304 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 
leaving the middle part of the tube as the forward convexity of the U. 
Thus the U formation in this case, and in Solenomya and lamellibranchs 
in general, is probably due to very different causes. For in the latter 
it appears that the anterior end of the pericardium with the 
nephrostomes has been the fixed point, and that the middle part of 
the renal tube has been drawn backwards by the general backward 
growth of the body. 
EXPLANATION OF PLATE X. 
Fie. 1.—Diagram of the right organ of Bojanus and neighbouring parts of Nucula 
nucleus, seen from the side. 
», 2.—Diagram showing the relation of the oviduct and organ of Bojanus. 
3.—Part of transverse section of Nucula nueleus, passing through the opening 
between the reno-pericardiae duct and the uro-genital cloaca. 
5, 4.—Diagram of the organs of Bojanus viewed from above. 
> 
arb. 1-3. arborescent processes of the organ of Bojanus. 
cv.c. cerebro-visceral connection. 
exe.a. excurrent arm of the organ of Bojanus. 
int. intestine. 
neph. nephrostome. 
od. oviduct. 
pe., pe. pericardial cavity and cut edge of pericardial floor. 
pe.a. pericardial arm of the organ of Bojanus. 
rect. rectum. 
rp.d. reno-pericardial duct. 
ug.c. uro-genital cloaca. 
x. opening between rp.d. and ug.c. 
