I EXCRETION AND RESPIRATION 235 



protoplasm combine with oxygen, 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 activity 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 sup- 

 ply 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 Amoeba while 

 carbon dioxide passes out into the water. This is the process 

 of breathing or respiration (p. 144), and it occurs in Amoeba 

 without the agency of lungs or other 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 Amoeba is doubtless 

 accompanied by an evolution of heat, as in higher animals 

 (p. 151), although this has never been proved. 



■\Ve thus see that a very elaborate series of chemical pro- 

 cesses is constantly going on in the' interior of Amceba, as in 

 the frog, the whole series of which is spoken of collectively 



