PLATE XVni. 



ISOPHYLLIA DIPSACEA Dana. 



Fi(i. 127. — Transverse .section througli tlie stomodiieal region of tlie same larva as tliat from which the section 

 represented on PI. XVII, tig. 126, was taken. At tiais level the ventral pair of mesenteries has already 

 disappeared, and the members of the dorsal pair are very rudimentary. " Reflected ectoderm " is seen on 

 the upper and lower borders of the stomodieum. Thi' larval cavity is occupied by a highly vacuolated 

 tissue containing many zooxanthelU'e. X 200. 



Fig. 128. — Transverse section through the game larva below the stomoda-al region. The members of erne [lair of 

 mesenteries are very strongly developed and bear filaments. The polypal cavity is still tilled with the 

 vacuolar tissue, which shows divisions here and there, especially around the mesenterial filaments. An 

 odd mesentery is strongly developed on the lower surface, and is probably to be regarded as an 

 irregularity. X 200. 



MANICINA AREOLATA Linnams. 



Fici. 129. — Transverse section through a complete mesentery toward the lowei' part of the stomoda^al region, including 

 the portion of the stomod;eal wall and skeletotrophic tissue to which it is attached. The mesogkcal folds 

 supporting the retractor muscle are simple, and mainly restricted toward the basal insertion of the 

 mesentery. The muscular fibrils on the other face of the mesentary are well developed, especially 

 toward the middle, where they are cut obliquely; elsewhere they are cut practically transversely. At 

 this level the skeletotrophic endoderm lining the calicinal wall is greatly thickened and vacuolated, 

 with very few protoplasmic contents, and the nuclei limited toward the margin; the skeletotrophic 

 endoderm of the septal invagination is, however, very narrow. There are practically no remains of the 

 calicoblast layer nor desmocytes, and the skeletotrophic mesogUea is a thin lamella. X 300. 



Fig. 130. — Transverse section through an incomplete mesentery, terminated at its free extremity by a rudimentary 

 mesenterial filament. The face of the mesoglica bearing the retractor muscle is very deeply plaited 

 almost throughout its extent. A comparison with fig. 12H shows how greatly the mesoghcal folds ujay 

 vary within the same species. X 300. 

 634 



