76 



OLWEN M. REES. 



large nuclei, and they are slightly granular and ciliated. Between the 

 ectoderm and mesogloea is a very delicate layer containing the nerve 

 cells. The mesogloea in this region is denser and more uniform than that 

 of the body wall, and it is not fibrillar. 



The ectoderm of the stomatodseum is distinctly corrugated. The 

 cells are large, and many of them contain colonies of zooxanthellse. The 

 whole wall of the oesophagus is thrown into ridges like those described 

 and figured by McMurrich in his description of Halcurias (3). A trans- 

 verse section through the lower region of the column shows the body 

 wall with twenty short mesenteries, each consisting only of an elongated 

 form of the parieto-basilar muscle. Longitudinal sections of the oral 

 disk were examined. The ectoderm is spongy and contains a few colonies 



Fig. 4. — Transverse sections through a tentacle, showing structure of wall : (a) near 

 the tip, {b) near the base ; cc. ectoderm, en. endoderm, on. niesogleea, m.f. nmscle fibres, 

 n. nematocysts, 7i.l. nerve layer, ;:. zooxanthellse. 



of zooxanthellse. No nematocysts are present. The mesogloea sends up 

 folds into the ectoderm, except at the points where the mesenteries are 

 attached to the disk. The endoderm contains a few scattered cells of 

 zooxanthellse. From longitudinal sections of a tentacle (Fig. 3) it is seen 

 that the ectoderm is very thick at the tip, and contains numerous nema- 

 tocysts. In this region zooxanthellse are almost absent. This ectoderm 

 causes the swollen tip of the tentacle (Fig. 4a). Lower down the 

 zooxanthellse become more numerous (Fig. 4&), and are seen in the 

 ectodermal cells as colonies of pigmented bodies of a greenish yellow 

 colour. The presence of these algal colonies accounts for the blotches 

 described on the exterior of the tentacles. Near the base the ectoderm 

 is less thick, and in places contains neither nematocysts nor 



