The most highly evolved class of the phylum Mollusca. The most common species of cephalopods are the Octopuses, Cuttlefish, and Squids. Typical is the circle of arms or "tentacles" with rows of suckers, on the head, which developed from the mollusk foot. They swim by jet propulsion: water is pressed out of the mantle cavity through a funnel. The shell, external in Nautiloidea, is in other cephalopods reduced and internal, or absent. Cephalopods have a mouth with a horny bill, and highly developed eyes. The skin bears chromatophores, which enable the animals to change their color quickly, and also light organs (see bioluminescence).