Zoology 250 Lecture 10
The BILATERIA
- 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) All are triploblastic (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
- 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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Copyright © 1999 by A. Richard Palmer. All rights reserved.
(revised Jan. 7, 1999)