Phylum Echinodermata

Echinoidea (sea urchins and sand dollars)
test (= corona) constructed of 20 radial plate rows (= 10 segments)
narrow segments = ambulacra, broad segments = interambulacra
interambulacral plates are large and tubercular, without perforations
ambulacral plates have pored plates through which the tube feet emerge
mouth with five hard teeth arranged in a circlet (known as Aristotle's lantern)
[reduced in derived forms; absent in stem group echinoids] (used for grazing)
move with tube feet or ”walk” on their spines
traditionally devided in regular (Regularia) and irregular echinoids (Irregularia)
Echinodermata – classification – Eleutherozoa – Echinozoa - Echinoidea