CIRCULATION, RESPIRATION AND EXCRETION 143 



quantity of nitrogenous material which must be dissolved and 

 in part upon the amount eliminated through the lungs and 

 skin. 



Most of the water excreted by animals is, of course, con- 

 sumed as such, but it includes also that formed by the oxida- 

 tion of organic hydrogen the so-called metabolic water. 

 Babcock has shown that in some classes of animals, notably 

 insects, this metabolic water suffices for all the needs of the 

 organism, so that they are not dependent upon a supply of 

 drinking water. 



