WORMS 401 



by an extremely thin shell of lime. When they die, their 

 shells sink to the bottom of the sea. So rapidly do these 

 animals multiply that their minute shells have made thick 

 layers of chalk like the famous chalk cliffs of the south 

 of England. 



Our chief interest in protozoa in the present study is that 

 certain of them are the cause of several kinds of disease which 

 can readily be prevented with proper care. Malaria and 

 the terrible African disease called sleeping sickness, and 

 probably yellow fever, are caused by these little animals. 

 We shall study them more fully later in connection with 

 harmful bacteria. 



Worms. Another class of invertebrates is the worms. 

 One of these, the earthworm, was found in the study of 

 soil making to be very important and should be considered 



EARTHWORM 

 A great helper to the farmer. 



in this place. If an earthworm is examined, it will be 

 seen that the body is made up of segments or rings, and 

 that it moves by successively shortening and elongating 

 its body. Extending through the middle of the body is 



