ROUND OR THREAD WORMS 



453 



carried by the lymphatic fluid into the blood-stream, and thence into the 

 muscles (muscular trichina:). Here they remain, living for some time 

 upon the constituents of the muscle, which they destroy, grow rapidly in 

 size, and after rolling themselves up spirally are finally enclosed in a 

 lemon-shaped capsule or cyst. In this condition they may remain for a 



considerable time without dying. If, 

 however, they are consumed with the 

 flesh of their hosts by man or one of 

 the animals mentioned — and this may 

 not happen till years afterwards — 

 they develop to the perfect sexually 

 mature parasite. 



In man trichinae produce a violent, 

 sometimes even fatal, disease known 

 as trichinosis. As a protection against 

 it, pork should never be eaten until it 

 has been pronounced free from trich- 

 inae after a microscopic examination, 

 and only then after having been pre- 

 viously thoroughly boiled, roasted, or 

 smoked, for it is by the pig alone that 

 this dreaded parasite is conveyed to 

 man. 



It appears that the true host of 

 the trichina is the rat, which occa- 

 sionally kills weaker members of its 

 own species and eats them, or feeds 

 on the corpses of its fellows, and thus 

 constantly reinfects itself, i.e., intro- 

 duces the parasite into its body. From 

 the rat it is conveyed to the pig, in the 

 the latter devouring the dead body of an infected 

 Hence these troublesome and dangerous vermin 



Mil 



Trichina. 



. Intestinal form ( x about 

 50 times). J., Young 

 trichinae escaping from 

 the body. II. Encysted 

 trichinae, muscular form 

 ( x about 100 times). 



event of 



rodent. 



should be carefully kept away from pig-sties. 



Other Thread-worms. 



The Common Round Worm (Ascaris lumbricoides) is a parasite 

 which inhabits the intestine of man, especially children. The female 

 may reach a length of nearly 16 inches. 



Another smaller worm, about £ inch in length, occurs in the rectum 

 in children, sometimes in great numbers {Oxyuris vermicularis). 



30—2 



