242 THE KIDNEYS 



it occurs in all known living genera, except Nautilus. The ink- 

 sac consists of a large bag generally divided into two portions, in 

 one of which the colouring matter is secreted, while the other 

 acts as a reservoir for its storage. A long tube connects the 

 bag with the end of the rectum, the mouth of the tube being 

 controlled, in Sepia, by a double set of sphincter muscles. 



The Kidneys 



The kidneys, nephridia,^ renal or excretory organs, consist 

 typically of two symmetrical glands, placed on the dorsal side of 

 the body in close connexion with the pericardimn. Each kidney 

 opens on the one hand into the mantle cavity, close to the anus 

 (see Fig. 64, p. 157), and on the other, into the pericardium. 

 The venous blood returning from the body passes through the 

 vascular walls of the kidneys, which are largely formed of cells 

 containing uric acid. The blood thus parts with its impurities 

 before it reaches the breathing organs. 



The kidneys are paired in all cases where the branchiae are 

 paired, and where the heart has two auricles, i.e. in the Amphi- 

 neura, the Diotocardia (with the exception of the Neritidae), the 

 Pelecypoda, and all Cephalopoda except Nautilus, which has 

 four branchiae, four amides, and four kidneys. In other Gastero- 

 poda only one kidney survives, corresponding to the left kidney 

 of Zygobranchiate Gasteropods. 



Besides their use as excretory organs the kidneys, in certain 

 groups of the MoUusca, stand in very close relation to the genital 

 glands. In some of the Amphineura the generative products, 

 instead of possessing a separate external orifice of their own, pass 

 from the genital gland into the pericardium and so out through 

 the kidneys (see Fig. 61 C, D, p. 154). In the Diotocardia it is 

 the right kidney alone which serves, besides its excretory func- 

 tions, as a duct for the emission of the generative products, the 

 left kidney being at the same time greatly reduced in size. 

 Thus in Patella the left nephridium is small, the right being 

 much larger ; both function as excretory organs, but the right 

 serves as a mode of conveyance for the seminal products as well. 

 In certain Pelecypoda (e.g. Yolclia, Avicula, Modiola, Pecten, 

 Spondylus) the genital glands communicate directly, and with a 



^ vecppjs, kidney. 



