PARASITISM 



173 



202. Life History of the Sheep Liver Fluke.— The egg cell is produced 

 in the ovary of the fluke and is passed into the oviduct where it is fertilized. 



Ora/ 



ce/Zs 



Yo/k „ 

 <re// B 



Fig. 85. — Diagram of the life history of a sheep liver fluke. A, the adult fluke, Fasciola 

 hepatica, in bile duct of sheep. B, egg, much enlarged (hatches in water). C, the mira- 

 cidium, enlarged (free swimming). D, Lymnaea (Galba) bulimoides, a snail, the inter- 

 mediate host. E, the sporocyst containing developing rediae, in liver of the snail. (C and 

 E from Leuckart, " Parasiten des Menschen.' ) F, rediae in liver of the snail; one con- 

 taining developing daughter rediae and the other developing cercariae. G, the cercaria which 

 emerges from the snail and is free swimming in the water. {From Leuckart.) H, encysted 

 cercaria on vegetation. 7, sheep, the final host, ingesting cysts on grass. (Figures arranged 

 by H. W. Manter.) 



It becomes enveloped by yolk cells derived from the yolk glands, and shell 

 forming material also received from the yolk glands is hardened into a shell 

 by a secretion from the shell gland (Fig. 82). The egg now passes to the 



