THE PROTOZOA 



In the preceding discussion of the classification of animals, it was pointed 

 out that the simplest animals are unicellular, and that on the basis of this 

 characteristic they may be classified as members of the subkingdom Protozoa, 

 containing only the phylum Protozoa. These unicellular animals may be 

 contrasted with all the phyla of multicellular animals constituting the sub- 

 kingdom Metazoo. The Protozoa are defined as single-celled animals, al- 

 though some protozoans form permanent colonial aggregations which approach 

 the simplest metazoans in complexity. The word Protozoa ('Tirst animals," 

 "primordial animals") is well chosen; although many protozoan cells are 

 very highly specialized, the single-celled condition is considered most prim- 

 itive of all the structural plans of animals. It seems reasonable to conclude 

 that the protozoans have descended, without changing their unicellular state, 

 from the primeval organisms that were also the ancestors of the Metazoa. 



The phylum Protozoa is divided into two subphyla: the subphylum Plasmo- 

 droma, which' contains the class Sarcodina, the class Flagellata, and the class 

 Sporozoo; and the subphylum Ciliophora, which includes the most complex of 

 the protozoans, along with many simpler types, in the class Ciliata and the 

 class Suctoria. 



The vast majority of protozoans are free living, but all members of the 

 class Sporozoa are parasitic, and there are many parasitic species in other 

 classes. Some of these parasites, such as the malaria organism, cause serious 

 diseases in man and animals. 



Save for exceptional species, the Protozoa are of microscopic size; for this 

 reason, among others, they were not clearly understood in relation to other 

 forms of life until rather late in the history of classification. Certain of the 

 ciliates were observed and recognizably described by Leeuwenhoek as early as 

 1675, and many other protozoans were discovered during the eighteenth and 

 nineteenth centuries. The unicellular nature of these animals, and the fact 

 that they might be compared with the cellular units of metazoans, was not 

 recognized until 1845, after the elucidation of the Cell Theory. Although 

 we do not now speak of digestive tracts and other organs in protozoans, as 

 Ehrenberg did in 1838, it must be recognized that within the limits of the 

 unicellular state some protozoans are very complex organisms. 



The structural complexity of many protozoans illustrates the fact that dif- 

 ferentiation in animals may occur at the cellular level as well as in tissues 

 or organs composed of large numbers of cells. The component cells of meta- 

 zoans, specialized to carry on particular functions, are physiologically un- 

 balanced. They can exist in such a state of specialization only because the 

 many-celled organism taken as a whole is a physiologically balanced unit. 

 The single cell constituting the body of a protozoan necessarily performs all 

 functions, and hence it must be physiologically balanced, whatever its degree of 

 specialization. Together with the complexity of many protozoans, this con- 

 dition of physiological balance, which enables the protozoan to be a complete 

 and independent individual, has led some zoologists to regard the Protozoa as 

 animals to which the cell concept does not apply. According to this view, 



229 



