The Collaborative International Dictionary of English v.0.48:

Gnathostoma \Gna*thos"to*ma\, n. pl. [NL., from Gr. gna`qos the
   jaw + ?, ?, the mouth.] (Zool.)
   A comprehensive division of vertebrates, including all that
   have distinct jaws, in contrast with the leptocardians and
   marsipobranchs (Cyclostoma), which lack them. [Written also
   Gnathostomata.]

WordNet (r) 3.0 (2006):

Gnathostomata
    n 1: comprising all vertebrates with upper and lower jaws [syn:
         Gnathostomata, superclass Gnathostomata]