Source : Webster's Revised Unabridged Dictionary (1913)

Gnathostoma \Gna*thos"to*ma\, n. pl. [NL., from Gr. ? the jaw +
   ?, ?, the mouth.] (Zo["o]l.)
   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}.]

Source : WordNet®

Gnathostomata
     n : comprising all vertebrates with upper and lower jaws [syn: {superclass
         Gnathostomata}]