Most of an octopus's neurons are in its arms

Two-thirds of an octopus's neurons sit outside its brain. Most of its nervous system lives in its arms, which can sense and act with a striking amount of local independence. The common octopus has around 500 million neurons — comparable to a dog — with about two-thirds of them in the arms rather than the central brain. The central brain holds roughly 170-180 million, while each arm carries its own bundle plus a brachial ganglion at its base. Severed arms, when electrically stimulated, still move in the same basic patterns as arms under normal control. Worth noting: a study suggests arms and brain are more connected than previously thought, so “nine brains” is a metaphor, not a literal claim.

Most of an octopus's neurons are in its arms

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