An octopus has 9 brains — one central and one in each arm
An octopus has a central brain that coordinates overall behaviour, but each of its eight arms also contains its own cluster of neurons — essentially a local brain — that can operate semi-independently. About two-thirds of an octopus's neurons are in its arms, not its central brain. An arm that has been severed from the body can continue to respond to stimuli and attempt to bring food to where the mouth used to be for up to an hour. The arms effectively 'think' locally, reducing the processing load on the central brain.
We conceptualise brains as singular centralised control centres. Discovering that an animal distributes its cognition across nine semi-independent processing units — with arms that can act intelligently without consulting the head — makes the concept of 'a brain' feel much less settled.
“An octopus has 9 brains — one central and one in each arm. Two-thirds of its neurons are in its arms. A severed arm will still try to bring food to where the mouth used to be. 🐙 #OddlyHuman”