Jellyfish have traditionally been considered simple and primitive. When you gaze at one in an aquarium tank, it is not hard to see why. Renaissance scholars considered them plants. Eighteenth-century naturalists grudgingly granted them admittance into the animal kingdom, but only just. They classified cnidarians as “zoophytes,” somewhere between animal and plant. In some ways, cnidarians are a better model for human biology than fruit flies. As strange as it may seem, gazing at a jellyfish in an aquarium is a lot like looking in the mirror.