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TEXTBOOK OF ZOOLOGY 



like pseudopodia and a shell composed largely of silica. The pseudo- 

 podia extend through the relatively large apertures in the shell. 



3. Class Infusoria (infuso'ria, crowded).— This group includes 

 those single-celled animals covered with small hairlike, cytoplasmic 

 processes known as cilia. They occur in both fresh and marine 

 waters as free-swimming organisms. There are a few parasitic forms, 

 notably Balantidium coli. Paramecium, Stentor, and Vorticella are 



Coleps 



STEHTOl^ 



VOKTlceiLA. 



Fig. 27. — Group of typical Infusoria. (Courtesy of General Biological Supply 



House.) 



the commonly studied infusorians. The class is now divided into two 

 subclasses, Ciliata and Sudoria. The first, Ciliata, is composed of 

 four orders, (a) Holotrichida, possess cilia of equal length over the 

 body, or they are restricted to particular regions in specialized forms ; 

 a cell-mouth is present in most forms. Paramecium is our most com- 

 mon genus living in fresh water. Didinium, Frontonia, Chilodon, 

 and Coleps are other common forms. Opalina is a well-known para- 



