Flatworms reproduce sexually and disperse via eggs. Beyond that, there is little in the way of common life cycle features in this diverse group. Life cycles may be direct, or have 1, 2, or 3 intermediate hosts. Autoinfection is possible for some species. Adults are usually parasites of vertebrate animals, while larval forms may be free-living or infect vertebrates or invertebrates. Some parasitic larval forms are capable of asexual proliferation. Life cycles may involve aquatic or terrestrial hosts, or both.
Parasitic forms enter their hosts via the digestive system or by penetration of the skin. They can be found in all host tissues and cavities. The most common site for adults is the lumen of the digestive system, but they can occur in other organs that provide some means for release of eggs. Larval stages usually infect tissues or tissue spaces.
Monogenean adults usually produce an egg, from which an oncomiracidium hatches. The oncomiracidium swims to find a new host, attaches and then develops into an adult.
Aspidogastrean adults produce an egg from which a larva hatches. The larva develops directly into a new adult.
Digenean adults produce an egg. Subsequent development follows myriad routes, depending on the species involved. A miracidium develops within the egg, and may hatch in water and swim until it can penetrate a mollusc first intermediate host, or the egg may be eaten by a mollusc and then hatch. The miracidium develops through mother (and possibly daughter) sporocyst stages, and/or mother (and possibly daughter) redia stages, all of which are capable of asexual proliferation in the mollusc. Whichever path is followed, the final product is a larval form called a cercaria, which is usually a swimming stage that may encyst on vegetation as a metacercaria, or may penetrate or be eaten by another host, and then develop in that host to the metacercaria stage. A few species have a mesocercaria stage between the cercaria and metacercaria. The metacercaria develops into an adult when it is eaten by a definitive host.
Cestodes produce an egg. Subsequent development follows myriad routes, depending on the species involved. An oncosphere develops in the egg. After being eaten by an intermediate host, there are three major routes of development. Some species hatch from the egg as a coracidium and then develop into a procercoid after being eaten by an invertebrate first intermediate host, then into a plerocercoid in a vertebrate second intermediate host, and finally into the adult in a vertebrate definitive host. Other species develop into a cysticercoid in an invertebrate intermediate host, then into an adult in a vertebrate definitive host. Yet other species develop into some form of bladder larva (cysticercus, coenurus, strobilocercus, hydatid) in a vertebrate intermediate host, then complete development into adults in a vertebrate definitive host. Some bladder larvae are capable of asexual proliferation. Eggs of all cestodes are dispersed into the external environment via the feces of their host, but all other transfers from host to host require that they be eaten. There are no free-living stages that seek out hosts.