An octopuses body can be split into three parts, the head, the mantle and the arms, the head being the location of the beak, brain and eyes, and the mantle being the sac above it holding the rest of it's organs. Octopuses and cuttlefish have the highest brain-to-body ratio of any invertebrate, which is also greater than many vertebrates.The octopus's nervous system is spread throughout it's body, and octopus arms that have been severed (either by artificially severing nerves or by the octopus amputating it as a defensive or reproductive mechanism) retain the ability to move and react to stimuli as they would in a healthy octopus. Two-thirds of the octopuses neurons are in it's eight arms. Octopuses color-changing cells, chromatophores, are each surrounded by muscles that expand and contract to change the visibility of the pigment. Light sensitive receptors in their skin called opsins help them choose a coloration that will best camoflauge them. Attatched to octopuses brains are two organs called statocysts that allow them to orient themselves relative to gravity and keep their pupils oriented horizontally. They may use the statocysts to hear sounds between 400 and 1000 Hz as well. Each of an octopus's suction cups have chemoreceptors so that they can taste what they touch, but it seems that they lack proprioreceptive ability and need to see their arms to locate them.