﻿98 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. X? 



STOMOD.EUM OR CESOPHAGUS 



The stomodjeum of all coral polyps fully investigated is found to 

 be without siphonoglyphs or gonidal grooves, a structure charac- 

 teristic of most actinians and alcyonarian polyps. Though the oesoph- 

 agus is oval or slit-like in section, the -extremities show no histo- 

 logical differences from the sides. In many species the stomodaeal 

 walls all round arc deeply ridged and grooved, in a manner unlike 

 anything which occurs in ordinary actinians ; the ridges correspond 

 with the insertion of the mesenteries, and where strongly developed 

 are histologically different from the grooves. 



MESENTERIES AND MESENTERIAL FILAMENTS 



The mesenteries, like the tentacles, conform to the hexactinian 

 cyclic plan — 6, 6, 12, 24, etc., either throughout or during only the 

 early stages of growth. Two great groups of corals, however, are 

 recognizable, according as the asexual growth of the colonies takes 

 place by budding or by fission. In the former the bud polyps are, 

 to all intents and purposes, new polyps, having the mesenteries 

 throughout arranged in hexameral alternating cycles, with two pairs 

 of directives, exactly as in polyps reared directly from larva?. 

 Though in the adult polyp the last cycle of mesenteries commenced 

 may not reach the number of pairs necessary to complete the hexac- 

 tinian plan, yet so far as the additions are made they follow the nor- 

 mal sequence. Where fissiparity is established, however, the cyclic 

 arrangement is irregular, the early hexamerism is altogether lost, 

 and no new pairs of directives arise. Whenever, as is the case in 

 some polyps, directives and cyclic hexamerism are wanting, it may 

 with good reason be assumed that the polyps are products of fission, 

 not of gemmation (p. 102). 



The mesenteries are restricted to the upper half or two-thirds of 

 the polyp, being resorbed in the lower region as the polyp grows 

 upward. Histologically the retractor and oblique muscles are always 

 feebly developed, parieto-basilar muscles seem to be absent, and no 

 evidence of mesenterial stomata is forthcoming. 



.Mesenterial filaments occur on nearly all the mesenteries, both 

 complete and incomplete, though sometimes they are merely incipient 

 on the youngest cycles. In all the species examined the filaments 

 arc simple, not trilobed as in most actinians. In the latter group two 

 lateral lobes are developed, in addition to the median lobe, and these 

 bear the ciliated bands, which are specially concerned in the circula- 

 tion phenomena of the polyp. In both groups, however, the histology 



