242 



THE KIDNEYS chap. 



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 pericardium. Each 

 kidne}^ opens on the one hand into the mantle cavity, close to 

 the anus (see Fig. 64, p. 157), and on the other, into the peri- 

 cardium. 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 auricles, and four kidneys. In other 

 Gasteropoda 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 Mollusca, stand in very close relation to the geni- 

 tal 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 kidne3^s (see Fig. 61 C, D, p. 154). In the Dioto- 

 cardia it is the right kidney alone which serves, besides its 

 excretory functions, as a duct for the emission of the generative 

 products, the left kidney being at tlie same time greatly reduced 

 in size. Thus in Patella the left nephridium is small, the right 

 being muph 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. Yoldia^ Avicida, Modiola, 

 Pecten^ Spondylus) the genital glands communicate directly, 



1 vc^/L y, kidney. 



