NRPHRTDIA OF TH R AOTINOTROCHA LARVA. 107 



side of the cesopliagnSj as descinbed by Miiller, Wagener, and 

 others. The wall of the ventral outgrowth differs but slightly 

 in histological structure from that of the remainder of the 

 ventral wall of the stomach ; but its granular cells are gene- 

 rally vacuolated, large droplets of fatty-looking fluid being 

 situated at their outer ends (figs. 4, 13). Masterman has 

 compared this gastric caecum of Actinotrocha to the noto- 

 chord of Vertebrates. The two organs seem to me funda- 

 mentally different both in their relations and in their 

 histological structure ; and I agree with Menon, Ikeda, 

 and de Selys Longchamps that Mastermau's theory as to its 

 homology is quite unjustified. I may repeat the statement 

 of Longchamps that the histological characters of the gastric 

 cascum " sont absolument differents de tout ce qui a jamais 

 ete observe dans une notochorde quelconque'' (16). 



We may now turn to those structures with which we are 

 more directly concerned — the cavities themselves. 



It is generally agreed, since the appearance of Caldwell's 

 paper, that there exists in the trunk region of the larva, 

 behind the ring of tentacles, a large ccelomic cavity in which 

 the gut lies attached to the body-wall by a median ventral 

 mesentery. It is in this cavity that the ventral pouch grows, 

 carrying the mesentery with it. This trunk cavity is limited 

 in front by a definite complete septum, which has an oblique 

 course, being attached much further forward dorsally than 

 ventrally. Concerning the posterior limits of the trunk 

 coelom there is some disagreement. Masterman described a 

 perianal sinus on the inner side of the perianal ciliated ring, 

 and added that ''at the perianal area are a pair of organs 

 which I have not fully made out, but they may be the 

 rudiments of the trunk nephridia. They lie in the hasmocele 

 space immediately below the ciliated band, and are thin 

 walled. They have an internal opening into the trunk 

 coelom, and appai-ently open to the exterior on either side of 

 the anus. Their walls are of the same natui'e as those of the 

 coelom, and appear to be portions of the coelom, with the 

 trunk nephridial tubes cut off from the rest of the trunk 



