Dis – double, Genos – race) is a class of trematodes in the Platyhelminthes phylum, consisting of parasitic flatworms (known as flukes) with a syncytial tegument and, usually, two suckers, one ventral and one oral.

Adults commonly live within the digestive tract, but occur throughout the organ systems of all classes of vertebrates.

A similar tegument is found in other members of the Neodermata; a group of platyhelminths comprising the Digenea, Aspidogastrea, Monogenea and Cestoda.

Digeneans possess a vermiform, unsegmented body-plan and have a solid parenchyma with no body cavity (coelom) as in all platyhelminths.

Electron microscopic studies have shown that the light microscopically visible germ balls consist of mitotically dividing cells which give rise to embryos and to a line of new germ cells that become included in these embryonic stages.

The exact conformation of these organs within the male terminal genitalia is taxonomically important at the familial and generic levels.

As adults, most digeneans possess a terminal or subterminal mouth, a muscular pharynx that provides the force for ingesting food, and a forked, blind digestive system consisting of two tubular sacs called caeca (sing.

In others the caeca may fuse with the body wall posteriorly to make one or more anuses, or with the excretory vesicle to form a uroproct.

Chemoreception plays an important role in the free-living miracidial larva recognising and locating its host.

There is a bewildering array of variation on the complex digenean life cycle, and plasticity in this trait is probably a key to the group's success.

[2] This has led to the inference that the ancestral digenean was a mollusc parasite and that vertebrate hosts were added subsequently.

The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by an annelid worm),[2] in which they proceed to hatch.

Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, the miracidium, which must locate and penetrate the body wall of the snail host.

After post-ingestion hatching or penetration of the snail, the miracidium metamorphoses into a simple, sac-like mother sporocyst.

The mother sporocyst undergoes a round of internal asexual reproduction, giving rise to either rediae (sing.

These in turn undergo further asexual reproduction, ultimately yielding large numbers of the second free-living stage, the cercaria (pl.

Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats.

The evolutionary origins of the Digenea have been debated for some time, but there appears general agreement that the proto-digenean was a parasite of a mollusc, possibly of the mantle cavity.

It is likely that more complex life cycles evolved through a process of terminal addition, whereby digeneans survived predation of their mollusc host, probably by a fish.

Other hosts were added by the same process until the modern bewildering diversity of life cycle patterns developed.