Chapter 34 Vertebrates 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 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 muscular post anal tail These chordate characteristics are a notochord a dorsal hollow nerve cord pharyngeal slits and a The notochord present in all chordate embryos is a longitudinal flexible rod located between the digestive tube and the nerve cord While the notochord persists in the adult stage of some invertebrate chordates and primitive vertebrates it 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 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 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 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 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 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 Tunicates Members of the subphylum Urochordata commonly called tunicates belong to the deepest branching lineage of chordates 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 2 3 4 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 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 Lancelets are suspension feeders feeding by trapping tiny particles on mucous nets secreted across the Lancelets Adult lancelets retain key chordate characteristics 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 which primarily occurs across the external body surface Though feeble swimmers their swimming mechanism resembles that of fishes through the coordinated A lancelet frequently leaves its burrow to swim to a new location 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 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 characters as adults 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 The degenerate adult stage of tunicates appears to be a derived trait that evolved only after the tunicate against it 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 develop the posterior part of its body axis Rather the anterior region is elongated and contains a heart and digestive system Research on lancelets has revealed important clues about the evolution of the chordate brain Rather than a full fledged brain