Genetic analysis shows that urchins' bodies contain light sensitive molecules, so the biologists speculated that their spines might be light sensitive and act like cells of a compound full-body eye. They put black disks of various sizes on the wall of a tank containing purple sea urchins, and when the disk was over 3.5 inches, the urchins showed a fight-or-flight reaction, with two-thirds "running" toward the disk, and the rest apparently fleeing it.
The researchers think that the more spines an urchin has, the sharper its vision is. The experiment also indicates that urchins may have compound brains as well. They don't have one organ that can be called a brain, but they do have a nervous system spread evenly through their bodies. The coordination of visual input from their spines into images, and acting upon them, suggests some processing is going on.