PARAMCECIUM 281 



their bodies out of inorganic materials. As they grow they 

 divide, and this capacity for multiphcation seems unlunited. 

 The Protista swarm upon tlie surface of ponds and of the sea. 

 They are devoured by mj^iads of Crustacea (Entomostraca), 

 whose capacity for reproduction seems, in turn, hmited only 

 by the number of Protista. These Entomostraca, however, 

 constitute, as shown in more detail in Chapter X, the main 

 food supply of many of our most important food fishes. Thus 

 the chain of relations is complete between the Protista and 

 man. 



The Protista that are without pigment occupy a very dif- 

 ferent niche in nature. They live on organic matter. Many 

 of them devour the decaying materials that are found on the 

 bottom of ponds. Some live on the surface of other organ- 

 isms, feeding upon the waste matters eliminated. But many 

 others have gone farther and live as parasites in the interior 

 of the body of other organisms. In the highly nutritious 

 medium in which they now find themselves they multiply 

 tremendously. As a result of this accelerated growth they 

 eliminate large quantities of substances into the body which 

 are often highly injurious, even bringing about the death of 

 the higher infested animal. Until a few years ago it was sup- 

 posed that the most dangerous of these parasitic Protista 

 were plants, but to-day we recognize that almost any group 

 of Protista may give rise to dangerous human parasites. 

 " Malaria," yellow fever, cholera, and man)' other tropical 

 diseases of man and domesticated animals are due to Protozoa. 

 Many of these parasitic Protozoa are conveyed from one indi- 

 vidual to another by means of insects that bite persons already 

 parasitized. The parasites imbibed with the blood of the 

 victim usually develop further within the insect and, when the 



