The Collaborative International Dictionary of English v.0.48:

Rhizostomata \Rhi`zo*stom"a*ta\, n. pl. [NL., fr. Gr. ??? a root
   + ???, ????, a mouth.] (Zool.)
   A suborder of Medusae which includes very large species
   without marginal tentacles, but having large mouth lobes
   closely united at the edges. See Illust. in Appendix.
   [1913 Webster]