BIOLOGY AND HUMAN WELFARE 



401 



Fig. 254. — Triehinella spiralis. A, larvae free among muscle fibers; B, a 

 single larva encysted among fibers; C, piece of pork containing many encysted 

 worms, natural size; D, adult worm, highly magnified. (From Leuckart.) 



The widespread distribution of the several species of Hook- 

 worms and their insidious effects make them also of great prac- 

 tical importance. Biological studies have demonstrated the 

 process by which the tiny worms, just visible to the naked eye 

 as whitish threads, are hatched in a warm moist soil, make their 

 way through the skin of the human foot, enter the circulatory 

 system, are carried to the lungs, and finally work to the intestine. 



Fig. 255. — The American Hookworm, Necator americanus, 



highly magnified. 



Here, as adults, they become attached, feed upon the blood of 

 their host and liberate eggs. These pass out with the host's feces 

 to become the source of infection for others. The spread of knowl- 

 edge of the essential facts and of vermifuges to expel the parasites 

 has been an important contribution of the International Health 

 Board which has carried on a campaign in over fifty countries. It 

 has been estimated that not less than two million persons are af- 

 flicted with the disease, but the recently acquired facts in regard 

 to the parasite should result eventually in its almost complete 

 eradication. (Fig. 255.) 



