154 O. C. GLASER. 



edge tenable. Erlanger ('92), though denying the excretory 

 function of the " Ansae " of Bitliyuia, conceded that on mor- 

 hological grounds these cells might be considered the equivalents 

 of the peculiar velar cells of the pulmonates and of the marine 

 prosobranchs. 



Heymons ('93) introduced another source of confusion by 

 attempting to homologize the excretory cells of opisthobranchs 

 with the external kidneys of prosobranchs. This homology 

 seems far-fetched, since the excretory cells of Umbrella are situ- 

 ated near the anus of the larva, though they originate much 

 further forward. This difference in position seemed of no im- 

 portance to Heymons, since, as he says, McMurrich had already 

 shown that the excretory cells may lie at various distances 

 behind the velum. Conklin ('97) says in reply : "this difference 

 in position seems to me, however, to be a very considerable one. 

 In all prosobranchs these cells lie close behind the velum, while 

 in Umbrella they are removed from that structure by almost the 

 whole diameter of the embryo. Further, the fact that they sink 

 into the interior in Umbrella would indicate that they are differ- 

 ent from the excretory cells of prosobranchs." 



Conklin ('97) finds that the "external excretory cells " of 

 Crepiditla have no connection with the velum. This is also true, 

 in a sense, of Fasciolaria. In this form the external kidneys 

 originate long before the velum, so that their ultimate connection 

 with that organ is not primitive but secondary, and due to their 

 origin near the place where the velum originates. In this respect 

 they are not fundamentally a part of the velum, any more than 

 the external kidneys of Crepidula. 



It is obvious from the literature considered so far that the dis- 

 cussion of homologies has involved at least three different kinds 

 of excretory organs. In order to emphasize the differences I 

 shall henceforth call those mesodermal, or possibly ectodermal, 

 structures of pulmonates and prosobranchs which open to the 

 exterior through a pore, primitive kidneys ; those modified ecto- 

 dermal excretory cells which may occur in addition to primitive 

 kidneys, external kidneys ; and finally such external organs as 

 Heymons and others have described in opisthobranchs, excretory 

 cells. 



