PARASITIC FLATWORMS 



371 



obscures all other structures in the body. The eggs are covered 

 with a firm chitinous shell which is often opaque but in other cases 

 is transparent enough to permit one 

 to follow the gradual development 

 of the enclosed embryo. 



The development of most ecto- 

 parasitic trematodes is simple and 

 not different from that of free-living 

 flatworms. There emerges from the 

 egg-shell in due time a ciliated larva 

 which swims about in the water until 

 it finds a new host to which it 

 attaches itself. In endoparasitic 

 trematodes the life cycle is more 

 complicated in all cases and ex- 

 tremely involved in some. Only a 

 general outline of conditions can be 

 given here. 



The eggs of the fluke reach the 

 external world in the feces or dis- 

 charges from the host. Within the 

 egg-shell is developed a minute 



, Fig. 652. Azygiasebago. Dorsal view. X 16. 



larva, the miracidium, evidentlv "^•'^.'"'i'^'^p'■°'^''?''^'^y^'^"''°^°^^^''^P'^'^'• 

 '' Semi-diagrammatic to siiow relation of organs 



adapted by its ciUated covering to HigUy mSSfilfd'T'^aceTabuw''i"r"ex: 

 a free existence. Sooner or later the « SH '^^nSSiS^JoS 

 egg arrives in water where the shell &^^,t^£;T^S^^d!'^'^ 



1,11 . • yr, yolk reservoir. (Original.) 



opens and the larva escapmg swims 



about in search of a new host. The latter is not the species which 

 shelters the adult but an intermediate host which for almost all flukes 

 is a mollusk, in the tissues of which the miracidium changes to an 

 irregular sac (sporocyst); this produces within itself a new gen- 

 eration (redia) which also in this host produces a third generation 

 (cercaria). The miracidium possesses an eye-spot (not always 

 pigmented) and often a boring apparatus at the anterior end. 

 These structures are lost in the metamorphosis into a sporocyst, 

 a stage so simply constructed that the young rediae escape by the 

 rupture of the wall. A redia is characterized by the presence of a 



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