108 ISABELLA M. DRUMMOND. 



the urinogenital organs in every way comparable to that which 

 obtains among the Amphinoma. 



Finally^ with regard to the origin of the single asymmetrical 

 gonad in the Gasteropoda, both Pelseneer and Haller seem 

 agreed that this has been formed by the fusion of the 

 originally separate gonads of both sides. Phylogenetically, 

 of course, this may have been the case, but ontogenetically 

 there is no trace of it, the existing gonad being formed 

 exclusively from the extreme left-hand corner of the original 

 left division of the pericardium. 



Summary. — The conclusions at which we have arrived 

 are as follows : 



(a) The embryology of Paludina demonstrates that the 

 functional kidney of the adult belongs morphologically to 

 the definitive left side of the body, as von Erlauger has 

 already pointed out. 



(fe) The definitive right kidney is not lost, as von Erlanger 

 describes, but persists as the genital duct. 



(c) An indication of the original coelomic connection 

 between gonad and kidney is found in the course of develop- 

 ment of Paludina as a thickened ridge of pericardial epi- 

 thelium, which finally becomes indistinguishable from the 

 gonad, and, after it has acquired a lumen, communicates with 

 the definitive left kidney close to the reno- pericardial 

 aperture, 



{d) The gonad arises as a solid proliferation of the 

 morphologically dorsal wall of the pericardium. It arises 

 fi'om the original left side only, and shows no sign of a 

 paired origin. 



Part II. — The Development of Paludina viewed in Connection 

 with Theories of Torsion. 



(a) Description of Development. 



Stage A (fig. lOj. — The youngest stage which I have 

 examined is a bilaterally symmetrical, oval embryo, with 



