TREMATODA 197 



emphasize the fact that the stages in the Hfe history of all parasites are 

 ultimately connected with environmental conditions. The egg of 

 Fasciola hepatica does not hatch unless the pH of the water in which 

 it is deposited is below 7-5, the optimum point apparently being 

 about pH 6-5. If the eggs are kept in water more alkaline than 

 pY{ 7-5 the embryo remains within the shell and eventually dies. 



The identification of a cercaria with an adult is a task which requires 

 great patience, and many cercaria are known which have not been as 



---glory. 



Fig- 155* Bucephalus larva (cercaria) of Gasterostomum fimbriatum. After 

 Benham. f.tail, forked tail; ^/.or^. glandular organ ; M. mouth ; /)Aar. pharynx. 



yet connected with an adult. Almost any mollusc, if dissected care- 

 fully under a hand lens, will provide specimens of rediae and cercariae, 

 although infected specimens may be more common in some localities 

 than in others. The tail of a cercaria is often an elaborate structure. 

 Some have rings and chitinous stiffenings, while the well-known 

 Bucephalus larva of Gasterostomum is a cercaria with a forked tail 

 (Fig. 155)- 



Class CESTODA 



The Cestoda may be defined as endoparasitic Platyhelminthes in 

 which the enteron is absent and the ciliated ectoderm has, in the adult, 

 been replaced by a thick cuticle. In the parenchyma lime cells occur 

 (see Fig. 156). Proglottides are usually formed. 



