[i] Protochordates and Agnathans

• Protochordates

  • Cilia and pharyngeal slits
  • Earliest form of respiration in vertebrates
  • Basal vertebrates use velum
  • Pharyngeal slits are involved
• Ammocoete larvae

  • Water enters buccal cavity and exit through pharyngeal slits
  • 7 pharyngeal slits
  • Velum or velar folds instead of cilia
  • Branchial valve prevents retrograde flow of water
  • Gill components:
  • Interbranchial septum
  • Primary gill filaments or lamellae
  • Secondary lamella

  
• Adult lampreys

  • Parasitic via suctorial orifice
  • Tidal, bidirectional flow of water
  • Oxygenated water through gill slits

  
• Hagfish

  • Branchial apparatus without major expansions or contractions
  • Velum - inverted T structure
  • Velum and branchial pouches produce unidirectional flow
  • Resemblance of respiration to modern day fishes