a) originate intracellularly (endoskeleton) & form as a single crystal
b) they take a remarkable variety of shapes and sizes:

a flexible strutwork (asteroids)
form an interlocking chain of elements (ophiuroid & crinoid arms)
fuse into a rigid test (echinoids)
be small and isolated (holothuroids)
may form the jaws of pedicellaria (some asteroids & echinoids)

a) the primary original function was feeding; locomotion arose later
b) connects to the outside (most taxa) via a porous plate (madreporite)
c) polian vesicles of the ring canal help maintain internal pressure
d) radial canals (5) extend down ambulacral grooves and may be open (crinoids, asteroids) or enclosed (other classes)
e) lateral canals lead to tube feet that may or may not bear suckers
f) tube feet are extended via hydrostatic pressure (via ampullae in many groups) and retracted or bent via longitudinal muscles
g) the form of the water vascular system varies among classes:

TRAIT	Crinoid	Asteroid	Ophiuroid	Echinoid	Holothuroid
madreporite	multiple pores	aboral	oral	aboral	internal (to perivisc.)
polian vesicles	N?	Y	Y	N?	Y
ambulac. groove	open	open	covered	covered	covered
tube feet suckers	N	Y/N	N	Y/N	Y/N
podia penetration	between plates	between plates	between plates	paired, thru plates	between plates
podia spacing	stagger	stagger	paired	stagger	stagger
ampullae	N	Y	N	Y	Y

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