DISEASES OE THE OX AND SHEEP. 459 



sporocyst, which in its turn gives rise to about ten redise. These 

 redise move about within the snail, and thereby cause so much 

 disturbance that but very few snails live for many weeks after 

 they have become infested. The next occurrence is that germs 

 called cercarisB, tadpole-like little animals possessed of a flat oval 

 body and a long slender tail, are formed within. Each cercaria 

 is possessed of two suckers corresponding to those of the adult 

 fluke. These cercarise escape through a special opening, and 

 then wriggle out of the snail, and swim about in the water, until 



V 



Fig. 59. — The Shell of Limn^us Truncatulus. 

 A magnified, B a little larger than the natural size. 



they come into contact with some plant or other solid substance. 

 Then each comes to rest, attaching itself perhaps to a blade of 

 grass or other similar substance, drawing up its body into the 

 form of a round ball, exuding a gummy substance containing 

 many granules, and throwing off" its tail. The gummy substance 

 hardens to form a snowy-white envelope or cyst, which is attached 

 to the grass or other object near at hand. Unless it is then 

 swallowed by a sheep or some other animal which is capable of 

 becoming a host for the adult fluke, or even by a human being, 



