BORN INFECTED g 



or never noticed. But tiny as they are, they were 

 hatched infected, and by their bite the spirochaetes 

 pass into man in the fluid excrement that enters the 

 wound. 



In Europe and America sheep are sometimes in- 

 fested with a skin parasite, popularly known as a 

 " sheep ked." This is really a wingless fly, Melo- 

 phagiis ovinus. It harbours a parasite that primarily 

 belongs to the alimentary canal, but it may leave the 

 gut, enter the ovaries, and penetrate the eggs. The 

 eggs develop within the mother into a grub, which is 

 passed from the mother as a sort of chrysalid or 

 pupa. As the eggs are infected, the puparia also are 

 infected, and when the fully developed insects emerge 

 in the wool of the sheep, they, too, contain the 

 parasites known as Crithidia melophagia. Here un- 

 doubted hereditary infection occurs, though the 

 adult insects may become infected by the casual 

 methods mentioned before. 



The main means by which protozoal diseases are 

 spread have already been indicated in the foregoing 

 section. But it must be noted that in many cases 

 the method of transmission of parasites from one 

 host to another is almost unknown. It is only 

 recently that much attention has been given to the 

 subject, and the work demands rigorous and 

 laborious attention. 



Two principal classes of animals appear to act as 

 transmitters of protozoal diseases. Leeches are the 

 chief agents in the transference of protozoal diseases 

 of fishes and amphibia, while insects and ticks are 

 responsible for the spread of parasitic diseases of 



