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CHAPTER 34 VERTEBRATE EVOLUTION AND DIVERSITY Introduction Humans and their closest relatives are vertebrates . This group includes other mammals, birds, lizards, snakes, turtles, amphibians, and the various classes of fishes. They share several unique features including a backbone, and a series of vertebrae. A. Invertebrate Chordates and the Origin of Vertebrates The vertebrates belong to one of the two major phyla in the Deuterostomia, the chordate s. The phylum Chordata includes three subphyla, the vertebrates and two phyla of invertebrates, the urochordates and the cephalochordates. 1. Four anatomical features characterize 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, postanal 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.
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While the notochord persists in the adult stage of some invertebrate chordates and primitive vertebrates, it remains as only 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 develops in the vertebrate embryo 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 gill slits connect the pharynx, just posterior to the mouth, to the outside of the animal. These slits 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. 2. Invertebrate chordates provide clues to the origin of vertebrates Most urochordates , commonly called tunicates , are sessile marine animals that adhere to rocks, docks, and boats.
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This note was uploaded on 08/28/2010 for the course SCIENCE 101 taught by Professor Wong during the Spring '10 term at Rutgers.

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