PARTS OF ANIMALS, III. vn.-viii. 



the diaphragm, though this is one of the parts that 

 are near the viscera. 



VIII. The bladder is not present in all animals : Bladder. 

 Nature seems to have intended only those animals 

 which have blood in their lung to have a bladder. 

 And this is quite reasonable, when we remember that 

 such animals have an excess of the natural substance 

 which constitutes the lung, and are therefore more 

 subject to thirst than any others ; i.e. they need a 

 larger amount of fluid food as well as of the ordinary 

 solid food, and the necessary result of this is that a 

 larger amount of residue also is produced, too large 

 in fact for all of it to be concocted by the stomach 

 and excreted with its own proper residue ; hence it 

 is necessary to have some part that will receive this 

 additional residue. This shows us why all animals 

 which have blood in their lung possess a bladder 

 too. As for those whose lung is spongy and which 

 therefore drink Httle, or which take fluids not as 

 something to drink but as food (e.g. insects and 

 fishes), or which are covered with feathers or scales 

 or scaly plates, not one of these has a bladder, owing 

 to the small amount of fluid which they take and 

 owing to the fact that the surplus residue goes 

 to form feathers or scales or scaly plates, as the 

 particular case may be. Exceptions to this are the 

 Tortoises : though scaly-plated they have a bladder. 

 In them the natural formation has simply been 

 stunted. The cause of this is that in the sea-varieties 

 the lung is fleshy and contains blood, and is similar to 

 the lung of the ox ; while in the land-varieties it is 

 disproportionately large. And whereas in birds and 

 snakes and the other scaly-plated creatures the 

 moisture exhales through the porous flesh, in these it 



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