A group of parasitic flatworms or flukes constituting a subclass or order of the class Trematoda, in the phylum Platyhelminthes. The name Digenea refers to the two types of generations in the life cycle: (1) the germinal sacs which parasitize the intermediate host (a mollusk or rarely an annelid) and reproduce asexually; and (2) the adult which is primarily endoparasitic in vertebrates and reproduces sexually. The adult usually is hermaphroditic, but many of the blood flukes and a few others are dioecious. Vertebrates of all classes, except the Cyclostomata, serve as definitive hosts. Those feeding on aquatic plants and animals harbor the greatest variety of digenetic trematodes, but several species occur in strictly terrestrial hosts. Effects on the vertebrate vary from no apparent harm to severe and even fatal injury. Control of flukes is by preventive measures, including interruption of their life cycles and treatment with drugs.