Zoology 250 (2002) KEY CHARACTERS ON THE ANIMAL 'TREE OF LIFE' (click on tree to see full cladogram)

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• 1) CELLULARITY (unicellular vs multicellular) Metazoans are multicellular; differences b/n 'multicellular' & 'colonial unicellular' are subtle

• 2) BODY SYMMETRY (radial vs bilateral)

  • radial symmetry refers to symmetry about a single axis, bilateral symmetry refers to symmetry about a plane (defined by 2 axes)
  • earliest metazoans had no symmetry; radial & bilateral arose later

• 3) CLEAVAGE TYPE (spiral vs radial) Radial cleavage is primitive in the Metazoa, spiral cleavage is a synapomorphy of the Protostomia

• 4) # TISSUE LAYERS (diploblastic vs triploblastic)

  • true tissues (several cell types work together at a single function) are a synapomorphy of the Eumetazoa
  • early eumetazoans had only 2 tissue layers (ectoderm & endoderm) surrounding a non-cellular middle layer (diploblastic)
  • higher metazoans (Bilateria) have 3 tissue layers: ectoderm, endoderm, and extensive middle layer with complex organs (triploblastic)

• 5) TYPE & ORIGIN OF GUT (blind vs complete) A blind gut (mouth, no anus) was primitive in Eumetazoa; a complete gut (mouth & anus) evolved twice: Protostomia & Deuterostomia

• 6) TYPE OF CILIATION (mono- vs multiciliated) Monociliated cells are primitive; multiciliated cells arose twice (Proto- & Deuterostomia)

• 7) COELOM TYPE (acoelomate vs pseudocoelomate vs coelomate)

  • a true coelom is a fluid-filled internal body cavity lined by epithelium
  • fluid-filled internal body cavities take several forms and arose independently in several higher groups of invertebrates

• 8) COELOM ORIGIN (schizocoely vs enterocoely) True coeloms arise developmentally via two different methods: schizocoely (splitting of mesenchyme), enterocoely (outpocketing of epithelium)

• 9) TYPE OF SKELETON (exoskeleton, endoskeleton, hydrostatic) An exoskeleton lies outside the body and is produced by ectoderm. An endoskeleton lies inside the body and is produced by mesoderm or mesoglea: may be rigid elements or fluid-filled (=hydrostatic skeleton)

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