the anterior part of the body of an animal when marked off by a difference in size, or by the constriction called a neck.
The presence or absence of a head was much used formerly as a character in classification. Thus Latreille divided animals into the headless, and those provided with a head; and these again formed two groups, the Vertebral animals, with heads properly so called, and Cephalidia, with small, indistinct, heads. But this classification would separate the oyster and all other lamellibranch mollusks from the snail, cuttlefish, etc.; it is, in fact, an artificial character. The mouth and principal nervous organs are the guides to the anterior end of the body, where the head, when recognizable, is situated.
The head of the vertebrated animals presents a regular series of increasing complexity from the lancelet upward. In that fish the most anterior part of the nervous cord is lodged in a canal scarcely distinct from the rest of it. Ascending in the series, it becomes evident that as the anterior nervous mass enlarges, and the ganglia increase in complexity, the anterior vertebrae change their character; as the brain becomes specialized so does the brain case or skull. In man the brain attains its highest development and the head its greatest complexity, the difference between skull and face being now most pronounced. The increasingly globular form of skull in the vertebrates is due to the greater increase of the cerebral hemispheres relatively to that of the base of the brain and axis of the skull; hence the brain comes in man to overhang the face. See SKULL.
Entry from Everybody's Cyclopedia