THE PROTOZOA 



125 



that of assimilation, which means that now that new food matter has 

 been digested and is within the body, there must be a rearrangement of 

 some kind to form new particles and to add them to those already exist- 

 ing. It is this ability to manufacture protoplasm from unorganized mat- 

 ter that is one of the very fundamental properties of living matter. 



Any movement or energy expended by an animal is due to the 

 breaking down of complex molecules by what is known as oxidation.* 

 The process of tearing down is called katabolism or dissimilation. This 

 is a slow combustion process giving out heat and producing energy by 

 which the animal can perform its various functions of life. The sub- 

 stance thus broken down and "used up" must also be accounted for in 

 any scientific study. This residual matter usually consists of solids and 

 fluids ; namely, water and some mineral substance, urea and carbon 

 dioxide (CO,). Under this heading we include all secretions, excre- 

 tions, and products of respiration. 



Whenever glands produce a substance which is to be used again by 

 the animal, such product is called a secretion, while substances which 

 are thrown out of the body entirely are called excretions. 



The contractile vacuole, by virtue of the fact of its containing urea, 

 is considered an excretory organ, and because CO 2 also makes its way 

 to the exterior of the organ, it is supposed to be respiratory likewise. 

 Amoeba, like any animal, grows more rapidly than otherwise if 



food is plentiful. The food being taken in- 

 ternally the growth therefore comes from 

 the center outward, in other words by intus- 

 susception. 



Whenever a cell reaches its full growth, 

 its outer shell, membrane, or whatever its 

 external covering may be called, not having 

 infinite possibilities in the way of extension 

 and stretching, usually breaks if more food 

 is taken. Cell division is the process by 

 which the plant or animal starts anew, thus 

 saving the parent cell from breaking. A 

 simple division into two (Fig. 47), is called 

 binary or amitotic division. It will be re- 

 membered that this mere splitting in two 

 parts is the shortest method by which cells 

 divide, but, which, as we have already said, 

 probably does not occur at all, and it is only 

 due to our observational methods not being 

 sufficiently delicate to note the exact processes, that we speak of ami- 

 totic division at all. In Amoeba proteus, however, which we are study- 



Fig. 47. 



So-called amitotic division of 

 Amoeba, showing the changes which 

 take place during division. The 

 dark body in each figure is the nu- 

 cleus ; the transparent circle, the 

 contractile vacuole ; the outer, clear 

 portion of the body, the ectoplasm ; 

 the granular portion, the endo- 

 plasm ; the granular masses, food 

 vacuoles. Much magnified. 



*Oxidation may be likened to a series of infinitesimal explosions, which could be detected if 

 we had instruments delicate enough for such a purpose. 



