Snails as parasite hosts

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 hosts in which it is able to reproduce asexually and to change into the next generation, the cercaries.


Miracidium and cercary of Schistosoma mansoni. Source: Bielefeld University.

The cercaries 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 enduration stage (cyste) or as movable stage.
In the final host, mostly a mammal or another warm blooded animal, the development from the cercary to the adult takes place. Afterwards there is sexual reproduction 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.

Fasciola hepatica

Dicrocoelium lanceolatum

Schistosoma mansoni

Leucochloridium paradoxum.