The size of the opening through which they can squeeze is only restricted by the size of their beaks.
A few octopuses in the Cirrina suborder do have internal shells though as well as two fins, though very little is yet known about these fascinating species.
The octopuses beak is the hardest part of most octopus bodies which is used primarily to break through the external skeletons of crustaceans and the shells of bivalves.
