EUGLENA, AMOEBA, AND PARAMECIUM ' 289 



of the decaying vegetation present. Amoebae may live several hours 

 in water from which the oxygen is removed before asphyxiation 

 occurs. The contractile vacuole likely assists in discharging COg. 



Catabolism or Dissimilation. — The chemical union of the oxygen 

 with the organic substance of the protoplasm liberates kinetic energy 

 and heat. This is known as oxidation and is a burning process which 

 goes on within the protoplasm. Water, some mineral matter, urea, 

 and carbon dioxide are residual products of this process. 



Excretion. — These by-products of metabolism in the form of waste 

 liquids must be disposed of. They cannot be allowed to accumulate 

 beyond certain limits in the living organism if life is to continue. 

 Urea and uric acid, which are protein by-products, excess water, and 

 salts, are discharged from the body of Amoeba by way of the con- 

 tractile vacuole along with some carbon dioxide. The contractile 

 vacuole is formed by the union of small droplets of liquid under 

 the plasma membrane. It fills out with liquid which is forced out 

 through the membrane as the vacuole disappears. Its location ap- 

 parently is not fixed in the cell but is often near the nucleus. The 

 contractile vacuole is absent in some forms, and in such cases, ex- 

 cretion occurs only by diffusion through the cell surface. There is 

 likely some excretion by this means in all Amoebae. 



Growth. — If there is increase in the volume of a body, this is 

 spoken of as growth. In all living organisms. growth is accomplished 

 by addition to the protoplasm. If food is plentiful, more material is 

 added to the protoplasm than is used up in the oxidation which pro- 

 duces active energy. In other words, growth occurs when the rate 

 of anabolism exceeds the rate of catabolism in the organism. 



Reproduction and Life Cycle 



The life history of the many-celled animals to be studied later 

 includes a series of changes from q^^, through embryo state, to 

 adult. In Amoeba the cycle is likely only partly known, because it 

 is difficult to maintain cultures in perfectly normal conditions for 

 sufficiently long periods to get this complete story. Ordinarily, the 

 animal grows when conditions are favorable until it attains a cer- 

 tain size; when this limit of size has been reached growth ceases. 

 Why does the cell cease to grow? Why should it not attain the 

 size of a man? Or why should a tree not continue to grow until it 

 reaches the sky, or a man take on the proportions of an elephant? 



