"Octopuses and their relatives have what Woods Hole researcher Roger Hanlon calls electric skin. For its color palette, the octopus uses three layers of three different types of cells near the skin's surface--all controlled in different ways. The deepest layer, containing the white leucophores, passively reflects background light. This process appears to involve no muscles or nerves. The middle layer contains the tiny iridophores, each 100 microns across. These also reflect light, including polarized light (which humans can't see, but a number of octopuses' predators, including birds, do). The iridophores create an array of glittering greens, blues, golds, and pinks. Some of these little organs seem to be passive, but other iridophores appear to be controlled by the nervous system. They are associated with the neurotransmitter acetylcholine, the first neurotransmitter to be identified in any animal. Acetylcholine helps with contraction of muscles; in humans, it is also important in memory, learning, and REM sleep. In octopuses, more of it "turns on" the greens and blues; less creates pinks and golds. The topmost layer of the octopus's skin contains chromatophores, tiny sacks of yellow, red, brown, and black pigment, each in an elastic container that can be opened or closed to reveal more or less color. Camouflaging the eye alone--with a variety of patterns including a bar, a bandit's mask, and a starburst pattern--can involve as many as 5 million chromatophores. Each chromatophore is regulated via an array of nerves and muscles, all under the octopus's voluntary control."