LIFE HISTOEY. 37 



near the anterior end, and with a pair of blunt pro- 

 cesses projecting from the hinder part of the ventral 

 surface, which aid in locomotion. The body-wall 

 resembles that of the sporocyst in structure, but is 

 more muscular, and has definite excretory canals 

 which commence as funnels with ' flame-shaped ' 

 bunches of cilia. 



The alimentary tract is a comparatively short 

 blind sac, with walls one cell thick : the mouth is 

 at the anterior end, and behind it the wall of the sac 

 is thickened to form a strong muscular pharynx. 



From the inner surface of the body-wall of the 

 redia, cells are budded off which develop into gastrulse 

 as ia the sporocyst : these may become rediae like 

 the parent, or may develop into cercarise. Both 

 redias and cercarise may be formed in the same 

 redia. 



C. The Third Form is a Cercaria (fig. 19). These are not 

 necessarily the third generation, for several generations of 

 redire 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 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, Limncsa 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. 



