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the intensified development of the cephalic portion of the body in bilaterally symmetric animals during the process of evolution. The head end of the body, which includes the oral orifice, is first to experience new objects in the environment. As a result, the sense organs (vision, olfaction, touch, and hearing, the latter being developed only in vertebrates) and the sections of the nervous system that regulate the sense organs and make up the brain are concentrated in the head end of the body. In vertebrates a skull has developed to protect the aforementioned organs. In invertebrates this protective function is performed by a hard external cover. In cephalopods the brain is protected by a cartilage capsule.