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.