VERTEBRATA (Neo-Lat. nom. pl. of Lat. t•ertebratus, jointed, articulated. from vertebra, joint, vertebra, from rent err, to turn). A sub division of the phylum Chordata, which includes the largest and most differentiated or highly de veloped of animals. The group called vertebrates, or back-boned animals, may be defined as fol lows: Segmented Metazoa provided at some stage of development with a gelatinous supporting rod or `ehorda' running through the long axis of the body, and with throat-elefts or They have a nmin nerve•tube lying near the dor sal surface of the body which enlarges a nti.riorly to a brain. There is an internal, metauueric, cartilaginous or bony skeleton, consisting typi sally of an axial series of parts replacing the chorda; of a box (cranium), unclosing the brain ; of a series of arches (neural arches) protecting the nerve-tube; and of supports to the body-walt (ribs). The heart is ventral to the alimentary tract, which begins in a ventrally mouth and ends in a ventral anus, placed near the base of the tail. Vertebrates are the only group of animals in which the body may be divided into head, trunk, and tail, the tail being postanal. They alone are characterized by having typically, except as lost by degeneration, four limbs. They have an outer skin that consists of more than a single layer of cells. They have an internal skeleton that is essentially living, containing active cells within it. Each pair of limbs, when present, is supported by a bony girdle, which may in extreme cases be rudimentary. The skull, ex cept iu Cyclostomi, consists of cranium and lower jaw. The jaws are typically provided with teeth, but these are lost in turtles and birds. The cen tral nervous system consists of a nerve-tube (spinal cord) and brain, with which latter the main sense•organs are connected. The brain is the special seat of sense-perception, of voluntary motor-impulses, and of the higher intellectual faculties. From brain and spinal cord a merically repeated series of nerves runs off to the skin, viscera, and muscles. The sense-organs con
sist of an olfactory pit usually paired; paired camera eyes, with lens; a pair of auditory or gans (pits or closed cavities) usually comprising three semicircular canals. In addition to the cerebrospinal system is a visceral system (the sympathetic). The digestive tract is a tube sep arated from the body-wall by the peritoneum lined body-cavity. The digestive tube is divided into oesophagus and stomach, the small intestine with liver and pancreas, and the large intestine. ( See SYSTEM.) Respiratory organs are typically present, either as gills in the aquatic vertebrates or as lungs. In some sala manders, however, respiration is by the throat and entire skin. The gills (q.v.) are thin plates typically placed in two rows on the gill-arches between the slits. The lungs, which arise, ap parently, as swimming bladders in fishes. come to lie ventral and to have a purely respiratory function in land vertebrates. The circulatory system (q.v.) consists of a set of vessels almost wholly cut off from the body-cavity, and carry red blood, due to the red blood-corpuscles. This fluid is propelled by a heart that consists essen tially of two parts, a thin-walled auricle that receives blood entering from the trunk behind, and a thick-walled muscular ventricle that forces the blood out forward, to the respiratory organs, head, and trunk. In addition to the blood-vessels are the lymph-vessels, spaces incompletely cut off from the and carrying chiefly white blood-corpuscles and special nutriment to the blood. Special excretory organs (see EXCRETORY SYSTEM) are present, essentially consisting of paired, metamerieally repeated tubules into which blood-vessels pour their waste material. The tubules empty into a common paired duct, which also may serve, in the male, to convey the sexual products to the exterior. Reproduction is always sexual and the sexes are usually separate. See