arrowworm

[ ar-oh-wurm ]
/ ˈær oʊˌwɜrm /

noun

any small, translucent marine worm of the phylum (or class) Chaetognatha, having lateral and caudal fins.
Also called glassworm.

Origin of arrowworm

First recorded in 1885–90; so called from a fancied resemblance to an arrow

British Dictionary definitions for arrowworm

arrowworm
/ (ˈærəʊˌwɜːm) /

noun

any small marine invertebrate of the genus Sagitta, having an elongated transparent body with fins and prehensile oral bristles: phylum Chaetognatha (chaetognaths)