262 MARTIN F. WOODWARD. 



it appears to me if this view be carefully followed out that it 

 is possible to derive the Monotocardia from such an existing 

 Diotocardian as Pleurotomaria. 



If we examine the condition of these organs in one of the 

 Azygobrauchia we shall find that with the loss of the right 

 auricle and gill the pericardium becomes displaced to the 

 left, and consequently the two kidneys approach one another 

 very nearly, so much so that Haller thought he found a com- 

 munication between the two. It appears, however, doubtful 

 if such a connection was present in the forms he examined, 

 but at the same time it seems extremely probable to me that 

 such a condition was attained in the early Monotocardia, as 

 the pericardium shifted further to the left to take up a posi- 

 tion at the end of the left gill, and the two kidneys conse- 

 quently came into close contact. Supposing a perforation to 

 occur in the wall intervening between the two kidneys, a 

 condition would be attained that would be of the greatest 

 advantage to the mollusc, as it would enable it to discharge 

 the secretion of the right kidney through the cavity of the 

 left, while the old right kidney-duct would now serve to 

 transmit the genital products unmixed with excreta. 



By a diminution in size of the glandular portion of the 

 papillary sac, and a complete severance of the right kidney 

 duct as a genital duct, we arrive at the condition of the 

 Monotocardia, in which we find a kidney situated in the 

 position of the right kidney, but whose cavity conimunicates 

 with the pericardium, and whose aperture suggests that of 

 the papillary sac; while packed in between this kidney and 

 the pericardium is the degenerate glandular portion of the 

 papillai-y sac forming the renal gland. 



I would thus regard with Perrier the single kidney of the 

 Monotocardia as representing the excretory part of the right 

 kidney of the Diotocardia plus the cavity, external aperture, 

 and reno-pericardial pore of the papillary sac ; Avhile the 

 glandular part of the latter structure persists as the renal 

 gland, and the duct of the right kidney becomes the genital 

 duct. 



