CHAPTER XVI 

 THE PROTOZOA OR THE SIMPLEST ANIMALS 



The lowest and simplest of all animals are the Protozoa. 

 The group differs from the animals that have been studied 

 in that the individual consists of but a single cell. Most 

 of the Protozoa are of microscopic size, and some are so 

 small that the highest powers of the microscope are neces- 

 sary to detect them. The commonly recognized classes 

 of the Protozoa may be separated by the following key: 



A . Protozoa, at least at some period of life, moving by 

 means of cilia ........ Infusoria. 



A A. Body devoid of cilia. 



B. Protozoa furnished with one or more flagella. . . . 



. .;, . * iJiv'.'-i Flagella ta. 



BB. Body devoid of flagella. 



C. Usually free forms, with pseudopodia. . Sarcodina. 

 CC. Exclusively parasitic forms, multiplying by means 



of spores and generally devoid of pseudopodia in 

 the mature state. Sporozoa. 



Of these classes the Infusoria have the most complex 

 organization. It is easy to obtain a typical infusorian in 

 the common slipper animalcule, Paramcecium, which 

 usually makes its appearance in infusions of hay or other 

 vegetable matter. If one places a quantity of hay in some 

 water coming from a pond or stream that contains more 

 or less plant life, it is probable that Paramcecia will make 

 their appearance in the mixture in the course of one or 

 more weeks. They may be recognized by their uniformly 



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