Two major divisions of the Bilateria (Protostomia & 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 & 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 & determinate cleavage
    • multiciliated cells
    • trochophore-like larva

  • DEUTEROSTOMIA:
    • radial & indeterminate cleavage (primitive state)
    • monociliated (primitively)
    • larva not trochophore-like

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