184 



ANIMAL LIFE 



two hosts for its completion. The eggs of the tape-worm 

 pass from the intestine with the excreta, and must be 

 taken into the body of some other animal in order to de- 

 velop. In the case of one of the several species of tape- 

 worms that infest man this other host must be the pig. 

 In the alimentary canal of the pig the young tape-worm 

 develops, and later bores its way through the walls of the 

 canal and becomes imbedded in the muscles. There it lies, 

 until it finds its way into the alimentary canal of man by 

 his eating the flesh of the pig. In the intestine of man 

 the tape-worm continues to develop 

 until it becomes full grown. 



In a lake in Yellowstone Park 

 the suckers are infested by one of 

 the flat-worms (Ligula) that at- 

 tains a size of nearly one fourth 

 the size of the fish in whose in- 

 testines it lives. If the tape-worm 

 of man attained such a compara- 

 tive size, a man of two hundred 

 pounds' weight would be infested by 

 a parasite of fifty pounds' weight. 



98. Trichina and other round- 

 worms. Another group of animals, 

 many of whose numbers are para- 

 sites, are the round-worms or thread- 

 worms (Xemathelminthes). The 

 free-living round-worms are active, 

 well - organized animals, but the 

 parasitic kinds all show a greater 

 or less degree of degeneration. One 

 of the most terrible parasites of man is a round-worm called 

 Trichina spiralis (Fig. 109). It is a minute worm, from 

 one to three millimetres long, which in its adult condition 

 lives in the intestine of man or of the pig or other mam- 

 mals. The young are born alive and bore through the walls 



FIG. 109. Trichina 



(after GLAUS), a, male ; b, 

 encysted form in muscle ; c, 

 female. 



