Overview: Half a Billion Years of Backbones
* Vertebrates are named for vertebrae, the series of bones that make up the vertebral column or backbone. * There are about 52,000 species of vertebrates, far fewer than the 1 million insect species on Earth. * Plant-eating dinosaurs, at 40,000 kg, were the heaviest animals to walk on land. * The biggest animal that ever existed is the blue whale, at 100,000 kg. * Humans and our closest relatives are vertebrates.
* This group includes other mammals, birds, lizards, snakes, turtles, amphibians, and the various classes of fishes. Concept 34.1 Chordates have a notochord and a dorsal, hollow nerve cord * The vertebrates belong to one of the two major phyla in the Deuterostomia, the chordates. * Chordates are bilaterian animals, belonging to the Deuterostomia. * The phylum Chordata includes three subphyla, the vertebrates and two phyla of invertebrates—the urochordates and the cephalochordates. Four derived characters define the phylum Chordata.
* Although chordates vary widely in appearance, all share the presence of four anatomical structures at some point in their lifetime. * These chordate characteristics are a notochord; a dorsal, hollow nerve cord; pharyngeal slits; and a muscular, post-anal tail. 1. The notochord, present in all chordate embryos, is a longitudinal, flexible rod located between the digestive tube and the nerve cord. * It is composed of large, fluid-filled cells encased in fairly stiff, fibrous tissue. * It provides skeletal support throughout most of the length of the animal. * While the notochord persists in the adult stage of some invertebrate chordates and primitive vertebrates, it remains only as a remnant in vertebrates with a more complex, jointed skeleton. * For example, it is the gelatinous material of the disks between vertebrae in humans. 2. The dorsal, hollow nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a tube dorsal to the notochord. * Other animal phyla have solid nerve cords, usually located ventrally. * The nerve cord of the chordate embryo develops into the central nervous system: the brain and spinal cord. 3. The digestive tube of chordates extends from the mouth to the anus. * The region posterior to the mouth is the pharynx. * In all chordate embryos, a series of pouches separated by grooves forms along the sides of the pharynx. * In most chordates, these grooves (known as pharyngeal clefts) develop into pharyngeal gill slits that allow water that enters the mouth to exit without continuing through the entire digestive tract. * In many invertebrate chordates, the pharyngeal gill slits function as suspension-feeding devices. * The slits and the structures that support them have become modified for gas exchange (in aquatic vertebrates), jaw support, hearing, and other functions during vertebrate evolution. 4. Most chordates have a muscular tail extending posterior to the anus. * In contrast, nonchordates have a digestive tract that extends nearly the whole length of the body. * The chordate tail contains skeletal elements and muscles. * It provides much of the propulsive force in many aquatic species. Invertebrate chordates provide clues to the origin of vertebrates. * Members of the subphylum Urochordata, commonly called tunicates, belong to the deepest-branching lineage of chordates. * They most resemble chordates during their larval stage, which may be brief. * The tunicate larva uses its tail muscles and notochord to swim through the water in search of a suitable substrate on which it can settle, guided by cues from light- and gravity-sensitive cells. * Tunicates undergo a radical metamorphosis to form a sessile adult with few chordate characteristics. * Its tail...