Sand dollars and heart urchins. The term sand dollar refers to species of extremely flattened, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits. Related animals include the sea urchins, sea cucumbers and starfish. The heart urchins or Spatangoida are an order of sea urchins. Sea urchins or urchins are small, spiny, globular animals that, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. Sea urchins move slowly, and feed on mostly algae. Sea otters, starfish, wolf eels, triggerfish, and other predators hunt and feed on sea urchins. Their roe is a delicacy in many cuisines. The name "urchin" is an old word for hedgehog, which sea urchins resemble. Undated, no illustrator credited.