Jellyfish are animals of the phylum Cnidaria. Most of them live in the oceans, in salt water, where they eat small sea animals like plankton and little fish, and float in the sea. Only a few jellyfish live in fresh water.
They have soft bodies and long, stinging, venomous tentacles that they use to catch their prey, usually small plankton animals or small crustaceans or tiny fish. Some jellyfish hunt other jellyfish. Venom is injected by stinging cells called nematocysts. A jellyfish is 97% water.
Most jellyfish have a bell-shaped body and long tentacles at the underside of the body. Tentacles are long “arms” with special stinging cells called nematocysts. They move by contracting their bodies, but they do not have much control over where they go: most of the time, they drift with the water current. The largest type of jellyfish is the Lion’s mane jellyfish, which has tentacles that can be as long as 60 meters, but most jellyfish are much smaller.
Jellyfish occur in four of the Cnidarian classes:
Scyphozoa: the true jellyfish
Cubozoa: the box jellyfish
Staurozoa: the stalked jellyfish
Hydrozoa: the hydroids
These four classes are sometimes linked as the sub-phylum Medusozoa.
There are many types of jellyfish. The smallest jellyfish are just a few inches across. The largest jellyfish is the Lion’s mane (Cyanea capillata), whose body can be over 3 feet (1 m) across, with much longer tentacles. Some jellyfish in the dark (this is called phosphorescence). Some of the most dangerous jelly fish include the box jelly (Genuses Chironex, Chiropsalmus and Carybdea) and the tiny, two-cm-across Irukandji jelly (Carukia barnesi); the venomous sting of these jellyfish can kill a person.
source : KidzSearch