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Some are longer than a blue whale. Others are barely larger than a grain of sand. One species unleashes one of the most deadly venoms on earth; another holds a secret that’s behind some of the greatest breakthroughs in biology. They’ve inhabited the ocean for at least half a billion years, and they’re still flourishing. David Gruber investigates the secret powers of jellyfish.

Lesson by David Gruber, animation by Silvia Prietov.