They don"t have brains, bones or even hearts, yet jellyfish have pulsed through Earth"s oceans for half a billion years. Don"t be fooled by the "fish" in their name: they belong to the invertebrate club called Cnidaria and they"re made up of about 95% water. With their bell-shaped bodies and trailing tentacles, they"re more like ocean drifters than deep-sea athletes. Some glow, like the one pictured here in the Pacific Ocean off Guerrero, Mexico, others pack stings and a select few, like the "immortal jellyfish," can reset their life cycle and start again.