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Trematodes are flattened oval or worm-like animals, usually no more than a few centimetres in length, although species as small as 1 millimetre (0.039 in) (Monogenea) and as large as 7 centimetres (2.8 in) (Fasciolopsis) are known. species that exemplifies the remarkable life history of the trematodes is the bird fluke, Leucochloridium paradoxum. The definitive hosts, in which the parasite reproduces, are various woodland birds, while the hosts in which the parasite multiplies (intermediate host) are various species of snail.
Most trematodes have complex life cycles, with larval stages parasitizing one or more species that are different from host of adults. Larval stages of some medically important species include miracidium, redia, cercaria, and metacercaria.Most trematodes are endoparasites.They include several parasites that have an enormous impact on human populations, such as human liver flukes and the blood flukes that cause schistosomiasis.arval stages of some medically important species include miracidium, redia, cercaria, and metacercaria. Most trematodes are endoparasites. They include several parasites that have an enormous impact on human populations, such as human liver flukes and the blood flukes that cause schistosomiasis.