The male system usually has a single testis, seminal vesicle and muscular penis. As in the Digenea there may be extra openings into the female reproductive system from the outside. The development of monogeneans is direct , i.e., there is no intermediate host such as a snail. Eggs are passed out into water where they hatch. The eggs of monogeneans are often remarkably large and are frequently equipped with long filaments, which are extensions of the egg envelope.
Some species use the filaments to stick the eggs on to the skin of fish.
In others, the eggs form a mass which may float or sink and get caught in fish respiratory currents. The hatched egg is called an oncomiracidium . This is a ciliated larva that can swim actively, has eyespots and a larval haptor. The oncomiracidia attach to their host by means of the haptor and promptly loose their ciliated coat. Growth to the adult stage is usually accompanied by an increase in the complexity of the haptor.