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Brief overview of Ctenophora ("comb-jellies"). Superficially like cnidarian medusae, but lack cnidocytes, are biradially symmetrical, have determinate cleavage, and are possibly triploblastic ('mesoglea' may be a true mesoderm based upon developmental origin; if so may be a sister group to the Deuterostomes). Share a common complex cell type: colloblast.

• 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 sensory organs & nervous system at anterior end) in mobile forms

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

  • c) Extensive mesoderm arises two ways developmentally, via: epithelial cells (enterocoely), mesenchyme cells (non-epithelial)

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  • d) Complex organs (organ-system level of organization: locomotory, circulatory, excretory, nervous, reproductive)

  • e) Most exhibit a well-developed coelom (fluid-filled cavity lined by mesodermally derived epithelium) and blood-vascular system

  • f) Gut is usually tubular and complete (mouth & anus)

  • g) Distinctive excretory structures: protonephridia (in small animals lacking coelom or blood vascular system), metanephridia (in large animals w/blood vascular system)

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

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  PROTOSTOMIA & DEUTEROSTOMIA

• 1) Two major subdivisions of the Bilateria exhibit many differences:

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

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

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