Zoology 250 Lecture 10
The BILATERIA
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-transport system; fluid-filled cavities may also have other functions (enclose gonads, locomotion)
- Most Bilateria exhibit a well-developed coelom (fluid-filled cavity lined by mesodermally derived epithelium) and blood-vascular system (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 coelom or blood vascular system; filtration, resorption & transport in one structure
- metanephridia: in large animals with a blood vascular system; filtration is separate from resorption & transport
- 5) Complete gut (mouth & anus). In bilaterians, the gut is solely for digestion & absorption, unlike in Cnidaria; anus evolved twice
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