308 



COMPARATIVE ANATOMY. 



within the pelvis, and their ventral flattened surface is usually 

 lobulated, and often penetrated by deep furrows and clefts in which 

 the veins lie embedded (Fig. 242, V, V} ; their posterior ends may 

 fuse together in the middle line, as in Lizards. 



There is not always a perfect symmetry between the organ of 

 either side, and this is most marked in Snakes, in which the 



FIG. 241. EXCRETORY APPARATUS OF Monitor indicus. 



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

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

 urinary bladder is not represented. 



N, A", kidneys ; SG, collecting tubes which open into the ureter (Ur t Ur 1 } ; Ur l 

 aperture of ureter into the cloaca. 



greatly lobulated kidneys, like those of limbless Lizards, are 

 elongated, narrow, and band-like, in correspondence with the form 

 of the body. 



A urinary bladder, arising from the ventral wall of the 

 cloaca, is present in Lizards and Chelonians ; it is usually bilobed, 

 as in Amphibia, and so points to a primitively paired condition. 

 A bfadder is wanting in Snakes, Crocodiles, and Birds. 



Mammals. The kidneys of Mammals are proportionately 

 small, and lie on the quadratus lumboruin muscle and ribs. They 

 usually possess a convex outer, and a concave inner border ; the 

 latter is called the hilum, and at this point the ureters arise and 

 the blood-vessels enter. The expanded proximal portion of the 

 ureter is divided up to form one or more calyces (Fig. 243, Co), 

 into which small papilliform processes of the pyramids (see 



