URINARY ORGANS 459 



Slight indications of a segmental arrangement are found only 

 in the anterior sexual portion of the kidney of Urodeles ; in the 

 posterior part, and in the entire kidney of Anura, all traces of 

 segmentation have disappeared. In both cases, however, the 

 nephrostomes remain throughout life in great numbers on the 

 ventral surface of the kidney, which is covered over by the 

 peritoneum (Fig. 344). The nephrostomes are connected with the 

 urinary tubules in larval Anura, but later on they become 

 separated from them, and open into the efferent renal veins. In 

 consequence of this, the body-cavity of adult Anura is a closed 

 lymph-sinus, as in the Amniota ; the peritoneal fluid, which in the 

 larva was carried to the exterior and lost, is in the adult poured 

 into the general circulation, like the rest of the lymph. 



Reptiles and Birds. In the Sauropsida, as in the Mammalia, 

 the mesonephros, so far as it is retained in the adult, is usually 

 entirely distinct from the functional excretory apparatus ; this 





FIG. 345. EXCRKTORY APPARATUS OK Monitor iii'tini*. 



The right kidney is shown in its natural position, while the left is turned on its 

 longitudinal axis, so that the ureter and collecting tubes are visible. 

 The urinary bladder is not represented. 



jV, N, kidneys ; SG, collecting tubes which open into the ureter ( Ur, Ur' 2 ) ; Ur 1 , 



aperture of ureter into the cloaca. 



consists of a metanephros, entirely wanting in nephrostomes 

 (cf. p. 446). 



The metanephros never extends so far along the body-cavity 

 as does the mesonephros ; as a rule it has the form of a small, 

 compact or lobulated organ, usually situated in the posterior 

 half of the body-cavity, or even entirely confined to the pelvic 



