Digenea

A large group of flukes (parasitic flatworms) within the class Trematoda, with some thousands of described and probably many thousands of not yet described species. The Digenea range from about 0.2 mm (0.008 in.) to over 1 m (3.3 ft) in length. They have an indirect life cycle, with at least one intermediate host and a final host. In contrast to the Aspidogastrea, larvae multiply in the first intermediate host, final hosts include all groups of marine, freshwater, and terrestrial vertebrates; first intermediate hosts are mollusks (snails and bivalves) and exceptionally an annelid; second intermediate hosts include all groups of vertebrates, mollusks, and other invertebrates. Most species infect the digestive tract, but many occur in the liver, lungs, blood system, and body cavity. They also are found under the scales of fish and in the nasal passages of marine mammals. Many species have great importance as agents of disease in humans, livestock, fowl, and marine and freshwater fish.