16 AMCEBA LESS. 



These waste matters or excretory products are given off 

 partly from the general surface of the body, but partly, it 

 would seem, through the agency of the contractile vacuole. 

 It appears that the water taken in with the food, together in 

 all probability with some of that formed by oxidation of 

 the protoplasm, makes its way to the vacuole, and is ex- 

 pelled by its contraction. We have here another function, 

 performed by Amceba, that of excretion, or the getting rid 

 of waste matters. 



In this connection the reader must be warned against a 

 possible misunderstanding arising from the fact that the 

 word excretion is often used in two senses. We often hear, 

 for instance, of solid and liquid "excreta." In Amceba 

 the solid excreta, or more correctly faces, consist of such 

 things as the indigestible cell-walls, starch grains, &c., of the 

 organisms upon which it feeds ; but the rejection of these 

 is no more a process of excretion than the spitting out of 

 a cherry-stone, since they are simply parts of the food 

 which have never been assimilated never formed part and 

 parcel of the organism. True excreta, on the other hand, 

 are invariably products of the waste or decomposition of 

 protoplasm. 1 



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 : on the other hand, 

 as we have seen, the protoplasm is continually forming 

 carbon dioxide. Now when two gases are separated from 

 one another by a porous partition, an interchange takes place 

 between them, each diffusing into the space occupied by the 



1 In the higher animals the distinction between excreta and faeces is 

 complicated by the fact that the latter always contain true excretory 

 products derived from the epithelium of the intestine and its glands. 



