Zoology 250 (2005) PROTOSTOMIA & DEUTEROSTOMIA (click on tree to see full cladogram)

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Two major divisions of the Bilateria (Protostomia and Deuterostomia) exhibit many differences:

• 1) Origin of mesoderm is from very different cell types:
  • mesenchyme cells (non-epithelial) migrate in (Protostomia)
  • epithelial cells inpocket from archenteron (Deuterostomia)

• 2) Origin of coelom is via two different mechanisms:
  • schizocoely: mesenchyme cells proliferate into balls of cells that then 'split' to form mesodermally lined coelom (Protostomia)
  • enterocoely outpockets from archenteron close off and form enclosed, mesodermally lined coelom (Deuterostomia)

• 3) Origins of mouth and anus differ. The blastopore gives rise to the:
  • mouth; the anus arises secondarily (Protostomia)
  • anus; the mouth arises secondarily (Deuterostomia)

• 4) Other differences include:
  • PROTOSTOMIA:
    • spiral and determinate cleavage
    • multiciliated cells

  • DEUTEROSTOMIA:
    • radial and indeterminate cleavage (primitive state)
    • monociliated (primitively)

• 5) Membership in the Deuterostomia & Protostomia is in a state of flux:
  • deuterostomes may only include echinoderms, hemichordates and chordates; the remaining bilaterian phyla may all be protostomes