Phyla Hemichordata and Chordata Overview

Overview of Phyla Hemichordata and Chordata

Learning Outcomes

  • Describe the general body structure of hemichordates.
  • Identify shared anatomical features of echinoderms and hemichordates.
  • Compare the nervous systems of hemichordates with annelids and arthropods.
  • List four unique anatomical features of chordates.
  • Explain why the evolution of jaws and paired fins is significant.
  • Understand counter-current exchange and its importance in physiology.

Phylum Hemichordata

  • Organisms: Acorn worms and pterobranchs.
  • Classification: Deuterostomes.
  • Body Structure
    • Tripartite body:
    • Proboscis: Anterior part that aids in feeding and burrowing.
    • Collar: Contains respiratory organs.
    • Trunk: Houses the gut and other internal organs.
    • Coelom: Tripartite coelom divides the body into three cavities.
    • Circulatory System: Open circulatory system.
    • Respiratory System: Features ciliated pharyngeal gill slits for respiration.
    • Nervous System: Dorsal nerve cord (sometimes hollow).

Class Enteropneusta

  • Characteristics:
    • Body plan includes a buccal diverticulum (for food intake) and a glomerulus (kidney-like structure).
    • Nerve cords can be dorsal and ventral.

Class Pterobranchia

  • Characteristics:
    • Body includes tentacles, arms, and a stalk, with organisms living in tubes for protection.
    • Known for colonial living, forming zooids from a single individual.

Phylum Chordata

  • Three Principal Groups:
    • Cephalochordata (lancelets)
    • Urochordata (sea squirts and tunicates)
    • Craniata (hagfishes and vertebrates)

Key Features of Chordates

  • Notochord: A flexible rod providing structural support.
  • Dorsal, Hollow Nerve Tube: Unique to chordates, part of the central nervous system.
  • Pharyngeal Slits: Functions in feeding and respiration.
  • Postanal Tail: Extends beyond the anus, used in movement.
  • Endostyle: A ciliated groove involved in filter feeding in some chordates.

Subphylum Cephalochordata

  • Features:
    • Contains a notochord, segmented muscles, and pharyngeal slits.
    • Body Structure:
    • Tentacles, mouth, tail, atrium, intestine, anus, and atriopore for excretion.

Subphylum Urochordata

  • Features:
    • Possess incurrent and excurrent siphons for water flow.
    • Includes sensory tentacles and an endostyle for filter feeding.
    • Larval Stage: Tadpole larva undergoes metamorphosis into the adult form.

Evolution of Jaws and Paired Fins

  • Jaws: Evolved from gill arches.
  • Middle Ear Bones: Derivatives of similar gill arches.
  • Paired Appendages: Evolution sheds light on adaptation.
  • Multiple Evidence: Supported by fossil records and genetic studies.

Counter-Current Exchange in Fishes

  • Mechanism:
    • Efficient transfer of oxygen and carbon dioxide.
    • Water flows across gill lamellae, creating a concentration gradient for gas exchange.
    • Unoxygenated blood enters the gills, passes through, and exits oxygenated.

Salt-Water Balance in Fishes

  • Adaptations for Salt Regulation:
    • Hyper-saline tolerant fish (e.g., silverside) manage water loss in salty environments.
    • Freshwater fish have specialized kidneys to filter excess water, maintaining low salt concentrations.
  • Marine Fish: Produce very concentrated urine to conserve water while excreting salt.

Key Concepts

  • The anatomical features of hemichordates share similarities with echinoderms.
  • Chordates possess unique characteristics that distinguish them from other invertebrate phyla.
  • The evolutionary significance of jaws and paired fins reflects the adaptive strategies of vertebrates.
  • Understanding physiological processes like counter-current exchange and osmoregulation is critical for comprehending the survival mechanisms of aquatic life.