THE GASTROPODA 



109 



3. Excretory Organs. In the Gastropoda the kidneys are the 

 essential organs of excretion, but the pericardial glands serve as 

 accessory excretory organs, as also certain parts of the body in 

 which the products of excretion are collected, forming veritable 

 accumulative kidneys. 



(1) The kidneys are originally paired, as in all other Mollusca, 

 and a single pair is found (Figs. 55, III, XIII; 81, /, h; 91, 

 127) in all the Aspidobranchia, except the Neritacea, including 

 the Neritidae and allied families. These two kidneys open one 

 on each side of the anus, but they do not retain their primitive 

 symmetry in any Gastropod, and although they are independent of 

 one another, the topographically left kidney is rudimentary, and 

 that of the right side alone is functional in almost every case. 



Fio. 90. 



Transverse section of the lung of Janetta, k, ureter ; pa.c, pallial or pulmonary cavity ; po, 

 pneumostome ; si, blood sinus ; tr, " tracheae " or diverticula of the pulmonary cavity. (After 

 Plate.) 



In the Neritacea (Neritidae, Titiscaniidae, Helicinidae, Hydro- 

 cenidae, and Proserpinidae) and in all the Pectinibranchia and 

 Euthyneura the topographically right kidney no longer exists. 

 In Paludina the two kidneys coexist during development, but in 

 the adult that of the topographical right side has disappeared. As 

 regards the position of these organs, their primitive situation is 

 wholly within the visceral mass (Docoglossa, Fig. 88, k), and their 

 migration outside the visceral mass is a specialisation which begins 

 to show itself in the Rhipidoglossa at any rate, in the case of the 

 left kidney (Fig. 127) and is completely realised in the case of the 

 single kidney in other Streptoneura and Tectibranchia, in which 

 the excretory organ is more and more localised in the mantle (Figs. 

 75, k ; 63, r). The kidney is always a dorsal organ, situated in the 

 neighbourhood of the pericardium, with which it communicates by 

 a ciliated aperture. In the detorted Aspidobranchs (Fissurellidae), 

 however, the very rudimentary left kidney has lost this pericardial 



