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 denned 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 

 of protozoa which once lived in the ocean. When they 

 died, their bones 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. X30 

 diameters. (After 

 Ostertag.) 



