478 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



their connection with the stomodreuin they are tipped with a tissue of like nature. Increasing in 

 extent, it is continued as the mesenterial filament along the edge of the rirst pair of mesenteries, 

 almost as far as their termination at the aboral end of the larvae, ceasing on one mesentery a little 

 in advance of the other (c/, also Pis. XVIII. XXV). 



At this early stage the mesenterial filament is not sharply marked off, except histologically, 

 from the rest of the mesenterial epithelium. Its numerous nuclei stain brilliantly in borax 

 carmine, and structurally it is indistinguishable from the stomodaeal ectoderm. This resem- 

 blance, combined with the absolute continuity of the two at the commencement of the filaments, 

 would seem to remove all doubt that the two — stomodaeal ectoderm and the mesenterial filaments — 

 are of one and the same origin. 



But the conclusion becomes less certain when the incomplete mesenteries are taken into 

 account, for along their free edge is a tissue of exactly similar nature; yet the mesenteries are in 

 no way connected with the stomodaeum, and there is no apparent means by which the incipient 

 filament can have been in unity with its ectoderm. 



The early appearance of the filaments on the second pair of mesenteries is represented in 

 fig. 112, from a section taken a little above the termination of the stomodaeum. At first only 

 one mesentery displayed any marginal modification, but in the figure the filament has appeared 

 on the other member of the pair, while below it is as strongly developed on both as on the first 

 pair of mesenteries, but disappears in advance of the filament of the latter. In some of the 

 sections coming below that represented in fig. 112 there is the feeblest hint of the filament on 

 the mesenteries of the third pair, which scarcely extend beyond the endodermal lining. 



It is manifest therefore that the filaments on the second pair of mesenteries originate 

 quite independently of any connection with the stomodaeum and of the reflected ectoderm. 

 From a study of the conditions in both the third and the second pairs it is inconceivable how at 

 any earlier stage, say before the middle embryonic tissues had broken down, that any such 

 connection could have been established. The reflected ectoderm passes backwardly but a short 

 distance along the ccelomic surface of the stomodaeum, and there is no possibility of its working 

 its way upward, across what represents the disk, and then downward along the free edge of the 

 mesentery. The presence of filaments on the second pair of mesenteries before union with the 

 stomodaeum would imply that a similar development may also take place on the third pair of 

 mesenteries before their union, and sections reveal that such actually occurs. It is also manifest 

 from the sections, that before the union of the mesenteries with the stomodaeum is effected, there 

 is no means by which the free margin of the former can have come into contact with the reflected 

 ectoderm. 



From the conditions represented in the larva? of corals generally, the conclusion is reached 

 that the mesenterial filaments may originate independently of any connection with the stomodaeal 

 ectoderm, and may therefore be assumed to be endodermal. 



The filaments on both the complete and incomplete mesenteries at the early stages of larval 

 development present but little histological differentiation, except in the case of the filament on 

 the first mesenterial pair. They consist mainly of supporting cells, and stain much more deeply 

 than the ordinary endodermal lining, but gland cells and nematocysts are scarcely determinable. 

 Similar details are often presented by the edge of adult mesenteries, which always remain free 

 from the stomodaeum, showing that in the orders beyond the primary the filaments originate in 

 the same manner. In these cases the filaments may never become fully developed, but remain 

 in an incipient or rudimentary condition. This is illustrated by the mesenteries of Orbicella 

 acrqpora. The six pairs of mesenteries of the first cycle are complete, and filaments are well 

 developed below; similar filaments appear on the mesenteries of the second cycle, which fail to 

 reach the stomodaeum; but on the third cycle of twelve pairs the mesenteries are merely tipped 

 with a deeply-staining tissue, which is indistinguishable from that on the filaments of larva'. 

 Further, some mesenteries bear incipient filaments only in their upper course, while the organs 

 are fully developed below. Cladocora arbuscula also affords similar illustrative examples (PI. 

 VII). The edge of a mesentery in its upper course is represented in fig. 57 a, and the filament is 

 seen to be quite rudimentary; below the stomodaeal region, however, the filament on the same 



