TRE DEVRLOPMRNT OF PALUDTNA VIVIPARA. 105 



place, a remnant of the primitive condition is found in the 

 frequent connection between gonad and kidney in existing 

 forms. 



While there is almost perfect agreement with regard to 

 the general features of the anatomy of the primitive form, 

 however, there is considerable divergence of opinion as to 

 the course which evolution has followed, and consequently 

 various interpretations are put upon the structures of 

 existing forms, while hitherto embryology has been almost 

 silent, and evidence has had to be almost entirely drawn 

 from the field of comparative anatomy. Pelseneer (12 and 

 13) and Haller (10 and 11) are among the chief writers who 

 deal with the coelom and its derivatives among the Proso- 

 branch Gasteropoda, and still uphold quite different views 

 upon many points, though they seem agreed in maintaining 

 that a gradual loosening of the connection between gonad 

 and kidney has taken place throughout the group. While 

 Pelseneer, however, only maintains that the point of com- 

 munication tends to shift away from its primitive position 

 by the reno-pericardial aperture nearer to the external 

 opening of the ureter, as is the case in the Lamellibranchs, 

 Haller regards the connection between gonad and kidney as 

 altogether severed among the higher Rhipidoglossa, a 

 portion of the coelom becoming specialised as the gonaduct. 

 Connected with this is the different view which these two 

 authors take of the homology of the existing kidney. Thus 

 Haller regards the functional kidney as in nil cases the 

 right one (after torsion). Ainong primitive forms (e.g. 

 Fissurella) this keeps its connection with the gonad, while 

 that on the left side of the body loses its connection with the 

 gonad and kidney, and is fast disappearing. Further stages 

 of evolution are shown by Haliotis, Trochus, and Paludina, 

 in all of which the left kidney has entirely disappeared, and 

 the connection between the gonad and right kidney is lost, 

 while the latter has more and more passed over to tlie left 

 side of the body. Pelseneer, on the other hand, has demon- 

 strated, though this is still denied by Haller, that in both 



