522 MR. A. HANCOCK ON THE STRUCTURE AND HOMOLOGIES OF THE 
mine which view is correct. Nevertheless enough has been ascertained to satisfy us that 
the renal organ in the Lamellibranchiata and Nudibranchiata correspond pretty closely, 
so far as we have, up to this moment, compared them. | 
Professor Rolleston and Mr. Robertson, however, have recently described*, in addition 
to the parts above alluded to, what they consider a water-vascular organ—a minutely 
ramified system of tubes or vessels in connexion with the reproductive organs, which 
seems not only for the discharge of the products of these organs, but also for that of 
water supposed to be drawn into the economy through the orifices leading into the 
organs of Bojanus by the agency of the pericardial sinus. As this system of aquiferous 
vessels opens, in numerous instances, into the organ of Bojanus, it may be looked upon 
anatomically, I presume, as a development of the kidney; and if so, we cannot but be 
reminded of the highly ramified condition of the renal chamber in Bornella and Scyllea. 
This may be a mere analogical resemblance, but it is very striking and worthy of a 
passing remark. : | 
. A still more important discrepancy, however, appears to exist. The above authors 
point out, in their valuable paper already alluded to, that the pericardial sinus is the 
instrument through which water from the exterior finds its way into the blood-vascular 
system. In the Nudibranchs we have seen that the pericardial chamber is closed, with 
the exception of the orifice leading into the pyriform vesicle. We should therefore 
scarcely expect that in the Lamellibranchiata this same chamber would communicate 
with the vascular system. 
Moreover it is not easy to understand how water absorbed in this way could escape 
being contaminated with the deleterious urinary or other excretory matters it would 
assuredly meet with in the organ of Bojanus. And there is also, apparently, a mechanical 
difficulty in the way of the pericardial sinus acting as a suction-apparatus. The heart 
in a living state so completely fills the pericardial sinus that there is no space for any 
considerable influx of fluid. In Azodon the ventricle rolls against the wall of the cavity 
with such force that, on laying the latter open during life, the former bulges out through 
the incision. Indeed the pericardial wall follows the action of the heart during its 
diastole and systole, as it does likewise in the Nudibranchs. The pericardium is conse- | 
quently always in a state of repletion; and the only effect the opening of the shell-valves 
could have would be to permit of a more unrestrained action of the heart. Nothing 
approaching to a vacuum can be formed by the action of an apparatus such as this. 
. In saying this I do not mean to deny the possibility of water entering directly into the 
blood-vascular system in the Lamellibranchs ; only it seems unlikely that it should be 
received in the manner described in the memoir referred to. M. Lacaze-Duthiers described 
some time ago the existence, in Dentalium and Pleurobranchus, of pores by which the 
vascular system communicates directly with the exterior+. In Plewrobranchus this 
opening is situated at the root of the branchial vein, and leads almost directly into it, 8 
can be proved by any one who will take the trouble to examine for himself. This is a 
very startling fact, and it is interesting to know that it coexists with a renal Sy stem 
* Phil. Trans. vol. chi. p. 29. : 
T Ann. des Sci. Nat. 4° série, Zool. t. vii. p. 5, and t. xi. p. 200. . 
