THE DEVELOPMENT OF rALUPTNA VIVIPAUA. 107 



cooloinic connection between gonad and kidney in the primi- 

 tive Rbipidoglossa similar to that which he describes for the 

 Docoglossa is not very clear. Most writers, however, liave 

 described the gonad as having become separated from the 

 ccelom nltogether, and having acquired a new opening into 

 the kidney. To this, and also to Pelseneer's view that this 

 opening occurs nearer to the external aperture in the higher 

 forms than in the lower, von Erlanger's description of 

 the course of development iu Paludiua lent strong support. 

 This, however, has completely failed, for the communication 

 between gonad and kidney has been shown to be close to the 

 reno-pericardial aperture in Paludina, as Pelseneer has de- 

 scribed in Fissurella and other primitive forms, while traces 

 of an original ccolomic connection between the two are found 

 in the thickened ridge of pericardial epithelium described 

 above, which can hardly be otherwise interpreted than ns 

 representing a groove in the coelomic floor along which, in 

 more primitive forms, the genital products passed to the 

 reno-pericardial aperture. That the latter still remains open 

 even after the communication between gonad and kidney is 

 established is no real hindrance to such an interpretation, for 

 the solid nature of the rudiments of both the gonad and the 

 coelomic connection shows that the ontogeny is abbreviated, 

 and gives no exact picture of the phylogenetic events. The 

 opening of this pericardial groove into the kidney must, it is 

 true, represent at least a portion of the reno-pericardial aper- 

 ture. Phylogenetically, we may believe, the edges of the 

 groove drew together and a tube was formed, opening at one 

 end into the gonad and at the other into pericardium and 

 kidney at once thi'ough the reno-pericardial aperture. 

 When, by abbreviation, this tube came to be formed in the 

 course of development as a solid rudiment, it is easy to 

 understand how the hollowing out and subsequent communi- 

 cation with the kidney might lead to the appearance of a 

 rupture of the kidney wall. If this interpretation be correct, 

 we have in Paludina, which has always been regarded as one 

 of the most specialised of the Rhipidoglossa, a condition of 



