Decapitated Worms Get Better, See Again
The way in which headless flatworms recover their sight provides important clues about the evolution of animal eyes.
For humans, decapitation is fatal. For a planarian flatworm, it’s a mild and temporary inconvenience.
These small animals are masters of regeneration. Cut off their heads, and a new one—sometimes two new ones—will regrow within a few days. Bisect them, and both halves will regenerate a full animal. Excise a small lump of tissue, and it too will produce a new worm. Transplant a onto a dying planarian and the donor cell will take over, creating skin, nerves, muscle, and eventually an entire body. As one 19th-century naturalist wrote, planarians could “almost be called immortal under the edge of a knife.”
