The octopus is a cephalopod mollusk in 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. Most octopuses have no internal or external skeleton, allowing them to squeeze through tight places. Octopuses are highly intelligent, probably the most intelligent of all invertebrates. The octopus inhabits many diverse regions of the ocean, especially coral reefs. For defense against predators, they hide, flee quickly, expel ink, or use color-changing camouflage. An octopus trails its eight arms behind it as it swims. All octopuses are venomous, but only the small blue-ringed octopuses are deadly to humans. In the larger sense, there are around 300 recognized octopus species, which is over one-third of the total number of known cephalopod species. The term octopus may also be used to refer only to those creatures in the genus Octopus.