Jellyfish swimming in the Pacific, Guerrero, Mexico (© Christian Vizl/TANDEM Stills + Motion)

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.