THE ROUND WORMS AND FLAT WORMS 



125 



in the liver, which is their usual abode, the host, whether a 

 sheep or a man, has a very uncomfortable time. The life 

 history of the liver fluke requires two hosts in addition to a 

 period of life in the water. The eggs are passed out of the 

 intestine and if they gain access to water they hatch into 

 ciliated, free-swimming embryos. Should an embryo, 

 in the course of its wanderings, encounter a water snail 



FIG. 104. Developmental stages of the liver fluke Fasciola hepatica. 

 a, free-swimming larva which develops in the body of a snail into a 

 sporocyst b. The latter produces internally other larval forms, the 

 rediae R. c, a redia which contains still other rediae R and a cercaria, C 

 or final larval form, d, a cercaria. The cercarise escape from the 

 snail and swim freely in the water. (After Leuckart.) 



it enters the body and there undergoes further develop- 

 ment. The larval form so produced may give rise to 

 numerous others by a process of parthenogenetic repro- 

 duction. Finally these larvae leave the snail, swim about 

 in the water, and frequently attach themselves to grass 

 or weeds near the water's edge. Here they encyst. If 

 now a sheep comes along, eats the grass that harbors the 

 encysted larva, the latter develops into the mature form 



