Lifecycle of the Medusa Jellyfish

Classification: Kingdom Animalia
Phylum Cnidaria (corals, jellyfish, sea anemones, hydrozoans)
Class Scyphozoa (Jellyfish)
many Orders, Families, Genera and Species.

Jellyfish are fish-eating animals that float in the sea - only a few jellyfish live in fresh water. They have soft bodies and long, stinging, poisonous tentacles that they use to catch fish. Venom is sent out through stinging cells called nematocysts. A jellyfish is 98% water.

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 glow in the dark (this is called phosphorescence). Some of the deadliest jellies include the box jelly (Genus Carybdea) and the tiny, two-cm-across Irukandji jelly (Carukia barnesi); the venomous sting of these jellyfish can kill a person.

Many animals eat jellyfish, including sea turtles and some fish (including the sun fish).

From : http://www.enchantedlearning.com