THE MOLLUSCA ~ 13 
open, not into the pericardium, but in the reno-pericardial duct, 
uniting the pericardium to the kidney. Such an arrangement is 
found in T’rochus, Solenomya, ete. (Fig. 5°"’, g, 7); and here we find 
that the genital and pericardial cavities are united to the kidney 
by a common duct of double origin, genital and pericardial. As a 
further differentiation, we find in a fairly large number of Lamelli- 
branchs and in the Scaphopods that the two distinct branches of 
this duct become longer (Fig. 5°", 7); then the common duct dis- 
appears, and the gonad opens directly into the renal sac (Fig. 
Be iat): 
The pericardial coelom always surrounds the heart except in the 
Octopoda and the Anomiidae, or is much reduced or absent. 
Sometimes prolongations, ramifications, or parts of this pericardial 
cavity have their walls much specialised to form an excretory 
apparatus, known as the pericardial glands. The pericardial 
coelom always communicates with the renal sacs or renal portion 
of the original coelom: in Nautilus alone the kidneys are no 
longer continuous with the pericardial cavity, and this latter opens 
directly to the exterior by ‘“coelomopores,” orifices peculiar to 
itself. 
The Cephalopods have a pair of coelomoducts leading directly 
from the genital cavity to the exterior. In the Aplacophora this 
genital space only communicates with the exterior through the 
intermedium of the pericardium and renal sacs. The polyplaco- 
phorous Amphineura have acquired two special genital canals, 
through which the sexual products are discharged, but they do not 
appear to be true primitive coelomoducts (Fig. 5°, e; Fig. 30, D). 
Finally, in the Lamellibranchs in general and in the Gastropods 
the genital ducts are formed at the expense of a portion of the 
renal sacs and ducts (on one side only in Gastropods); but the 
male ducts of the hermaphrodite Lamellibranchs, the Anatinacea, 
are neomorphs and an exception to the general rule. 
5. Excretory Organs.—The essential organs of excretion are the 
renal sacs or urocoeles, whose morphological nature requires 
further elucidation. They consist of paired canals, more or less 
modified, which open to the exterior on the surface of the body 
and internally into the pericardium, except in the case of Nautilus, 
in which, as described above, the pericardial coelom has its own 
separate orifices, and in Dentaliwm. The reno-pericardial apertures 
are more or less elongate ciliated funnels whose cilia create a 
current in the direction of the kidney. In Elysia alone does the 
kidney possess multiple reno-pericardial apertures, to the number 
of about ten (Fig. 92). True “nephridia” (Lankester) only occur 
in the young stages of certain Gastropods (Pulmonates (Fig. 118), 
Paludina, etc.) and in Lamellibranchs; they are described below 
under the head of Embryology (p. 136). 
