Echinocardium is a heart urchin, bilaterally rather than radially organised on the outside despite the echinoderm five-part underlying plan. Living within the sediment poses two problems: oxygen and food. It solves both with modified tube feet that are far longer than the animal's body.
One set of podia constructs and maintains a respiratory funnel running from the burrow up to the sand surface, lining it with mucus so it holds its shape. Water drawn down that shaft supplies oxygen. Other tube feet, on the underside, reach out to collect organic particles from the surrounding sand and pass them to the mouth. A separate sanitary drain is kept behind the animal for waste.
The pale, fragile shells found on beaches are the tests of dead animals and are often mistaken for a kind of shell or a sand dollar. The species is also widely used as an ecological indicator of sediment quality, though how far its distribution tracks pollution rather than grain size and hydrodynamics is debated.