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Any of various marine mollusks of the class Cephalopoda, having long tentacles around the mouth, a large head, a pair of large eyes, and a sharp beak. Cephalopods have the most highly developed nervous system of all invertebrates. Many cephalopods squirt a cloud of dark inky liquid to confuse predators. Cephalopods include the octopus, squid, cuttlefish, and nautilus, and the extinct ammonites, belemnites, and other nautiloids.