- Kingdom: Animalia
- Phylum: Echinodermata
- Subphylum: Echinozoa
- Class: Echinoidea
- Order: Spatangoida
- Family: Loveniidae
- Scientific name: Echinocardium cordatum
- Norwegian: sjømus
The color is grey or light brown. The spines are of variable length. The diameter is usually no more then 5 cm, but individuals as large as 9 cm have been reported. In contrast to the two other species of this family, E. flavescens (yellow sea potato) and E. pennatifidum, it has a groove at the anterior end.
It lives buried, just below the surface (usually 8 to 15 cm), in sandy substrate. It depends on sheltered locations as the shell is very fragile. It is easily crushed if the waves were to dig it up from the sand. It has been reported on depths from 5 to 230 meters.
The common heart urchin is common around the world, including South Africa, Australia, Japan, the Mediterranean and Europe. It is not reported from Iceland and Greenland.