A head is the cephalic part of an animal, which usually comprises the eyes, ears, nose and mouth, each of which aid in various sensory functions, such as sight, hearing, smell, and taste. Some very simple animals may not have a head, but many bilaterally symmetric forms do. Heads develop in animals by an evolutionary trend known as cephalization. In bilaterally symmetrical animals, nerve tissues concentrate at the anterior region, forming structures responsible for information processing. Through biological evolution, sense organs and feeding structures also concentrate into the interior region; these collectively form the hed.