Jellyfish are free-swimming marine animals composed of more than 95 percent water, with bodies consisting of an umbrella-shaped bell and trailing tentacles. Jellyfish are radially symmetrical with a central axis through the length of their bodies. As a result, they have a top and bottom, but no left or right sides, as do most animals whose bodies are bilaterally symmetrical.
Because their bodies are soft and lack a skeletal structure or outer shell, jellyfish are easily damaged. Their bodies require water for support, and jellyfish die when removed from water for too long. To move, jellyfish take water into the bell and squirt it out behind them, or simply drift on the currents.
The jellyfish's digestive system is simple. Food is taken in through a mouth on the underside of the bell and digested in a sac-like organ. Waste is then expelled back out through the mouth.
Jellyfish have very primitive senses that detect light and prey. Nematocytes are located on the feeding arms, tentacles and mouth of the jellyfish. When potential prey brushes against trigger hairs, the nematocytes propel a coiled barb and venom into the prey, paralyzing it. The jellyfish then draws the prey into its mouth and into the digestive organ.
There are about 200 species of true jellyfish.