Source : Webster's Revised Unabridged Dictionary (1913)

Rhizostomata \Rhi`zo*stom"a*ta\, n.pl. [NL., fr. Gr. ??? a root
   + ???, ????, a mouth.] (Zo["o]l.)
   A suborder of Medus[ae] which includes very large species
   without marginal tentacles, but having large mouth lobes
   closely united at the edges. See Illust. in Appendix.