An octopus is a cephalopod mollusc of the order Octopoda. It has two eyes and four pairs of arms and, like other cephalopods, it is bilaterally symmetric. An octopus has a hard beak, with its mouth at the centre point of the arms. An octopus has no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles), allowing it to squeeze through tight places. Octopuses are among the most intelligent and behaviourally flexible of all invertebrates. They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water, and their ability to hide. All octopuses are venomous, but only one group, the blue-ringed octopus, is known to be deadly to humans. Engraving captioned: Octopus (Octopus vulgaris) 1878.