NEREIS AND ITS ALLIES 



155 



FIG. 144. Showing abnormal forms 

 resulting from mutilation of Pla- 

 narians. After Van Duyne. 



encysted stage the worm is 

 known as a "sporocyst," 

 because it is full of germs 

 (spores) of a new genera- 

 tion. 1 The spores develop 

 in the snail into curious 

 organisms, a sort of second- 

 ary larvae known as redia. 2 

 The redise may produce, by 

 a kind of internal budding, 

 new redise, and so on re- 

 peatedly, until at last, on 

 the death of the snail, or 

 from some other cause, the 

 last generation of rediae 

 produces liver-flukes. 3 The 

 young liver-flukes wriggle out of the snail, attach them- 

 selves to damp grass, lose their tails, and encyst them- 

 selves. If these cysts be eaten by a 

 sheep, they develop in the sheep's 

 body into an adult liver-fluke (Fig. 

 145). Thus the stages which we can 

 recognize in the liver-fluke are : 



First generation : egg from liver-fluke, larva, 



and adult sporocyst. 

 Second generation : redia (this may be several 



times repeated). 

 Third generation : " cercaria " larva, encysted 



larva, and adult liver-fluke. 



1 Fig. 146, B. 2 Fig. 146, C. 



3 These, while young, have tails, and are called 

 "cercaria. 



FIG. 145. Distomum, 

 the liver-fluke. Nat. 

 size. Excr., excretory 

 pore; mo., mouth; repr., 

 reproductive aperture ; 

 sckr., posterior sucker. 

 From Parker and Has- 

 well. 



" 



