CHAPTER II 

 PHYLUM PROTOZOA • 



The Cell as an Individual. — The Protozoa represent the 

 simplest organization of animal protoplasm to form independent 

 units or individuals. Though in the typical examples the indi- 

 viduals comprise single cells (Figs. 15, 19, 20), the cytoplasm 

 of these cells has undergone various lines of differentiation and 

 specialization. Thus these cells are far from simple or "undif- 

 ferentiated" when considered from the point of view of adapta- 

 tion for numerous different functions (see Frontispiece). The 

 chief point of distinction between the cell of a protozoan and one 

 of a many-celled animal (metazoan) lies in the fact that in the 

 former all of the functions of a living animal are executed by a 

 single mass of protoplasm. Any specialization for different func- 

 tions in the single cell must be restricted to specialization of a 

 part within that cell. On the other hand, the Metazoa are com- 

 posed of numerous cells, groups of which have become specialized 

 for limited functions. Consequently, for the rest of the func- 

 tions characteristic of living matter these cells of limited function 

 in the Metazoa become more or less dependent upon the other 

 cells united with them to form an aggregate termed an individual. 

 It thus becomes obvious that differentiation in Protozoa usually 

 involves specialization of the parts within a single cell, while in 

 the Metazoa entire cells become specialized for limited functions. 

 In this light it is readily understood that "undifferentiated" as 

 applied to a protozoan cell does not imply "unorganized" for 

 the organization found there is frequently much higher (see 

 Frontispiece) than in many kinds of metazoan cells which have 

 undergone histological differentiation (Figs. 37-46). 



Protozoa as Non-cellular Organisms. — Since the concept of the 

 Protozoa as simple organisms has led to such widespread mis- 

 understanding of the true nature of these complicated, though 

 minute, animals, some protozoologists have chosen to consider 

 Protozoa as non-cellular. The advocates of this position hold 

 that the body of a protozoan with its specialized regions repre- 



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