Any worm belonging to the Platyhelminthes; also, sometimes applied to the planarians.

a member of Phylum Platyhelminthes. Includes non-parasitic Planaria, as well as parasitic forms.

parasitic or free-living worms having a flattened body

Invertebrate with flat, worm-like body (e.g. tapeworm).

The flatworms (Platyhelminthes, Greek "platy"': flat; "helminth": worm) are a phylum of relatively simple soft-bodied invertebrate animals. With about 25,000 known species they are the largest phylum of acoelomates. Flatworms are found in marine, freshwater, and even damp terrestrial environments.