Zoology 250 (2009) 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 and nervous system at anterior end)

• 2) Third tissue layer. Triploblastic (ectoderm, mesoderm, endoderm)
  
  a) Ectoderm, mesoderm and endoderm are germ layers; they refer to embryonic origin not to structure or function
  b) The mesoderm is fully cellular, usually extensive, and yields complex organs: different tissues working together as a single unit ("organ-system level of organization")
  c) In soft-bodied forms, circular (outer) and longitudinal (inner) body wall muscles act antagonistically during movement and locomotion

• 3) Compartmentation. Large body size and extensive mesoderm require some kind of internal fluid-filled cavities are fully enclosed and under control of organism (unlike cnidarian gastrovascular cavity); cavities may take on other functions (house gonads, locomotion)
  
  a) 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)
  b) 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
  • a)An excretory system includes 3 steps: i) ultrafiltration (typically across the basal lamina), ii) resorption (of ions and valuable biomolecules), and iii) transport (move wastes out of body).
  • b) Nephridial systems come in two general forms:
    • protonephridia: in small animals lacking a coelom or blood vascular system; filtration, resorption and transport occur in one structure; mostly depend on diffusion for ultrafiltration
    • metanephridia: in large animals with a blood vascular system; filtration is separate from resorption and transport; depends on pressure differences for ultrafiltration

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