DISEASES CAUSED BY ONE-CELLED ANIMALS 125 



have a very simple sort of life-history, the disease-producing 

 Protozoa have usually a very complicated life-history and 

 one that requires two kinds of hosts for its completion. 

 The bacteria or bacilli that cause typhoid fever for example, 

 multiply in the body by simple division repeated indefi- 

 nitely, forming generation after generation of bacilli all alike 

 and of the same habits. The Protozoa that cause malaria 

 multiply for a number of generations in the body, somewhat 

 as the bacteria do, but then gradually cease multiplying and 

 either die or lie more or less inert in the blood until they are 

 sucked up with some of this blood into the stomach of a 

 mosquito when they renew their active life and their process 

 of multiplication but in a way very different from their 

 former way. The very shape and appearance of the germs 

 become so changed that they could not be recognized as 

 belonging to the same kind if the actual process of the 

 changes had not been clearly observed. 



In the mosquito's stomach some of the little round in- 

 active bodies suddenly put forth five or six long slender 

 lash-like processes that break off and go swimming about 

 like little snakes. These find some of the inactive bodies 

 which have become somewhat swollen and fuse with them 

 and the new body formed by this fusion becomes active 

 and moves toward the wall of the stomach and there burrows 

 into this wall as far as its outer coating. Here the parasite 

 comes to rest and begins to grow rapidly until it forms a 

 little nodule on the outer surface of the stomach. Inside 

 this nodule the body stuff of the parasite divides into many 

 hundred minute spindle-shaped bodies. Finally the walls 

 of the nodule, which now projects into the body-cavity of 

 the mosquito, break and the hundreds of new active germs 

 escape into the blood of the insect which flows freely all 

 through its body-cavity. From the blood they migrate 

 forward into the neck and head and finally lodge in the 

 salivary glands where they remain. 



