ON THE PAIRED NEPHRIDIA OF PROSOBRANCHS. 618 
gations denied that the organ hitherto described as heart really 
represented the blood-propelling organ, has in the full account 
of his researches admitted that the heart really exists, but in 
a very rudimentary form. Nevertheless he refuses to admit 
that the pouch, in the roof of which the heart forms a gutter- 
like invagination, really represents the pericardium. His 
chief objection is that the epithelial lining of this pouch pro- 
duces the ova and the spermatozoa. The conception that the 
cavity of the pericardium in molluscs represents the reduced 
ccelom, and that the cavity of the gonad originally formed a 
part of this coelom, is evidently unknown to Mr. Pruvot. The 
fact that the epithelial lining of the pericardial cavity repre- 
sents the gonad or genital epithelium is the best proof that the 
Neomenia show a very primitive condition in this point, 
reminding one strongly of the condition to be met with in a 
great number of Annelids. I therefore further conclude 
that Pruvot’s oviducts are really the nephridial ducts which 
possibly have lost their connection with the glandular part of 
the renal organ. 
Having discussed Pruvot’s rather isolated conception of the 
uro-poetic organs of Neomenia, I think it advisable to com- 
pare the results which I have obtained by the study of Proso- 
branchs with those of other anatomists on the corresponding 
organs of the remaining molluscs. 
In an account of the development of Paludina I have shown 
that the gonad arises as an evagination of the pericardium or 
ceelom, which gradually becomes constricted off and acquires a 
special duct leading into the mantle cavity. The glandular por- 
tion of both renal organs has a similar mode of formation, and 
the original lumen of the evagination leading into the peri- 
cardium becomes the reno-pericardiac funnel. The cavity of 
the gonad and those of both nephridia are, therefore, parts of 
the ceelom. 
In Paludina the actual right kidney has disappeared, and 
its duct, formed by an introflection of the ectoderm, becomes the 
efferent duct of the gonad. The actual left kidney persists, 
and remains in communication with the pericardium. 
