Heart urchins are irregular sea urchins because they have flattened bodies, regular sea urchins have round bodies. Other irregular sea urchins are sand dollars and cake urchins. You won't see a heart urchin when walking in the beach because they burrow in the sand and are hidden. 
The heart urchin body consist of tentacles, one mouth, pharyngeal slits, an atrium, an intestine, an atriopore, one notochord, a dorsal hollow nerve cord, segmental muscles, an anus and a tail.  The body is covered in a number of tiny but sharp spikes.
Heart urchins reproduce in the summer to the fall on the coasts of Ireland and Britain. It is not hard to find a mate, usually heart urchins live within a dense population of each other. Sexual reproduction is done after the animals fourth birthday in the water column. The male will release sperm and the female will release an egg, into this said water column, they will meet and a echinopluteus larvae is formed. Shortly following this both male and female heart urchins die. 20-40 days after the sea urchin is conceived it becomes a juvenile sea urchin.
Heart urchins like to bury themselves deep into muddy sand. They commonly found on coast of Ireland and Britain. Once buried I the sand, heart urchins have protection against predators and have food. Heart urchins eat detritus which is commonly found in between the grains of sand. Detritus is waste and heart urchins are herbivores so they will find plant waste. How they eat this is they use their suction cups on the bottom of their feet to digest food.
Defences against Predators
Heart urchins are covered in sharp spikes that can move toward a predator. 
(Echinocardium cordatum )
- Echinocardium australe Wikipedia