Other Minds: The Octopus and the Evolution of Intelligent Life

by Peter Godfrey-Smith

The skin of a cephalopod is a layered screen controlled directly by the brain. Neurons reach from the brain through the body into the skin, where they control muscles. The muscles, in turn, control millions of pixel-like sacs of color. A cuttlefish senses or decides something, and its color changes in an instant. Here is how it works. The skin has an outer layer, a dermis, that acts as a covering. The next layer down contains the chromatophores, the most important of the color-control devices. A single chromatophore unit contains several different kinds of cells. One cell holds a sac of a colored chemical. Around it are muscle cells, one or two dozen of them, which pull the sac into different shapes. Those muscles are controlled by the brain. They stretch the sac to make its color visible, or relax it for the opposite effect. Each chromatophore contains just one color. Different cephalopod species use different colors, and usually the animal has three kinds. In a giant cuttlefish, the chromatophores are red, yellow, and black/brown. Each is much less than a millimeter in diameter.