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Cnidarians of class Scyphozoa have a predominant jellyfish stage. They are characterized by a cellular mesoglea and tentacles in their gastrovascular cavity. All of the largest jellyfish belong to this class. The common Aurelia aurita is seen in bays and harbors, sometimes in large numbers. It is pallid, unlike some of the more colorful species in the genus Cyanea. Stalked jellyfish, the Stauromedusae, are unusual members of the Scyphozoa; they are found attached to seaweed, especially in cooler marine habitats. The order Rhizostomea includes jellyfish in which the original mouth has closed, and which have many subsidiary mouths found in frilled oral arms. Cassiopaeia is a well-known example, living in warmer, shallow waters, where it is often found lying on the bottom upside down, exposing its green algal symbionts to the sun.
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