Jellyfish, a water-dwelling animal. It is not a fish but a cnidarian, a spineless, soft-bodied animal. The jellyfish is one of two basic body forms that the animal has during its life cycle; the other is called a polyp. There is no common name for the animal in both its forms.
Jellyfish capture food with their tentacles.The jellyfish has a semitransparent, bell-shaped, sac-like body fringed around the bottom edges by slender, dangling tentacles. The body is composed of a jelly-like material that gives the animal both shape and buoyancy. The name “medusa” comes from the tentacles that hang from the edge of the body, suggesting the snakes that grew from the head of Medusa, a monster in Greek mythology.
Jellyfish range in diameter from less than one inch (2.5 cm) to about 12 feet (3.7 m), depending on the species. They may be pinkish, bluish, brownish, or almost colorless. Some jellyfish are found in freshwater, but most are marine and live mainly in coastal seawaters.The jellyfish moves vertically through the water, alternately sinking and then rising to the surface by rhythmically contracting its body. In this manner it drifts about, carried by the water current. It usually lives in large groups, or schools. Jellyfish are sometimes washed ashore by tides and storms.