532 A. M. <;AlMi SAUNDEKS AND MAI{(!Ai{i;T I'OOLE. 



IV. Secondary kidneys. Several large vacuolated ectoderm 

 cells opening to the exterior by a short duct : Opistho- 

 branchs. 



The origin of Type I has been diiferently described by 

 several investigators, and though there seems to be much 

 evidence of its arising from the ectoderm, yet a mesodermal 

 origin has been ascribed to it by Erlanger in Paludina, 

 Stauffacher in Cyclas, and Rabl and Holmes in Planorbis. 

 If the ectodermal origin of these larval excretory organs 

 should be proved beyond dispute, we should fairly be able 

 to compare them v^^ith the Annelid nephridia. Nevertheless, 

 if these organs are taken as representing ancestral nephridia, 

 and thus indicating a relationship between the Annelid and 

 Molluscan phyla, it is remarkable that they have never been 

 found in the primitive groups of the latter, the Amphineura 

 and the Aspidobranchia, and yet are present in the highly 

 specialised Pulmonates. The second type of larval excretory 

 organ is of no special significance. It has obviously been 

 developed to meet some special need during the early stages 

 of ontoo-eny. Type 111 may possibly be of a similar nature 

 to the preceding, but it certainly stands apart fiom the others, 

 and until we know whether it is ectodermal or mesodermal, 

 as has been asserted by Mazzarelli, it is impossible to compare 

 it with any other form of excretory organ. The fourth type 

 of kidney, which we have called the secondary kidney of 

 Opisthobranchs, offers some difficult problems both as regards 

 its homology and its ultimate destiny. The position it occupies 

 is very similar to that of the definitive kidney in the adult. 

 Mazzarelli, when he originally described it as arising from 

 the mesoderm, believed it to be the rudiment of that organ. 

 This idea he has abandoned in his later work on the free- 

 swimming larvfB of Opisthobranchs, and is inclined to believe 

 that the organ disappears in the metamorphosis, and is in no 

 way connected with the adult organ. He produces no evidence 

 for this view, though, as will appear later, we think it has 

 much justification. Casteel thinks it probable that the 

 secondary kidney persists through the metamorphosis and 



