PLATYHELMINTHES 163 



cysts, upon reaching the stomach, are dissolved, setting free the par- 

 asites which, passing to the intestine, enter the bile ducts and there 

 become mature. After laying their eggs the majority of the flukes pass 

 down the bile ducts to the mtestine where, under the hifluence of the 

 digestive juices, they shrivel and die. 



The period of time occupied ])y the entire cycle is so influenced by 

 climatic conditions that no definite estimate as to it can be given. 

 As a rough approximation, twelve weeks may be given as about the 

 time required under such favorable conditions as usually prevail during 

 the summer season. 



The life histories of Dicrocoelium lanceatum and Fasciola magna are 

 probably essentially similar to that of F. hepatica, but are as yet not 

 definitelv known. 



Tabular Review^ of Life History of Fasciola Hepatica 

 Adult Fluke. — In bile duct of liver of rmninant. 



I 

 Egg.— Free. 



I . 

 ^liracidium. — Free. 



I 

 Sporocyst. — In puhnonarj' chamber of snail. 



I ^1 



Sporocysts 5 to 8 Redise. 



[ I 

 Redise. — In liver or other organ of snail. 



Daughter Rediae 15 to 20 Cercarise 



Cercarise. — Free. 



I 

 Cvsts. — Upon grass stalks or other vegetation. 



Adult Flukes. — In bile duct of Uver of ruminants after 

 ingestion of c^'sts. 



Prevalence. — The loss from hepatic fascioliasis in England was for 

 a time in the neighborhood of three million head of sheep annually. 

 It was principally for this reason that investigations were made by 

 which the life history of the parasite was determined, and by which 

 was revealed the essential alternation between the sheep and snail 

 host. This pointed the way for measures of control consisting mainly 

 in the elimination of snails by the drainage of pastures or in the limiting 



