VERTEBRATA, a group name, no longer significant in classification, for all those animals (the vertebrates) considered collectively which possess a backbone, composed of vertehrm. (See ANAmmv; OSTEOLOGY). This category would include mammals, birds, reptiles, am phibians and fishes. These have important characteristics in common that distinguish them from mollusks, insects, crustaceans, worms and other animals of simpler type. Yet it was not until 1797 that the distinctive char acteristics were stated by Lamarck, who drew a firm line between "back-boned,D or vertebrate and Thackboneless," or' invertebrate, forms. Anatomists and embryologists have since made the distinctions that Lamarck pointed out more precise, and the more import ant characteristics may be surruned up as fol lows: (1) In vertebrates the central nervous system, namely, the brain and the spinal cord, lies on the dorsal surface of the body, and is tubular in structure. (2) In all young verte brates there is formed along the dorsal sur face of the gut, and, therefore, of hypoblastic origin, a supporting rod or ttotochord, which in the simpler forms may persist throughout life, but in higher forms is more or less completely replaced by the backbone— an axis developed from the mesoblastic sheath of the notochord. (3) In almost all young vertebrates several pairs of slits or clefts open from the pharynx to the exterior; in some amphibians, all fishes and simpler forms they persist throughout life as respiratory organs, and are usually associated with feathery. gills; in most amphibians th disappear dunng adolescence; in reptiles, birds and mammals they are practically functionless vestigial organs, which in a few cases do not even open. (4) A great part— for example,
the retina— of the vertebrate eye arises as an outgrowth from the brain, whereas the eye of invertebrates develops as a direct insinking of the sldn. (5) In vertebrates the heart is formed on the ventral surface, while that of invertebrates is dorsal. (6) Finally, vertebrates agree with annelids and arthropods among the invertebrates in being bilaterally symmetrical segmented animals. The segmentation is shown by the distribution of the nerves and ganglia, by the gill-clefts by the series of vertebrm, by the muscle-segments and nephridia (kidney tubes) in embryonic life at least.
But, while our knowledge of these charac teristics has become more precise, it is no longer possible to draw a boundary line between vertebrates and invertebrates with a firm hand. It can no longer be said that fishes form the base of the vertebrate series, for hag and lam prey (Cyclostomi), though in many ways more primitive, are certainly vertebrates; the lancelet (Amphioxus), though perhaps degenerate, can not be excluded from the alliance; the Tunicata (q.v.), though almost always degenerate in adult life, are all vertebrates in their youth, and the worm-like Balanoglossus (q.v.) has also certain hardly disputable vertebrate char acters. The possession of a bacicbone is, there fore, no longer so exclusively definite a mark of the group, as it once was, but instead, the presence of a notochord. The term vertebrate has, therefore, ceased to have scientific signifi cance although it remains convenient for popu lar use to designate the groups more properly styled CHORDATA.