These are an extremely important group of animals from an economic and medical viewpoint. They are seeming ubiquitous parasites of all vertebrates, and many are serious pathogens of humans and domestic livestock. Clonorchis - the human liver fluke. Paragonium - the human lung fluke. As their name suggests, members of this class are single host parasites, generally living on the external surface of their hosts.
Development is far less complex than that of the digeneans.
They have no real medical or economic importance, although monogeneans are known to infect large stocks of intensively farmed fish. The damage to farmed salmon fish stocks in Scottish waters has lead to the suggestion for an organic fish farming approach. The oral suckers characteristic of digeneans are largely absent from this class, although a complex caudal sucker is normal. This organ develops extensively in response to the continual buffeting suffered by the adults as the fish swims through the water. The closest relatives of the monogenea are thought to be the Cestode worms.