thumb|Fossilized stranded scyphozoans on a Cambrian tidal flat at [[Blackberry Hill, Wisconsin.]]

The Scyphozoa are an exclusively marine class of the phylum Cnidaria, referred to as the true jellyfish (or "true jellies").

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

Scyphozoans have existed from the earliest Cambrian to the present. They typically range from in diameter, but the largest species, Cyanea capillata can reach across. 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 four or eight oral arms that hang down from the central mouth. Some species, however, are instead filter feeders, using their tentacles to strain plankton from the water.

Anatomy

Scyphozoans usually display a four-part symmetry and have an internal gelatinous material called mesoglea, which provides the same structural integrity as a skeleton. The mesoglea includes mobile amoeboid cells originating from the epidermis.

Scyphozoans have no durable hard parts, including no head, no skeleton, and no specialized organs for respiration or excretion. Marine jellyfish can consist of as much as 98% water, so are rarely found in fossil form.

Unlike the hydrozoan jellyfish, Hydromedusae, Scyphomedusae lack a vellum, which is a circular membrane beneath the umbrella that helps propel the (usually smaller) Hydromedusae through the water. However, a ring of muscle fibres is present within the mesoglea around the rim of the dome, and the jellyfish swims by alternately contracting and relaxing these muscles. The periodic contracting and relaxing propels the jellyfish through the water, allowing it to escape predation or catch its prey.

The mouth opens into a central stomach, from which four interconnected diverticula radiate outwards. In many species, this is further elaborated by a system of radial canals, with or without an additional ring canal towards the edge of the dome. Some genera, such as Cassiopea, even have additional, smaller mouths in the oral arms. The lining of the digestive system includes further stinging nematocysts, along with cells that secrete digestive enzymes. in the order Semaeostomeae, and the enormous Nemopilema nomurai, in the order Rhizostomeae, found between Japan and China and which in some years causes major fisheries disruptions.

The jellyfish fished commercially for food are Scyphomedusae in the order Rhizostomeae. Most rhizostome jellyfish live in warm water. About 200 extant species are recognized at present, but the true diversity is likely to be at least 400 species.

:::* Family Pelagiidae

:::* Family Phacellophoridae

:::* Family Ulmaridae

:* Order Byroniida (possible stem coronatids)

References

  • The Classification and Distribution of the Class Scyphozoa