316 APPLIED BIOLOGY 



277. Useful Protozoa. The emphasis upon certain 

 protozoans as causes of disease tends to leave the false im- 

 pression that all protozoa are harmful, or at least useless. 



A large number of common protozoans are important in the 

 food-supply of somewhat larger animals, these in turn of still 

 larger animals, and so on to aquatic animals such as fishes. 



Thus even the minute bacteria eaten by some protozoans 

 may indirectly, through a series of animals of increasing size, 

 finally come to be of use in the food-supply of man himself. 

 We cannot accurately estimate the relation of the smallest 

 animals in the food-supply of larger animals ; but vast num- 

 bers of animals certainly depend directly and indirectly upon 

 protozoans as food. 



Many of the protozoans are important as scavengers, as- 

 sisting bacteria in breaking up dead organic bodies ( 116, 

 117) and preparing the organic material for use again in the 

 cycle of organic matter. For example, a protozoan may eat 

 some particles from the body of a dead animal or plant, build 

 these particles into its own protoplasm, which may later serve 

 as protein food for some larger animal ; or these particles 

 may soon be oxidized to excretions which may serve as ma- 

 terials for food of plants. 



Another example of the usefulness of protozoans is that a 

 large amount of chalk in the great deposits in England and 

 elsewhere is composed of the shells of certain protozoans. 

 In some places these shells are very abundant in the mud of 

 the ocean bottom. 



278. Colonial Protozoa. Amceba and Paramecium rep- 

 resent the common kinds of protozoans in that all individuals 

 live free and independently of each other. We shall examine 

 briefly groups or colonies of certain protozoans. 



One of the most common colonial protozoans is a tree-like 

 form. It consists of a delicate and much-branched stalk, 

 which is attached to some object, and at the end of each 

 branch is a one-celled animal of bell-like form (similar to Fig. 



