CHAPTER XIII. 



PHYLUM I: PROTOZOA. 



171. General Survey of the Group. These are the 

 minutest of the animals and cannot be studied in the 

 laboratory without the compound microscope. The 

 following facts make them of special interest: 



1. While they are very small, they are perfect cells 

 and have all the powers necessary for successful life. 

 They take food and digest and assimilate it so that it 

 builds up the body; they move and change their form; 

 they are sensitive; and they reproduce. So perfect are 

 these powers that they increase very rapidly, if the food 

 and other conditions are favorable, so that the water 

 literally swarms with them. If conditions become 

 unfavorable, they as quickly disappear. 



2. They are found in any waters on the earth where 

 life is possible at all; though they are most abundant 

 in stagnant waters where there is much decaying matter. 

 They are able to use matter so fine and so nearly de- 

 cayed that it would not be used by higher animals. They, 

 in turn, become food for animals somewhat higher up. 

 In this way they prevent much organic matter becoming 

 inorganic and useless to other animals. 



3. Some of them live as parasites in the bodies of 

 other animals. They are so small that they may live 

 within the cells. The parasite which produces malaria 

 lives and multiplies in the human red blood corpuscles. 

 Other human diseases are due to parasitic protozoa. 



