Jellyfish

The jellyfish is a water-dwelling animal. It is not a fish but a coelenterate, a spineless, soft-bodied animal. The jellyfish is actually 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. The jellyfish has a semitransparent, bellshaped, 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-and its common name. Its other 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 to about 12 feet, depending on the species. Jellyfish may be pinkish, bluish, brownish, or almost colorless in color.

Some jellyfish are found in fresh water, but most are marine and live mainly in coastal sea waters. 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. In coastal areas, it is often washed ashore by strong tides and storms.

The tentacles are used to seize prey, and stinging organs on both the body and tentacles are used to stun the prey. In many species, these organs can cause painful injuries to swimmers or to barefoot persons who accidently step on one of these animals. The mouth, located on the underside, leads to a stomachlike cavity. A jellyjish feeds on other small aquatic animals, including small fish.

Jellyfish reproduce sexually. The female medusae produce eggs, which are fertilized within the stomach cavity. The eggs develop into larvae, which are shed into the water. The larvae settle down and attach themselves to a rock or a plant and develop into the tubular forms called polyps. Each polyp multiplies itsef by asexual budding and later, through fission, divides into many small medusae.

Jellyfish belong to the class Scyphozoa or the class Hydrozoa. All are members of the phylum Coelenterata.