52 



ZOOLOGY. 



eaten by the hog are developed in its body into the larval 

 tape-worm (called in this species Cysticercus celluloses, Fig. 

 54). The head with its suckers is formed, and the body 

 becomes flask-shaped; the Cysticerci then 

 bury themselves in the liver or the flesh of 

 pork, and are transferred living in uncooked 

 pork to the intestines of man. The body 

 now elongates and new joints arise behind 

 the head until the form of the tape-worm is 

 attained, as in Fig. 53. 



The hinder joints then become filled with 

 eggs and break off, becoming independent 

 joints comparable with the " parent-nurses" 

 of the Cercarias, except that they are not 

 contained in the body of the Taenia (as in 

 ^^^la^Ii the Cercaria), but are set free. The inde- 

 Tape-worm. pendent joint is called a ''proglottis." It 

 escapes from the alimentary tract of its human host, and 

 the eggs set free, in and about privies, are swallowed by 

 that unclean animal, the pig, and the cycle of generations 

 begins anew.* 



Class II. — Nematelminthes {Bound- or Thread-worms). 



General Characters of Round-worms. — In these worms the 

 body is round and thread-like, not being jointed. Many 

 are parasitic: such are the Ascarids. 



The round-worm most dangerous to human life is the 

 Trichina spiralis (Fig. 55). It is very minute, the female 

 being 3mm. in length, and the male worm half as long. The 

 female is capable of producing a thousand young. The 

 eggs are eaten by rats, dead rats are sometimes devoured by 

 pigs, and pork thus infested when eaten by man, either 

 raw or partly cooked, often causes the death of their hu- 

 man host. 



The hair-worms (Gordius aquaticus, Fig. 56) resemble a 

 piece of a horse's hair, and are so-called because they are 



* See Van Beneden's Animal Parasites and Messmates, 1876. 



