Cephalization

Cephalization is considered an evolutionary trend, whereby nervous tissue, over many generations, becomes concentrated toward one end of an organism. This process eventually produces a head region with sensory organs.

Cephalization is intrinsically connected with a change in symmetry. It accompanied the move to bilateral symmetry made in flatworms, with ocelli and pinnae placed in the head region. In addition to a concentration of sense organs, all animals from annelids on also place the mouth in the head region. This process is also tied to the development of an anterior brain in the chordates from the notochord. A notable exception to the trend of cephalization throughout evolutionary advancement is phylum Echinodermata, which, although having a bilateral ancestor, as evidenced by their embryology, develop into a pentaradial animal with no concentrated neural ganglia or sensory head region. However, some echinoderms have developed bilateral symmetry secondarily.