ON THE PAIRED NEPHRIDIA OF PROSOBRANCHS. 609 
lobules situated on either side of the anus: they have been 
described by Haller (18). On their surface can be descried 
numerous canals, having the appearance of blood-vessels. 
These canals, however, are by no means blood-vessels. They 
unite from both sides and run into a large sinus which enve- 
lops the rectum. The sinus opens to the right of the anus 
by a large lip-like orifice. This opening must not be mis- 
taken for the female genital opening, which lies to the left of 
it. The anal kidney does not communicate with the peri- 
cardium, while the actual left kidney is plainly seen to possess 
a reno-pericardiac duct.! 
In this instance also it would be highly desirable to study 
the development of the left kidney, and of the anal kidney 
of Dolium, as under the present state of our knowledge 
it is doubtful, although probable, that the so-called anal 
kidney really represents the modified right kidney of 
Dolium. 
To return to R. Perrier’s hypothesis, this author is ready 
to admit that his ideas on the homology of the only re- 
maining kidney of Monotocards can easily be reconciled 
with my own. He was led to believe that one kidney had 
disappeared in these forms. The only circumstance which 
induced him to suppose that the remaining kidney was the 
actual right nephridium was the predominance assumed by 
the right kidney in Diotocards, an argument derived from 
analogy, and “ peu probant,” as Perrier himself wrote to me. 
It would be necessary to ascertain whether the nephridial 
gland really represents one or the other kidney. I hope to be 
able to solve this problem in the course of time, having 
already collected materials for the study of the development 
of Cassidaria, which Perrier chose as a type for his descrip- 
tion of the nephridial gland. If according to Perrier the 
nephridial gland represents the actual left nephridium, we 
should be led to conclude that in Paludina and Bythinia, 
where no such gland exists, the actual right kidney has 
1 Dr. Schiemenz had the kindness to show me the organ just described in 
several specimens of Dolium preserved in spirits. 
