i9os; 



EXCRETORY ORGANS OF METAZOA. 579 



where the nephridia function as discharge ducts for the genital 

 products. . . . And again in some Lamelhbranchs, Diotocardians 

 and the Scaphopods there exist relations between sex glands and 

 nephridia in that the sex glands open into the nephridia, so that a 

 shorter or longer portion of the latter functions not only as kidney 

 or ureter but also as discharge duct for the genital products ' 

 (Hescheler, 1900). In those prosobranchs with only one adult 

 nephridium, Drummond (1902) has shown for Paludina, and after 

 a full discussion of the literature, that the right nephridium of the 

 embryo persists as the left one of the adult, in agreement with 

 Erlanger, but contrary to his results she finds the left nephridium 

 of the embryo does not disappear but becomes the genital duct. 



Larval Nephridia. — These are known only in Gasteropods and 

 Pelecypods (Lamelhbranchs), and it will be most convenient to 

 treat separately the groups in which they occur. 



(a) Prosobraiicli Gasteropods. — Two kinds of these have been 

 described, (i) External nephridia (Aussennieren, excretory cells). 

 These are ectoblastic, unicellular or multicellular organs, usually 

 projecting from the surface of the body just behind the velum; 

 there is one pair of them, and their cavity communicates with the 

 blastocoel; sometimes they have an opening to the exterior. They 

 have been described most carefully for Crepidula (Conklin, 1897) 

 and Fasciolaria (Glaser, 1905), also for Nassa, Natica, Fustis 

 (Bobretzky, 1877), Paludina and Bythinia (Sarasin, 1882, who 

 calls them "ansce"), Fasciolaria and Fulgur (McMurrich, 1886), 

 Fissurclla (Boutan, 1885), and Capuhis (Erlanger, 1892a). Glaser 

 has demonstrated that they are first digestive, later serve as reser- 

 voirs for waste products, and subsequently fall off from the surface 

 of the larva ; Sarasin and AIc^Murrich supposed they were originally 

 parts of the preoral velum, and that with excretory specialization 

 they separated off from it; but Conklin and Glaser show that they 

 arise independently of and before the velum. As " secondary 

 outer kidneys " Glaser has described certain excretory cells placed 

 in the velum and the head vesicle. 



(2) The second kind of larval excretory organs of the proso- 

 branchs are mesoblastic. These arise from a mesoblastic anlage 

 that is at first solid, while more or less of the duct is ectoblastic; 



