THE ECONOMIC IMPORTANCE OF ANIMALS 217 



THE RELATIONS OF ANIMALS TO DISEASE 



The Cause of Malaria. The study of the life history and 

 habits of the Protozoa has resulted in the finding of many parasitic 

 forms, and the consequent expla- 

 nation of some kinds of disease. 

 One parasitic protozoan like an 

 amosba is called Plasmodium ma- 

 larice. It causes the disease 

 known as malaria. When a mos- 

 quito (the anopheles) sucks the 

 blood from a person having mala- 

 ria this parasite passes into the 

 stomach of the 

 mosquito. Af- 

 ter completing 

 a part of its life 

 history within 

 the mosquito's 

 body the para- 

 site establishes 

 itself within the 

 glands which 

 secrete the sa- 

 liva of the mos- 

 quito. After 

 about eight 

 days, if the in- 

 fected mosquito 

 bites a person, 

 some of the 

 parasites are 

 introduced into 



the blood along with the saliva. These parasites enter the cor- 

 puscles of the blood, increase in size, and then form spores. The 

 rapid process of spore formation results in the breaking down 

 of the blood corpuscles and the release of the spores, and the 



The life history of the malarial parasite. This cut of the 

 malarial parasite shows parts of the body of the mosquito 

 and of man. To understand the life history begin at the 

 point where the mosquito injects the crescent-shaped 

 bodies into the blood of man. Notice that after the spores 

 are released from the corpuscles of man two kinds of cells 

 may be formed. These are probably a sexual stage. Devel- 

 opment within the body of the mosquito will only take 

 place when the parasite is taken into its body at this 

 sexual stage. 



