44 THE LIVER-FLUKE. 



From the inner surface of the body-wall of the 

 redia, cells arise which develop into gastrulse as 

 in the sporocyst : these may become redise like the 

 parent, or may develop into cercarise. Both rediae 

 and cercarise may be formed in the same redia. 



C. The Third Form is a Cercaria (fig. 21). These are not 

 necessarily the third generation, for several genera- 

 tions of rediee may intervene between them and 

 the sporocyst. 



1. Within the parent redia the embryo develops a long tail 



near its hinder end, an anterior sucker round the 

 mouth, and a posterior sucker on the ventral surface. 

 Its alimentary tract, which is at first solid, becomes 

 bifurcated to form the two limbs of the intestine, 

 the portion in front of the bifurcation forming a 

 pharynx and a short oesophagus. The rudiments 

 of the genital organs can be recognised. A single 

 redia may contain about twenty such cercarise at 

 one time. 



2. The ripe cercarise, which measure nearly 1 mm. in 



length, including the tail, escape from the redia by 

 an aperture just behind the collar. At first they 

 are very active, and work their way out of the snail. 

 As this snail, Limncea truncatula, is amphibious, they 

 may be set free either in water or on damp grass. 

 In either case they shortly lose their tails, and 

 encyst on grass or some other plant. 



3. In the encysted condition the cercarise are swallowed 



by sheep with the grass. They then again become 

 active, escaping from their cysts, and working their 

 way along the bile-ducts, where they grow rapidly, 

 and develop in about six weeks into sexually mature 

 flukes. 



