THE PROTOZOA 



cretion in Amoeba may be compared, again, either with that of a component cell 

 of a higher animal, or with that of the higher animal as an organism. For in a 

 vertebrate excretion occurs initially at the cellular level, waste products being 

 released by cells into the lymph, eventually to find their way to the specialized 

 organs of excretion through which they will ultimately be eliminated from the 

 body. At the amoeba's unicellular level of organization, the initial passage of 

 wastes from the cell into the surrounding water completes the process of excretion. 



The contractile vacuole of Protozoa has long been considered as primarily an 

 organelle (the intracellular counterpart of a multicellular organ) specialized 

 for the elimination of excreta. Nitrogenous substances have actually been de- 

 tected in samples of fluid ingeniously removed from contractile vacuoles. How- 

 ever, the chief function of these structures probably involves the removal of 

 excess water from the cell. As a result of the osmotic gradient between the 

 external medium and the cell contents, water tends constantly to enter the cell, 

 and provision must be made for its elimination. The contractile vacuole may be 

 thought of as a pump, operating continuously to maintain the proper environ- 

 mental conditions within the cell. The water removed by this action contains a 

 certain amount of excretory material in solution, and to this extent the con- 

 tractile vacuole may be regarded as an excretory organelle. Many protozoans 

 (for example, most marine forms) lack contractile vacuoles and must depend en- 

 tirely on difTusion through the general cell surface for the elimination of wastes. 

 Undoubtedly most excreta are removed from Amoeba in this manner. 



From these considerations it is apparent that the metabolic processes of this 

 very simple, unicellular animal and of a complex vertebrate are essentially 

 analogous. The fundamental requirements of both protozoan and metazoan 

 cells are everywhere comparable: food, providing energy sources and essential 

 basic substances; oxygen, for the combustion of nutrients in an ordered manner, 

 releasing energy for synthetic activities; and a provision for the maintenance of 

 favorable internal conditions by the elimination of metabolic wastes. The chief 

 differences displayed concern the fact that in each species the cell synthesizes the 

 particular specific compounds characteristic of its kind. 



Life Cycle and Reproduction. The life cycle, or life history, of a many- 

 celled animal is the series of changes from egg to adult that occurs in each 

 generation. Many protozoans, including some members of the Sarcodina, also 

 exhibit serial changes of form which constitute their life cycles. In the com- 

 mon amoebas, however, the life history seems to involve nothing but an endless 



Fig. 8.5. Amoeba: successive stages in division. (Adapted from K. von Frisch, 1952, Bwlogie, 

 vol. 1, printed by permission of Bayerischer Schulbuch-V'erlag.) 



235 



