THE DEVELOPMENT OF FASCIOLAKIA. 155 



Anyone who has seen Stauffacher's ('98) beautiful figures of 

 the primitive kidney of the trochophore of Cyclas cornea can never 

 compare similar structures with external kidneys in prosobranchs 

 and pulmonates, or excretory cells in opisthobranchs. Meisen- 

 heimer ('98) introduced a complication however, for his very 

 complete study of the development of Liinax ina.viunts leaves 

 almost no doubt that the primitive kidney of this gastropod is of 

 purely ectodermal origin. The reasons which Stauffacher has 

 advanced that the primitive kidney of Cyclas is mesodermal, are 

 as convincing as Meisenheimer's that in Liinax it is ectodermal, 

 so that it is necessary for the present to subdivide the group of 

 primitive kidneys into ectodermal primitive kidneys and meso- 

 dermal primitive kidneys. If the conclusions of Stauffacher and 

 Meisenheimer be indeed correct, then two sets of homologies can 

 be granted ; .the mesodermal primitive kidneys of prosobranchs, 

 pulmonates and lamellibranchs may be homologous ; and the 

 ectodermal primitive kidneys in the same groups may be homolo- 

 gous. No homology however can be granted between a primi- 

 tive kidney of mesodermal and one of ectodermal origin without 

 doing violence to the whole conception of homologies. Whether 

 a conception which separates organs as much alike in structure 

 and probably in function as the ectodermal and mesodermal 

 primitive kidneys of larval molluscs should be violated, is a ques-" 

 tion which at present I do not feel able to discuss. 



Are the external kidneys homologous with ectodermal primi- 

 tive kidneys ? I believe that there is no more reason to homol- 

 ogize external kidneys with ectodermal primitive than with meso- 

 dermal primitive kidneys, for the differences between external 

 kidneys and primitive kidneys, of whatever layer, are the same. 

 That in the one case both organs should originate from the same 

 germ layer is no criterion on which to base homologies, for if it 

 were we should be logically driven to homologize not only all 

 ectodermal structures, but all structures of whatever origin. 



In considering the relations of the larval excretory systems of 

 molluscs it seems to me to be of great importance to keep the 

 differences which I have tried to emphasize constantly in mind, 

 but the distinctions once made, the task of recognizing true 

 homologies is by no means a simple one. Possibly all meso- 



