526 MR. A. HANCOCK ON THE STRUCTURE AND HOMOLOGIES OF THE 
eases, however, the function is apparently the same. The renal chamber proper, in the 
two groups, is characterized by its glandular nature. In the Cephalopods the renal 
glands are specialized in connexion with the venæ cave, the rest of the chamber being 
for the purpose of receiving the extravasated fluids from the contained organs. In the 
Nudibranchs this chamber is modified both in its form and function. In some it is 
reduced to’ a delicately branched gland, and is consequently a mere secreting organ; in 
others it is partly glandular and partly a receptacle for extravasated fluids, either from 
the organs with which it is connected or from vascular ramifications that are spread over 
its walls. : 
Such being the condition of the renal organs in these two groups of mollusks, it seems 
evident enough that they are not designed for the reception of water from the exterior ; 
and indeed in neither the Cephalopod nor the Nudibranch could it possibly penetrate 
further than the first or renal chamber, as the passages connecting it with the other 
chamber in both these forms would be closed, on the slightest pressure of the contained 
fluid, much in the same manner that the ureters are in the bladder of the higher animals. 
Fluid, however, may be easily made to pass in the contrary direction, namely, from the 
genital chamber in the one, or from the pericardial chamber in the'other, to the renal 
chamber, and hence to the exterior through the external orifices. "Thus an irresistible 
inference is raised in favour of the view that these chambers are for the purpose of 
throwing fluid out of the system, not for the purpose of taking it in. 
Should there therefore exist any water-system as generally understood in these Mol- 
lusks, it is not in connexion with the renal organs. Some special apparatus may be 
provided for such a purpose; but if so, it has hitherto escaped my observation. At the 
same time the possibility of something of the kind must be allowed since the discovery 
that in at least Plewrobranchus the blood-vascular system opens externally: in this 
highly organized mollusk the external water can apparently be drawn in and mingled 
directly with the circulating noarishing fluid. I have searched in vain, however, for a 
similar opening in the Nudibranchs; and until such be found, it may be fairly assumed 
that the fluid which pervades the tissues of these animals, as it does the tissues of all 
mollusks, is admitted in the ordinary way through the agency of the alimentary tube. 
EXPLANATION OF THE PLATES. 
The following letters indicate the same parts in all the figures :— 
a, the so-called pericardium; a’, dorsal wall of ditto; a”, floor of ditto; b, ventricle; €, auricle ; 
d, pyriform vesicle; e, orifice of ditto leading into pericardium ; f, laminated plicæ in the in- 
terior of the vesicle ; J, glandular appendages lining the tubular prolongation of ditto; و7‎ tubu- 
r prolongation of ditto ; û, orifice of prolongation, leading into the renal chamber; 7, similar 
orifice at the apex of the vesicle when the prolongation is wanting; j, renal chamber proper; 
17 dorsal wall of ditto; k, anterior bifurcation of ditto ; L lateral branches of ditto; m, artery 
