Zoology 250 (2002) The BILATERIA (click on tree to see full cladogram)

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The Bilateria includes all remaining phyla. Key innovations include:

• 1) Bilateral symmetry. Probably arose initially in a benthic, creeping form (planula-like?); associated with cephalization (concentration of sense organs & nervous system at anterior end)

• 2) Third tissue layer. Triploblastic (ectoderm, mesoderm, endoderm)
  • The mesoderm is fully cellular, usually very extensive, and yields complex organs: different tissues working together as a single unit ("organ-system level of organization")
  • In soft-bodied forms, circular (outer) & longitudinal (inner) body wall muscles act antagonistically during movement & locomotion

• 3) Compartmentation. Large body size and extensive mesoderm require some kind of internal fluid-filled cavities are fully enclosed & under control of organism (unlike cnidarian gastrovascular cavity); cavities may take on other functions (house gonads, locomotion)
  • Most Bilateria exhibit a well-developed coelom (fluid-filled cavity lined by mesodermally derived epithelium) and blood-vascular system (fluid filled channels in connective tissue= persistent larval blastocoel)
  • A pseudocoelom is simply a blood-vascular system lacking a heart

• 4) Excretory organs. Large body size and extensive mesoderm also require some kind of excretory system
  • An excretory system includes 3 steps: i) ultrafiltration (typically across the basal lamina), ii) resorption (of ions + valuable biomolecules), and iii) transport (move wastes out of body).
  • Nephridial systems come in two general forms:
    • protonephridia: in small animals lacking a coelom or blood vascular system; filtration, resorption & transport occur in one structure
    • metanephridia: in large animals with a blood vascular system; filtration is separate from resorption & transport; depend on pressure differences for ultrafiltration

• 5) Complete gut (mouth & anus). In bilaterians, the gut is solely for digestion & absorption, unlike in Cnidaria; anus evolved twice

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