fluke, parasitic flatworm of the trematoda class, related to the tapeworm. Instead of the cilia, external sense organs, and epidermis of the free-living flatworms, adult flukes have sucking disks with which they cling to their hosts and an external cuticle that resists digestion by the host. In most species the adult worms absorb nutriment through the digestive system; in a few whose digestive system is reduced or completely absent, food is absorbed through the cuticle. Adult flukes are commonly hermaphroditic, that is, each individual possesses both male and female reproductive organs; however, they reproduce sexually. One worm may produce over 500,000 embryos. Species of the order Monogenea are external parasites on the skin and gills of fish; their simple life cycle is completed in a single host. The order Digenea includes the internal parasites, many of which have complicated life cycles, the various asexual stages living in mollusks and the sexual stages invading the internal organs of vertebrates; more than 35 species are known to inhabit humans. The human liver fluke, Clonorchis sinensis, has a life cycle that requires two intermediate hosts, snails and fish. The eggs pass out of humans via the feces. They survive if they are deposited in water and eaten by snails. The larvae invade the soft tissues of the snail from the digestive tract where they pass through several stages and reproduce asexually; they emerge from the snail as free-swimming larvae. If they manage to encounter fish, they penetrate into the flesh and encyst; if the raw fish is eaten by humans, the young flukes are released in the intestines. They then crawl up the bile duct, attach by their suckers, mature, reproduce sexually, and begin to shed eggs. In addition to the infestation of the liver by Clonorchis, which is prevalent in East Asia, many other disorders are caused by flukes. The Asian and African blood fluke disease, schistosomiasis, is caused by adults of the genus Schistosoma that burrow into the skin of humans and animals and lodge in the blood vessels. Lung flukes, common in East Asia, infest uncooked crab meat and encapsulate as adults in the lungs of humans. Liver rot, fatal to sheep and other herbivorous animals, is caused by a liver fluke, Fasciola hepatica, whose larvae encyst in grasses after leaving the snail host. A species of fluke prevalent in lakes of the N central United States causes a rash called "swimmer's itch." The name fluke is also applied to species of flatfish. Flukes are classified in the phylum Platyhelminthes, class Trematoda.