Jellyfish have drifted along on ocean currents for millions of years, even before dinosaurs lived on the Earth. The jellylike creatures pulse along on ocean currents and are abundant in cold and warm ocean water, in deep water, and along coastlines. But despite their name, jellyfish aren't actually fish—they're invertebrates , or animals with no backbones. They squirt water from their mouths to propel themselves forward as their tentacles hang down from their smooth baglike bodies and sting their prey.