100 



portant difference is the inability of animals to resynthesize the or- 

 ganic waste products of respiration into substances that may be 



again utilized as nutrients This is especially the case with the 



soluble products arising from protein metabolism. With most animals 

 these nitrogenous products are excreted in solution through the kid- 

 neys, chiefly as urea, but birds, reptiles, and all insects excrete most 

 of the nitrogenous waste matter as uric acid, or its ammonia salt, which 

 being practically insoluble in the body fluids, is voided in a solid con- 

 dition." (Page 6i) : "The need for water is much less for ani- 

 mals that excrete uric acid than for those that excrete urea, since 

 uric acid, being practically insoluble in the the body fluids, is not so 

 poisonous as urea and is voided solid with a minmum loss of water. 

 Many animals that excrete uric acid instead of urea never have access 

 to water and subsist in every stage of their development upon air dried 

 food which usually contains less than lo per cent water. The most 

 striking illustrations of this kind are found among insects such- as the 

 clothes moths, the grain weevils, the dry wood borers, the bee moths, 

 etc. The larvae of these insects contain a high per cent of water, and 

 the mature forms, in spite of the development of wings which are rela- 

 tively dry, rarely contain less than 50 per cent of water." (Pp. 171- 

 172) : "Serpents and other reptiles that live in arid regions and rarely 

 if ever have access to water, except that contained in their food, are 

 said by Vauquelin to excrete all of the waste nitrogen as salts of uric 

 acid. The same is true of birds that live on desert islands where only 

 salt water is available. It is essential that animals of these types should 

 produce as much metabolic water as possible from the assimilated food, 

 and the waste of water through the excretions should be reduced to a 

 minimum. Since the food is largely protein both of these ends are at- 

 tained by the excretion of uric acid which, as already stated, contains 

 the least hydrogen of any nitrogenous substance excreted by animals so 

 that the maximum amount of metabolic water has been derived from 

 the food consumed." (Pp. 174-175) : "There are many animals that 

 are able to go long periods without having access to water except that 

 contained in their food, in which water usually amounts to less than 

 20 per cent of total weight, and the metabolic water derived from oxi- 

 dation of organic nutrients. A notable example of this is the prairie 

 dog which thrives in semi-arid regions. These small animals feed 

 upon the native herbage which for months at a time is as dry as hay. 

 It has been surmised that the burrows in which they live extend to 

 underground water courses, but this does not seem likely since in many 

 of these regions wells must be sunk hundreds of feet before water is 

 reached. It is more probable that they depend chiefly upon metabolic 

 water. They feed mostly at night when the temperature is low and 



