50 



ZOOLOG 7. 



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 cercarise 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 

 Fm.52.— Fasciola he- drinking or grazing, when snail-shells may 

 a^ranchecThftes- 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 verj' numerous, sometimes over a thousand 



% n number. 



The common human tape-worm, Tamia. solium-, varies 



