[3] Origin of Chordates

• 5 Chordate Characteristics:

  1. Notochord
  2. Pharyngeal slits
  3. Endostyle or thyroid gland
  4. Dorsal hollow nerve cord
  5. Post-anal tail
• Notochord

  • Develops from mesoderm
  • Dorsal to coelom, parallel to central nervous system
  • Elastic rod-like properties: Does not collapse axially, but has lateral flexing
• Pharyngeal slit

  1. Arose in Hemichordata
  2. Pharynx - part of digestive tract located posterior to the mouth
  3. Longitudinal series of openings, no role in respiration
  4. Walls of pharynx form pharyngeal pouches
  5. Early chordates - pharynx expands to become a pharyngeal basket
• Endostyle

  1. Glandular groove in the floor of the pharynx; used in filter feeding
  2. Thyroid gland derived from the endostyle
  3. Larval lamprey possess true endostyle 

  • This becomes a true thyroid gland as they mature
• Dorsal nerve cord (DNC)

  • Synonymous with neural tube
  • Derived from ectoderm
  • Forms through invagination, where ectoderm hardens into neural plate which forms a tube
  • Dorsal to notochord
• Post-anal tail

  1. Posterior elongation
  2. Extension of chordate locomotor apparatus, musculature and notochord
  3. Aids in swimming