All octopuses lack an external shell. Additionally, most lack an internal shell although few species still have a small remnant shell in their mantle. The species that have a small internal shell all belong to the group of octopus known as the Cirrina, or "deep-sea octopuses".
Octopuses have eight arms that are covered with circular suckers (suction cups). Their suckers enable them to cling to surfaces as well as to grasp prey.
Octopuses, like all cephalopods, are bilaterally symmetrical. They have two eyes and a beak-like structure around their mouth that enables them to bite and tear their food.