344 APPLIED BIOLOGY 



of liver-flukes, the only way to avoid infecting sheep is to 

 keep them away from that stream for a number of years. 

 Ultimately the fluke larvae would disappear from a pasture 

 if there were no sheep in which the parasites could complete 

 the life-history. 



297. Round Worms. A good example of a harmless 

 round worm is the vinegar-eel, which is often abundant in 

 unfiltered vinegar, and especially in " mother of vinegar." 

 They should be examined with the low power of a microscope. 

 Many species of the round worms are parasitic in intestines 

 of man and animals. Some are slender threads half an inch 

 long, while others may be a foot long. 



The most terrible round worm is the trichina. The adult 

 worms live in the intestine of man, pig, and other mammals. 

 The males are about 1 mm. (^V inch) and the females 3 mm. 

 long. The eggs develop inside the female and the young 

 (1000 or more) are born alive; that is, discharged as young 

 worms. These pass through the walls 

 of the intestine of their host, and reach 

 such muscles as those of the arms, legs, 

 and back. Each worm embeds itself in 

 a muscle-fiber (Fig. 109), and a cyst 

 forms around it. It may remain en- 

 cysted for years. An ounce of infected 

 pork may have 80,000 such cysts. If 

 the flesh be eaten by another animal or 

 by man, the cysts dissolve in the diges- 

 FIG. 109. Trichinae ^ive organs, the young worms develop, 



encysted in muscle. , , - ,.,. -, 



(After Leuckart.) form eggs and sperms, and fertilized eggs 

 develop into young worms which become 

 encysted in muscles. In a town in Germany, in 1884, the 

 flesh of one pig infected 364 persons, and 57 died within a 

 month. This large number of infections is easily understood 

 when we calculate from the figures above. A single ounce of 

 pork might introduce into a human stomach 80,000 trichinae . 



