308 SOME COMMON INTERNAL PAKASITES [CH. IX 



behind a circular thickening of muscle known as the 

 collar. If the temperature conditions be favourable several 

 generations of redise are produced. Eventually, in the 

 autumn, all the rediae give birth to other forms known as 

 the cercarice. Of these each redia produces from fifteen to 

 twenty. A cercaria is about O28 mm. long, and is rather 

 like a tadpole in outline. The anterior part is flat and 

 heart-shaped, and possesses two suckers, a mouth, pharynx 

 and forked digestive tract ; the posterior portion, which is 

 about double the length of the anterior, consists of a very 

 muscular and contractile tail. In this stage the parasite 

 makes its way out of the body of the snail and swims in 

 the water or wriggles along among the leaves of plants. 

 Soon it comes to rest upon a blade of grass or other similar 

 object, shakes off its tail, and secretes over itself a white 

 calcareous case. In this condition the cercaria can remain, 

 encysted upon the herbage, for some time. It develops 

 no further except it pass into the stomach of a sheep or 

 some other herbivore. Here the wall of the cyst is 

 dissolved by the gastric juice, and the liberated cercaria 

 makes its way up the bile-duct into the liver where it 

 rapidly grows and becomes sexually mature in about six 

 weeks. Within the liver they can certainly live for three 

 years and probably longer. 



It is to be noted that L. truncatula, though an aquatic 

 snail, frequently leaves the water and is capable of with- 

 standing considerable drought. Hence the infection may 

 linger in a pasture for a surprisingly long time. The fact 

 that sheep suffer from " rot " far more than cows is due to 

 the encystment of the cercaria occurring upon the lower 



