Food. The kind and amount of food available in a given body 

 of water also controls the distribution of Protozoa. The food is 

 ordinarily one of the deciding factors of the number of Protozoa 

 in a natural habitat. Species of Paramecium and many other holo- 

 zoic protozoans cannot live in waters in which bacteria or minute 

 protozoans do not occur. If other conditions are favorable, then the 

 greater the number of food bacteria, the greater the number of 

 protozoa. Noland (1925) studied more than 65 species of fresh-water 

 ciliates with respect to various factors and came to the conclusion 

 that the nature and amount of available food has more to do with 

 the distribution of these organisms than any other one factor. Di- 

 dinium nasutum feeds almost exclusively on paramecia; therefore, it 

 cannot live in the absence of the latter ciliate. As a rule, euryphagous 

 Protozoa which feed on a variety of food organisms are widely dis- 

 tributed, while stenophagous forms that feed on a few species of food 

 organisms are limited in their distribution. 



In nature, Protozoa live in association with diverse organisms. 

 The interrelationships which exist among them are not understood 

 in most cases. For example, the relationship between Entamoeba 

 histolytica and certain bacteria in successful in-vitro cultivation has 



