1) The Echinodermata ("spiny-skinned animals") is an ancient marine phylum of uncertain origin
a) perhaps the most bizarre of living animal phyla
b) five main living classes: Crinoidea (feather stars, sea lilies), Asteroidea (starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchins, sand dollars), Holothuroidea (sea cucumbers)
2) Pentaradial symmetry was linked to a sessile lifestyle (primitively)
a) 'oral' & 'aboral' are more useful terms than 'anterior' & 'posterior'
b) the oral surface is oriented up in attached forms and down in most mobile forms
3) Unique calcareous, dermal ossicles give the phylum its name
a) originate intracellularly (endoskeleton) & form as a single crystal
b) they take a remarkable variety of shapes and sizes
c) they may: form a flexible strutwork (asteroids), form an interlocking chain of elements (ophiuroid & crinoid arms), fuse into a rigid test (echinoids), be small and isolated (holothuroids)
d) may form the jaws of pedicellaria (some asteroids & echinoids)