Flatworms

Species in the class Monogenea are parasites, completing their life cycles within the body of a single living host, such as fish, frogs, turtles, and squid. Most of the 400 species in this class are ectoparasites, meaning that they cling to the outside of their host, for example, to the gills, fins, or skin of fish. Their bodies are covered by a protective cuticle and have adhesive suckers at each end. They eat by sucking blood through their mouths, which open beside their suckers.