ON THE PAIRED NEPHRIDIA OF PROSOBRANCHS. 599 
that it has entirely atrophied in Fiss. nubecula, where even 
the papilla has disappeared. 
Fig. 23 shows a transverse section through the pericardium 
and both kidneys of Fiss. greca. Dorsally and in the middle 
line the cavity of the pericardium (Pc.) can be easily seen. 
In it lies the ventricle (V.) surrounding the rectum (R.), to 
the right of which an artery is seen in transverse section. 
The section is not absolutely transverse, so that only one 
auricle, the left one (au/.), is to be seen. Fig. 10 had shown 
that both auricles are of the same size, and situated symme- 
trically on either side of the ventricle and the longitudinal 
body axis. On the ventral side of the pericardium lies the 
basibranchial sinus (das.) ; to its right lies the beginning of 
the right kidney (Nr.), with the commencement of the genital 
duct (Gd.) situated to the right of the latter. Fig. 26, a part 
. of a transverse section through Fissurella nubecula, shows 
the genital duct (Gd.), containing young ova, opening into 
the beginning of the right kidney (N*.), which also contains 
some ova. This shows that the sexual products are really 
discharged through the right nephridium. To the left of the 
basibranchial sinus (das.) is the left nephridium (N/.). The 
figure shows the tremendous difference in the size of the 
nephridia. The section of the left is the largest one in the 
whole series, and still it is surprisingly small, compared to the 
right kidney, which extends from one side to the other of the 
section, and insinuates itself between the different viscera 
(Nr., Nr.). Fig. 21 is a transverse section through the papilla 
of the right kidney in Fiss. gibberula, showing the external 
opening of the right kidney (Xr.), situated between the genital . 
duct (Gd.) and the beginning of the right kidney cavity (Nr.). 
Von Jhering, Haller, and Perrier are unanimous in saying 
that the left kidney does not open into the pericardium. 
Whilst Jhering and Boutan failed to detect a reno-pericardiac 
duct in the right kidney, Haller and Perrier have described 
a right reno-pericardiac canal. 
I have carefully studied a great number of transverse and 
frontal series through Emarginula and Puncturella, and 
