The Collaborative International Dictionary of English v.0.48:
Arrowworm \Ar"row*worm`\, n. (Zool.) A peculiar transparent worm of the genus Sagitta, living at the surface of the sea. See Sagitta. [1913 Webster]WordNet (r) 3.0 (2006):
arrowworm n 1: 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: arrowworm, chaetognath]