Jellyfish predate dinosaurs. How have they survived so long?

Gelatinous drifters that move in a mesmerizing way, jellyfish are beautiful creatures. But take a closer look and you'll see there's more to these creatures than a pretty picture. Some shoot a deadly venom, while others produce a luminescent glow that sparked one of the most important biological discoveries.  They're also ancient, having lived in the ocean for roughly half a billion years.