• 1) CELLULARITY (unicellular vs multicellular) Metazoans are all multicellular, but the difference between 'multicellular' and 'colonial unicellular' can be subtle (will review when discussing sponges).

• 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 two axes)
  • the earliest metazoans were radially symmetrical

• 3) # TISSUE LAYERS (diploblastic vs triploblastic)

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

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

  • a 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

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

• 6) TYPE & ORIGIN OF GUT (blind vs complete) A blind gut (mouth, no anus) was primitive in the Eumetazoa; a complete gut (mouth & anus) arose twice independently (Protostomia & Deuterostomia)

• 7) TYPE OF SKELETON (hydrostatic, endoskeleton, exoskeleton) An exoskeleton lies outside the body and is produced by ectoderm, an endoskeleton lies inside the body and is produced by mesoderm or mesoglea, a hydrostatic skeleton is fluid-filled and lies inside the body

• 8) TYPE OF CILIATION (mono- vs multiciliated) Monociliated cells are primitive in the Metazoa, multiciliated cells arose twice independently (Protostomia & Deuterostomia)

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