Vertebrata.

Vertebrata. In popular as well as in more exact classification it has been long recognised that mammals, birds, reptiles, amphibians, and fishes have certain important characteristics in common which distinguish them from molluscs, insects, crustaceans, worms, and other animals of simpler type. Yet it was not until 1797 that the distinctive characteristics were stated with some precision by Lamarck, who drew a firm line between 'backboned,' or vertebrate, and 'backboneless,' or invertebrate, forms. During the 19th century anatomists and embryologists have made the distinctions which Lamarck pointed out yet more precise, and the more important characteristics may be summed up as follows. (1) In vertebrates the central nervous system, viz. the brain and the spinal cord, lies on the dorsal surface of the body, and is tubular in structure. (2) In all young vertebrates there is formed along the dorsal surface of the gut, and therefore of hypoblastic origin, a supporting rod or notochord, 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 they disappear during 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—e.g. 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 skin. (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 vertebrae, by the muscle-segments and nephridia (kidney-tubes) in embryonic life at least.

But, while our knowledge of these characteristics 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 lamprey (Cyclostomata), though in many ways more primitive, are certainly vertebrates; the lancelet (Amphioxus), though perhaps degenerate, cannot be excluded from the alliance; the tunicates, though almost always degenerate in adult life, are all vertebrates in their youth, and the worm-like Balanoglossus has also certain hardly disputable vertebrate characters. Moreover the influence of evolutionary conceptions has led zoologists to disbelieve in the rigid apartness of any type—a disbelief which is, moreover, strengthened by the discovery of vertebrate affinities among various invertebrates—e.g. annelids, nemerteans, and arthropods. In regard to the precise origin of Vertebrata there are several rival theories, but none of these has as yet found a solid foundation. See the articles SPINAL COLUMN and SKELETON, and those on the several divisions of vertebrates.

Classification of Vertebrata or Chordata.