Green-banded Broodsac (Leucochloridium paradoxum) – A species of parasitic flatworm that is found in Europe and North America. L.paradoxum begins its life style inside the digestive system of a snail. It will then develop inside the snail (Usually S.putris) and take over its body and place its ‘broodsac’ inside the snail’s tentacle (usually the the left). Then it will ‘drive’ its host to an open area and put on a display in order to attract birds. Once inside the bird the flatworm will lay its eggs which will be passed when the bird defecates. A snail will then find and eat the excrement and the life cycle will repeat itself.
In fact you can clearly see the green parasite inside the snail’s body in this picture..