The jellyfishes in this order are among the most familiar members of the class, being those usually seen in temperate waters, and they are also the most typical. There are numerous tentacles round the bell margin and the oral arms are long and frilly. The musculature is usually obvious and there are numerous simple or branched canals.
The order is found in coastal waters of all seas, warm and temperate, while some species of Cyanea are found in polar regions. Pelagia occurs in deeper waters and lacks the fixed scyphistoma, the planulae developing directly into small medusae. Jellyfishes vary in size from five to forty cm. in diameter; species of Cyanea with a diameter of over two metres have been recorded. They are often coloured in patches and streaks. In many instances the gonads are highly coloured and many jellyfishes fluoresce in the dark. The Common Jellyfish (genus Aurelia) is seven to twenty cm. in diameter and sometimes larger, with numerous short tentacles round the bell margin giving it a frilly appearance. In Aurelia aurita, which is often found washed ashore on beaches, the four crescent-shaped gonads are purple and very noticeable.
Chrysaora isoceles, found occasionally off the coasts of Britain and with related American species, is a whitish jellyfish with long tentacles hanging from a lobed edge, each lobe having a dark brown spot. There are long, frilly, brown oral arms, a brown spot in the centre of the exumbrella and twenty-four brown triangular patches radiating from it.
Species of Cyanea are found off the coasts of Britain and America and can be identified by the lobed bell margin with eight bundles of tentacles arranged round it. Small specimens are often thirty cm. in diameter and even bigger individuals have been recorded from the North Atlantic.
They are variously coloured, ranging from yellows to purples and blue. Species of Pelagia, especially P. noctiluca, are luminescent. They have a scalloped bell margin and long, thin tentacles. In addition the oral arms are very long and the bell is spotted with purples and red-browns.