CHAPTER XII 

 PHYLUM PROTOZOA 



Protozoa are single-celled animals. In many species 

 great numbers of cells are grouped together to form colonies, 

 but in such cases there is no division of labor between the 

 cells; tissues are not formed, as in true many-celled animals. 

 A protozoon, then, has all the characteristics of single cells 

 as described in Chapter IV. Each protozoon, though con- 

 sisting of but a single cell, is able to feed, escape dangers, 

 reproduce, and carry on all the activities characteristic of 

 living things (Chapter III). Most protozoans are very 

 small and can usually not be seen without the aid of a 

 microscope. But what they lack in size is made up in num- 

 bers, and there is no bit of natural water or soil without 

 them. They often swarm by myriads in fields, lakes, rivers, 

 puddles, eave troughs, cisterns, water pipes, and other wet 

 places. Many species live as accidental guests or as para- 

 sites in the bodies of many-celled animals, some even caus- 

 ing important diseases of man and the domestic animals. 



There are four classes of Protozoa, arranged primarily 

 according to methods of locomotion. They are as follows: 



1. Sarcodina (Rhizopoda). Protozoa in which locomotion is by 

 means of rather blunt protoplasmic projections called pseudopodia. 



2. Mastigophophora. Protozoa which have whip-like flagella for 

 locomotion. 



3. Sporozoa. Parasitic protozoans possessing no special locomotor 

 organs, and in which there is at times a formation of many spores for 

 reproductive purposes. 



4. Infusoria. Protozoa which have the body covered with many 

 hair-like cilia for locomotion. 



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