ON THE PAIRED NEPHRIDIA OF PROSOBRANCHS. 9597 
been removed the right kidney is seen surrounding the peri- 
cardium on the right side. It sends a prolongation to the 
right, which extends further forwards than the pericardium 
itself, and another one to the left, so that the pericardium is 
entirely surrounded by the right renal organ, which extends 
far back nearly to the posterior end of the animal. In order 
to see the left kidney it is necessary to remove the mantle 
funnel which projects through the slit of the shell (fig. 22, F.), 
with the underlying sinuses of the funnel, also to open the 
pericardium dorsally and to lift up the left auricle. It is 
better to remove the heart entirely by cutting the rectum 
anteriorly and posteriorly to the veutricle, the two auricles at 
the origin of the branchial efferent veins, and the aorta (qao.), 
which issues from the ventricle posteriorly, ventrally, and on 
the left. 
Such a dissection is to be seen in fig. 5. The bottom of 
the pericardial cavity is displayed (Pc.) ; on the right a part of 
the right kidney (Vr.) is seen by transparency through the 
ventral wall of the pericardium. MR. and R&. are the two cut 
ends of the rectum on its way through the pericardium ; qo. is 
a section of the aorta, which has just left the heart and 
pierces the ventral wall of the pericardium. ‘The left kidney 
(N/.) is exceedingly small compared with the huge right renal 
organ. It lies embedded in the anterior wall of the peri- 
cardium, between this and the basibranchial sinus. It generally 
has the shape of a sac (fig. 19) provided with a bent duct 
leading into the terminal papilla. Sometimes, however, as in 
fig. 5, it is slightly ramified, 
In order to see the anterior openings of both kidneys and 
their nipples or papillz it is best to make a transverse section 
of the mantle cavity just in front of the anus. ThisI did ona 
well-hardened specimen, first making a horizontal section with 
a sharp razor, passing just below the bottom of the mantle 
cavity, then a transverse one just in front of the anus. 
Fig. 6 shows a preparation obtained by this method. The 
mantle cavity is seen to be divided into four portions or nooks 
by the branchial supports (47.) on either side. One may, 
