THE LIVER-FLUKE OF SHEEP. 



745 



from the time the encysted cercaria is swallowed by the sheep, before 

 the fluke is fully grown and begins to lay eggs. The flukes do not 

 always pass from the sheep in summer-time, as was once thought by 

 leading veterinarians. There is no time of year when sheep-livers 



Fig. 10. 



Fig. 11. 



containing flukes can not be obtained. Cases are known where the 

 flukes have been known to live for more than a year. 



The summary of the life-history of the liver-fluke is given as fol- 

 lows : " The adult fluke in the liver of the sheep produces enormous 

 numbers of eggs, which are distributed with the droppings of the 

 sheep. If these eggs have moisture and a suitable degree of warmth, 

 they continue to live, and in each is formed an embryo. The embryo 

 leaves the egg and swims in search of the particular snail (Zimnceus 

 truncatulus) within which its future life and growth take place. The 

 embryo bores into the snail, and then grows into the form which is 

 called a sporocyst. The sporocyst gives rise to the second generation. 

 This generation is known as the redia. The rediaa in turn produce the 

 third generation, which has the form of a tadpole, and is called cer- 

 caria. The cercaria? quit the snail and inclose themselves in envelopes 

 or cysts, which are attached to the grass. "When the grass to which 

 the cysts adhere is eaten by sheep or other suitable hosts, the young 

 liver-fluke comes out of the cyst and takes up its abode in the liver of 

 its host, and the fatal circle is thus completed." 



The fluke-disease alternates between a kind of snail and the sheep. 

 One sheep can not contract the "rot" directly from another member 

 of the flock, and one snail can not take it from its neighbor. The 

 sheep drop the eggs, and hatching, the embryos, find the snails, and 



