482 ELEMENTARY ANATOMY. [LESS. 



end, as in Proteus ; or for their entire length, as in the 

 Spoonbill. 



Their surface may be marked by convolutions like that of 

 the human cerebrum, as in Sharks and Chelonians. 



The opening of the ureters may be placed behind the pos- 

 terior termination of the alimentary canal. This is the case 

 in osseous Fishes, and consequently a true bladder must be 

 wanting in them. Each ureter may, however, dilate into a 

 large bladder-like structure, as in Amia^ or there may be a 

 single, median vesicle. It is possible for several distinct 

 urinary ducts to open into this vesicle, as in the Stickleback. 

 Every sort of bladder-like structure may be absent, as in 

 Cobitis, and in all Serpents and Birds. A true bladder may, 

 however, be developed in Ichthyopsidans, as in the Batra- 

 chia, where it is placed in front of the termination of the 

 alimentary tube and of that common chamber (the cloaca) into 

 which that tube opens, as well as the bladder or rena! ducts. 



The right renal organ may be much longer and placed 

 more forward than the left, as in Serpents. 



The ureters may open, not into the bladder, but into the 

 cloaca, as is the case in the Monotremes, or Ornitkodclphia, 

 and in Reptiles. 



These ducts may terminate (in the cloaca) behind the 

 alimentary canal, or may come rather to the side, and in 

 the OrnitJiodelphia rather towards the front of the cloacal 

 chamber. 



These circumstances may serve to explain the difference 

 between the position of the alimentary and urinary outlets in 



man's class, and the situation 

 of those apertures in osseous 

 Fishes where the alimentary 

 outlet is anterior. The modi- 

 fication by which the urinary 

 outlet comes to be anterior does 

 not result from any change 

 in the position of the termi- 

 FIG. 407. KIDNEY OF THE SEAL nation of the alimentary tube, 

 condSon. g US lobulated butin the gradual production 

 forwards, on each side of it, ot 



the ureters, till they come in non-Ornithodelphous Mammals 

 to end (through the intervention of the bladder and urethra) 

 in a canal opening altogether in front of the rectal aperture. 



The kidneys may be more symmetrical in position than 

 in man. This is the case in most Birds, where they are 



