Trematodes

Trematodes, a class of flat-worms whose members are parasitic in or on a great variety of animals. The body is unsegmented, leaf-like, or more or less cylindrical, and provided with adhesive suckers. In one set, mostly ceteroparasitic, the embryo develops directly into the adult sexual forms; of this 'monogenetic' life-history Polystromum (e.g. in the frog), Deplozoon (on minnows), Gyrodactylus (on various fresh-water fishes), Calicotyle (on rays), Aspidogaster (in the fresh-water mussel) are illustrative types. In another set, mostly endoparasitic in vertebrates, the embryo gives rise to one or more generations of non-sexual forms; of this 'digenetic' life-history the liver-fluke (Distomum), Monostomum, and Bilharzia are illustrative. All are hermaphrodite except Bilharzia, and self-impregnation is common. See FLUKE, BILHARZIA, &c.