ON THE PAIRED NEPHRIDIA OF PROSOBRANCHS. 611 
treatise nor my own paper on the development of Paludina, 
in which I was the first to maintain the opposite view. 
Of course, I am quite prepared to admit that many facts and 
statements must needs escape the attention of the author of a 
text-book comprising the whole of the animal kingdom, 
especially when the writer had not previously himself worked 
for some time upon the group he describes. 
I now propose to compare the results afforded by the study 
of Prosobranchs with those obtained by other workers from 
the remaining groups of Gasteropods and molluscs in general. 
The Heteropods are now almost generally considered as 
modified Prosobranchs. As was to be expected accordingly, 
the nephridium is situated to the left of the anus and rectum. 
We now come to the so-called Kuthyneura. I have 
already stated that the development of Planorbis (10) con- 
firmed my view of the homologies of the only remaining 
nephridium, which is also supported by the evidence collected 
from several other papers by different investigators dealing 
with the same question! with regard to Pulmonates. Of all 
Opisthobranchs, the Tectibranchiata are certainly the 
least modified forms and the most nearly related to Proso- 
branchs. The anatomy and a part of the embryology of 
Aplysia has been recently studied by my friend Signor 
Mazzarelli. According to him the external opening of the 
renal organ in the adult lies ventrally and to the left of the 
anus and rectum. ‘he kidney in the larva or veliger stage 
most probably corresponds to an organ which has been de- 
scribed as an eye by Lacaze-Duthiers. The position of the 
organ, which in the larva lies to the left of the rectum and 
anus (after the torsion has taken place in a leiotropic direc- 
tion), shows that it must be the actual left kidney. The 
development shows that the mode of formation of this organ 
closely agrees with that of the kidney of Paludina, Bythinia, 
and Planorbis. Before the torsion it lies to the right of 
the rectum. Its glandular part has a mesodermic origin ; the 
1 T must refer the reader to the list of embryological papers given in my 
memoir on ‘ Paludina’ (8). 
