Chordate Study Notes

LECTURE 1&2: INTRODUCTION TO CHORDATES

OVERVIEW OF THE ANIMAL KINGDOM

  • Sub-kingdoms:
    • Nonchordata: Animals without a notochord.
    • Chordata: Animals that possess a notochord or chorda dorsalis at some stage in their life cycle.
    • Importance: Includes vertebrates, such as humans, and some invertebrates.

GENERAL CHARACTERISTICS OF CHORDATES

  • Habitat: Aquatic, aerial, or terrestrial.
  • Symmetry: Bilaterally symmetrical and metamerically segmented.
  • Skeletal Systems: Exoskeleton may be present and well-developed in many vertebrates.
  • Germ Layers: Triploblastic with three germinal layers:
    • Ectoderm
    • Mesoderm
    • Endoderm
  • Body Cavity: Coelomate with true coelom, which may be enterocoelic or schizocoelic in origin.
  • Notochord: Present at some stage of the life cycle.
  • Digestive System: Complete system with digestive glands.
  • Circulatory System: Closed blood vascular system.
    • Heart is located ventrally with dorsal and ventral blood vessels.
  • Hepatic Portal System: Well-developed for nutrient processing.
  • Excretory System: Includes proto-, meso-, or meta-nephric kidneys.

ADDITIONAL FEATURES OF CHORDATES

  • Muscle Blocks: Bilateral blocks of muscle surrounding the notochord and nerve cord.
  • Limbs: Typically have four appendages (legs, arms, wings, or fins).
  • Endoskeleton: Internal skeleton with a backbone in vertebrates.
  • Nervous System: Highly developed brain and nervous system.
  • Respiration: Through lungs or gills.
  • Reproductive System: Primarily sexual reproduction.
  • Excretion: Through kidneys and intestines.

STRUCTURAL OUTLINE OF CHORDATES

  • Ventral Heart: Chambered and located ventrally to the alimentary canal.
  • Blood Vascular System: A continuous closed system consisting of arteries, capillaries, and veins.
  • Hepatic Portal System: Nutrient-rich blood from the digestive tract passes through liver capillaries before reaching the heart.

DEFINING FEATURES OF CHORDATES

  1. Notochord:

    • Definition: A cartilaginous rod that runs beneath and supports the nerve cord.
    • Characteristics: Dorsal, solid, stiff, elongated, and supports the neural tube.
    • In vertebrates: Replaced by the vertebral column in adulthood.

  2. Dorsal Tubular Nerve Cord:

    • Definition: A bundle of nerve fibers running dorsally.
    • Function: Connects the brain with muscles and organs.
    • Origin: Ectodermal, forms the brain and spinal cord.

  3. Pharyngeal Gill Slits:

    • Definition: Openings at the throat region.
    • Function: In aquatic chordates, these are used for gill support.
    • Terrestrial chordates: Present only during embryonic stages; develop jaw and ear bones in adulthood.

  4. Post-anal Tail:

    • Definition: An extension from the body water beyond the anal opening.
    • Function: Often used for locomotion in aquatic organisms; provides balance in terrestrial forms.
    • In humans: Present only during embryonic development.

COMPARISON: NON-CHORDATES VS. CHORDATES

FeatureNon-ChordatesChordates
NotochordCompletely absentPresent at some life stages
Nervous SystemSolid, ventral nerve cordHollow, dorsal nerve cord
GutDorsalVentral
RespirationBody surface, gills, tracheaGills or lungs
HeartAbsent or dorsal/lateral alignmentVentral heart
SkeletonExoskeletonBoth endoskeleton and exoskeleton
Circulatory SystemOpen systemClosed system
Pharyngeal Gill SlitsAbsentPresent at some stage
Post-anal TailAbsentPresent at some stage
CoelomAcoelomate or pseudocoelomateAlways coelomate

ORIGIN AND ANCESTRY OF CHORDATES

  • Possible Invertebrate Ancestors:
    • Annalids, arthropods, echinoderms.
  • Theories of Origin:
    • Coelenterate Theory: Suggests chordates evolved from coelenterates, but deemed unviable.
    • Annelid Theory: Proposes evolution from annelids; presents difficulties due to anatomical differences.
    • Echinoderm-Hemichordate Theory: Posits a common ancestor for chordates, hemichordates, and echinoderms based on evolutionary evidence.
    • Embryological Evidence:
    • Common characteristics between echinoderm larvae and chordate embryos, such as enterocoelic coelom and similar nervous system development.

CLASSIFICATION IN PHYLUM CHORDATA

  • Subphyla:
    • Hemichordata: Doubtful chordates, grouped with non-chordates.
    • Urochordata: Tail chordates exhibiting chordate features in larval stages.
    • Cephalochordata: Typical chordates showing chordate structures in both larval and adult forms.
    • Vertebrata: Backbone chordates.
    • Group Craniata (Chordates with skull).
      • Division AGNATHA: Jawless fishes (e.g., lampreys, hagfishes).
      • Class Ostracodermi: Extinct jawless fish.
      • Class Cyclostomata: Current jawless fish.
        • Order Myxinoidea: Examples include hagfish (e.g., Myxine, Bdellostoma).
        • Order Petromyzontia: Examples include lampreys (e.g., Petromyzon).
    • Division GNATHOSTOMATA: Vertebrates with jaws.
      • Super Class PISCES:
      • Class Placodermi: Extinct spiny sharks.
      • Class Chondrichthyes: Cartilaginous fishes (Sharks, rays).
        • Subclass Elasmobranchii: 850 species of sharks, rays, and skates.
        • Subclass Holocephali: 30 species of ratfish.
      • Class Osteichthyes: Bony fishes, over 20,000 species.
        • Subclass Actinopterygii: Ray-finned fishes.
        • Superorder Teleostei: Includes diverse species of bony fishes.
        • Subclass Crossopterygii: Includes coelacanths.
        • Class Choanachthys (Dipnoi): Lungfishes.
      • Superclass TETRAPODA: Includes land vertebrates with paired appendages.

UROCHORDATES

  • General Information:
    • Commonly known as tunicates or sea squirts.
    • Adult forms are simple and sack-like with two siphons.
    • 1,300 species exist, primarily marine.
    • Larval Characteristics:
    • Possess notochord, dorsal hollow nerve cord, gill slits, and postanal tail.
  • Development:
    • Life cycle includes extensive changes from larva to adult form (metamorphosis).
    • Adults resemble invertebrates more than chordates.

GENERAL CHARACTERISTICS OF UROCHORDATES

  • Body Structure:
    • Unsegmented and lacks a tail in adults; covered by tunicine (cellulose-like material).
    • Atrial cavity present which encloses the pharynx.
    • Respiratory and Circulatory Systems:
    • Gills present in the pharyngeal wall; open-type circulatory system.
    • Heart is ventral and periodically reverses its pumping function.
  • Reproductive Systems:
    • Asexual reproduction via budding; hermaphroditic with external fertilization common.
  • Metamorphosis: Describes retrogressive metamorphosis, where larval forms have advanced characteristics that are lost upon maturation.

COMPARISON WITH OTHER CHORDATES

  • Similarities with Cephalochordates:
    • Presence of notochord, dorsal tubular nerve cord, gill slits, and atrial features.
  • Differences:
    • The unique retrogressive metamorphosis and absence of segmentation distinguish urochordates from other chordates.
    • Urochordates are sometimes considered ancestral forms of modern chordates due to their basic structural features.