CHAPTER V. 



PROTOZOA.* 



[Limited to the Study of Pathogenic Forms]. 



INTRODUCTION. 



Most of those diseases which are known to be due to an infecting 

 agent are caused by bacteria; but some of them are caused by protozoa. 

 The bacteria belong to the vegetable kingdom. The protozoa are 

 minute animals; they are extremely numerous, and they 

 are very widely distributed throughout nature. 



From a zoological point of view, the protozoa consti- 

 tute an important sub-kingdom. It is sometimes diffi- 

 cult to say whether one of these minute organisms is a 

 plant or an animal. For this reason, the unicellular 

 organisms are sometimes classified by themselves, as 

 Protista, in a kingdom which belongs to neither the 

 animal nor the vegetable kingdom; usually, however, 

 the protozoa are placed in the animal kingdom and 

 they are defined as organisms which have the following 

 characteristics: "They are unicellular; they reproduce 

 by various methods of division and, often, in addition, 

 by conjugation; they may be solitary or united in 

 colonies, free living or parasitic; with some exceptions, 

 they do not possess chlorophyl." 



Many protozoa live in fresh water. Others live in 

 the sea; chalk is formed from the skeletons of myriads 

 diameters. (After o f protozoa which once lived in the ocean. When they 

 died, their b^nes fell to the bottom and formed chalk. 

 Most of the protozoa are free-living; but others are parasitic on 

 animals and plants. Some of the parasitic protozoa are harmless and 

 do no injury to the hosts which support them; others produce severe 

 diseases. Before mentioning those which cause disease (see p. 667) it 



* Prepared by J. L. Todd. 



68 



FIG. 43. Mie- 

 scher's sac from 

 the musculature 

 of a hog. X3O 



