Cephalization

Cephalization is an evolutionary trend in which sensory and nervous organs are placed in the anterior region of an organism. This creates a “cephalic region” at the anterior, forming a head and a brain. Due to the focus of an anterior (and by proxy, a posterior) region, cephalization is commonly seen in organisms alongside bilateral symmetry. Bilaterally symmetric organisms locomote head-first and thus require their sensory and nervous organs to be at their anterior. As such, it is one of the characteristic traits of members of the clade Bilatera.