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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.
Trematoda is a class within the phylum Platyhelminthes. It includes two groups of parasitic flatworms, known as flukes. They are internal parasites of molluscs and vertebrates. Most trematodes have a complex life cycle with at least two hosts. The primary host, where the flukes sexually reproduce, is a vertebrate. The intermediate host, in which asexual reproduction occurs, is usually a snail.