THE ANIMAL SERIES 385 



is a form of breathing. During heavy showers, when the 

 worms' holes in the soil become filled with water, the 

 worms come out to the surface because they cannot get 

 sufficient oxygen while covered' with water. For this 

 reason great numbers can be found on the streets and 

 sidewalks or on the surface of lawns and parks after a 

 heavy rain. The worm has no definite part of its body 

 for seeing, yet it is sensitive to light and also sensitive to 

 touch. 



The economic value of the earthworm to the farmer is 

 very great. The holes which it makes in the soil give 

 opportunity for soil ventilation or aeration. It also carries 

 large quantities of earth to the surface and thus keeps the 

 soil in continuous circulation or movement, making it 

 more fertile. The food of the earthworm is mostly the 

 root hairs of growing plants or any other organic 

 matter in the soil which it swallows while burrowing. 



There are also a great number of other worms, such as 

 sandworms. These can be found along the seacoast and 

 sometimes along rivers and lakes. There are also a great 

 number of worms which are parasites to man. These 

 will be studied later. 



266. Insects. Of all the animals now living, the 

 insect is probably the most completely fitted for its 

 environment, and on this account it is winning its way in 

 the struggle for life. An insect usually has six legs and 

 three divisions of its body. The three divisions are 

 known as the head, the thorax, and the abdomen. The 

 legs are all attached to the thorax or the middle division. 

 Most insects have two pairs of wings. Those which 

 have only one pair of wings prominent have two rudi- 

 mentary wings called " balancers." These can easily be 

 found on the common housefly. 



