VERTEBRATE

also called Craniata, any animal of the subphylum Vertebrata, the predominant subphylum of the phylum Chordata. They have backbones, from which they derive their name. The vertebrates are also characterized by a muscular system consisting pimarily of bilaterally paired masses and a central nervous system partly enclosed within the backbone. The subphylum is one of the best known of all groups of animals. Its members include the classes Agnatha, Chondrichthyes, and Osteichthyes (all fishes); Amphibia (amphibians); Reptilia (reptiles); Aves (birds); and Mammalia (mammals). any animal of the subphylum Vertebrata, having a vertebral column, or backbone. The vertebrates, which include the fishes, amphibians, reptiles, birds, and mammals, are the predominant members of the Chordate phylum. In addition to the backbone from which they derive their name, the vertebrates are characterized by (1) a muscular system consisting primarily of bilaterally paired masses and (2) a central nervous system partly enclosed within the backbone. Vertebrates are unique in possessing an internal skeleton formed of either cartilage or bone, or both. By its ability to provide support during growth, this skeleton allows vertebrates to achieve large size, so that most vertebrates are bigger than most invertebrates. Except in the most primitive forms, the skeleton consists of skull, vertebral column, and two pairs of limb elements, although one or both pairs of limbs are absent from a few higher forms (e.g., snakes, whales) that have lost limbs in the course of evolution. The skull, by providing a secure housing for brain, eyes, ears, and olfactory organs, has facilitated the evolution of intelligence and a high degree of responsiveness to the environment. The vertebral column and limb skeleton provide support for the body as a whole: movement is effected by the action of muscles that are attached to the bones or cartilage. The mass of musculature forms the contours of the body. The outer surface is covered by skin that protects the inner parts not only by providing a general covering but also by forming structures of specific protective value, such as bony or horny scales, feathers, and fur. Internally the vertebrate trunk is a hollow cavity in which the visceral organs are suspended. The heart and respiratory organs are closely associated. The heart lies just behind the gills or between the lungs; it sends unoxygenated blood directly into these organs for oxygenation and removal of carbon dioxide. The digestive tract includes esophagus, stomach, and intestine and usually terminates in a cloaca, a chamber common to the digestive, genital, and urinary systems. In mammals, however, the digestive tract acquires a separate terminal opening. The central nervous system consists of brain and spinal cord. Although both of these become very thick-walled, they always retain a small central canal; i.e., they are hollow. The brain in lower vertebrates is devoted largely to serving the sense organs of the head. During the course of evolution, however, the brain becomes much larger, relative to body size, through the development of association areas that permit more intensive interchange of information among the parts of the brain. The spinal cord, extending backward from the base of the skull, gives off pairs of nerves at repeated intervals. These nerves run to the skin, muscles, and internal organs. The brain also gives off a series of nerves, not regularly arranged, of which one passes through the neck to innervate the heart, lungs, and other viscera. Vertebrate history may be traced back to the Silurian period, approximately 438 million to 408 million years ago. Additional reading Charles G. Crispens, Jr., The Vertebrates, Their Forms and Functions (1978); J.Z. Young, The Life of Vertebrates, 3rd ed. (1981); Libbie H. Hyman, Hyman's Comparative Vertebrate Anatomy, 3rd ed., edited by Marvalee H. Wake (1979); Edwin H. Colbert, Evolution of the Vertebrates: A History of the Backboned Animals Through Time, 3rd ed. (1980); Alfred Sherwood Romer and Thomas S. Parsons, The Vertebrate Body, 6th ed. (1986); Leonard B. Radinsky, The Evolution of Vertebrate Design (1987); Robert L. Carroll, Vertebrate Paleontology and Evolution (1988); and F. Harvey Pough, John B. Heiser, and William N. McFarland, Vertebrate Life, 3rd ed. (1989). Malcolm T. Jollie