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The Distributed Octopus
Most of an octopus's neurons never see the inside of its head.
An octopus has around 500 million neurons — a dog's worth — but two-thirds of them are not in its head. They are distributed through its eight arms, each of which can taste, touch, and execute complex motions with a degree of independence.
Sever the connection to the central brain (as happens in nature, since arms are sometimes lost) and an arm will still reach, grasp, and withdraw from painful stimuli. The octopus is the closest thing Earth has produced to a genuinely different architecture for a mind, which is why it fascinates roboticists as much as biologists: it is an existence proof that central control is optional.
Photo: Diego Delso, via Wikimedia Commons (CC BY-SA 4.0).