CHAPTER III 



SUBKINGDOM I. PROTOZOA 



TYPE I. PROTOZOA 



The Protozoa (Gr. irpwros, first, and i;wov, animal) are the 

 simplest organisms which clearly belong to the animal kingdom. 

 They live either in fresh or salt water, or in damp places, or, in 

 the case of those which are parasitic, within the bodies of higher 

 animals. The characteristic which distinguishes them from all 

 other groups of animals is the fact that each protozoon consists 

 of a single cell. In most cases each individual leads a free life, 

 independent of its neighbors, but not a few live associated 

 together in groups or colonies. The cells composing these 

 colonies, however, represent each a complete individual, — ■ as 

 a rule physiologically complete, -- though in a few exceptional 

 cases there is a slight difference of function among the individ- 

 uals in a colony. Thus the protozoan colonies differ from the 

 multicellular animals, for in the latter there is always a division 

 of labor corresponding to a structural differentiation of cells. 



They are for the most part extremely small, so that a micro- 

 scope is necessary to determine their structure, though some are 

 large enough to be seen with the naked eye, and some of those 

 which have been preserved as fossils are decidedly macroscopic. 

 Yet, despite the fact that the Protozoa are minute and unicellu- 

 lar, and, as we say, the simplest animals, they exhibit a consid- 

 erable differentiation of structure, as one might well infer, since 

 here a single cell has to perform all the functions of the animal 

 body, such as motion, alimentation, respiration, excretion, and 

 reproduction, which in the higher animals are severally per- 

 formed by different groups of cells, conforming to the principle 

 of a division of labor. Such a division of labor is on the whole 



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