The octopus (plural octopuses) is a soft-bodied, eight-limbed mollusc of the order Octopoda (/ɒktˈɑːpədə/, ahk-TAH-pə-də). Around 300 species are recognised, and the order is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like other cephalopods, the octopus is bilaterally symmetric with two eyes and a beak, with its mouth at the center point of the eight limbs ("tentacle" is used as an umbrella term for cephalopod limbs; however, within a teuthological context, "arm" is used to refer to such limbs while "tentacle" is reserved for feeding appendages not found on octopuses). The soft body can rapidly alter its shape, enabling octopuses to squeeze through small gaps. They trail their eight appendages behind them as they swim. The siphon is used both for respiration and for locomotion, by expelling a jet of water. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse of all invertebrates.