations of the six primary exo- 

 septa, or, less likely, as arising anew with each cycle of entosepta. Regarded 

 as the persistent representatives of the primary exosepta, they more nearly 

 conform to the law of substitution of the exotentacles in actinians as estab- 

 lished by Lacaze-Duthiers and Faurot. In actinians generally it is found 

 that after the protocnemic stage the tentacles appear two at a time, one ento- 

 coelic and one exoccelic, corresponding with the two chambers formed upon 

 the appearance of a new pair of mesenteries ; sometimes the entotentacles 

 appear in advance of the exotentacles, the reverse of what happens in Side- 

 rastrea radians. The entotentacles are always larger than the exotentacles, 

 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 

 exotentacles all attain an equal length and are all relegated to the outermost 

 cycle, whatever be the cycle of entotentacles with which they appeared. They 

 constitute a single cycle of which the members are always smaller than those 

 of the cycle of entotentacles last to appear, and the number of exotentacles 

 in the last cycle is always half the total number of tentacles, and, of course, 

 equal to the number of entotentacles. 



As new entotentacles are added the exotentacles become pushed aside so 

 as to occupy different radii at different times. The calcareous septa being 

 hard, fixed structures, do not admit of such rearrangement ; the new septal 

 growth has to be adapted to the old, resulting in the fusion of the new ento- 

 septa with the old exosepta. 



