THE PROTOZOA 



The Protozoan Cell 



Returning to the Protozoa, we may conclude that the members of this 

 phylum are single cells which exhibit the fundamental capacities of metab- 

 olism, responsiveness, and reproduction; they are, therefore, capable of going 

 about the business of living as individuals which are single cells. Structurally 

 speaking, an amoeba is an animal reduced to essentially the simplest terms. 



It should be recalled that, alternatively, the Protozoa may be regarded as 

 non-cellular animals, as animals whose bodies are not subdivided into cells. 

 However, the bodies of protozoans exhibit fundamental similarities to the cells 

 of other organisms, both in their basic structure and in their physiological 

 activities. The question of the nature of protozoans is largely a philosophical 

 one, but it seems reasonable to believe that both Protozoa and Metazoa arose 

 by evolution from a common ancestry of single-celled forms. In their descent 

 from such ancestry, the Protozoa have undergone specialization within the 

 limits of a single cell, except insofar as species have arisen which consist of 

 colonial aggregations of cells or individuals. These colonial forms will be 

 discussed in the next chapter. The Metazoa, on the other hand, have special- 

 ized as many-celled individuals in which there is a division of labor between 

 cells, and hence an unbalanced physiological state for the individual cell. 

 There are species among the Protozoa whose cellular organization is far more 

 complex than that of any metazoan cell, because specialization within the 

 limits of the unicellular state is the unique direction in which the Protozoa 

 have evolved. Yet the most complex of these protozoan cells can be regarded 

 as single cells thus specialized and need not be considered as organisms 

 having nothing in common with the cells of metazoans. 



Unfortunately, there is no fossil record which shows, like the record of 

 vertebrate evolution, how unicellular and multicellular animals arose, or from 

 what ancestry they were derived. The record does show that protozoans such 

 as the Radiolaria were in existence at the period represented by the oldest 

 known rocks containing animal fossils, and that the Foraminifera are only a 

 little younger as a group. From this fact it may be presumed that there were 

 simpler protozoans aeons before that early period, because morphological 

 simplicity must logically antedate such complexity as that of the shelled 

 amoeboid forms. A one-celled animal, no matter how specialized it may be- 

 come as a cell, is obviously simpler as an animal than a many-celled one. 

 From this standpoint the Protozoa are the simplest of animals, and they seem 

 to be more like the ancestors of all animal life than any other animals now 

 in existence. 



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