484 EDWIN S. GOODRICH. 



Cyclas (Ziegler, 116). In other cases, such as Paludina, and 

 Bythinia (v. Erlanger, 32, 33), the nephridium appears not to 

 open internally, but to remain in the pronephridial stage. 



The first stages in the development of the nephridium of the 

 Mollusca have, unfortunately, not yet been satisfactorily 

 worked out. Rabl (88a) derives the nephridium in Planorbis 

 from a large cell which also gives rise to the mesoblast. 

 Wolfson traced that of Lymngeus, which, he says, is derived 

 from a large in-wandering epiblastic velar cell on either side 

 (111). Hatschek (50), v. Erlanger (32, 33), and others trace 

 it to cells which they consider to be of mesodermal character, 

 but the exact origin of which is not clear.^ In some cases a 

 late epidermal invagination is said to take place at the nephri- 

 diopore which forms the peripheral end of the duct (v. Erlanger, 

 32, 34) . 



We must now examine the development of the excretory 

 organs of the adult Mollusca, which appear to be nothing but 

 peritoneal funnels. An accurate description of the develop- 

 ment of Paludina has recently been given by R. von Erlanger, 

 and although the ontogeny is much modified owing to the 

 asymmetry of the adult, I shall begin with it as it is the only 

 detailed account we have. The genital follicles or coelomic 

 sacs (pericardium) arise as a cavity on either side, a hollowing 

 out of the mesoblast (fig. 20). These cavities enlarge and fuse 

 below the gut. On either side a thickening takes place on the 

 ventral wall of the ccelom, which here grows out in the form of 

 a typical peritoneal funnel (fig. 21). The right peritoneal 

 funnel then enlarges and fuses with an epidermal invagination, 

 which forms the outer end of the excretory organ. " Was die 

 Niere anbelangt,'^ says von Erlanger, "so bin ich der Ansicht, 

 dass der secernirende Abschnitt derselben aus dem Mesoderm 



1 I think we may safely say that there is nothing which precludes the pos- 

 sibility of the nephridia being primitively derived from the epiblast, as they 

 appear to be in the Platyhelminths, Rotifers, and Chsetopods; although the fore- 

 casts may sink in at a very early stage and thus become included in that " An- 

 lagecomplex " which we call mesoblast. In Helix, de Meuron derives them 

 from the epiblast, but he is not positive about the internal extremity (79). 



