an extensive order of parasitic worms. they are found in the internal cavities of animals belonging to all classes. many species are found, also, on the gills and skin of fishes. a few species are parasitic on man, and some, of which the fluke is the most important, are injurious parasites of domestic animals. the trematodes usually have a flattened body covered with a chitinous skin, and are furnished with two or more suckers for adhesion. most of the species are hermaphrodite. called also trematoda, and trematoidea. see fluke, tristoma, and cercaria.