Zoology 250 (2017) 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 body layer well developed. Triploblastic
  a) Ectoderm, mesoderm and endoderm are germ layers; they refer to embryonic origin not to a structure or function
  b) The mesoderm layer 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 aid locomotion

• 3) Compartmentation. Large body size & mesoderm require some kind of internal fluid-transport system; fluid-filled cavities are fully enclosed & under control of organism (unlike cnidarian GV cavity); cavities may take on other functions (house gonads, aid locomotion)
  a) Most Bilateria have 2 fluid-filled systems: 1) spacious coelom(cavity lined by mesodermally derived epithelium) & 2) blood-vascular system(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:
    • metanephridia: typically in large animals with a blood vascular system; filtration occurs in a structure separate from resorption & transport; depend on pressure differences for ultrafiltration
    • protonephridia: primitive; typically in small animals lacking coelom or blood vascular system; filtration, resorption & transport all occur in one structure; ultrafiltration is mainly by diffusion

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