MORPHOLOGY OF THE MADKEPORARIA. 97 



precise significance as to their relationships at different stages 

 may be forthcoming than is possible in Sidcrastrca. There .are 

 indications that in some forms an entoseptum and an exoseptum 

 arise together, thus more closely recalling the method followed 

 in the appearance of the tentacles. 



The relationships now proved to exist between the entosepta. 

 and exosepta of corals involve important considerations when the 

 cyclic hexameral sequence is not completed in the mature coral- 

 lite, a condition which almost invariably happens in 5. radians, 

 as well as in numerous other corals. As regards both septa and 

 mesenteries it is found in such cases that the last cycle is rarely 

 a multiple of six, but some irregular number, resulting from the 

 fact that at maturity the polyp does not complete the last cycle 

 begun. Exosepta have been shown to appear always in associa- 

 tion with entosepta, whatever be the number making up a coral- 

 lite, and, as often remarked, the two series are equal in number 

 and the exosepta outermost in position. Hence it follows that 

 in the cyclic incompletion of the mature corallites of Siderastrea 

 the third entoccelic cycle and fourth exocoelic cycle of septa vary 

 in the same degree ; whatever number of entosepta be lacking to 

 form the complete third cycle of twelve a like number will be 

 wanting from the twenty-four exosepta which should form the 

 fourth cycle. 



When describing the number of septal cycles within a calice 

 of which the cyclic hexameral plan is incomplete it is usual in 

 systematic works on corals to regard the hexameral multiples as 

 completed so far as the number of septa will permit, and then to 

 relegate to the last cycle all the surplus septa not included in the 

 hexameral formula. The cycles are all supposed to be hexam- 

 erously complete with the exception of the last. Thus, with 

 regard to 6". radians, Milne- Ed wards states : " Three cycles of 

 septa complete, and, in general, a variable number of a fourth 

 cycle"; likewise Verrill (1901, p. 133), 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 now established between the entosepta and 



