CHAPTER VI 

 PROTOZOA* 

 Introduction 



Many of the diseases which are known to be due to an infecting 

 agent are caused by bacteria; but others are caused by protozoa. 



The bacteria belong to the vegetable kingdom. The protozoa are 

 unicellular animals; they are extremely numerous and are very widely 

 distributed in nature, occurring in water, soil, and in the bodies of most 

 animals. 



Froni a zoological point of view, the protozoa constitute an impor- 

 tant sub-kingdom. It is sometimes difficult to say whether a minute 

 organism is a plant or an animal. For this reason, primitive unicellular 

 organisms are sometimes classified by themselves, ss Protista (page 114), 

 a kingdom which thus includes not only primitive organisms which 

 have not yet been definitely established in either group but also certain 

 unicellular animals and plants. It appears important, however, to 

 determine as far as possible the genetic relationship of various or- 

 ganisrns and, by the study- of their physiology and modes of develop- 

 ment to differentiate between those which are plant-hke and those 

 which are animal-Uke in character. The protozoa are thus included 

 in the animal kingdom and have been defined as "unicellular animals." 

 They are to be distinguished, on the one hand from primitive forms such 

 as bacteria which lacking differentiation of nucleus and cytoplasm 

 do not conform to the type of structure of true cells, on the other hand, 

 from unicellular organisms of plant-like character such as algae and 

 fungi which are included with the Protophyta. 



Many protozoa Uve in fresh water. Others live in the sea; chalk is 

 formed from the skeletons of mjTriads of protozoa which once lived in 

 the ocean. While a large proportion of the protozoa are free-living, 

 others are parasitic on animals and plants. Some of the parasitic 

 protozoa are practically harmless and do no apparent injury to the 



• Prepared by J. L. Todd. Revised by E. E. Tyzzer. 



120 



