PARASITISM 



825 



expense of the vertebrates. The most dreaded of the round 

 worms is the Trichina living in the flesh of hogs, rats, dogs, 

 and man. One animal becomes infected by eating the flesh 

 of another containing the parasites which upon reaching 

 the stomach of the new host bring forth many 

 young ones. These migrate through the 

 walls of the alimentary canal into the mus^ 

 cles and remain there in a living condition 

 many months, never again becoming active 

 until eaten by a new host. The government 

 meat inspectors have found as many as 

 seventeen thousand hogs containing this par- 

 asite in one year. A microscopic worm, 

 Filaria, occurring in the blood of man in the 

 tropical regions, produces various disorders. 

 Among the larger parasites is the stomach 

 worm, Ascaris lumbricoides, from three to six 

 inches long, affecting children, and various 

 other species of Ascaris are common in nearly 

 all mammals. They hold on to the mucous 

 coat of the stomach or intestine by means 

 of a hook at the hind end, and feed on the 

 digested food. Eggs are produced at the rate 

 of about fifteen thousand daily, and passing 

 out with the excrement may reach the water 

 supply and infect new individuals. Strongylus is a genus 

 containing many species of minute, round worms parasitic 

 in the mammals, birds, and frogs. Some kinds occupy 

 the digestive system while others are present only in the 

 respiratory organs. Nine out of every ten cats harbor 

 round worms of some kind. 



FIG. 365. Tri- 

 china. 1, fe- 

 male ; 2, male. 

 Enlarged fifty 

 times. 



