66 COMPARATIVE ANATOMY. 



highly developed animals there are also present various or- 

 ganic bodies, such as urea, creatin, and creatinin, as well as 

 inorganic compounds, as phosphates of ammonia and mag- 

 nesia. Secretion commonly elaborated from venous blood, 

 but in Mammalia partly from arterial also. 



No true kidney below Mollusca. 



MOLLUSCA. Organ absent in Polyzoa, Brachiopoda,* and 

 Tunicata. 



Lamellibranchiata. The kidneys (organ of Bojanus) al- 

 ways double. Each is an oblong gland of a brown or brown- 

 ish-yellow color, placed upon dorsal region of body, near the 

 hinge line, beneath pericardium and inferior adductor muscle. 

 The usually very thin walls of these two sacs have numerous 

 folds or plicae, which form compartments or areolse, all of 

 which are covered with ciliated epithelium. (Siebold.) The 

 short excretory duct opens into mantle upon side of base of 

 foot, and is generally isolated from genital pore, or is in com- 

 mon with it. Secretion solid. 



Gasteropoda. Organ lamellose without cilia, and placed 

 near branchial vein; in Doris at region of membrane about 

 heart ; in pulmonic types at posterior and upper part of res- 

 piratory chamber between the heart and rectum. It is pro- 

 vided with excretory canal opening by the side of the pneu- 

 matic orifice, as in Limax, or within the respiratory chamber, 

 <as in Helix. Secretion solid. 



Cephalopoda. The kidneys (spongy bodies) are append- 

 ages to main venous canal. The renal cells are placed upon 

 the sides of everted follicles, the bases of which are formed 

 by the walls of the vein itself. Each cell is granulated, and 

 possesses well-defined nucleolar elements and commonly a 

 crystalline product. The urine passes along an imperfectly 

 defined duct into the cavity of the mantle, thence to escape 

 from the body through the syphon. 



ARTICULATA. Kidneys absent below Myriapoda; func- 

 tion with some supplied through water-vascular system. 



* Hancock looks upon the 'great perivisceral chamber with its extensive 

 ramifications' of Brachiopoda as having a renal significance. 



