Pelagic Octopus. Octopuses have 2 eyes and 4 pairs of arms and are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms. Octopus lack an internal or external skeleton allowing them to squeeze through tight places. Octopuses are among the most intelligent and behaviorally flexible of all invertebrates.
The Argonauts are a group of pelagic octopuses. They are also called paper nautiluses, referring to the paper-thin eggcase that females secrete. This structure lacks the gas-filled chambers present in chambered nautilus shells and is not a true cephalopod shell, but rather an evolutionary innovation unique to the genus Argonauta. It is used as a brood chamber and for trapped surface air to maintain buoyancy.