62 
ZOOLOGY. 
euten by tlie hog are developed m its body into the larval 
tape-worm (called in this species Cysticerciis celluloscB, 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 
^^'^cul'!''Z?^i^rZ\ ^^^^ 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 {Round- 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 ((?or^7m5 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. 
