Jellyfish are neither jelly nor fish. Jellyfish lack backbones, heart, blood, brain, or gills and, in fact, are over 95% water. Jellyfish posess stinging cells located on the tentacles and other body parts. Jellyfish usually just drift with the ocean currents, but occasionally swim by pulsing their umbrella or bell. Even jellyfish that have washed ashore should not be handled, since the stinging cells may still fire. The stings of some jellyfish can be quite painful, causing reactions ranging from mild rashes to death (in extreme allergic-reaction instances.) Although feared by swimmers, most jellyfish are harmless to human beings.
Jellyfish drifters from the National Aquarium
Marine Aquarium Design in Los Angeles