35 



Descriptive Zoology. 



wall of the intestine into the muscles or other tissues. 

 Here it becomes flask-shaped (bladder worm) and develops 

 a head with hooks and suckers ; but in this condition it 

 must remain unless eaten by some other animal. If the 

 flesh be cooked, the bladder worm (cysticercus) will be 

 killed. The danger comes in eating raw or half-raw meat. 

 If taken into the intestine of another animal, it attaches 

 by hooks, or suckers, or both, and the body elongates by 

 the formation of segments as above noted. 



Kinds of Tapeworms. Man is infested by at least 

 three different kinds of tapeworms, one obtained from 

 pork, another from beef, and a third from fish; but the 

 latter kind is seldom, if ever, found in this country. 



Various animals have their own kinds of tapeworms, 

 usually getting them from a certain kind of animal on 

 which they prey ; thus the dog has tapeworms which 

 have passed their larval stage in the muscles of the rabbit. 

 The cat gets a tapeworm from the mouse. 



The Liver Fluke. This is another of the flatworms. It 

 is a parasite in the liver of the sheep, living in the larger 

 bile ducts. The eggs develop into embryos which escape 



with the excrement. 

 Then they pass into the 

 bodies of snails. Here 

 they reach the larval 

 stage. After leaving the 

 snail, they attach them- 

 selves to damp grass 

 under water. If eaten 



by sheep, they become fully developed worms like the adult 

 from whose eggs they developed. In England it was esti- 

 mated that 3,000,000 sheep were killed by the liver fluke 

 1880, and that the average yearly loss is 1,000,000. 



FIG. 197. LIVER FLUKE. 



A trematode worm. 



