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 Trochus, Solenomya, etc. (Fig. 5 bis , g, i) 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 Lk "lelli- 

 branchs and in the Scaphopods that the two distinct branches of 

 this duct become longer (Fig. 5 u \ j) ; then the common duct dis- 

 appears, and the gonad opens directly into the renal sac (Fig. 

 5** h, k). 



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 ofily 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 Dentalium. 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). 



