arrowworm

from The Century Dictionary.

• noun An animal of the genus Sagitta (which see).

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) A peculiar transparent worm of the genus Sagitta, living at the surface of the sea. See Sagitta.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology A transparent marine worm of the phylum Chaetognatha, found in plankton.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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