Monogenetic flukes spend their entire life cycle as parasites on a single host, often on the gills and skin of fish; they include no human parasites. They hold on to the fish by the use of hooks and attachment organs at the posterior end. Most of the parasite's body space is devoted to the hermaphroditic reproductive system. The egg on hatching releases a ciliated larva that enables the parasite to reach a new host. Species of the genus Gyrodactylus can can be serious pests in hatcheries, particularly since a single worm can give rise to more than one hundred descendants in three weeks.