The Clypeasteroida includes a number of familiar sea urchin groups, including the sand dollars, sea biscuits and cake urchins. They are mainly shallow-water forms inhabiting sandy substrata. The sand dollars are mostly infaunal, living immediately underneath the sediment-water interface often in beach-face or shallow shoal settings. Clypeasteroids are unique in having large numbers of tiny tube feet and this has allowed them to harvest the small organic particles found amongst sand-sized grains. No other echinoid group has tube-feet of such minute proportions. Although all have a functioning lantern, the teeth are internal and are used solely for crushing and biting particles internally. A general review of clypeasteroid evolutionary biology was given by Ghiold (1984).
Ghiold, J. 1984. Adaptive shifts in clypeasteroid evolution - feeding strategies in the soft-bottom realm. Neues Jahrbuch fur Geologie und Palaontologie Abhandlungen 169, 41-73.