Cephalization

Cephalization is the evolutionary tendency for nervous tissue and sensory organs to become concentrated at the anterior end of an animal, forming a head region that typically houses a brain and specialized sense organs. It is most prominent among bilaterians with active, forward-directed movement and centralized nervous systems. In contrast, many cnidarians and other organisms retain diffuse nerve nets with little or no true cephalization.

During ontogeny and evolution, the anterior nerve cords expand and differentiate into ganglia or a brain, while sensory structures such as eyes, olfactory organs, statocysts, and antennae become localized at the head.

Cephalization is often quantified by a cephalization quotient (CQ) or cephalization index, which compare actual brain mass to that expected for an animal's body mass; higher values indicate greater concentration of nervous tissue in the head.

Functionally, cephalization supports improved sensory processing, decision making, and coordinated locomotion, enabling active predation, complex behaviors, and enhanced environmental interaction. While pronounced in many mollusks, arthropods, and vertebrates, cephalization is not universal and may be reduced in sessile or simple lifestyle organisms. The pattern has evolved independently in multiple lineages, reflecting convergent functional benefits rather than a single ancestral condition.