Development in Intermediary. 



The reproduction of digenetic trematodes takes 

 Hetekogony. the form known as heterogenesis or heterogonj', of 

 which the chief features are : («) an alternation in 

 the modes of reproduction, an asexual phase alternating with a 

 sexual phase ; (b) the asexual individuals differ in shape and in 

 internal structure from the sexual forms ; (c) the two kinds of 

 reproducing individuals live under conditions which are more 

 or less radically different. All digenetic trematodes have their 

 asexual phase in the mollusca, and for the most part in the 

 gastropoda. The sexual phase is attained in a vertebrate. The 

 asexual phase has one or more generations. The egg on hatch- 

 ing gives rise to a ciliated larva, the " miracidium," which dies 

 in about twenty-four hours unless it has been able to harbour 

 in a suitable mollusc. Within this mollusc the miracidium, 



HOST Transition INTERMEDIATE HOST Transition HOST 



/SPOROCYST 



ENCYSTED 



SPOROCYST— DAUGHTER-SPOROCYSTS- 



EGG - MIRACIDIUM - jg^^^oCYST-REDI^E ' "''<=*'''^-i 



\SPOROCYST-REDI>E-DAUGHTER.RE01/E-| FREE-SWIMMING' 



Fig. 40. — Life-cycle of a digenetic trematode. 



usually after migrating to the digestive gland or " liver," becomes 

 changed into a smooth-walled .sac called a sporocyst. Budding 

 from the wall of the sporocyst results in bodies which may be 

 of three kinds : (a) cercariae ; (b) daughter-sporocysts ; (c) redias. 

 The cercariae are typically immature adults provided with certain 

 larval structures to enable them the better to invade the definitive 

 host. They are the infective forms. 



The daughter-sporocysts are elongated sacs resembling the 

 mother-cysts, and are not provided with alimentary canal or 

 suckers. They migrate from the mother-cyst to other parts of 

 the digestive gland, and later give rise by budding to cercariae. 



Kediae are characterized by the presence of a single sucker and 

 a simple sac-like alimentary canal. These give rise by internal 



