Studies on the Development of Larvai Nephridia. 493 



tliey arise iu intimate connection witli the mesenchyme. Tiieir 

 canals are also according- to Ms observatious formed from the sinking 

 in of the anal pit and its division into diverticula. Regarding 

 the oiigin of the solenocytes he made no direct observations , but 

 thinks that Ikeda has advanced no conclusive proof of their 

 origin from mesenchyme cells. 



It is to Goodrich (11) that we owc the most complete résumé 

 and description of the nephridia and their relations to the body 

 cavities. In the fully grown Actinotrocha larva the nephridia do not 

 open into the body cavity. The excretory cells fouud at the ends 

 of the nephridial cauals are similar in ali respects to the solenocytes 

 foimd in connection with the nephridia of Polychaets. The nephridia 

 open to the exterior ventrally behind the septum dividing the closed 

 posterior trunk coeloni from the anterior preseptal haemocoel, into 

 which they project and end blindly. During metamorphosis the 

 nephridia probably lose their solenocytes and acquire openiugs into 

 the coelomic cavities by means of peritoneal funnels. This last 

 however he did not determine by direct Observation. 



CowLES (6) in A. architecta has been able to confìrm most of 

 the points brought forward by Goodrich. The nephridia do not 

 oi)en into the eoliar cavity bat end in thiu walled bulbs to which 

 the excretory cells are attached. These along with the canal of 

 the nephridium are probably of ectodermal origin. 



Thus there is a more or less unanimous agreement that the 

 nephridia arise in some way from the posterior pouches, that they 

 do not open into the body cavity, are furnished with solenocytes, 

 which may or may not be derived from the mesoderm, and that 

 the fiinnel openings of the adult ue})hridia into the body cavity are 

 a secondarily acquired feature, possibly of peritoneal origin. 

 As regards the body cavities there is stili considerable diversity of 

 opinion and much remains to be done in elucidating their rela- 

 tionships. 



2. The early origin of the mesoderm. 



As the first appearance of the larvai nephridia is essentially 

 connected with the early origin of the mesoderm and the development 

 of the boLly cavities, I bave followed the process of gastrulation 

 and the first appearance of the mesoderm with some care. Both 

 the species^ which I bave studied agree with one another in ali 



1 For material and methods see end of paper. 



33* 



