The Collaborative International Dictionary of English v.0.48:

Flatworm \Flat"worm`\, n. (Zool.)
   Any worm belonging to the Platyhelminthes; also, sometimes
   applied to the planarians.
   [1913 Webster]

WordNet (r) 3.0 (2006):

flatworm
    n 1: parasitic or free-living worms having a flattened body
         [syn: flatworm, platyhelminth]