Octopus niveus; Print; The octopus is a soft-bodied; eight-limbed mollusc of the order Octopoda. 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.