1) The DEUTEROSTOMIA form the second 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 and body plan, monociliated cells
2) Phylogenetic relations among them remain a hotly debated topic!
ECHINODERMATA (I): Overview, symmetry, body plans
1) The ECHINODERMATA ("spiny-skinned animals") approx. 6,000 living spp; an ancient, solely marine phylum of approx. 20 classes, but only 6 living!
2) Most bizarre of all living animal phyla: "I salute the echinoderms as a noble group designed to puzzle the zoologist" (L.H. Hyman 1955)
a) have an endoskeleton of thousands(!) of mobile elements
b) large skeletal ossicles behave like single optical crystals of calcium carbonate
c) move using thousands of independently mobile tube feet=podia d) the five rows of tube feet point in different directions
e) have many complex behaviors but unusually simple nervous system
f) have five complete sets of coelomic systems, often intermingled
g) dorsal surface is covered with thousands of pincers (2 classes)
h) many extinct classes with bizarre body forms
4) Pentaradial symmetry was linked to a sessile lifestyle (primitively)
'oral' and 'aboral' (= opposite to mouth) are more useful terms than 'anterior' and 'posterior'; the oral surface is oriented up in attached forms and down in most mobile forms
5) Though phylogenetic relations are controversial, 2 body plans prevail:
a) discrete arms with ambulacra (a V-shaped groove containing the radial canal and 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)