THK DEVEL0P3IENT OF APLYSIA PUNCTATA. 533 



becomes the kidney of the adult in spite of its ectodermal 

 origin ; he supports his view by referring" to Meisenheimer's 

 account of the "ectodermal" origin of the common rudiment 

 of heart, kidney, pericardium, and gonad in Dreissensia 

 and other forms. Heymons considered the kidney to be 

 merely a larval organ ; he further compared it to the external 

 ectodermal kidneys of Prosobranchs, our Type II. This 

 suggested homology seems to us very far fetched. To begin 

 with, these Prosobranch kidneys are variable in position in 

 the same species, while the Opisthobranch kidneys are derived 

 from almost the same cell in the three forms, Aplysia, Fiona, 

 and Umbrella. Further, the Prosobranch kidney is an 

 external protruding organ, while the Opisthobranch kidney 

 sinks well below the surface epithelium. 



Our own conclusion is that the secondary kidney of 

 Opisthobranch s cannot be homologised with any of the other 

 various molluscan kidneys. We have already given our 

 reason for believing that it cannot be homologised with 

 our Type II. The fundamental difference between it and 

 our Type I is that cilia are absent. It is possible that 

 in such advanced forms as the Opisthobranchs the cilia might 

 have been lost and the nephridium reduced to some such 

 condition as that which we find in the secondary kidney. 

 But the posterior position of the organ makes it unlikely that 

 it has anything to do with the Annelid nephridium, the 

 representative of which in Molluscs is always found close 

 up under the velum. In the embryo of terrestrial Pulmonates, 

 also, the nephridium is preserved, although they are more 

 modified than the Opisthobi-anchs. 



The position of the secondary kidney suggests at first sight 

 that it is the rudiment of the definitive kidney. But in all 

 those cases in which the origin of the definitive kidney is 

 known for certain, and in which it has been traced from the 

 embryo to the adult, it has been found to arise as an 

 evagination from the coelomic epithelium, which joins an 

 ectodermal invagination and so reaches the exterior. A 

 communication between the coelom and the kidney is present 



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