From WordNet (r) 2.0 (August 2003) [wn]:
flatworm
n : parasitic or free-living worms having a flattened body [syn:
platyhelminth]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Flatworm \Flat"worm`\, n. (Zool.)
Any worm belonging to the Platyhelminthes; also, sometimes
applied to the planarians.
[1913 Webster]