Udonellida

a group of flatworms previously assigned to the class Monogenoidea, later distinguished as a separate class of the phylum Platyhelminthes, and now regarded as one of the orders of the class Turbellaria. The cylindrical body measures as much as 3 mm in length. On the anterior end is a pair of glandular organs, and on the posterior end a disklike adhesive sucker without hooks. The Udonellida live as commensals on the bodies of parasitic copepod crustaceans (family Caligidae), which in turn inhabit the bodies and gills of marine fishes, mainly of the family Pleuronectidae. The Udonellida are hermaphroditic. They attach their large, oval eggs in groups to the host by a long stalk.