Describe Class Echinoidea in animal kingdom?

Sand dollars and sea urchins either have flattened disc-like bodies, or are somewhat spherical. While they lack the typical five-arm pattern, they do have five rows of tube feet that line the bottom of their shells. Sand dollars use their tube feet to walk on the ocean bottom, and sea urchins have movable spines that are used in locomotion. Echinoids feed by grazing on algae.