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Read chapter 2 of the text Phylum Chordata The chordates are a group of particular interest to us as we belong to it, being members of the subphylum Vertebrata. – PowerPoint PPT presentation

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Title: Read chapter 2 of the text


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  • Read chapter 2 of the text

2
Phylum Chordata
  • The chordates are a group of particular interest
    to us as we belong to it, being members of the
    subphylum Vertebrata.
  • The chordates include all of the vertebrates
    (fish, amphibians, reptiles, mammals and birds),
    but also two non-vertebrate subphyla the
    Urochordata and the Cephalochordata.

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Phylum Chordata
  • The chordates were in the 19th century considered
    to have been derived from protostome ancestors
    (the annelid, mollusc, arthropod group).
  • However, a better understanding of embryology
    shows that chordates are deuterostomes and the
    invertebrates they are most closely related to
    are the Echinodermata (sea stars, sand dollars,
    sea urchins) and the Hemichordata (acorn worms).

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Protostomes and Deuterostomes
  • Within the eucoelomates there are two major
    evolutionary lineages that split early in the
    history of animals and follow quite different
    developmental pathways.
  • These are the protostomes mouth first and
    deuterostomes mouth second.

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Important differences in development between
protostomes and deuterostomes
  • The differences in development that distinguish
    the protostomes and deuterostomes include
  • Whether cleavage of cells in the early zygote is
    spiral or radial.
  • Whether or not, if the early blastomere is
    separated, each cell can develop into a normal
    larva or not.
  • Whether the blastopore ultimately forms the mouth
    or anus of the organism.
  • Whether or not the organism possesses a coelom
    and how that coelom is formed.

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Protostomes and Deuterostomes
  • Protostomes include the annelids, mollusks, and
    arthropods.
  • Deuterostomes include the echinoderms and
    Chordates, which includes the vertebrates.

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Characteristics of the Chordata
  • Chordates are
  • bilaterally symmetrical
  • triploblastic
  • have a well developed coelom
  • have a complete digestive system

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Germ layers of a triploblastic organism
  • Endoderm innermost germ layer of an embryo.
    Forms the gut, liver, pancreas.
  • Ectoderm Outer layer of cells in early embryo.
    Surrounds the blastocoel. Forms outer epithelium
    of body and nervous system.
  • Mesoderm Third germ layer formed in gastrula
    between ectoderm and endoderm. The coelom forms
    in the mesoderm. Mesoderm gives rise to
    connective tissue, muscle, urogenital and
    vascular systems and peritoneum.

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Coeloms
  • The coelom is a cavity entirely surrounded by
    mesoderm.
  • A coelom provides a tube-within-a-tube
    arrangement which has many advantages
  • Allows visceral organs to grow independently of
    the body wall
  • fluid-filled coelom acts as a hydrostatic
    skeleton in some animals (e.g. earthworms).
  • In mammals the pericardial, peritoneal, and
    pleural cavities are formed from the coelom.

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Coeloms
  • Triploblastic organisms (organisms with three
    germ layers including mesoderm fall into one of
    three different coelomic states
  • Acoelomate mesoderm fills the blastoceol, no
    cavity occurs in the mesoderm. Flatworms and
    nemerteans.
  • Pseudocoelomate mesoderm lines only outer edge
    of blastocoel. No peritoneal lining develops.
    Nematodes and rotifers.
  • Eucoelomate Have a true coelom derived from
    mesoderm and lined with peritoneum. Arthropods,
    annelids, mollusks, echinoderms, vertebrates.

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Five distinctive characteristics of the chordates
  • Five distinctive characteristics separate the
    chordates from all other phyla
  • Notochord
  • Single, dorsal, tubular nerve cord
  • Pharyngeal pouches or slits
  • Endostyle
  • Postanal tail
  • Not all of these characteristics are apparent in
    adult organisms and may appear only in the
    embryonic or larval stages.

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Notochord
  • Notochord the notochord is a flexible, rodlike
    structure. It extends the length of the body and
    is an anchor point for muscles.
  • The notochord bends without shortening so it
    permits the animal to undulate.

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Notochord
  • In nonvertebrates and the jawless vertebrates the
    notochord is present throughout life.
  • However, in the jawed vertebrates it is replaced
    by the vertebral column the remnants of the
    notochord being found in the intervertebral
    disks.

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Single, dorsal, tubular nerve cord
  • In most invertebrates the nerve cord, if present,
    is ventral to the gut.
  • In chordates, in contrast, the nerve cord is
    dorsal to the gut and notochord. The nerve cord
    passes through the neural arches of the
    vertebrae, which protect it.
  • The nerve cord is enlarged in vertebrates into a
    brain, which is surrounded by a bony or
    cartilaginous cranium.

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Pharyngeal pouches and slits
  • Pharyngeal slits occur in aquatic chordates and
    lead from the pharyngeal cavity to the outside.
  • The pharyngeal slits are used as a filter feeding
    device in protochordates (i.e., Urochordata
    (Tunicates)) and Cephalochordata (lancelets e.g.
    Amphioxus).
  • Water containing food is drawn in through the
    mouth by cilia and exits via the pharyngeal slits
    where the particles are trapped in mucus.

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Amphioxus
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Pharyngeal pouches and slits
  • In vertebrates the pharyngeal arches have been
    modified into gills by the addition of a rich
    blood supply and thin gas permeable walls.
  • The contraction of muscles in the pharynx drive
    water through the gills.

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Pharyngeal pouches and slits
  • In amniotes an opening may not form and rather
    than slits only grooves called pharyngeal pouches
    develop.
  • In tetrapods these pouches give rise during
    development to a variety of structures including
    the middle ear cavity, eustachian tube, and
    tonsils.

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Endostyle or thyroid gland
  • The endostyle is found in protochordates and in
    lamprey larvae. It is located on the floor of
    the pharynx and secretes mucus, which is used to
    trap particles.
  • The endostyle works with the pharyngeal slits in
    filter feeding.

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Endostyle or thyroid gland
  • Some cells in the endostyle secrete iodinated
    proteins and are homologous with
    iodinated-hormone secreting thyroid gland, which
    is found in adult lampreys and vertebrates.

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Postanal tail
  • The postanal tail, some musculataure and the
    notochord enable larval tunicates and amphioxus
    to swim.
  • The postanal tail evolved to allow organisms to
    swim and its efficiency has been enhanced by the
    addition of fins. The postanal tail is present
    only in vestigial form in humans (the coccyx)
    although tails as a whole are widespread among
    vertebrates.

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Amphioxus
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Classification of the Chordata
  • There are three subphyla in the Chordata
  • Subphylum Urochordata tunicates
  • Subphylum Cephalochordata lancelets
  • Subphylum Vertebrata fish, amphibians, reptiles,
    birds, mammals, etc.

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Subphylum Urochordata
  • The Urochordata (tunicates named for the tough
    tunic that surrounds the adult) look like most
    unpromising candidates to be chordates and
    relatives of the vertebrates.
  • The largest group, the ascidians or sea squirts
    (Class Ascidiacea) as adults are marine, sessile,
    filter feeding organisms that live either
    solitarily or in colonies.

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Ciona intestinalis (a solitary sea squirt)
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Synoicum pulmonaria  a colonial sea squirt
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Ascidians
  • Adult ascidians lack a notochord and there is
    only a single ganglion in place of the dorsal
    nerve cord.
  • Of the five characteristics of chordates adults
    possess only two pharyngeal gill slits and an
    endostyle, both of which they use in filter
    feeding.

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Ascidians
  • The adult sea squirt draws water in through an
    incurrent siphon and pushes it back out an
    excurrent one.
  • Food particles are filtered out in the pharyngeal
    slits with mucus from the endostyle used to trap
    particles.

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Larval Ascidian
  • Even though the adult ascidian hardly resembles a
    chordate its larva does.
  • Larval ascidians are very small and tadpole-like
    and possess all five chordate characteristics
    previously outlined.

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Young larval ascidian
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Larval Ascidian
  • The larval ascidians role is to disperse and to
    achieve this it is free swimming. However, it
    has only a short larval life (minutes to a couple
    of days) and does not feed during this time.
  • Instead it searches for a place to settle and
    then attaches and metamorphoses into an adult.

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Ascidian metamorphosis
  • During metamorphosis the notochord disappears,
    the nerve cord is reduced to a single nerve
    ganglion and a couple of nerves.

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Other Urochordate classes
  • Besides the ascidians there are two other classes
    of the Urochordata the Larvacea and Thaliacea.
  • Both are small, transparent planktonic forms.
    Thaliaceans are cylindrical or spindle shaped
    whereas larvaceans are tadpolelike and resemble
    an ascidian larva.

45
Garstangs hypothesis of chordate larval evolution
  • In the 1920s it was proposed that the
    vertebrates were derived from an ancestral
    ascidian that retained its characteristics into
    adulthood (the process by which juvenile
    characteristics are retained into adulthood is
    referred to as paedomorphosis).

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Garstangs hypothesis of chordate larval evolution
  • Garstangs hypothesis is supported by
    embryological evidence, but more recently
    molecular analyses have suggested that sessile
    ascidians are a derived form and that the
    free-living larvaceans are more likely to be the
    closest relatives of the chordates.

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Subphylum Cephalochordata
  • The cephalochordates are the lancelets, which are
    small (3-7 cm long) laterally compressed fishlike
    animals that inhabit sandy sediments of coastal
    waters. They lack a distinct head and have no
    cranium.
  • They are commonly referred to as Amphioxus as
    this was the original genus name. There are 29
    species, five of which occur in North American
    coastal waters.

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Amphioxus
  • Amphioxus is a filter feeder.
  • Water enters the mouth and then is moved by
    beating cilia through the pharyngeal slits, where
    food is trapped in mucus. Cilia then move the
    food to the gut.

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Amphioxus
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Amphioxus
  • Amphioxus is interesting because it displays the
    basic chordate characteristics in a simple and
    obvious form because of its transparency.
  • Amphioxus is considered to be the closest living
    relative of the vertebrates because it shares
    several characteristics with vertebrates that
    Urochordates do not possess.

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Amphioxus characteristics shared with vertebrates
  • Characteristics Amphioxus shares with vertebrates
    include
  • Segmented myomeres (blocks of striated muscle
    separated by connective tissue)
  • Dorsal and ventral aortas
  • Branchial (gill) arches (blood vessels running
    over the gills).

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Amphioxus characteristics not shared with
vertebrates
  • Amphioxus however lacks several characteristics
    that biologists think the ancestor of vertebrates
    possessed. These include
  • Tripartite brain (with forebrain, midbrain and
    hindbrain) protected by a cranium (skull)
  • Chambered heart
  • Closed circulatory system
  • Muscular gut and pharynx (food moved through gut
    by ciliary action not peristalsis)
  • List continues on next slide

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Amphioxus characteristics not shared with
vertebrates
  • Various special sensory organs (eyes, chemical
    and pressure receptors, nose, inner ear)
  • Neural crest (ectodermal cells that are found on
    the embryonic neural tube and are engaged in the
    formation of the cranium, tooth dentine, some
    endocrine glands and Schwann cells, which provide
    myelin insulation to nerve cells).

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Significance of differences between amphioxus
and vertebrates
  • The differences between non-vertebrate chordates
    such as Amphioxus and early (and modern)
    vertebrates are a result of the increased size
    and activity of vertebrates.

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Significance of differences between amphioxus
and vertebrates
  • Because vertebrates are large they cannot depend
    on slow processes such as diffusion and ciliary
    action to support them.
  • As a result, specialized organ systems are needed
    to carry out physiological tasks at a much faster
    rate.

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Significance of differences between amphioxus
and vertebrates
  • Vertebrates are also much more active than
    nonvertebrate chordates and are predators or at
    least grazers (which is predation on plants).
  • Activity requires a suite of traits to support
    it.
  • sensory systems so you can seek things out
  • complex nervous system to coordinate activity
  • more efficient circulatory, respiratory and
    digestive systems to fuel the activity
  • muscles and skeleton to facilitate movement.
  • The transition from nonvertebrate chordate to
    vertebrate thus is closely related to the
    adoption of a more active lifestyle.

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Haikouella
  • An early stage in the transition to vertebrates
    is marked by the Cambrian era (520mya) fossil
    Haikouella
  • Haikouella is likely similar to the common
    ancestor of the vertebrates and it possesses a
    muscular pharynx, which implies it pumped water
    across its gills, which implies in turn a more
    active lifestyle, although it also possesses a
    filter-feeding apparatus.

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  • Once physiological systems began to develop that
    enabled vertebrates to be more active and sense
    their environments we would expect ecological
    competition and selection to have rapidly driven
    the evolution of vertebrate diversity.
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