PARASITIC PROTOZOA 



365 



microscopical examination, are shown to be free from the para- 

 site are sent to the growers. 



Epidemics Among Fishes. Sometimes in the United States, 

 and more frequently in Europe, destructive epidemics appear 

 among the fishes in lakes or rivers. Hundreds of thousands 

 of fish may die and be washed upon the shores and the particu- 

 lar species attacked may be almost exterminated in some 

 regions. The diseases responsible for these epidemics are 

 caused by various species of Myxosporida, which attack and 

 destroy the tissues of their hosts, often causing many serious 

 tumors over the body. Young fish are especially apt to suffer 

 from the attacks of these parasites. 



DISEASES CAUSED BY INVISIBLE MICROORGANISMS 



There are a number of diseases the causative agents of which 

 are not yet definitely known. Nevertheless it has been demon- 



FIG. 155. Stable-fly, Stomoxys calcitrans, after engorging itself 

 blood. (About three times natural size.) 



with 



strated that many of these are caused by living organisms, 

 probably too small to be seen by any microscopes that have 

 yet been invented. More refined microscopic methods may 

 sometime reveal some of these to us, and we may then be able 

 to determine their relation to other known forms. 



That yellow fever is caused by such an organism has been 

 definitely shown, and although we do not know the parasite 



