246 COMPARATIVE ZOOLOGY 



7. The waste products of this oxidation taken up by 

 the blood must be got rid of ; some from the lungs (car- 

 bon dioxide, water), some from the kidneys (water, urea, 

 mainly), some from the skin (water, salines). (Respira- 

 tion in part, excretion.) 



The mechanism to accomplish all this in the lowest 

 forms of life is exceedingly simple, a single cavity and 

 surface performing all the functions. But in the major- 

 ity of animals the apparatus is very complicated : there 

 is a set of organs for the prehension of food ; another 

 for digestion ; a third, for absorption ; a fourth, for dis- 

 tribution ; and a fifth, for purification. 



