376 MOLLUSCA. 



pericardium, and in other Gastropods it has its own special duct, 

 which has been interpreted as, and probably is, a persistent portion 

 of the otherwise absent right (primitive left) nephridium. The 

 developmental evidence in favour of this view is not very strong, 

 but we may hold it at present as a provisional view. In no 

 Gastropod is the gonadial section of the coelom in direct connection 

 with the pericardial. 



Excretory organs. The kidneys, or organs of Bojanus, of the 

 Gastropoda are typically paired and symmetrical, somewhat dorsally 

 placed near the pericardium, and open externally on each side 

 into the mantle - cavity, generally not far from the anus, and 

 internally into the pericardium by the reno-pericardial openings. 

 This typical condition is found in the Chitonidae (Fig. 308) alone. 

 In all other Gastropods the kidneys are of unequal size, and in all 

 but the Aspidobranchiata there is only a single kidney, the right 

 one (primitive left) being absent (Fig. 297). 



In the Aspidobranchs there are two kidneys (except the Neritidae) ; 

 of these the right is always larger than the left, and in addition to 

 its proper function discharges that of generative duct, inasmuch as 

 the genital gland opens into it (in Haliotis by a large slit, and in 

 Fissurella on a papilla not far from the external opening, Fig. 297 

 5and c). They both open into the mantle-cavity, one on each side 

 of the anus; but with regard to their pericardial opening there is 

 some variation. In Trochus, Turbo, and Haliotis the small left sac 

 opens into the pericardium, while the right kidney is without the peri- 

 cardial opening. In Fissurella^ Emarginula, Patella, etc., it is said 

 by some authors that neither of the kidneys have the reno-pericardial 

 opening; whereas in Lottia, and according to other authors Patella 

 and Fissurella, the right kidney sac alone has a pericardial opening* 

 (Fig. 297 b). In other Gastropods there is only one kidney, the left 

 (primitive right) ; it opens externally near the anus, generally into 

 the mantle-cavity, but sometimes it opens with the anus, with or 

 without a long ureter, and there is a common cloaca (some Pulmo- 

 nates, Pteropoda Gymnosomata) ; internally it opens by a ciliated 

 orifice into the pericardium (Fig. 297 e). In Paludina there are 

 two kidneys in the embryo, but the one on the right side disappears 

 in the course of development. 



In the simplest cases the kidney is a sac with glandular walls, but 

 it may acquire a spongy texture, or it may consist of a non-glandular 



* R. v. Erlanger, "On the paired nephridia of Prosobranchs," Q.J.M.S. 33, 



p. 587. 



