468 BIOLOGY FOR BEGINNERS 



Certain protozoa cause disease (see Chap. 18 and 25) and some 

 parasitic worms (Chap. 20) also do considerable harm. Many 

 insects live upon the plants that man also uses for food and in this 

 way cause serious destruction to crops, while others transmit 

 disease (Chap. 25). To a very small extent " wild animals " harm 

 man directly and also destroy some of his domestic animals, but 

 this is of comparatively little importance. 



Economic Value of Animals. In dealing with the economic im- 



FIG. 148. A common bath sponge. From Kellogg and Doane. 



portance of animals we shall take them up by groups beginning 

 with the simplest first, namely the protozoa. 



Protozoa. These minute one-celled forms are of vast importance 

 to man insomuch as they are the source of food for higher animals 

 and these in turn finally provide man with nourishment, by way 

 of such important sources of food as clams, oysters, crustaceans, 

 and fishes, many of which find, in protozoa, their chief food supply. 



Certain protozoa develop minute shells and the deposits of these 



