FLATWORMS, ROUNDWORMS, AND ROTIFERS 



69 



resemble hairs that some people think they are transformed 

 horse hairs. The guinea worm, which is a parasite of 

 man in tropical countries and 

 which sometimes attains a length 

 of several feet, is a member of 

 . the group of roundworms. 



Trichinella spiralis. — This is 

 one of the most important round- 

 worms, economically speaking, in 

 the United States. The adult 

 worm is very small, scarcely vis- 

 ible to the naked eye, and is 

 found only in the intestines of 

 mammals, as, man, pig, rat, etc. 

 It is probable that the pig be- 

 comes infested with this worm 

 by eating the 

 flesh of some 



animal, as the rat. Of course man be- 

 comes infested by eating raw or half- 

 cooked pork. In this way the worms, 

 in a larval condition, are taken into the 

 alimentary canal. There they soon be- 

 come mature, and the young are produced 

 in immense numbers. These soon work 

 their way through the walls of the intes- 

 tines and finally reach the muscles (Fig. 

 37), where they become surrounded with 

 a cyst, and there they remain. Before 

 Fjg. 37. —Trichinella forming the cyst, the embryos feed upon 

 spiralis imbedded ^j^g muscular tissue, causing serious com- 



in human muscle. '^ , 



After Leuckart. pUcations that of ten result m the death 



Pig. 36. — Hairworm. 



