Monogenea
- 1,100 species of this ectoparasite
- Host: aquatic vertebrates
- Mostly external parasites of fish by attaching to the gill filaments and feed on epithelial cells, mucus, or blood (Opisthapor facilitates attachment)
- One adults develops from one egg.
- Adults: produce and release eggs with a sticky thread to attach them to the fish gills, they attach eggs to the fish's gills and the eggs hatch into oncomiracidium that swims to another host fish and attaches by its opisthaptor and develops into an adult
- More closely related to tape worms