Monogeneans are almost exclusively ectoparasites on aquatic vertebrates. Many are quite serious pathogens in aquaculture operations.
All monogeneans have a direct life cycle, with an adult that produces an egg. An oncomiracidium emerges from the egg and swims to find a new host. There it grows directly into another adult.
The body is usually flat and oval, with various attachment structures at the anterior end which collectively are called a prohaptor, and a well developed posterior attachment structure called an opisthaptor. There is a mouth, leading to a blind-ending intestine that may be diverticulate. Individuals are hermaphroditic with a single set of reproductive organs, althogh there may be multiple testes.