This is a minor group of endoparasitic flukes with morphological similarities to the Digenea and Monogenea, but possessing relatively unsophisticated adaptations to parasitism. It has been speculated that they are a "missing link" between these two taxa, but they possess several unique features not found in the other two. This group is also commonly known as the Aspidogastrea.
These are primarily parasites of molluscs but may infect some fishes and reptiles. They have a direct life cycle, but details for most species are poorly known.
All species have a prominent mouth and a blind-ending intestine. The reproductive system has typical platyhelminth components. A prominent feature is a ventral sucker which takes the form of a multiloculated disk, or a series of separate suckers. Another unique structure is the connective tissue septum which partially divides the body into dorsal and ventral sections.