PetalsPetal-shaped parts of the ambulacra, spread like a star. Their rows of pores carried tube feet used for breathing.
Front groove (anterior ambulacrum)A sunken groove from the front notch to the apical disc. It is the ambulacrum at the front, between the two front petals.
Apical discA small group of plates at the top of the test. The petals and the front groove all start here.
Inner fascioleA narrow band that loops around the disc and the front groove. In life it carries very small, close-set spines.
TuberclesKnobs on the plates, each the base of one spine. The larger ones beside the front groove carried longer spines.
InterambulacraThe areas of plates between the ambulacra. They carry tubercles and spines but no tube feet.
TestThe test is the urchin's shell, built of plates. On a beach it is usually found bare, without its spines.
The test from below 5
Mouth (peristome)The mouth opens toward the front of the underside. A heart urchin has no jaws and no teeth.
LabrumThe lip plate just behind the mouth. It is the first plate of the plastron.
PlastronA shield of large plates behind the mouth. Its spines are widened at the tip like spades.
Subanal fascioleA ring-shaped band of very small spines at the rear. It lies just below the anus, which is on the rear face.
SpinesA felt of fine spines that mostly point backward. Each one stands on a tubercle.
How this list was compiled: Every name was checked on 2026-10-01 in at least two sources, for facts only and with nothing copied: (1) the glossary of a natural history museum's directory of echinoids, read in an archived copy, https://web.archive.org/web/2014/http://www.nhm.ac.uk/research-curation/research/projects/echinoid-directory/taxa/glossary.jsp; (2) Th. Mortensen, Handbook of the Echinoderms of the British Isles, 1927, the order of the heart urchins and the genus Echinocardium, https://archive.org/details/handbookofechino00mort; (3) a marine biological station's species page for the sea potato, https://www.marlin.ac.uk/species/detail/1417; (4) Wikipedia, Spatangoida, Echinocardium cordatum, Sea urchin and Sand dollar.; (5) E. Forbes, A History of British Starfishes, 1841, the heart urchins, https://archive.org/details/historyofbritish00forb_0; (6) a university museum's animal diversity page on Echinoidea, https://animaldiversity.org/accounts/Echinoidea/. A name only one of them gives is not a part of this page. Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work
How it works
This heart urchin diagram shows the test, the shell, of the sea potato, the common heart urchin that lives buried in sand. The front is at the top: first the bare test from above, then the underside.
Read the full guide
From above
A notch marks the front. A sunken groove runs from it back to the apical disc, and the petals spread from the disc like a star. A narrow band, the inner fasciole, loops around the disc and the groove.
From below
The mouth is near the front, not in the middle as in a round sea urchin. Behind it lie the labrum, a lip plate, and the plastron, a long shield of plates. The subanal fasciole rings the rear end, just under the anus.
Front is up in both views: the notch on top and the mouth underneath are at the same end.
Questions and answers
What is a heart urchin?
A sea urchin of the order Spatangoida. Its body is an oval or heart shape: the mouth is toward one end and the anus toward the other. A regular sea urchin is round, with the mouth in the middle underneath.
Is a sea potato a heart urchin?
Yes. Sea potato is a common name of Echinocardium cordatum, the common heart urchin. It lives buried in sand, covered in a felt of fine spines that mostly point backward.
Do heart urchins have teeth?
No. A regular sea urchin chews with a set of jaws called Aristotle's lantern. Heart urchins have no lantern and no teeth.
What are the petals on a heart urchin?
They are the upper parts of the ambulacra, the bands that carry the tube feet. Each petal has rows of pores, and the tube feet that stood on them were used for breathing.