Octopus granulatus, a South African species.—Anthony BannisterThe Natural History Photographic Agency/EB Inc.
In general, any eight-armed cephalopod of the order Octopoda; specifically, members of a large, widely distributed group (genus Octopus) of shallow-water species. Species range from about 2 in. (5 cm) to 18 ft (5.5 m) long with an arm span up to 30 ft (9 m). The head is usually only slightly demarcated from the saccular body. Each arm is contractile and bears fleshy suckers. Two sharp beaks and a filelike organ in the mouth drill crustacean shells and rasp away flesh. Most octopuses crawl along the bottom; when alarmed, they may jet-propel themselves backward, and they sometimes eject an inky substance to cloud the water and protect themselves from predators. They can change colour rapidly, a reflection of their environment or mood. The common octopus (O. vulgaris) is thought to be the most intelligent of all invertebrates.