Monogeneans are usually ectoparasites on the skin and gills of fish. The mouth may be equipped with a sucker. Monogeneans also have a posterior attachment organ armed with suckers and hooks called a haptor. The life cycle involves only one host, for the adult. New hosts are located and infected by free-living larvae.
Clockwise from top left :
Monopisthocotylea entobdellae from a sole,
Polyopisthocotylea polystoma from a frog,
Diplozoon paradoxum from fish.