The kidneys, nephridia,[331] 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 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 Amphineura, 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 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 functions, 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._ _Yoldia_, _Avicula_, _Modiola_, _Pecten_, _Spondylus_) the genital glands communicate directly, and with a similar object, with the renal pouch on the same side of the body, but in the majority of cases the orifices are distinct. * * * * * The following memoirs will be found useful for further study of this portion of the subject:-- =D. Barfurth=, Ueber den Bau und die Thätigkeit der Gasteropodenleber: Arch. Mikr. Anat. xxii. (1883), pp. 473–524. =Th. Behme=, Beiträge zur Anatomie und Entwickelungsgeschichte des Harnapparates der Lungenschnecken: Arch. Naturges. iv. (1889), pp. 1–28. =R. Bergh=, Semper’s Reisen im Archipelago der Philippinen; Nudibranchiata: Theil ii. Band ii. (1870–78), Band iii. (1880–1892). =W. G. Binney=, Terrestrial Air-breathing Mollusks of the United States: Bull. Mus. C. Z. Harv. iv. (1878), 450 pp. „ On the Jaw and Lingual Membrane of North American Terrestrial Pulmonata: Proc. Ac. Nat. Sc. Philad. (1875), pp. 140–243. =J. T. Cunningham=, The renal organs (Nephridia) of Patella: Quart. Journ. Micr. Sc. xxiii. (1883), pp. 369–375.