underwater as soon as the tide recedes to avoid desiccation. Once the snails are eaten by herring gulls, the metacercariae hatch into mature flukes. These mate and produce eggs which are shed with the bird's feces. Their adult lifespan is a mere two weeks; several generations are thus produced each summer. It is unknown whether M. piriformes spends the winter as resting eggs in the habitat, or as sporocysts in already-infected periwinkles. In any case, snails harboring metacercariae generally die of desiccation before winter if they are not eaten. See also Other parasites that modify intermediate host behavior to facilitate transmission: Dicrocoelium dendriticum, a fluke Corynosoma constrictum, a thorny-headed worm Gynaecotyla adunca, a fluke Hymenolepis diminuta, a tapeworm Polymorphus paradoxus, a thorny-headed worm Footnotes References (2000): A transmissible trematode affects the direction and rhythm of movement in a marine gastropod. Animal Behaviour 59(6): 1161–1166. (HTML abstract) Molluscs described in 1983 Parasites of molluscs Parasites of birds Plagiorchiida