88 TEXTBOOK OF ZOOLOGY 



vacuoles with the food in the process of digestion circulate so 

 widely in the endoplasm that all parts of the cell may receive 

 nourishment by direct absorption. 



Egestion. — Indigestible material or debris that has been ingested 

 with the food is carried to the surface of the cell and cast out or 

 egested by simply being left behind as the animal moves away. 



Assimilation. — This is the process of transforming the digested 

 food material into protoplasm. In Amoeba the digested food mate- 

 rial is absorbed directly from the food vacuoles by the surrounding 

 cytoplasm. Since the vacuoles move rather generally through the 

 endosarc, most of the protoplasm of the cell is in rather close con- 

 tact with the dissolved food. 



Respiration. — This is a process whereby the gas, carbon dioxide 

 (CO2), leaving the protoplasm, is exchanged for oxygen (O2) en- 

 tering it. Such a process is essential to all living protoplasm. In 

 Amoeba this exchange is carried on primarily through the general 

 body surface. The water in which the animal lives must contain 

 dissolved oxygen in order that this diffusion may go on. Amoebae, 

 however, are able to and do live in rather foul water where the 

 oxygen content is rather low and the carbon dioxide high because 

 of the decaying vegetation present. Amoebae may live several 

 hours in water from which the oxygen is removed before asphyxia- 

 tion occurs. The contractile vacuole likely assists in discharging CO2. 



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- 



