Only a few invertebrates such as sponges, jellyfish, adult sea squirts and starfish do not have a brain; diffuse or localised nerve nets are present instead.

The brain is located in the head, usually close to the primary sensory organs for such senses as vision, hearing, balance,taste, and smell. The brain is the most complex organ in a vertebrate’s body. In a typical human, the cerebral cortex (the largest part) is estimated to contain 15–33 billion neurons, each connected by synapses to several thousand other neurons.