Metazoa Rana. 



295 



FIG. 157. KIDNEYS AND SPERMARIA OF 

 FROG. (Wiedersheim.) 



CrAo 



FK 



Renal system. It will now be necessary to glance at 

 the mechanism by which the water and soluble nitrogenous 

 waste are removed from the blood and got rid of. That duty 

 is performed by two organs, the kidneys, which have already 

 been mentioned in connection with the circulatory system 

 as lying dorsally one on either side of the dorsal aorta, 

 and receiving branches 

 from that vessel. The kid- 

 neys are closely attached 

 to the dorsal body-wall, 

 and are covered on the 

 ventral aspect by a layer of 

 peritoneum. Each kidney 

 is a flat elongated body of 

 a dull red colour. Enter- 

 ing the outer edge of each 

 is the renal-portal vein, 

 and leaving the same edge, 

 a little more than half-way 

 down, is a delicate tube 

 known as the ureter, by 

 which the renal excreta are 

 got rid of. The ureters open 

 into the common chamber, 

 or cloaca, behind the open- 

 ing of the rectum. 



In structure the frog's 

 kidney is essentially an im- 

 mense number of nephridia ^ fkidne3?; Ur> Ur , ur eters ; t, their 



Closely packed together P int , of ongin; S,S', their opening imo 



1 ' the cloaca, Ll\ HO, spermaria ; FA, fat 



Supported by Connective body; AO, aorta; Cv, vena cava ; Vr t 



. . . efferent veins. 



tissue and permeated by 



the capillary endings of the renal artery and renal and 

 renal-portal veins. Each nephridium consists essentially 

 of a terminal double-walled sac, containing a tuft of capil- 

 laries communicating on the one hand with a small artery 



