Octopuses have no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles), allowing them to squeeze through tight places. Octopuses are among the most intelligent and behaviorally flexible of all invertebrates.
The beak, similar in shape to a parrot’s beak, and made of chitin, is the only hard part of their bodies. This enables them to squeeze through very narrow slits between underwater rocks.
An octopus has a highly complex nervous system, only part of which is localized in its brain. Two-thirds of an octopus’s neurons are found in the nerve cords of its arms, which have limited functional autonomy.
Octopus arms show a variety of complex reflex actions that persist even when they have no input from the brain.
The neurological autonomy of the arms means the octopus has great difficulty learning about the detailed effects of its motions. The brain may issue a high-level command to the arms, but the nerve cords in the arms execute the details.
There is no neurological path for the brain to receive proprioceptive feedback about just how its command was executed by the arms; the only way it knows just what motions were made is by observing the arms visually, i.e. exteroception.