336 FOUNDATIONS OF BIOLOGY 



exhibited by parasites in becoming fitted for this apparently 

 highly successful method of gaining a livelihood, and by the 

 hosts in bearing the burden with the least outlay. Generally 

 speaking, the effect on the parasite consists in a simplifica- 

 tion of the various organs of the body devoted to food- 

 getting, locomotion, etc., since their duties are foisted upon 

 the host; while the organs and methods of reproduction are 

 highly specialized and elaborated, owing to the necessity of 

 producing enough offspring to compensate for the hazards 

 involved in reaching a proper host. For in the majority of 

 cases a parasite is adapted to live in a specific host, and death 

 ensues if this is not attained at the proper time. 



Probably the most generally interesting example of para- 

 sitism is the cause of the disease known as MALARIA. Man is 

 subject to at least three types of malaria, each the result of 

 infection by a different malarial organism. The malarial 

 parasites are all unicellular animals, Protozoa, with compli- 

 cated life histories which are adaptations to the specific exi- 

 gencies of their parasitic existence. (Fig. 181.) One part 

 of the life history, the asexual, is passed in the red blood 

 corpuscles of Man; while the other, the sexual, occurs in the 

 digestive tract of certain species of Mosquitoes. A single 

 parasite inoculated into the human system by the bite of an 

 infected Mosquito enters a red blood corpuscle and multi- 

 plies. The progeny, liberated from the destroyed corpuscle, 

 similarly attack other corpuscles and multiply until a very 

 large number of blood corpuscles are destroyed. And the liber- 

 ation of poisonous products of the life processes of the parasites 

 provoke the chills and fevers characteristic of the disease. 



But the parasites must make their escape before the human 

 host successfully combats the toxic substances, kills the 

 parasites by taking quinine, or succumbs to them. The get- 

 away is accomplished, if at all, by a Mosquito biting the host 



