8 



Definitive host — Supports the sexually mature form. 



Passive transfer — Ex., embryos taken with food or drinking water. 



Active transfer — Ex., larval hookworms boring into the skin. 



Alternation of generations, one or more asexual generations followed 

 by a sexual generation, very common among parasites. 



Simple alternation of generations illustrated by the life history of 

 the malarial parasite, Plasmodium praecox. Sexual stage in mosqui- 

 toes of the genus Anopheles; asexual stage in blood of man. 



A complicated type, introducing several generations into the life- 

 cycle, illustrated by the life-history of the liver-fluke of the sheep, 

 Fasciola hepatica. Adult in liver of sheep; ciliated embryo in water; 

 sporocyst in body of snail, giving rise to several generations of rediae; 

 development of cercariae, which pass into the water, become encysted 

 on herbage, and are taken up by sheep, in which they become sexually 

 mature. 



The reproduction of parasites — Complicated life-history a disad- 

 vantage in that it enormously lessens the chances of the species per- 

 petuating itself. Compensating advantages. The handicap met by : 



Enormous fertility typical — Estimated that the beef tape worm 

 of man produces 150,000,000 eggs per year. Polyembryony of certain 

 parasitic insects, a hundred or more embryos developing from a single 

 egg. 



Resistance of eggs and larvce of many species — Thick-shelled eggs 

 of tapeworms, whipworms. Living embryos have been found in 

 Ascaris eggs preserved for months in strong formalin and other 

 chemicals. Trichinae may live for months after the infested hog is 

 killed. 



Hermaphroditism commonly met with in parasites, providing 

 for perpetuation of species by a single individual. 



Action of parasites upon their host — Extreme divergence of 

 views as to injuries caused by parasites. The prevailing views 

 among medical men and veterinarians who have not especially 

 studied the question, underestimate possible effects. 



Seriousness of direct action largely dependent upon the situation 

 of the parasite — Multiceps coenurus in the brain causing death of 

 sheep. 



Drain on nourishment of the host — Direct, or by taking blood. 



