POSTLARVAL DEVELOPMENT. 107 



Partial studies on other corals, as well as considerations on the tentacular 

 development in actinians, suggest that the exosepta may arise in different ways 

 in different species, and that a more precise significance as to their relation- 

 ships at different stages may be forthcoming than is possible in Siderastrea. 

 There are indications that in some forms an entoseptum and an exoseptum 

 arise together, thus more closely recalling the method followed by the tenta- 

 cles. Regarded as arising anew in each cycle, the two exosepta in Siderastrea 

 appear somewhat in advance of the entoseptum which is included between them. 



The relationships proved to exist between entosepta and exosepta involve 

 important considerations when the cyclic hexameral sequence is not completed 

 in the mature corallite, as almost invariably happens in S. radians, as well 

 as in many other species of corals. As regards both the septa and mesen- 

 teries it is found that the last cycle is rarely a multiple of 6, but some 

 irregular number from i to 12, resulting from the fact that at maturity the 

 polyp does not complete the last cycle begun. Exosepta have been shown 

 to appear always in close association with entosepta, whatever be the number 

 making up a corallite ; and, as often remarked, the two series are equal in 

 number and the exosepta always outermost in position. 



If we regard a septal cycle as made up only of entosepta, or of exosepta, 

 then in mature corallites of S. radians the third entocoelic cycle and fourth 

 exoccelic cycle of septa will vary in the same degree. Whatever number of 

 entosepta be lacking from the third cycle to form the complete cycle of 12, 

 a like number of exosepta will be wanting from the fourth cycle. 



When describing the number of septal cycles within a calice, the cyclic 

 hexameral plan of which is incomplete, it is usual in systematic works on 

 corals to regard the hexameral multiples as completed as far as the number 

 of septa will permit, and then to relegate to the last cycle all the remaining 

 septa not included in the hexameral formula. The cycles are all supposed 

 to be hexamerously complete with the exception of the last. Thus, with 

 regard to 6". radians, Milne-Edwards states : " Three cycles of septa com- 

 plete, and, in general, a variable number of a fourth cycle." Likewise Verrill 

 (1901, p. 153), describing the same species, says: "They [the septa] form 

 three complete cycles, with part of the fourth cycle developed, so that the 

 number is usually 36 to 40." 



The relationships proved to exist between the entosepta and exosepta in- 

 dicate that the above formulae do not express the true morphological charac- 

 ter of the septa. Any hexameral incompletion in the number of septa mak- 

 ing up a corallite affects both the entosepta and the exosepta, that is, both 

 the penultimate and the last cycles. If any septa are wanting to complete 



