.MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



42.') 



the cycles are widely apart, and each individual tentacle is distant from theothers. In these two 

 genera the tentaculiferous area comprises nearly the whole of the exposed polypal surface (p. 427). 

 In polyps whose asexual method of reproduction is by fission, the hexameral plan, character- 

 istic of larval polyps, and of adult polyps whose reproduction is by gemmation, is lost after 

 fission is established, and even the cyclic arrangement becomes obscure. In Favia, IsophylMa, 

 and Manicina individual tentacles belonging to several cycles can he made out. hut not with any 

 regularity all round. The tentacles in the young polyps are found to he arranged hexamerously, 

 hut this is altogether lost on mature colonies. Where fission is never or rarely completed, as in 

 Mseandrina and Pectinia, the tentacles are arranged in meandering systems, and only two rows 

 are developed, an inner entocoelic series and an outer alternating exoccelic series. 



Tentacular plan of a polyp of OrbUn Ua radfata. The Roman numerals indicate the orders of mesenteries; D, the directives. The innermost 

 cycle "i tentacles comprises twelve members, which are the smallest of the series. I hey arise from tic entocceles of the first and 

 second orders of mesenteries, which constitute the first[cycle of twelve pairs of complete mesenteries. The second, middle cycle 

 consists of twelve alternating tentacles, which are tin- largest represented. They are outgrowths of the entocoeles of the third order of 

 mesenteries, which form the actual second mesenterial cycle. The third or outermost cycle i- formed of twenty-four tentacles, which 



alternate with tic- members of the two previous cycles, and arise from exuc.elic chambers. Tic s| ies is exceptional in that the 



members of tin- innermost cycle are the smallest 'if the series, and also in tin- wide distance apart of the cycles. 



When the polyps retract, the tentacles do the same, and become greatly shortened. In addition. 

 many species of corals exhibit the curious phenomenon of complete introversion of the tentacles, 

 even during full expansion of the polyps, as well as on retraction. Both phases are clearly 

 illustrated in the tentacles of Parties (\\^>. '-'>'K 33, 40). At times the polyps in this genus will he 

 expanded to their full degree, without tiny display of tentacles; twelve small, more opaque, 

 circular areas, sometimes showing a central aperture, are all the external indications of their 

 presence, while the Local increased opacity indicates that they have become invaginated within 

 the ccelenteric cavity. At other times one or more individual tentacles of a polyp may be 



