Cephalization is the evolutionary tendency of concentration of the nervous command in central structures in which there are grouping of neurons (i.e, brain and ganglia formation).
Evolutionarily the cephalization process begins with the appearance of ganglia (group of neurons) in platyhelminthes and reaches an apex in vertebrates, animals with a cranial box to protect the well-developed brain.
With lateral symmetry the body can be divided into lateral portions, superior, inferior, anterior and posterior. These portions must be integrated and controlled in some manner and this need stimulated the appearance of ganglial complexity and of beings with a head, a privileged extremity of the bilateral body where the nervous central command and important sensory organs are located.