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common name for the free-swimming stage (see polyp and medusapolyp and medusa,
names for the two body forms, one nonmotile and one typically free swimming, found in the aquatic invertebrate phylum Cnidaria (the coelenterates). Some animals of this group are always polyps, some are always medusae, and some exhibit both a polyp and a medusa
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), of certain invertebrate animals of the phylum CnidariaCnidaria
or Coelenterata
, phylum of invertebrate animals comprising the sea anemones, corals, jellyfish, and hydroids. Cnidarians are radially symmetrical (see symmetry, biological).
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 (the coelenterates). The body of a jellyfish is shaped like a bell or umbrella, with a clear, jellylike material filling most of the space between the upper and lower surfaces. A mouth is located in the center of the undersurface and tentacles dangle from the bell margin. Many jellyfish are colored, with pink or orange internal structures visible through the colorless or delicately tinted bell, and all are exquisitely designed; they are among the most beautiful of animal types.

Typically, jellyfish catch their prey with the aid of stinging cells located in the tentacles; many jellyfish can cause irritating or even dangerous stings to humans. Food is carried by the tentacles to the mouth, then is moved into the stomach and is distributed to the body through radial canals. Jellyfish move up and down by contracting and relaxing the bell, using muscles that circle the bell margin; they are carried horizontally by waves and currents.

Jellyfish of the class Hydrozoa are small, ranging from 1-8 in. (0.32 cm) to several inches in diameter, and usually have four tentacles. They have several (often four) unbranched radial canals and simple sense organs. In this group the polyp, or attached stage, is often larger and more conspicuous than the medusa.

Jellyfish of the class Scyphozoa, sometimes called true jellyfish, are larger and often have numerous tentacles; they have branched radial canals and complex sense organs. In this group the medusa is the prominent form and the polyp is reduced to a small larval stage. Scyphozoan jellyfish are commonly 3-4 in. to 16 in. (2–40 cm) in diameter, though the lion's mane jellyfish, Cyanea capillata, which is found in cold northern seas can exceed 6 ft (1.8 m) across and have tentacles over 100 ft (30 m) long; a similar jellyfish is also found in the waters off New Zealand and Australia. Aurelia, or moon jellyfish, the flattened species common along North American coasts, are usually 1 ft (30 cm) or less across.

Tiny Craspedacusta, a hydrozoan jellyfish less than 1 in. (2.5 cm) long, occurs in freshwater lakes and ponds, but all other jellyfish are marine, living in ocean depths as well as along the coasts. The hydrozoan Physalia, or Portuguese man-of-war, is actually a large colony of modified individuals, some medusalike and some polyplike; a large gas-filled sac acts as a float for the colony. The tentacles of such a colony may extend 60 ft (18 m) into the water and can cause severe injuries to swimmers. Physalia is usually bright blue, sometimes with tints of pink and orange. The purple sail, Velella, a floating hydrozoan colony 1 to 3 in. (2.5–7.5 cm) across, may be blue or purple.

Jellyfish are classified in the phylum Cnidaria.


See L. Gershwin, Stung! On Jellyfish Blooms and the Future of the Ocean (2013).



(medusae), the sexually reproducing form of various, usually marine coelenterates that are for the most part free swimming. They form from buddings on the asexual generation of polyps, which lead a sedentary life. Jellyfish are a stage in the life cycle of scyphozoans and many hydrozoans (except the hydras and certain Leptolida, whose underdeveloped medusoid reproductive structures remain attached to the polyp colony). In the Trachylida (among the hydrozoans) and Pelagia (among the scyphozoans), the alternation of generations has disappeared, and the jellyfish develop directly from eggs.

Jellyfish consist of an umbrella-shaped or bell-shaped mass (bell) and measure from several mm to 2.3 m in diameter. They are semitransparent as a result of the extensive formation of mesoglea, and their sense organs are located on the margin of the body. The mouth, which is in the center of the underside of the bell, is often surrounded with mouth arms. Radially situated gastrovascular canals originate at the stomach. The nervous system is more developed than in the polyps. In addition to the highly developed nerve network in the tentacles and the underside of the bell, there are two nerve rings located at the bell margin. The gonads occur close to the stomach or the radial canals; the eggs and sperm are released into the water, where fertilization occurs and the jellyfish develop. Only the scyphomedusae Aure/ia and Cyanea and a few hydromedusae fertilize the eggs in the mother organism, producing a larval form, the planula.

Jellyfish move by means of contractions, which force the water out of the bell cavity. Only a few scyphozoans (the stauromedusans) live on the bottom, attached to the substrate. Jellyfish feed on plankton. The stinging capsules, or nematocysts of certain jellyfish can cause inflammations.


Naumov, D. V. Gidroidy i gidromeduzy morskikh solonovatovodnykh i presnovodnykh basseinov SSSR. Moscow-Leningrad, 1960.
Naumov, D. V. Stsifoidnye meduzy morei SSSR. Moscow-Leningrad, 1961.


What does it mean when you dream about a jellyfish?

Jellyfish are creatures of the sea, a frequent symbol of the emotions or of the unconscious mind. Thus, jellyfish in a dream can represent unconscious, painful emotions.


(invertebrate zoology)
Any of various free-swimming marine cnidarians belonging to the Hydrozoa or Scyphozoa and having a bell- or bowl-shaped body. Also known as medusa.


1. any marine medusoid coelenterate of the class Scyphozoa, having a gelatinous umbrella-shaped body with trailing tentacles
2. any other medusoid coelenterate
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