Truncatulus harbours the Cercaria of Fasciola hepatica, the liver-fluke, which causes rot in sheep. Ancylus, which occurs in rivers, has a minute limpet-like shell.
What determines the origin of the cercaria rather than a new generation of rediae is unknown.
The cercaria is just visible to the naked eye and has an oval or discoidal body and usually a long tail of variable form.
The cercaria swims freely for a time and either encysts directly on grass or weeds or it enters a second host which may be another mollusc, an insect, crustacean or fish, and then encysts.
The further development of the cercaria is dependent on the weed or animal in which it lies being eaten by the final host which is usually a predaceous fish or one of the higher vertebrates.
It escapes into the adjacent tissue and there gives rise either to one or more generations of rediae or at once to a new type of organism - the cercaria.
D, An adult redia, containing a daughter-redia, two almost mature cercariae, and germs. E, A free cercaria.
By a process of infolding, the thicker end is partially invaginated, the middle portion or " hind-body " and the organism may now present a superficial likeness to a cercaria.