384 GENERAL SCIENCE 



very useful to man but very difficult to obtain. Some of 

 these have lived in such great numbers that the skeleton- 

 like forms of their bodies have formed great deposits in 

 various parts of the world. The most widely used of 

 these deposits is chalk, of which crayon for writing on the 

 blackboard is made. There are great deposits of chalk 

 in the central and southern United States and also in 

 England. Another one-celled animal forms deposits of 

 quartz-like material. 



265. Worms. The worm which is most easily found 

 and studied is the common earthworm. It lives in the 



Intestine Gizzard Crop Gullet Pharynx 



outh 



FOOD TUBE or AN EARTHWORM AND THE RINGS OR SEGMENTS 



INTO WHICH ITS BODY IS DIVIDED 



soil but is found in greater abundance where the soil is 

 fertile. It burrows into the earth by making holes and 

 swallowing the earth as it goes. It is able to move by two 

 actions of its muscles. It has a layer of muscles run- 

 ning lengthwise of its body, which shorten the worm when 

 they contract. Another layer of muscles runs around 

 its body and lengthens the worm when they contract. It 

 also has projections on the lower part of its body which 

 can be directed forward or backward and prevent the 

 worm from sliding except in the direction in which it 

 wants to go. The worm is covered with a slimy, mucus- 

 like secretion which keeps its body moist, and on account 

 of this moisture it is able to take oxygen from the air; this 



