Chapter 34 VertebratesOverview: 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 (bilaterally symmetrical) animals, belonging to the clade 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 thedigestive 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. Pharyngeal Slits or Clefts 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.Tunicates- 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 and notochord are resorbed, its nervous system degenerates, and its organs rotate 90 degrees.- Tunicates are suspension feeders. Seawater passes inside the animal via an incurrent siphon, through the pharyngeal gill slits, and into a ciliated chamber, the atrium.1 Food filtered from the water is trapped by a mucous net that is passed by cilia into the intestine. Filtered water and feces exit through an anus that empties into an excurrent siphon.Lancelets- Lancelets (members of the subphylum Cephalochordata) are blade-like in shape. As larvae, lancelets develop a notochord; dorsal, hollow nerve cord; numerous gill slits; and post-anal tail. They live with their posterior end buried in the sand and the anterior end exposed for feeding.- Adult lancelets retain key chordate characteristics.- Lancelets are suspension feeders, feeding by trapping tiny particles on mucous nets secreted across the pharyngeal slits. Ciliary pumping creates a flow of water with suspended food particles into the mouth and out the gill slits. In lancelets, the pharynx and gill slits are feeding structures and play only a minor role in respiration, whichprimarily occurs across the external body surface.- A lancelet frequently leaves its burrow to swim to a new location.- Though feeble swimmers, their swimming mechanism resembles that of fishes through the coordinated contraction of serial muscle blocks. Contraction of chevron-shaped muscles flexes the notochord and produces lateral undulations that thrust the body forward. The muscle segments develop from blocks of mesoderm, called somites, arranged serially along each side of the notochord of the embryo.- Tunicates and lancelets may provide clues about the evolutionary origin of the vertebrate body plan.- Tunicates display a number of chordate characteristics only as larvae, while lancelets retain those characters as adults. Thus, an adult lancelet looks more like a larval tunicate than like an adult tunicate.- In the 1920s, biologist William Garstang suggested that tunicates represent an early stage in chordate evolution. This stage may have occurred through paedogenesis, the precocious development of sexual maturity in a larva. Garstang proposed that ancestral chordates became sexually mature while still in the larval stage.- The paedogenetic hypothesis is deduced from comparing modern forms, but the weight of evidence is against it.- The degenerate adult stage of tunicates appears to be a derived trait that evolved only after the tunicate lineage branched off from other chordates. Even the tunicate larva appears to be highly derived. Studies of Hox gene expression suggest that the tunicate larva does not