Squid chromatophores

YouTube link.

Squid rapidly change colour when the muscles surrounding a chromatophore-filled sac contracts and expands -- when it contracts, the pigment appears denser, when it expands the colour becomes muted and disappears. The squid changes its colour according to exterior factors -- such as a threat or the presence of a potential mate - because each chromatophore is linked to a nerve ending.

Via Wired and Neatorama.  The background version of Pachelbel seems rather plodding, but the visual images are interesting.