Chordates:
Chordates have two groups
(1)
Uro-chordates
(2)
Cephalochordates
Phylogenetic Relationship:-
Duetrostrome—Ancestor origin starts from Echinodermates and ends upon chordates
Origin
  • Echinodermates
  • Protochordate
  • Chordates
General Characters:
  • Bilateral Symmetrical
  • Deutrostrome Animals
  • Having a complete digestive tract
  • Having ventral contractile blood vessels(heart)
    Chordates have a unique combination of 5 unique or distinguished characters present at some stage of development.
(1)Notochords
(2)Pharyngeal slits
(3)Tubular nerve chord
(4)Post anal tail
(5)Endostyle/thyroid gland
Figure: Showing all 5 Characters
1. Notochord:
  • Noton means Back and Chor means ROD
  • It is a supportive rod that extends most of the length of the animal dorsal to the body cavity and tail
Composition:
  • It consists of a connective tissues sheet that encloses cells, each of which contains a large fluid-filled vacuoles
  • This arrangement gives the notochords some turgidity which prevents expression along the anteroposterior axis.
  • The notochord is flexible enough to allow lateral bending
2. Pharyngeal Slits:
  • Pharyngeal slits are a series of openings in the pharyngeal region between the digestive tract and outside the body.
  • These slits are not unique to chordates
  • But they are adopted for use in an important distributed way in the pharynx.
  • In some chordates, diverticula from the gut are present in the pharyngeal regions.
  • These diverticula do allow an open passageway to the outside. These diverticula are then called pharyngeal punches.
Functions:
  • The earliest chordates and some live chordates used them for feeding as a filter feeder.
  • Others chordates developed gills in the pharyngeal pouches for gaseous exchange.
  • Pharyngeal slits in terrestrial vertebrates are mainly, embryonic features and may be incomplete.
3. Tubular Nerve chord:
  • The tubular nerve chord is an associated structure that is responsible for the chordate's success.
  • The nerve chords run the longitudinal axis of the body, just dorsal to the notochords. It also expands anteriorly as a brain.
  • The central nervous system (CNS) is associated with the development of the complex system, used for sensory receptors, integration, and motor responses.
4. Post Anal Tail:
  • A post-anal tail extends posteriorly beyond the anal opening.
  • The notochords and vertebral column support the tail.
5. Endostyle or Thyroid Gland:
  • The 5th unique characteristic of the chordates is the presence of endostyle or thyroid glands.
  • It is present on the ventral aspect of the pharynx of tunicates, cephalochordates, and larval lampreys.
  • It secretes mucus that helps to trap food particles during filter feeding.
  • In adults and other chordates, the endostyle is transformed into an endocrine structure called the Thyroid gland.
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