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Scyphozoa
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Scyphozoa

Scyphozoa is a class within the phylum Cnidaria^[1], sometimes referred to as the "true jellyfish".

The class name Scyphozoa comes from the Greek word skyphos (σκύφος), denoting a kind of drinking cup and alluding to the cup shape of the organism.

Scyphozoans range in geological time from the Ediacaran period^{[citation needed]} through the Recent. Most species of Scyphozoa have two life history phases, including the planktonic medusa or jellyfish form, which is most evident in the warm summer months, and an inconspicuous, but longer-lived bottom-dwelling polyp, which seasonally gives rise to new medusae. Most of the large, often colorful, and conspicuous jellyfish found in coastal waters throughout the world are Scyphozoa.^[2] Scyphomedusae are found throughout the world's oceans, from the surface to great depths; no Scyphozoa occur in freshwater (or on land).

As medusae, they eat a variety of crustaceans and fish, which they capture using stinging cells called nematocysts. The nematocysts are located throughout the tentacles that radiate downward from the edge of the umbrella dome, and also cover the oral arms that hang down from the central mouth. Scyphozoans usually display a four-part symmetry and have an internal gelatinous material called mesoglea, which provides the same structural integrity as a skeleton. Scyphozoans have no durable hard parts, including no head, no skeleton and no specialized organs for respiration or excretion.^[3][4] Marine jellyfish can consist of as much as 99% water and therefore are rarely found in fossil form.

Unlike the hydrozoan jellyfish, hydromedusae, scyphomedusae lack a velum, which is a circular membrane beneath the umbrella that helps propels the (usually smaller) hydromedusae through the water. While scyphozoans lack this specialized locomotive device, they are able to move through the water by contracting and relaxing the muscles of their umbrella.^[5] The periodic contracting and relaxing propels the jellyfish through the water, allowing it to escape predation or catch its prey.

Scyphozoa includes the moon jelly Aurelia aurita^[6], in the order Semaeostomeae, and the enormous Nemopilema nomurai, in the Order Rhizosomeae, found between Japan and China and which in some years causes major fisheries disruptions.

Most of the jellyfish that are fished commercially for food are scyphomedusae in the order Rhizostomeae.^[7] Most rhizostome jellyfish live in warm water.^[8]

Orders

• Order Coronatae
• Order Semaeostomeae
• Order Rhizostomeae

References

External links

• The Classification and Distribution of the Class Scyphozoa

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