cephalization

thumb|upright=1.5|A lobster is heavily cephalized, with eyes, antennae, multiple mouthparts, and the brain (inside the armoured [[exoskeleton), all concentrated at the animal's head end.]]

Cephalization is an evolutionary trend in bilaterian animals in which, over a sufficient number of generations, special sense organs and nerve ganglia concentrate towards the front of the body, often producing an enlarged head. This is associated with the animal's movement direction and bilateral symmetry. Cephalization of the nervous system has led to the formation of a brain with varying degrees of functional centralization in three phyla of bilaterian animals, namely the arthropods, cephalopod molluscs, and vertebrates. Hox genes organise aspects of cephalization in the bilaterians.