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- 
5 worms (Nemathelminthes). The 
neta are syiraiy tree-living round-worms are active, 
(after Cras). a, male; >, well-organized animals, but the 
greveted form inmusele: ¢ Harasitic 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 manor of the pig or other mam- 
mals. The young are born alive and bore through the walls 
