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a parasitic organism that lives on the surface of the body of the host. Ectoparasites make up the largest group of organisms that live on the surface of other organisms. Some ectoparasites are permanently attached to the host, such as copepods living on the skin and gills of fish. However, most move along the host’s body, for example, Urcelariidae and monogenetic flatworms, which parasitize fish, and lice and bird lice, which parasitize birds and occasionally mammals. Various and frequently complex organs of attachment, such as suckers, suction disks, and hooks, are the principal means by which they adapt to existence on the host.