ARROWWORM
arrowworm, chaetognath
(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
Source: WordNet® 3.1
Etymology
Noun
arrowworm (plural arrowworms)
(zoology) A transparent marine worm of the phylum Chaetognatha, found in plankton.
Source: Wiktionary
Ar"row*worm`, n. (Zoöl.)
Definition: A peculiar transparent worm of the genus Sagitta, living at the
surface of the sea. See Sagitta.
Source: Webster’s Unabridged Dictionary 1913 Edition