In animals, the brain is the control center of the central nervous system, responsible for behavior. The brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, equilibrioception (balance), sense of taste, and olfaction (smell). While all vertebrates have a brain, most invertebrates have either a centralized brain or collections of individual ganglia. Some animals such as cnidarians and echinoderms do not have a centralized brain, instead have a decentralized nervous system, while animals such as sponges lack both a brain and nervous system entirely. Brains can be extremely complex. For example, the human brain contains roughly 23 billion neurons, each linked to as many as 10,000 other neurons.