50 AN INTRODUCTION TO ZOOLOGY 



katabolism. The products of this slow combustion are the energy 

 of movement, heat, and residual matter. Ordinarily this con- 

 sists of solids and fluids, mainly water, some mineral substances, 

 urea and CO 2 . Secretions, excretions, and the products of respi- 

 ration are included in this list. 



Secretion. We have already noted that an acid is poured 

 into the gastric vacuole by the surrounding protoplasm. Such a 

 product of dissimilation, which is of use in the economy of the 

 animal, is known as a secretion. 



Excretion. Materials representing the final reduction of 

 substances in the process of katabolism are called excretions. 

 These are deposited either within or outside of the body. A large 

 part of the excretory matter, including urea and CO 2 , passes 

 through the general surface of the body. The fluid content of 

 the contractile vacuole is known to contain urea, therefore this 

 organ is excretory in function. It is also respiratory, since CO 2 

 probably makes its way to the exterior by way of this organ. 

 Oxygen dissolved in water is taken in through the surface of the 

 body. This gas is necessary for the life of the animal; if replaced 

 by hydrogen, movements cease after twenty-four hours; if air is 

 then introduced, movements begin again; if not, death ensues (42). 



Growth. If food is plentiful, more substance is added to the 

 living protoplasm of the Ameba than is used up in its various 

 physical activities. The result is an increase in the volume of 

 the animal. This is growth, and, as in all other living organisms, 

 growth by the addition of new particles among the preexisting 

 particles, i.e. growth by intussusception. 



Reproduction. There is, however, a limit with regard to the 

 size that may be attained by A meba proteus, as it rarely exceeds 

 .25 mm. ( T -^g- inch) in diameter. When this limit is reached, the 

 animal divides into two parts. Why should there be such a 

 limit ? The following explanation is given by Herbert Spencer 

 and others. The volume of an organism varies as the cube of 

 its diameter, the surface as the square. Thus, as an animal 

 grows, the ratio between surface and volume decreases; and, since 



