Miracidium and cercary of
Source: Bielefeld University.
Flatworms (Plathelminthes) develop in a change of generations during which they changes hosts several times. From the egg the miracidia hatch, a simple larval stage that is ciliated and needs a host in which it is able to reproduce asexually and to change into the next generation, the cercariae.
The cercariae usually have already an outer appearance similar to the adult
stage of the worm and mainly represent a distribution stage that, after leaving
the host, waits to infect a suitable final host either as an endurance stage (cyst)
or as movable stage.
In the final host, mostly a mammal or another warm blooded animal, the development from the cercaria to the adult takes place. Afterwards sexual reproduction takes place and eggs are laid. The eggs are then excreted by the host and, the host moving, are distributed that way.
Among the parasites using snails as intermediate host, is for example the large (Fasciola hepatica) and the small liver fluke (Dicrocoelium lanceolatum), the pair fluke (Schistosoma mansoni, causing the bilharziosis illness), as well as Leucochloridium paradoxum afflicting amber snails.