An octopus is a soft-bodied, eight-limbed mollusk of the order Octopoda, which is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. The order consists of some 300 species. Octopuses are bilaterally symmetric with two eyes and a beaked mouth at the center point of the eight limbs. They are capable of radically altering their shape, enabling them to squeeze through small gaps. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviorally diverse of all invertebrates. They are solitary creatures that live alone in dens built from rocks, which the octopus moves into place using its powerful arms. Octopuses inhabit various regions of the ocean, including coral reefs, pelagic waters, and the seabed; some live in the intertidal zone and others at abyssal depths.