• 1) The Bilateria includes all remaining animal phyla

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

  • b) Gut is usually complete (mouth & anus); anus evolved twice

  • c) Triploblastic organization (ectoderm, mesoderm,endoderm)

  • d) Mesoderm yields complex organs: different tissues working together as a single unit ("organ-system level of organization")

  • e) Most exhibit a well-developed coelom (fluid-filled cavity lined by mesodermally derived epithelium) and blood-vascular system (persistant larval blastocoel); a pseudocoelom is simply a blood-vascular system lacking a heart
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  • f) Extensive mesoderm arises two ways developmentally: mesenchyme cells (non-epithelial) migrate in, epithelial cells inpocket

    • each yields a different coelom origin: schizocoely & enterocoely

  • g) Distinctive excretory structures:

    • protonephridia: in small animals lacking coelom or blood vascular system; filtration, resorption & transport in one structure

    • metanephridia: in large animals with a blood vascular system; filtration separate from resorption & transport

• 2) In soft-bodied forms, circular (outer) & longitudinal (inner) body wall muscles act antagonistically during movement & locomotion

• 3) Two major divisions of the Bilateria exhibit many differences:

  • PROTOSTOMIA: spiral & determinate cleavage; coelom (if present) via schizocoely; mouth (and sometimes anus) from blastopore; multiciliated cells; trochophore-like larva

  • DEUTEROSTOMIA: radial & indeterminate cleavage (primitive state); coelom via enterocoely; mouth not from blastopore; monociliated (primitively); larva not trochophore-like

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