A class of the Platyhelminthes which are ectoparasites of the gills, skin, and orifices of fishes and, less frequently, of the esophageal tracts and bladders of amphibians and turtles. They have conspicuous anterior and posterior holdfasts, the latter usually armed. The terminal genitalia are frequently sclerotized. The group is characterized by sexual reproduction, direct development, and a single host in the life cycle. Egg capsules have terminal filaments.