THE FLATWORMS 



187 



capes through one end of the eggshell and swims about. This 

 larva is called a miracidium. It swims about until it encounters 

 a certain fresh-water snail, but if no snail is found within eight 

 hours, the larva dies. 



When a snail is reached, the larva bores its way into the soft 

 parts of the body. Here in about two weeks it changes into a 



-c 



Fig. 108. — Stages in the life-history of the liver fluke. 



a, miracidium (ciliated embryo) ; b, sporocyst containing rediae (R) ; c, a 

 redia ; C, cercaria; D, gut; K, germ-cells; R, redia ; d, cercaria. (From 

 Sedgwick.) 



saclike sporocyst (Fig. 108, b). Each germ cell within the 

 sporocyst develops into a second kind of larva, called a redia 

 (Fig. 108, b, R ; c). The redias soon break through the wall of 

 the sporocyst and by means of germ cells (Fig. 108, c, K) usu- 

 ally give rise to one or more generations of daughter rediae (Fig. 

 108, c, R), after which they produce a third kind of larva known 

 as a cercaria (Fig. 108, c, C). The cercariae (Fig. 108, d) leave 

 the body of the snail, swim about in the water for a time, and 



