The bird fluke, Leucochloridium paradoxum, is a Trematode.
The host in which the parasite multiplies will be a bird.
The intermediate host (seen here) in which the parasite grows is a snail.
The adult parasite in the bird's gut produces eggs which end up on the ground in the bird's faeces.
An egg gets swallowed by a snail and hatches into a miracidium - a tiny, transparent larva.
The larva grows and change into sporocyst with a sac-like appearance.
This has a central body in the snail's digestive tract extending into a brood sac in its head, muscular foot and eye-stalks.
The parasite changes colour and pulsates to attract a predatory bird, and the snail seems to become suicidally fond of exposed positions on tall grasses or reeds.
See my snail movie
See also Fauna of Trematodes

See also Snail's Eye View