I EXCRETION AND RESPIRATION 235 



forming waste or excretory matters carbon dioxide, 

 water, and certain nitrogenous substances of simpler 

 constitution than proteids, such as urea. These products 

 of excretion, formed in the case of Amoeba without the 

 agency of any special excretory organs (e.g., kidneys), 

 are given off partly from its general surface, but partly, 

 it would seem, by the agency of the contractile vacuole, 

 by means of which the water taken in with the food is 

 also got rid of. 



With this breaking down of proteids the vital 

 activities of all organisms are invariably connected. 

 Just as useful mechanical work may be done by the fall 

 of a weight from a given height to the level of the 

 ground, so the work done by the organism is a result of 

 its complex proteids falling, so to speak, to the level of 

 simpler substances. In both instances potential energy 

 or energy of position is converted into kinetic or actual 

 energy. 



The statement just made that the protoplasm of 

 Amceba constantly undergoes oxidation presupposes a 

 constant supply of oxygen. The water in which the 

 animalcule lives invariably contains that gas in solution, 

 and diffusion takes place, oxygen passing into the 

 interior of the Amceba while carbon dioxide passes out 

 into the water. This is the process of breathing or 

 respiration (p. 1-43), and it occurs in Amceba without 

 the agency of special respiratory organs. Thus the 

 carbon dioxide is got rid of, and at the same time a 

 supply of oxygen is obtained for further combustion. 

 The oxidation of the protoplasm of the Amceba is 

 doubtless accompanied by an evolution of heat, as 

 in higher animals (p. 152), although this has never been 

 proved. 



We thus see that a very elaborate series of chemical 

 processes is constantly going on in the interior of 

 Amceba, as in the frog ; the whole series of which is 



