Chapter 2 



iNTRODUCTION 

 TO 



THE PROTOZOA 



Protozoa form the most primitive 

 group in the animal kingdom. The bodies 

 of all other animals are composed of 

 many units, or cells, but those of the 

 protozoa are a single cell. No matter 

 how complex their bodies may be, and 

 many of them are very much so, all the 

 different structures are contained in a 

 single cell. This complexity has made 

 some investigators maintain that, instead 

 of being considered single cells, protozoa 

 should be thought of as non-cellular (see, 

 for example, Boyden, 1957). This argu- 

 ment is essentially a verbal issue--a 

 matter of how one wants to define "cell. " 



Protozoa are microscopic in size, 

 only a few being visible to the naked eye. 

 They differ from the Metazoa in being 

 unicellular, but this difference is not as 

 clearcut as might be supposed. Some 

 protozoa have a syncytial stage in their 

 life cycle in which there are no cell walls 

 between the nuclei, and some species form 

 colonies which swim as a unit and which 

 contain somatic and reproductive organ- 

 isms which look different. The difference 

 between these and Metazoa is again partly 

 a matter of definition, and gives a clue to 

 how the Metazoa could have arisen. 



The boundary between the Protozoa 

 and certain one-celled plants, too, is not 

 clearcut. For example, the whole group 

 of slime molds are considered by proto- 

 zoologists to be protozoa and assigned to 

 the order Mycetozoorida in the class 

 Sarcodasida, but botanists consider them 

 fungi and assign them to the class Myxo- 

 mycetes. 



A still more confusing situation in- 

 volves the plant-like protozoa which con- 

 tain chlorophyll. Protozoologists assign 

 them to the subclass Phytomastigasina, 

 but botanists consider them green algae. 

 The problem is that there are many spe- 

 cies of colorless protozoa which differ 

 from green ones only in that they lack 

 chromatophores. Loss of chromatophores 

 can be produced experimentally. It has 



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