--------------- Wed. April 8 -------------

• 1) Chordates are a morphologically diverse group of three subphyla: Urochordata (sea squirts), Cephalochordata (lancelets), Vertebrata

  a) they are united by five key traits: pharyngeal gill slits, endostyle, notochord, dorsal hollow nerve cord, and post-anal tail

• 2) Adult urochordates bear little resemblance to other chordates

  a) most are sessile, are covered by a complex tunic, have a pharynx with many gill slits, and lack a notochord & nerve cord in adults

  b) the 'tadpole' larvae possesses the distinctive chordate characters

  c) Cl. Ascidiacea (sea squirts): sessile solitary or colonial filter feeders

  d) Cl. Larvacea (appendicularians): pelagic makers of mucous houses

• 3) Adult cephalochordates resemble the larvae of primitive vertebrates and possess all the chordate characters; their large pharynx with many U-shaped gill slits also closely resembles that of hemichordates

• 1) The Tree of Life reveals that life has a history: a grand vision

• 2) The Tree of Life can be read partly from traits of living organisms:

  a) The earliest multicellular 'animals' were colonial protists

  b) Early branches yielded cellular, then tissue (diploblastic, radial), then organ-system (triploblastic, bilateral) grades of organization

  c) The earliest bilaterians split into two major branches (Protostomia & Deuterostomia) within which coeloms evolved independently

• 3) The Tree of Life can also be read partly from fossils and molecules:

  a) A literal reading of the fossil record suggests animal phyla arose in an 'explosion' near the beginning of the Cambrian (approx. 580 MYA)

  b) But recent molecular evidence (Oct. 1996; 8 molecules) suggests protostomes diverged from deuterostomes nearly 1,200 MYA!

• 4) The study of comparative anatomy and development continues to yield essential insights into the evolution of animal form & diversity

• 5) Major questions about the Tree of Life remain (e.g., placement of ctenophores, 'pseudocoelomates', lophophorates, origin of chordates)

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