Liver-flukes (Distomidfe), a group of parasitic worms, the best known of which causes "rot" in sheep. The life-history is very complex, as the animal passes different periods of its'life in different hosts. The adult form inhabits the bile ducts of various domestic animals, most commonly the sheep. The eggs produced by the adults pass out of the body of the host, and when they happen to get into a pool of fresh water give rise to a small free-swimming embryo, which makes its way into the body of a pond-snail (Lymiueus). There the embryo becomes a sac-like body (sporocyst), which gives rise within itself to numerous Redise, a form differing much from the adult. These Redias may produce other generations like themselves, or may give rise to the next stage, the Cercaria. This is more like the adult, as it already possesses suckers, but differs from it in the presence of a powerful swimming tail, by means of which, having now bored its way out of the snail, it moves rapidly about in the water. After a time it creeps up a blade of grass and forms a kind of little cocoon; this being eaten by a sheep, the covering is digested, and the freed larva bores its' way into the animals liver and there becomes sexually mature.