THE ANATOMY OF PLEQKOTOMARIA BEYRICHJI. 261 



for it is no more — that the genital duct of Paludina arises 

 from the right secondary ureter, a structure which is not 

 known to be present in any liviug mollusc, and whose 

 existence we have no reason to presuppose. 



If, however, we may rely upon these ontogenetic reseai'ches, 

 theu the single kidney of the adult Monotocavdia would be 

 the left kidney or papillary sac of the Diotocardia, a vieAv 

 which is supported by the presence of a reno-pericardial pore. 



Against this view we have the position of the kidney in 

 relation to the stomach and pericardium in the majority of 

 the Monotocardia (Paludina being an exception), and the 

 necessity, if we accept it, to seek our ancestral Monoto- 

 cardian in some very archaic Diotocardian, one in which the 

 left kidney has not attained the specialised character of a 

 papillary sac.^ Moreover the acceptance of this view does 

 not explain the presence of the peculiar reual gland in the 

 Monotocardia, which has much the character of, and which 

 possesses the peculiar vascular relation of the papillary sac. 



Perrier, who made a very exhaustive investigation on the 

 molluscan kidney, believes that the single kidney of the 

 Monocardia contains representatives of both the kidneys of 

 the Diotocardia, and he sees in the renal gland of the former 

 group the representative of the papillary sac of the latter 

 group. This view, which is an extremely suggestive one, 

 has not met the consideration which it deserves, most zoo- 

 logists apparently acceptiiig Erlanger's statements on the 

 development of these organs in Paludina as conclusively 

 proving that the monotocardian kidney is the papillary sac. 



It is, however, possible to approach this subject from ano- 

 ther standpoint, and to endeavour to reconstruct the stages 

 which must have occurred in the displacement of the kidney 

 following upon the disappearance of the right gill and the 

 consequent displacement of the heart and pericardium, and 



' It might bethought that Fissurella or Patella amoug liviug Dioto- 

 cardians presented us with the couditiou we want, but these forms are too 

 obviously specialised in other respects to serve as the ancestors of the Mono- 

 tocardia. 



