Zoology 250 (2002) DEUTEROSTOMIA, ECHINODERMATA I (Echinoderm study images; click on tree to see full cladogram)

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• 1) The DEUTEROSTOMIA form the 2nd major branch in the Tree of Life; they share several traits: radial, indeterminate cleavage, mesoderm from epithelial cells, coelom via enterocoely, blastopore yields anus, tripartite coelom & body plan, monociliated cells

• 2) Phylogenetic relations among them remain a hotly debated topic!

• 1) The ECHINODERMATA ("spiny-skinned animals") ~6,500 living spp; an ancient, exclusively marine phylum of uncertain origin

• 2) Perhaps the most bizarre of all living animal phyla:
  
  a) have an endoskeleton of thousands(!) of mobile elements
  b) large skeletal ossicles behave like single optical crystals of CaCO3
  c) move using thousands of independently mobile tube feet
  d) the five rows of tube feet point in different directions
  e) have five complete sets of coelomic systems, often intermingled
  f) dorsal surface is covered with thousands of pincers (2 classes)

• 3) Five main living classes: CRINOIDEA (feather stars, sea lilies; ~75 spp.), ASTEROIDEA (starfish; ~1,800 spp.), OPHIUROIDEA (brittle stars; ~2,000 spp.), ECHINOIDEA (sea urchins, sand dollars; ~900 spp.), HOLOTHUROIDEA (sea cucumbers; ~1,100 spp.)

• 4) Pentaradial symmetry was linked to a sessile lifestyle (primitively)
  
  'oral' & 'aboral' are more useful terms than 'anterior' & 'posterior'

• 5) Though phylogenetic relations are controversial, 2 body plans prevail:
  
  a) discrete arms with ambulacra (a V-shaped groove containing the radial canal & primary podia) restricted to the oral surface, and clear pentaradial symmetry (crinoids, asteroids, ophiuroids)
  b) arms absent, and ambulacra extend from the oral to aboral pole; varying degrees of bilateral symmetry (echinoids, holothuroids)

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