jellyfish, common name for the free-swimming stage (see polyp and medusa), of certain invertebrate animals of the phylum Cnidaria (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 one species of Cyanea found in cold northern seas may reach 6 ft (1.8 m) across and have tentacles over 100 ft (30 m) long. Aurelia, the flattened jellyfish common along North American coasts, is 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).