Zoology 250 (2002) Phylum CHORDATA, Vertebrate Origins (protochordate study image; click on tree to see full cladogram)

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• 1) Chordates are a morphologically diverse group of three subphyla united by five traits:
  
  a) pharyngeal gill slits
  b) endostyle
  c) notochord
  d) dorsal hollow nerve cord
  e) post-anal tail

• 2) SubPh. UROCHORDATA (sea squirts & relatives) ~1500 marine spp; are four classes but we will only consider two
  
  a) adults bear little resemblance to other chordates
  b) most are sessile & covered by a complex tunic, have a pharynx with many gill slits, and lack a notochord & nerve cord in adults
  c) the 'tadpole' larva possesses the distinctive chordate characters

  Cl. ASCIDIACEA (sea squirts):

  sessile, solitary or colonial, filter feeders
  an endostyle secretes an iodinated mucous net that functions as a fine seive; cilia propel it across the pharynx and eventually to the mouth

  Cl. LARVACEA (appendicularians):

  pelagic; make amazing mucous houses to suspension feed; make several houses per day (4 - 16); can filter particles < 0.1 micron!
  the likely sister group of all remaining urochordates

• 3) SubPh. CEPHALOCHORDATA (lancelets) ~25 marine spp
  
  a) adults resemble the larvae of primitive vertebrates and possess all the chordate characters
  b) a large pharynx with many U-shaped gill slits also closely resembles that of hemichordates
  c) like sea squirts, an endostyle secretes a mucous net for filtering; cilia propel it across the pharynx and eventually to the mouth

• 4) SubPh. VERTEBRATA (vertebrates) >43,000 marine, freshwater & terrestrial spp.

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