Phylum Chordata

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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.
• In mammals the pericardial, peritoneal, and
  pleural cavities are formed from the coelom.

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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.

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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 passes 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 (muscle blocks)
• Dorsal and ventral aortas
• Branchial (gill) arches (blood vessels running
  over the gills).

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Characteristics of vertebrates not possessed by
Amphioxus

• 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
• Chambered heart
• Muscular gut and pharynx
• List continues on next slide

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Characteristics of vertebrates not possessed by
Amphioxus

• Various special sensory organs (eyes, chemical
  and pressure receptors)
• 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, provide
  myelin insulation to nerve cells).