LIFE-HISTORY 35 



C. The Third Form is a cercaria (fig. 12). These are not 

 necessarily the third generation, for several generations of 

 rediae 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. A single redia may contain 

 about twenty such cercariae at one time. 



2. The ripe cercariae, 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 cercarias are swallowed 



by sheep with the grass. They then again become 

 active, escaping from their cysts, and working their 

 way along the bile-ducts to the liver, where they 

 grow rapidly, and develop in about six weeks into 

 sexually mature flukes. 



D 2 



