Evolution
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. Evidence for this comes from the ubiquity of molluscs as first intermediate hosts for digeneans, and the fact that most aspidogastreans (the sister group to the Digenea) also have mollusc associations. It is thought that the early trematodes (the collective name for digeneans and aspidogastreans) likely evolved from rhabdocoel turbellarians that colonised the open mantle cavity of early molluscs.
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 until the modern bewildering diversity of life cycle patterns developed.
Read more about this topic: Digenea
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