THE WORM-LIKE ANIMALS 343 



Tape-worms are troublesome parasites in that they inter- 

 fere with the nutrition of the animals which they inhabit. 

 They are difficult to remove, because the head is so firmly 

 attached to the lining of the intestine. All the segments 

 might be dislodged by powerful drugs, but if the head re- 

 mained, it might continue to grow new segments. 



The bladder-worms embedded in human tissues cause an 

 uncommon disease known as hydatids. 



Prevention of tape- worms is a simple matter; namely, eat 

 no meat " cooked very rare." In the case of pork there is an 

 additional reason for this rule, in that the far more dangerous 

 parasite Trichina ( 297) may be present. With increasing 

 attention to sanitation and the disposal of sewage by bacterial 

 methods described in 258, /, there will be less chance of tape- 

 worm embryos getting into pigs and cattle and then indirectly 

 entering human beings. The rarity of human tape-worms 

 in United States is probably in part due to the fact that our 

 farms are more sanitary, and being larger than those of Europe 

 farm animals live farther away from human dwellings. 



296. Other Parasitic Flat Worms. One of the best 

 known flat worms whose habits of life resemble those of the 

 tape-worm is the liver-fluke, which lives in the bile-ducts of 

 sheep. It is a flat worm about one inch long and one fourth 

 inch broad, with two suckers for attachment. The eggs 

 escape into the sheep's intestine through the bile-duct, and 

 after being discharged from the intestine, an egg develops 

 into a larva covered with cilia. This larva swims, and if it 

 reaches a pond-snail, bores into it and becomes an elongated 

 sac. Inside this sac are formed many new larvae, and inside 

 each of these larvae are formed many more. Thus one larva 

 entering a snail produces a large number of larvae of different 

 forms; and each one of these may leave the snail, become 

 encysted on grass, and when eaten by a sheep will enter the 

 bile-duct and develop into a liver-fluke. When once a damp 

 pasture along a given stream has become infested with larvae 



