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Chapter III. 



THE LIVEE-FLUKE OP THE SHEEP. 



Fasciola (Distomum) hepatica. 



The adult liver-fluke is a flat unsegmented worm, about an 

 inch and a half long, living m the bile-ducts of certain 

 domestic animals, notably in those of the sheep, ia which it 

 gives rise to the destructive disease known as liver-rot. It 

 may occur in man. 



The animal is hermaphrodite, and its eggs, which have 

 thick chitinous shells, are deposited in enormous numbers 

 in the bile-ducts of the sheep or other host, from which 

 they pass into the alimentary canal, ultimately escaping 

 with the fffices. From these eggs, if deposited in damp 

 places or in water, embryos are produced which lead a free 

 existence for a short time, but very soon become parasitic 

 within water- snails. Two or more asexually produced 

 generations now succeed, the last of which, leaving the water, 

 encyst on leaves of grass. In this condition they are swal- 

 lowed by sheep, within whose bile-ducts they become the 

 adult sexually mature form. 



This alternation of sexual and asexual generations, living 

 parasitically within different hosts, is a very characteristic 

 feature in the life history of the typical parasitic worms. 



I. THE MATUEE LIVEE-FLUKE. 



Slit open the bile-ducts in the liver of an infected sheep, 

 and transfer the living flukes to a dish of salt-solution ('TS 

 per cent.) to clean them. 



§ 1. Inject the excretory system of one fluke, and the 



