Arrowworm

noun

1. An animal of the genus Sagitta (which see).
2. A peculiar transparent worm of the genus Sagitta, living at the surface of the sea. See Sagitta.
3. A transparent marine worm of the phylum Chaetognatha, found in plankton.
4. 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