Lecture Notes: Animals - The Chordate Phylum

Introduction to Chordates

• Chordates include vertebrates, such as humans.
• Differences between protostomes and deuterostomes:
  • Fertilization leads to a zygote, which undergoes cleavage during early development.
  • The blastopore becomes the mouth in protostomes and the anus in deuterostomes.

Chordate Characteristics

• Four main characteristics:
  • Notochord
  • Dorsal nerve cord
  • Pharyngeal gill slits
  • Post-anal tail
• Subphyla of Chordata:
  • Urochordata (e.g., sea squirts or tunicates)
    • Adults are sessile; larval forms retain chordate features.
  • Cephalochordata (e.g., lancelets)
    • Retain all chordate characteristics throughout life.
  • Vertebrata
    • Distinctive for having a vertebral column.

Primitive Vertebrates

• Hagfish
  • No jaws and poorly developed skull.
  • Feed on dead/dying fish and invertebrates.
• Lampreys
  • Have skull and vertebrae but no jaws.
  • Parasitic lifestyle.

Jawed Vertebrates - Chondrichthyes

• Cartilaginous Fish (e.g., sharks, rays, skates)
  • First group with jaws and paired fins.
  • No swim bladder; use asymmetrical tails for lift.
  • Presence of a cartilaginous skeleton.

Bony Fish - Osteichthyes

• Most species-rich group of vertebrates.
• Key Features:
  • Swim bladder for buoyancy.
  • Symmetrical tail and rotational fins for agility.
• Major Groups:
  • Ray-finned fish
  • Lobe-finned fish
    • Ancestors of amphibians.

Amphibians

• First vertebrates to transition onto land.
• Include frogs, toads, salamanders, and caecilians.
• Challenges of land living:
  • Support and movement, respiration, desiccation, reproduction.
• Adaptations:
  • Three-chambered heart and basic lungs.
  • Moist skin and external fertilization linked to water.

Reptiles

• Dominated Earth for millions of years.
• Overcame land challenges with adaptations:
  • Watertight skin and amniotic eggs.
  • Internal fertilization and more efficient respiratory systems.
• Diversity:
  • Turtles, lizards, snakes, crocodiles.

Birds

• Evolved from reptiles.
• Key Features:
  • Endothermy and adaptations for flight (e.g., hollow bones, feathers).
  • Specializations in beaks, wings, and feet.
  • Efficiencies in respiration and circulation.

Mammals

• Became successful post-dinosaur extinction.
• Unique Traits:
  • Hair, mammary glands, specialized teeth.
  • Efficient heart and respiratory systems.
• Diversity:
  • Monotremes (egg-laying), marsupials (pouched), and placentals.

Conclusion

• Chordates are a diverse and complex group with a wide range of adaptations allowing them to live in various environments.