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Monogenea (phylum Platyhelminthes) A class (or in some classifications an order of the class Trematoda) of parasitic worms most of which have a gut with an intestine and pharynx. An anterior sucker may be present, flanking the mouth. Mature individuals lack cilia. Most are ectoparasitic; fish are the main hosts. Their name is derived from the fact that their life cycle involves only one host (from the Greek monos, ‘alone’, and genes, ‘of a particular kind’); they are distinguishable from Digenea by their possession of a large, posterior, adhesive organ (an opisthaptor).