The segments of the anterior end of the body tend to become more and more distinctly separated from the rest of the body forming what is termed head. This differentiation of the head consists chiefly of the localization within the head of the main parts of the nervous system, the brain and organs of special sense. These structures control, to a very large degree, the activities and responses of the rest of the body, as such the head has become the dominant part of the organism. This localization of nervous structures and functions in the head with accompanying dominance of the head is called cephalization, and is more and more marked as the higher one asccnds in the animal kingdom. In the vertebrates cephalization is particularly well marked.
Cephalization is correlated with the advance of hctcronomous condition. As it progresses more and more anterior segments are appropriated by the head, and in general it may be said that higher the degree of cephalization, the greater is the number of segments composing the head. In advanced cephalization, such as is possessed by the vertebrates, it is very difficult, to find out the number and boundaries of the segments actually forming the head.