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 single adult cell 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.”
“The more you get to know about planarians, the more you fall in love with them,” says Akash Gulyani, who works with them at the National Center for Biological Sciences in Bangalore. Although most scientists who study planarians do so to understand how they regenerate so well, Gulyani had a different motivation. He wanted to know how these animals see the world—and how their senses recover as their eyes, brains, and heads regrow.
If you look at a planarian’s head, you’ll likely see two black dots. Those are its eyes, and they’re about as simple as true eyes can be. Each is a small cup lined with light-detecting cells. It can sense the presence and direction of light, but with no focusing lens, it only provides its owner with a blurry, low-resolution view of the world. And that view is processed by two clusters of neurons that can only loosely be called a brain. With this set-up, it seems likely that planarians are capable of only the simplest visual behaviors, like avoiding bright lights.