THE CLASSIFICATION OF PROTOZOA 



The one characteristic which the examples that we have discussed 

 have in common is that, whether they have one nucleus or several, 

 no nucleus is separated from another by a cell wall ; or, in 

 physiological rather than morphological terms, no nucleus ever 

 exerts its controlling influence on a part only of the cytoplasm. 

 This is true of all the animals which are commonly called Protozoa 

 with the exception of a small group called Cnidosporidia, which 

 have peculiar structures called pole capsules which seem to have 

 their own nuclei. As we have seen, a formal distinction between 

 animal and plant flagellates is impossible, but if we begin by 

 saying that the Protozoa are animals, the criterion of absence of 

 cell walls makes a workable, if not entirely adequate, definition. 

 It avoids the necessity of defining the cell, and so the necessity 

 of deciding whether the Protozoa are all non-cellular or not. It 

 is clear that they are not all unicellular, for all definitions of a 

 cell agree that it has but one nucleus. 



At some point, however, we shall have to consider the mieaning 

 of the word cell, and it may be said here that the two views 

 which have been current differ rather in their metaphysical 

 outlook than in their morphological terminology. According to 

 one view, a cell is a part of an organism, and as such it can have 

 no fundamental similarity to a protozoan such as Trypanosoma. 

 The other view, which looks on uninucleate Protozoa as being 

 single cells, is historically connected with the evolutionary view 

 that all other animals are derived from Protozoa by a series of 

 fissions after which the daughter units remained in contact, as they 

 do in the embryology of higher forms (Chap. 28). How^ever 

 probable on general grounds one may choose to consider this 

 hypothesis, there is not a single piece of evidence for it, and it is 

 therefore unwise to make much of it. As was first stressed by 

 Dobell, a protozoan carries out all the ordinary activities of the 

 so-called higher animals, and differs from them only in the organs 

 used ; our increased knowledge now makes this even clearer. 

 For some reason or other Amoeba is generally venerated as the 

 first ancestor of all animals, but for this view also there 



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