any chordate animal possessing a segmented spinal column in the adult stage. In many widely accepted systems of classification, these animals are grouped into the subphylum Vertebrata, a division only slightly less comprehensive than the phylum Chordata (see CHORDATE,). The subphylum includes mammals (including humans), birds, reptiles, amphibians, fish, rays, and lampreys. The vertebrate animals have a long evolutionary history; the earliest well-differentiated vertebrate fossils occur in the rock strata laid down during the Ordovician period of the Paleozoic era. See ANIMAL,; EVOLUTION,; PALEONTOLOGY,.

The vertebrate body is typically elongated and exhibits bilateral symmetry. Two pairs of appendages are usually present, but these may be greatly modified to adapt the animals to a highly specialized habitat: for example, the wings of birds, needed for flying, and the fins of fish, needed for swimming. Alternatively, these appendages may be altogether undeveloped, as in the lampreys. The central nervous system, made up of a spinal cord that is enlarged and highly differentiated anteriorly to form a brain, is a tube that lies dorsal to the notochord or vertebral column (see below) and parallel to the long axis of the body. In the head of most vertebrates the brain is enclosed within a bony skull called the cranium. The head, at the anterior end of the body, also contains the main sensory organs of sight, sound, and smell and the mouth.

At some stage in the development of vertebrate animals, the notochord, a supporting rod, develops along the median dorsal axis of the body; in most vertebrates this notochord is later replaced by a segmented spinal column consisting of a series of small bones with articulating surfaces. The spinal column, or backbone, provides great flexibility to the body. The development of vertebrates is accompanied by the formation of a series of slits, or gill clefts, in the walls of the pharynx. In aquatic forms these gill clefts become associated with highly vascular filamentous gills, which function together throughout life as organs of respiration. Terrestrial and amphibious vertebrates breathe by means of lungs; the gill slits function as breathing apparatus in those animals having a larval stage in their development but are vestigial in all the adult animals.

The urogenital system and the excretory end of the digestive tract have ventral openings near the posterior end of the body. The heart and main circulatory vessels are located dorsal to the digestive tract. The trunk of the body contains a body cavity, or coelom, lined with peritoneum into which most of the visceral organs are suspended by means of mesenteries. The muscular system is composed of segmented blocks of muscle tissues and is characterized by bilateral symmetry. This segmentation and symmetry is repeated in the nervous, circulatory, and urogenital systems, and in the skeletal framework of the body. All vertebrate REPRODUCTION, (q.v.) is sexual, and the sexes are almost always separate. Protective structures, such as scales, feathers, and hair, form as outgrowths from the skin.        K.A.C., KENNETH A. CHAMBERS, M.S.

For further information on this topic, see the Bibliography, sections 438. Biology, 458. Zoology.