56 BIOLOGY 



waste product, which must be eliminated. The Amoeba is, 

 therefore, obliged to absorb oxygen gas from some source and 

 to eliminate carbon dioxid gas. This process of absorbing and 

 eliminating gases is known as respiration. In the Amoeba there 

 appear to be no special respiratory organs, although possibly 

 the contractile vacuole performs this function. But the body 

 of the animal is sp small that special respiratory organs are 

 unnecessary, since gas is readily absorbed directly through the 

 surface of the body from the water in which the animal lives, 

 and carbon dioxid is as readily eliminated into the water. A 

 respiratory function is thus developed, but no distinct respira- 

 tory organs. The elimination of carbon dioxid gas, since it is 

 the getting rid of a waste product of metabolism, is not only 

 part of the function of respiration, but belongs also to the func- 

 tion of excretion. 



Excretion. As the result of this disintegration there arise 

 in the Amoeba disintegration products which are waste materials 

 and must be eliminated from the body. These products are 

 primarily three : carbon dioxid gas, water, and a product contain- 

 ing nitrogen, and related to urea which is excreted by the kid- 

 neys of higher animals. The function of getting rid of these 

 waste products is called excretion. In Amoeba the gas and 

 the water are excreted directly into the surrounding water, 

 either through the general surface of the body or by the contrac- 

 tile vacuole. The urea is probably eliminated by the contractile 

 vacuole. 



It should be clearly recognized that -the elimination of the un- 

 digested portions of the food, mentioned on page 55, is not 

 excretion. These undigested parts of the food, though sometimes 

 called "excreta," have never become part of the Amoeba's body 

 and are simply foreign bodies that have been rejected as useless. 

 True excretion, on the other hand, always refers to the elimina- 

 tion of the products of dissimilation. 



Relation to Water. Protoplasm requires water for its activi- 

 ties. Ordinary active living matter contains 60% to 80% of 



