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This phylum's 'place'
relative to other phyla:
Selected taxa within this taxon:
  • Subphylum Urochordata
  • Subphylum Cephalochordata
  • (Subphylum Vertebrata)
  • 48,000 species, 1400 of which are invertebrates by virtue of not possessing a backbone

    Body Plan


    Natural history

    Diverse morphologically and relationship with vertebrates is not always obvious.

    Subphylum Urochordata

  • Ascidians -   Sessile tunicates with tough protein and polysaccharide tunic or test.

  • Suspension feeders

    Sessile form is without a notochord, so why is this considered a chordate? 

     Notochord reabsorbed at a dramatic metamorphosis.
  • Larvaceans- Tadpole-like zooplankton that secretes a gelatinous 'house' (appear to be an important and overlook pathway in ocean carbon cycles) . Tail with notochord drives water movement for suspension feeding,  Notochord retained through out life, but adults are considered neotonous.

  •  Thaliaceans- Highly transparent, planktonic salps and doliolids.  May form aggregates that reproduce asexually.

    Subphylum Cephalochordata (amphioxous or lancelets)



    Based on one fossil group (the "calcichordates"), an argument has been made that vertebrates evolved from a clacified echinoderm-like ancestor.  However the proponderance of evidence unites the chordates as a single clade, excluding echinoderms.

    Traditionally, numerous morphological similarities used to argue Cephalochordata as the sister group of Vertebrata.

    However, recent arguments suggest that Urochordata may be more closely related to vertebrates based on more recent molecular evidence and hox-gene, cellular, and tissue level evidence.

     Is the metameric chordate body plan as a late-comer on the evolutionary scene?                                                                   

    So, what is left is the animals that are not 'invertebrates'


    Subphylum "Non-invertebrata"
    The remaining 'non-invertebrate' groups (the Subphylum Vertebrata or Craniata) represent a mere 1/20 of the splendid kingdom of animals!

    However, it does contain one species whose activities have impacted the diversity of life more than any other species since the  beginning of life. But this species also has the ability to consciously decide to be stewards over that creation.


    Lecture Sources:
    • Pechenik. Jan A.  2000. Biology of the Invertebrates.  McGraw-Hill, New York.

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