The question of which animals possess “more than one brain” is a fascinating entry point into the sheer complexity and diversity of the animal kingdom’s nervous systems. While humans and other vertebrates feature one highly centralized brain, many invertebrates have evolved radically different neurological architectures. These systems often distribute processing power across the body, creating structures that function as local control centers, which people often mistake for multiple brains. Exploring these decentralized designs reveals that the concept of a single, dominant brain is more of an evolutionary specialization than a universal requirement for coordinated movement.
Defining the “Multiple Brain” Concept
The common understanding of a brain as a single, centralized organ is based largely on the vertebrate model. A true brain is a highly concentrated mass of nervous tissue, typically located in the head, responsible for integrating sensory input and coordinating advanced behaviors. A ganglion, on the other hand, is a much simpler cluster of nerve cell bodies and associated tissue that acts as a local processing center. Ganglia are generally found outside the central nervous system and primarily function as relay stations, managing specific body segments or functions. When people speak of animals with “multiple brains,” they are referring to a body plan that utilizes many of these ganglia to distribute decision-making, rather than having several full-scale, centralized brains.
The Cephalopod Model: Central and Autonomous Control
The octopus is the most prominent example of an animal whose nervous system is so distributed that it appears to have multiple brains, though it possesses a single, highly developed central brain. Its neural architecture is unique among invertebrates, featuring approximately 500 million neurons, two-thirds of which are spread throughout the eight arms in large nerve cords. This arrangement results in significant arm autonomy, allowing each arm to make decisions locally without constant input from the central brain. The arms can perform complex motor tasks, such as tasting, sensing, and grasping, even after being disconnected from the brain. The central brain sends high-level commands, but the arm’s nerve cords handle the minute details of execution, including coordinating the hundreds of suckers.
Segmented Structures: The Role of Ganglia Chains
Decentralized control is also found in segmented animals, such as earthworms, leeches, and insects. These creatures build their nervous systems around a linear, repeating chain of ganglia that corresponds to their body segments. In an earthworm, nearly every segment of the body contains its own dedicated ganglion cluster along a ventral nerve cord. While a main supraesophageal ganglion (the “brain”) manages anterior sensory organs and overall direction of movement, local segmental ganglia handle motor control and reflex actions. If a particular segment of a leech is touched, the local ganglion for that segment can initiate a withdrawal reflex without needing to wait for a signal to travel all the way to the head and back.
Simple Radial Systems
The simplest distributed nervous systems are seen in organisms with radial symmetry, such as the sea star. These animals lack a centralized brain and cephalization—the concentration of nervous tissue in a head region. Their nervous system is organized around a nerve ring that encircles the mouth, which branches into a radial nerve running the length of each arm. Decision-making and sensory processing are distributed throughout this network, with the radial nerves handling most information. When a sea star moves, the arm sensing an attractive stimulus often takes temporary command, leading the direction of movement. This system allows for slow, coordinated movement and sensory input, where the entire nervous network functions as a unified, decentralized unit.