ANIMAL PARASITES. iii 



and that development cannot take place in insects and 

 animals which do not have this relationship to the 

 parasites. One illustration will suffice; no other 

 mosquito besides the genus stegomyia fasciata can 

 harbor or convey yellow fever, and none besides the 

 genus anopheles, malaria. In the stomach of other 

 varieties of mosquitoes than anopheles the Plasmodia 

 malaricE are digested. 



PROTOZOA. 



The smallest forms of animal life parasitic upon man 

 belong to the protozoa. The latter constitute that 

 class of organisms the individuals of which are com- 

 posed of only a single cell. Like bacteria, all parasitic 

 protozoa are microscopic in size. Their structure, 

 however, is far more complex, and their life-cycle is 

 bizarre in the extreme. Thus, some are known to 

 develop partly in water and partly' in a host; others 

 entirely within the bodies of two or more hosts. 



Those which have part of a cycle in water, and the 

 other in man, usually gain entrance into the body in the 

 drinking-water. 



Examples of this class are found in the amoeba of 

 dysentery and the coccidium hominis. The last men- 

 tioned is probably the cause, also, of that peculiar tumor 

 known as epithelioma contagiosum. On the other 

 hand, in the case of such varieties as never reach the 

 external world, but spend their whole existence in 

 various special hosts, these are usually conveyed to 



