The Octopus is a cephalopod mollusc of the order Octopoda . Octopuses have two eyes and four pairs of arms, and like other cephalopods, they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms. Octopuses have no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles), allowing them to squeeze through tight places. Octopuses are among the most intelligent and behaviorally flexible of all invertebrates. Octopuses have a relatively short life expectancy, and some species live for as few as six months. A group of octopuses is called a consortium.
The octopus inhabits many diverse regions of the ocean, including coral reefs, pelagic waters, and the ocean floor. They also live in deep-water habitats, while incirrate octopus species are found in reefs and other shallower seafloor habitats.
Octopuses are characterized by their eight arms, usually bearing suction cups. The arms of octopuses are often distinguished from the pair of feeding tentacles found in squid and cuttlefish. Both types of limbs are muscular hydrostats. Unlike most other cephalopods, the majority of octopuses – those in the suborder most commonly known, Incirrina – have almost entirely soft bodies with no internal skeleton. They have neither a protective outer shell like the nautilus, nor any vestige of an internal shell or bones, like cuttlefish or squid. A beak, similar in shape to a parrot's beak, is the only hard part of their bodies. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from moray eels or other predatory fish. The octopuses in the less familiar Cirrina suborder have two fins and an internal shell, generally reducing their ability to squeeze into small spaces.
Bottom-dwelling octopuses eat mainly crabs, polychaete worms, and other molluscs such as whelks and clams. Open-ocean octopuses eat mainly prawns, fish and other cephalopods. They usually inject their prey with a paralysing saliva before dismembering it into small pieces with their beaks. Octopuses feed on shelled molluscs either by using force, or by drilling a hole in the shell, injecting a secretion into the hole, and then extracting the soft body of the mollusc.