PLATE XVIII. 



CSOPHYLLIA DIPSACEA Dana. 



Fig. 127. — Transverse section through the stomodseal region of the same larva as that from which the section 

 represented on PI. XVII, fig. 12(5, was taken. At this 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. The larval cavity is occupied by a highly vacuolated 

 tissue containing many zooxanthellse. X 200. 



Fig. 128. — Transverse section through the same larva below the stomodwal region. The members of one pair of 

 mesenteries are very strongly developed and bear filaments. The polypal cavity is still filled 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. 



MANICTNA AREOLATA Linnieus. 



Fig. 129. — Transverse section through a complete mesentery toward the lower part of the stomodseal region, including 

 the portion of the stomodneal wall and skeletotrophic tissue to which it is attached. The mesogUeal 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 mesentarv 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 mesogloea 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 mesogkea bearing the retractor muscle is very deeply plaited 

 almost throughout its extent. A comparison with fig. 129 shows how greatly the mesoglueal folds may 

 vary within the same species. X 300. 

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