60 
ZOOLOGY. 
ing ciliated, while there is a large sucker on the under side 
of the body. These are called fluke-worms. Fasciola he- 
patica (Fig. 52) lives in the liver of the sheep, causing the 
disease known as rot." It is most abundant in the spring, 
several hundred occurring in the liver of a single sheep. 
At this time it passes into the intestine, and thence is car- 
ried out with the excrement. The eggs or flukes in many 
cases drop into pools, ditches, or ponds; 
here the ciliated young is liberated, and 
soon the ciliae are absorbed, when it be- 
comes inert, and probably soon afterward 
enters the body of a pond snail {LimncBus), 
where it transforms into a large sac, and 
develops new larvae in its interior. This 
sac-like larva is called a " nurse," or, when 
more highly developed, a " redia,'^^ The 
progeny of the redia is termed a *^cerca- 
ria." The cercariae are tadpole-like, and 
are restless, migrating from the bodies of 
their snail-host, and have been known in a 
few instances to penetrate the skin' of hu- 
man beings. They are probably more usu- 
ally swallowed by sheep and cattle while 
¥10.52.— Fascioiahe- drinking or grazing, when snail-shells may 
a, branched iiH;es- be accidentally swallowed. From the di- 
gestive canal of sheep, etc., the cercaria 
penetrates into the liver, where it probably loses its tail 
and becomes encysted, after many weeks or even months 
becoming a mature fluke-worm. From the liver it passes 
out through the liver-ducts into the intestine, and is finally 
expelled, thus completing its cycle of life. 
The tape-worms represent the order of Cestodes. They 
are large parasitic worms, with no mouth or digestive tract; 
the joints are very numerous, sometimes over a thousand 
\i number. 
The Qommon human tape-worm, Tcenia solium^ varies 
