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an order of coelenterates of class Scyphozoa.
The medusae of Rhizostomae (to 65 cm in diameter) lack marginal tentacles. The edges of the mouth extend into oral lobes with numerous folds; these, in turn, fuse to form numerous secondary oral openings. The primary oral opening is thus hidden. Contact with the stinging cells on the oral lobes can cause painful burns.
Rhizostome polyps are small. They form one medusa each by strobilation. There are about 80 species of Rhizostomae, living mainly in tropical seas (less frequently, temperate). There are two species in the USSR: Rhizostoma pulmo, common in the Black and Azov seas, and Rhopilema asamushi, found in the Sea of Japan.