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 (Nemathelminthes). 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 spiralis 

 (after GLAUS), a, male ; ft, 

 encysted form in muscle ; c, 

 female. 



