Cuttlefish, small cephalopod (family Sepiidae) of Old World coastal waters. Its head is attached to 8 short arms and a pair of long tentacles for catching prey. Inside the body there is a flat, chalky cuttlebone that is used as a support and a buoyancy mechanism. The cuttlefish is a good swimmer. When alarmed, it protects itself by shooting out a jet of inky fluid and jerking violently backwards.