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Zoology 250 (2007)
The BILATERIA
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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)
- Ectoderm, mesoderm and endoderm are germ layers; they refer to embryonic origin not to structure or function
- The mesoderm is fully cellular, usually extensive, and yields complex organs: different tissues working together as a single unit ("organ-system level of organization")
- 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)
- 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 and 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 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
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Copyright (c) 2007 by A. Richard Palmer. All rights reserved.
(revised Jan. 28, 2007)