Source : Webster's Revised Unabridged Dictionary (1913)

Arrowworm \Ar"row*worm`\, n. (Zo["o]l.)
   A peculiar transparent worm of the genus {Sagitta}, living at
   the surface of the sea. See {Sagitta}.

Source : WordNet®

arrowworm
     n : any worm of the Chaetognatha; transparent marine worm with
         horizontal lateral and caudal fins and a row of movable
         curved spines at each side of the mouth [syn: {chaetognath}]