Studies on the Development of Larvai Nephridia. 509 



beiug passed tliroug-li in a veiy sliort time. The material I have 

 obtained always shows these structures already formed or probably 

 sooü to appear. The criticai stage takes place some time after 

 metamorphosis when the young worm has almost assumed the adult 

 form. The nephridial canals are fouud just after metamorphosis, 

 as Ikeda (14) and Goodrich (11) have observed, one on either side 

 of the auiis somewhat reduced in size, in the position they hold in 

 the adult worm. The solenocytes have disappeared, there is no 

 doubt that they drop off into thc collar haemocoel before the ne- 

 phridia lose connection with this space. 



In Ä. hmncliiata after metamorphosis the larvai fuunel-like ends 

 of the nephridia are seen unchanged, although the solenocytes have 

 disappeared. It is possible the coelomic funnels may be formed by 

 the further growth of these structures, it is certain at this time no 

 trace of the funnels can be seen as separate growths of the coelomic 

 wall. I hope however to return to this point in a future paper. 



6. Summary and conclusion. 



It has been shown that in the young larva of Phoronis the 

 nephridia develop as outgrowths of the diverticula into which thc 

 nephridial or anal pit divides, that the solenocytes form as direct 

 outgrowths of certain cells of the sides and ends of the nephridial 

 canals. As the nephridial pit is eutirely an ectodermal structure, 

 the nephridia and solenocytes, as outgrowths of it, are also of 

 ectodermal origin. In the early stages the nephridial canals are 

 long and slcnder openings at the posterior end of the larva on either 

 side of the anus. Duriug development there is a considerable 

 shorteniug and thickeniug of these canals, and their external openings 

 move forward until in the Actinotrocha larva they open behind the 

 ring of tentacles on the anterior end of the trunk, where they pro- 

 ject inwards and forwards between the preseptal coelom and the 

 gut wall, into the haemoeoelic space of the collar region. They 

 are closed, never communicating with the blastocoelic space in which 

 they lie. During metamorphosis the canals of the larvai organs 

 persist as the canals of the adult nephridia, which acquire openings 

 into the coelom by means of ciliated funnels of unknown origin. 

 The main coelomic cavity of the larva, the body cavity of the adult, 

 appears a little after the nephridia as a small space on the dorsal 

 side of the rectum and is from the first unpaired. Only after meta- 

 morphosis do the nephridia come into relation with it. 



34* 



