478 K. iUMUNNI MENON. 



corpuscles are formed in tlie adult Plioronis from tlie endo- 

 thelium of the blood-vessels. 



To return to the body cavities — the trunk cavity is divided 

 into right and left halves by a complete ventral mesentery 

 and by a dorsal mesentery, which is only represented by a 

 small strand at the posterior end. The somatopleure is 

 separated from the ectoderm by a thick layer of chondroid 

 tissue (also developed in the collar, though not to the same 

 extent), from which it is detached in preparations, and often 

 thrown into irregular folds. At the anterior boundary of the 

 perianal band the somatopleure leaves the body-wall and 

 runs inwards and backwards to meet the gut wall, and become 

 continuous with the splanchnopleure. There is thus left a 

 space surrounding the posterior part of the intestine and 

 between the coelomic epithelium in front, the ectoderm on 

 the outside, and the endoderm on the inside. The ectoderm 

 forming part of the wall of this space is highly vacuolated, 

 the vacuoles being large and regularly arranged, and the cell 

 substance is reduced to an external and an internal limiting 

 layer and strands between the vacuoles. The nuclei of these 

 cells are arranged in the external and internal layers, but 

 mainly in the former. The arrangement of the parts will 

 be made clear by fig. 7. The space under the perianal band 

 thus appears to have a lining of its own. If the ccelomic wall 

 of this space were thrown into a fold projecting into the 

 trunk coelom we could get a section (without the internal 

 opening, which may be artificial) pretty much like the one 

 shown in Masterman's drawing of the posterior nephridia. I 

 venture to suggest this as a possible explanation of the 

 posterior nephridia, which I have not found as such, especially 

 as Masterman himself seems to be doubtful about them. 



(2) Nephridia. — Masterman describes for his Actino- 

 trocha proboscis pores, collar nephridia, and trunk nephridia. 

 I have carefully .looked, both in the living larva and in 

 sections, for the cellular tubes and the pores which Masterman 

 homologises with the proboscis pores, but have found no 

 trace of them. There is nothing to be added to what has 



