272 



ESSENTIALS OF ZOOLOGY 



3. Class Infusoria (in fii so' ri ^, 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 Yorticella are 

 the commonly studied Infusorians. The class may be divided into two 

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



Coleps 



Bidimum 



STEHTOK 



VOI^TICCLLA. 



Fig. 89. — Group of typical Infusoria. (Courtesy of General Biological Supply 



House. ) 







four orders (or five by some), (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 common genus living in fresh water. Didinium, Frontonia, 

 Chilodon, and Coleps are other common forms. Opalina is a well- 

 known parasitic genus which inhabits the large intestine of the frog, 

 (b) Heterotrichida possess a well-developed undulating membrane 

 in the cytopharynx. The body cilia are small or partially absent, 



