THE PLANARIANS, FLUKES, AND TAPEWORMS 263 



-Mouth 



Ventral sucker 



Eggs 



— Ovary 



been the sheep liver fluke {Fasci'ola hepat'ica). This animal 

 (Fig. 137) lives in the liver and bile ducts of sheep, pro- 

 ducing a disease known as liver rot. This disease is not 

 uniformly present over the 

 country because of certain 

 peculiarities in the life his- 

 tory of the worm. It was early 

 noted that sheep grazing on 

 lowlands were apt to contract 

 this disease, while those kept 

 in upland pastures away from 

 water were free from attack. 

 This is due to the fact that 

 the parasite cannot be passed 

 directly from one sheep to 

 another. The adult trema- 

 tode produces eggs which are 

 carried from the body of the 

 sheep along with the drop- 

 pings. From each egg a mi- 

 croscopic larva is hatched. 

 This larva dies unless it finds 

 a certain kind of snail into 

 whose body it can bore its 

 way and carry on the next 

 stage of its development. 

 After two additional larval 

 generations in the body of 

 the snail, great numbers of minute tailed larvae are pro- 

 duced and finally liberated into the water. These minute 

 worms, after a period of free-swimming life, come to rest on 

 blades of grass and other vegetation. Each forms a pro- 

 tective shell about itself and becomes an inactive cvst. If 

 such a cyst is eaten by a grazing sheep, the young fluke is 



Testes 



-Intestine 



Fig. 137. Photograph of a sheep 



liver fluke stained for microscopic 



study. (x8) 



