PROTOZOA 121 



hosts which support them; others produce severe diseases. Before 

 mentioning those especially which cause disease (see page 822) it will 

 be well to consider the protozoa as a class and to discuss the characters 

 which all have in common. 



Structure of the S'rotozoa 



Most protozoa are so small as to be visible only by the aid of the 

 microscope but certain species are visible to the naked eye as individuals, 



Fig. 90. — Amaba vespertilio. (After Doflein.) 



or as agglomerated masses of individuals. For example, the Sarco- 

 sporidia, which occur in the muscles of mice and other animals, can 

 easily be seen without a micrqgcope, and the huge plasmodial masses 

 of Mycetozoa, which are sometimes seen on rotting wood or in tan 

 pits may measure many centimeters in breadth. 



Like aU living things, the protozoa are composed of protojjlasm (page 

 18) and its products. Protoplasm is a complex mixture of various sub- 

 stances in a colloidal condition. When studied by appropriate methods, 



