Alex Mustard / Hearth heart urchin (Maretia planulata) which usually lives buried in the sand, crawls along the seabed. Dauin, Dumaguete, Negros, Philippines. Bohol Sea, tropical west Pacific Ocean.

Constantinos Petrinos / Longspine heart urchin (Maretia planulata) burrowing into the sand, Lembeh Strait, North Sulawesi, Indonesia.

Brandon Cole / Red Heart Urchin (Meoma ventricosa) exposed at night whilst foraging. Dominica, tropical Caribbean Sea.

Sue Daly / Shell of Purple heart urchin (Spatangus purpureus) Channel Islands, UK

Nick Upton / Common heart urchin / Sea potato (Echinocardium cordatum), a burrowing urchin, washed up on a sandy beach, St.Bees, Cumbria, UK, July.

Nick Upton / Common heart urchin / Sea potato (Echinocardium cordatum), a burrowing urchin, washed up on a sandy beach, St.Bees, Cumbria, UK, July.

Nick Upton / Common heart urchin / Sea potato (Echinocardium cordatum), a burrowing urchin, washed up on a sandy beach, St.Bees, Cumbria, UK, July.

Jurgen Freund / Heart urchin (Echinocardium sp) of the family Loveniidae. These sea urchins are named for their shape. Lembeh Strait, North Sulawesi, Indonesia

Georgette Douwma / Heart urchin (Lovenia elongata) lives buried in the sand during the day. Rinca, Komodo National Park, Indonesia, October

Philippe Clement / Sea potato / Heart urchin (Echinocardium cordatum) shell on beach, Belgium

Philippe Clement / Sea potato / Heart urchin (Echinocardium cordatum) shell on beach, Belgium

Philippe Clement / Sea potato / Heart urchin (Echinocardium cordatum) shell, Belgium

Philippe Clement / Sea potato / Heart urchin (Echinocardium cordatum) shell, Belgium

Kim Taylor / Common heart urchin skeleton {Echinocardium caudatum}, Atlantic.

Brandon Cole / Heart sea urchin {Maretia planulata} Indonesia, IndoPacifi

Adrian Davies / Common heart urchin specimen (Sea potato) (Echinocardium caudatum)

Visuals Unlimited / Heart urchin (Lovenia cordiformis), California, USA.