Source : Webster's Revised Unabridged Dictionary (1913)

Flatworm \Flat"worm`\, n. (Zo["o]l.)
   Any worm belonging to the Plathelminthes; also, sometimes
   applied to the planarians.

Source : WordNet®

flatworm
     n : parasitic or free-living worms having a flattened body [syn:
          {platyhelminth}]