330 UTILITARIAN ZOOLOGY 



burrows in the ground help on the work of drainage and aeration. 

 In short, the habits of these animals benefit the land in much the 

 same way as the operations of ploughing, harrowing, and the like. 

 Beneficial Parasitic Worms. — At first sight one is rather 

 apt to imagine that the members of the several groups of these 

 not altogether pleasing creatures do nothing but harm. This is 

 not, however, strictly true, for many of them pay special atten- 

 tion to noxious animals, and assist considerably in keeping down 

 their numbers. 



