Hydrozoa, the primitive fern-like creatures, many of which produce small jellyfish, Scyphozoa, the large jellyfish, or scyphomedusae, and their polyps, and Anthozoa, or sea anemones. Although many of these animals produce a jellyfish part of their life cycle, there are only about 200 species of large jellyfish that people are likely to notice - most of these are in the Scyphozoa. A jellyfish is an invertebrate made up mostly of water, it has no heart, brain or bones.
Jellyfish live in all the world's oceans.
They swim by jet propulsion. The jellyfish will expand then quickly contract its bell-shaped body, which forces water away from the bell and pushes the jelly in the opposite direction. When light levels fall, the ocelli of some jellyfish start to pulse, helping them to remain at the ocean's surface. Jellyfish have sensory organs called rhopalia, which form a row of small round structures along the rim of the bell. The rhopalia include sensory organs called statocysts that help maintain the jellyfish's balance.