Cuttlefish are not fish. Rather, they are of the order Sepiida and are cousins to the squid and octopus. They have eight short-ish arms and two longer tentacles. Inside the body is the cuttlebone. It is composed of calcium carbonate and is porous. The animal uses the cuttlebone to maintain its buoyancy in the water column, adjusting the gas-to-liquid ratio as appropriate. In the center of the arms is a sharp beak. They have eyes on the side of their heads. Cuttlefish are color-blind in our sense of the word, but are able to perceive the coloration and pattern of their surrounding in great detail. They use this perception to change the color of their skin to closely match the color, contrast, and texture of the substrate or other background even in darkness. They achieve this camouflage by adjusting the groups of pigmented chromatophores in their skin, having up to 200 of these specialized pigment cells per square millimeter (equivalent to almost 130,000 per square inch). Cuttlefish can also adjust the polarization of reflected light. These abilities have resulted in cuttlefish being called the chameleons of the sea. The structure of the eye, while superficially like that of vertebrates, is totally different. The pupil resembles a curving letter W. The two concentrated sensor cells on the retina look in somewhat different directions. Focus is achieved by moving the lens, rather than by reshaping it. There is no blind spot because the optic nerve is located behind the retina. Cuttlefish blood is greenish blue in color because it uses the copper-bearing protein hemocyanin to carry oxygen rather than the iron-bearing protein hemoglobin. But, since hemocyanin is less efficient than hemoglobin, the blood must flow faster. To accomplish this, cuttlefish have three hearts – one for each pair of gills and the third for the rest of the body. Like squid and octopus, cuttlefish produce ink, used to evade predators. The ink is brown in color and was named by the Greeks as sepia, which has evolved in English to mean a brown pigment. Cuttlefish are found is tropical and temperate coastal marine waters off Europe, Asia, Africa, and Australia, but not the New World. They eat crustaceans and small fish and are in turn eaten by larger fish, sharks, and seals. Cuttlefish are also popular food items of humans throughout their range. The cuttlebone is often used by jewelers as a mold for casting small objects, but is more widely given to parakeets and other caged birds as a source of dietary calcium.