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.  [1913 Webster]