356 



COMPARATIVE AXATOMY 



Reptiles and Birds. — In the Sauropsida, as ia the Mammalia, 

 the mesonephros, so far as it is retaiHed in the adult, is entirely 

 separate from the functional excretory apparatus ; this consists 

 of a metanephros, entirely wanting in nephrostomes (compare 

 p. 346 and Fig. 273). 



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 

 region : it has the latter position, for instance, in most Reptiles 



Fig. 2S2.— IJxcretoby Apparatus of Monitor iwlkus. 



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

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

 urinary bladder is not represented. 



N, X, kidneys ; <9(?, collecting tubes which open into the ureter (Ur, Ur"^) ■ l'r\ 

 aperture of ureter into the cloaca. 



(Figs. 282, 294 and 295) and all Birds (Fig. 283). The posterior 

 end of the kidney, which is generally narrower than the rest, 

 may even extend under the root of the tail, as in Lacerta, in 

 which region there is a fusion of the organ of either side. 



Thus, according to the position of the kidneys, the ureters 

 (metanephric ducts) either do not extend freely alono- the. 

 body-cavity, or they may have a longer or shorter free course. 

 The latter is the case, for instance, in Crocodiles, and more 

 especially in Birds (Fig. 283) : in the latter the kidneys 

 are closely embedded within the pelvis, and their ventral 

 flattened surface, which is usually divided into three lobes, is 



