Members of this order closely resemble the semacostome jellyfishes, the differences being the lack of tentacles round the bell margin and the great increase in size and fusion of the oral arms. These are much-branched and the original grooves of the arms close over to form a canal.
The main mouth generally becomes part of the fused system, and the numerous branches lead to the so-called suctorial mouths. In some genera additional mouth-bearing outgrowths, the shoulder ruffles, occur on the outer edges of the arms. The arms also bear elongated appendages with many nematocysts, assisting in the capture of food.
The fused mouth leads into a central stomach region, and numerous branched radial canals extend to the bell margin. They are generally good swimmers with well developed muscles round the bell margin and usually live in warm shallow waters, although species of Rhizostoma may extend into temperate waters. Little is known about their development, but it is thought there is a scyphistoma which produces ephyrae.
Rhizostoma is a widely distributed genus with eight long terminal appendages and is occasionally found on the south and west coasts of the British Isles. R. octopus is grey-green in colour, up to sixty cm. in diameter, and its bell has a purple edge. Often associated with it in the subumbrella is an amphipod crustacean, or Sandhopper (Hypera galba).
Cassiopeia, common round Florida and the West Indies, contains more sluggish forms which have eight branching mouth arms with suctorial mouths. A flattened bell with a marginal ridge gives the appearance of a saucer. These jellyfishes lie upside-down in shallow lagoons with the exumbrella facing downwards and adhering by the bell ridge. Pulsations of the bell cause water currents to flow over the outstretched arms, bringing food and oxygen.