CHAPTER 2 



PROTOZOA AS CELLS 



With major emphasis being placed today upon "the cell" as a 

 structural and functional entity, it is wise to bear in mind Hyman's 

 (1940, 1959) insistent warning that a protozoon is better to be 

 compared with a whole metazoan organism than with any of its 

 component cells. As significant as this concept is, however, there 

 are contexts in which it becomes unmanageable, and the chief of 

 these is cytology (a point well made by Grimstone, 1961, in 

 concluding an admirable short review of protozoan ultrastructure). 

 The electron microscope has demonstrated that the fine structure 

 of protozoa is directly and inescapably comparable with that of 

 cells of multicellular organisms. Where the greater versatility 

 and self-sufficiency of protozoa is accompanied by a higher degree 

 of structural elaboration, this will become apparent — even 

 startlingly so — in a comparison of such protozoa with other 

 cells. In other instances, what is startling is the slight morpho- 

 logical differentiation of protozoan organisms. The morphologist 

 has to start out by admitting that protozoa are, at the least, cells. 

 In this chapter the morphological properties that are common 

 to many or most protozoa — and to many or most other kinds 

 of cells — are considered. Particular emphasis is given to 

 organelles such as contractile and food vacuoles that are especially, 

 though not uniquely, protozoan. Ameboid locomotion — an 

 unquestionably fundamental phenomenon — will, however, be 

 discussed in the following chapter, for the practical reason that it 

 is not possible to discuss it without describing the structure of 

 the entire ameba. 



Membranes and the Cytoplasmic Matrix 



Not surprisingly, the surfaces of protozoan cells often are 

 peculiarly and elaborately specialized on an ultramicroscopic 



