The Collaborative International Dictionary of English v.0.48:

Chaetognatha \Chae*tog"na*tha\, Chaetognatha
\Ch[ae]*tog"na*tha\, n. pl. [NL., from Gr. ? hair + ? jaw.]
   (Zool)
   a phylum consisting of the arrowworms, a group of small
   active transparent free-swimming marine worms, of which the
   genus Sagitta is the type. They have groups of curved
   spines on each side of the head.

   Syn: phylum Chaetognatha.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

Chaetognatha
    n 1: arrowworms: a group of small active transparent marine
         worms [syn: Chaetognatha, phylum Chaetognatha]