324 



INVERTEBRATA 



CHAP. 



one or a pair of dark excretory vesicles situated extremely far back in 

 the neighbourhood of the anus. This vesicle, in Aplysia, has been 

 supposed by Mazzarelli (1898) to be the rudiment of the permanent 

 kidney, but Holmes (1900) declares that in Fiona the main part of it 

 is formed from a large cell which he identifies as 3c ul . In Umbrella 

 Heymons (1893) finds a pair of these organs which arise from 3c n 

 and 3d 11 . They thus roughly correspond to the archinephridia of 

 Polygordius in position. 



The whole uncertainty in the matter arises from the fact that 

 in no single species of Opisthobranchiata have we the complete 

 developmental history of the organ from its earliest origin in the 

 embryo until the larva has metamorphosed into the adult ; and 



a 



FIG. 254. Embryo of Limiiaca staynalis viewed from right side as a transparent object 



in order to show the larval kidney. (Alter Erlanger. ) 



a, anus ; c.o, external opening of the larval kidney : /, foot ; hrp, lobes of liver ; /., .larval kidney ; 

 n.c, nerve collar; o, mouth; r.s, radula sac; sh, shell ; sol, solcnocyte of the larval kidney; st, 

 stomach. 



Comparative Embryology, so long as it is based on bits and scraps 

 of development, is bound to be full of obscurities and apparent 

 contradictions. 



In Marine Prosobranchiata there are frequently present two 

 external protuberances of ectoderm cells, situated on each side and 

 behind the velum. These become filled with excreta, and are 

 eventually cast off. When it is remembered that the archinephridia 

 of Polygordius owe their origin to ectoderrnal cells, it will be seen 

 that it is quite possible to regard these external nephridia as 



homologous with them. 



SOLENOGASTRES 



We now pass on to consider the developmental history in other 

 classes of Mollusca. The Solenogastres are an extremely primitive 



