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 

 compHcated 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.6s2. Azygia sebago. Dorsal view. X 16. 

 larva, the miracidium, evidently ^.^male reproductive system in dorsal aspect. 

 ' ' -' Semi-diagrammatic to snow relation of organs 



p ovarian complex, ovarv' drawn in outline only. 



acetabulum; exb, e.x- 



intes- 



adapted by its ciliated covering to JJ«Jf; ^^^^-^^^^ 



a tree existence. Sooner or later the tin^; u, Laurer's canai- oo, ootype-, ot. ovar>-; 



, 1 i-l- T- 11 5g, shell gland; to, anterior testis; /p, posterior 



egg arrives m water where the shell testis; «/, uterus; t/.vitciiaria; w.yoikduct; 



, , , . . yr, yolk reser\-oir. (Original.) 



opens and the larva escaping 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 and often a 

 boring apparatus at the anterior end. These structures are lost 

 in the metamorphosis into a sporocyst, a stage so simply con- 

 structed that the young rediae escape by the rupture of the wall. 

 A redia is characterized by the presence of a rhabdocoel intestine 



