1889.] NATURAL SCIENCES OF PHILADELPHIA. 115 



plasm of which, with the exception of the nucleus, does not stain 

 with carmine. Otherwise the mesoglcea is homogeneous. The en- 

 doderm is thick, and is richly supplied with zooxanthellse. In struct- 

 ure the disc resembles the tentacles, possessing, like them, the 

 peculiar yellowish-green granular cells. 



The mesoglcea of the stomatodreum is homogeneous. I cannot 

 make any statements as to the histology of the ectoderm of this re- 

 gion, as it had macerated into a mass of a characteristic appearance 

 which cannot easily be described. Transverse sections show that the 

 gonidial groove, to which the macro or ventral directives are 

 attached, is very shallow, and indeed can hardly be said to exist. 



The mesenteries are arranged on the microtypus J . Their mes- 

 oglcea is for the most part very thin but thickens towards the base 

 where it contains a canal. (PI. VII, fig. 4, be.) A second canal, cir- 

 cular in section and packed with cells occurs in the thin region, the 

 mesoglcea splitting to form its walls. The muscle layers are only 

 slightly developed. 



A very peculiar arrangement occurs in connection with the mes- 

 enterial filaments of the perfect mesenteries. Immediately below 

 the stomatodseum the mesenterial filament is triradiate (PI. VII, fig. 

 3), the central ray being short and stout, the lateral rays longer and 

 recurved. The epithelium covering the central ray and that face of 

 the lateral rays which looks towards it resembles in structure that of 

 the stomatodreum. The outer surface of the lateral rays is, however, 

 covered with cells similar to those which line the general surface of 

 the mesentery. In a section which passes through the stomatoda?um 

 a little above its extremity, the intervals between the perfect mesen- 

 teries is occupied by macerated tissue resembling the ectoderm of 

 the stomatodaHim. Apparently it lines the surfaces of the mesenteries 

 for a short distance outwards from their point of attachment to the 

 stomatodreum, and also the outer surface of the latter for a short 

 distance above its inner opening. It looks as if the ectoderm of 

 the stomatodaBum were reflected upwards, so as to cover its endo- 

 dermal surface and the adjacent surfaces of the perfect mesenteries. 

 Further down (PL VII, fig. 4) the two lateral processes of the mesen- 

 terial filaments disappear, the central one alone persisting. It is 

 evidently the " glandular streak " of the filament. The cells which 

 cover the surface of the mesentery for some distance outward from 

 this towards the column-wall are very peculiar. (PI. VII, fig. 4 di.) 



1 See Erdmann loc. cit. 



