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
To label the heart urchin, write a name on the line for each number. Numbers 1 to 7 are on the bare test from above; 8 to 12 are on the underside. The front is at the top in both.
From above, down the left side: 2 ends in the sunken band, 4 on the narrow loop round it, 3 on the small round plate where the leaf shapes meet. Down the right side: 5 ends on the small knobs, 1 on a leaf shape, 6 on the plates between two leaf shapes; 7 ends on plain shell. From below, 8 ends on the curved slit, 9 on the plate just behind it, 10 on the long shield, 12 on the bristles and 11 on the ring at the rear.
When marking, ambulacra is right on line 1, apical system on line 3, shell on line 7 and peristome alone on line 8.
Two names end in fasciole: 4, the loop on top, is the inner one, and 11, the ring underneath, is the subanal one.
The bristles are drawn along one side of the underside only. In life they cover the whole test.
Questions and answers
Why is it called a heart urchin?
For its shape. The mouth lies toward one end and the anus toward the other, so the test is heart-shaped, not round. On the sea potato, the heart urchin drawn here, the dent is at the front, where number 2's band begins.
What do heart urchins eat?
The sea potato, the common heart urchin, eats detritus: organic debris that collects in a small pit in the sand above its burrow. Long tube feet at the front of the animal pass it down.
Where do heart urchins live?
The sea potato, the common heart urchin, lives in a burrow in sand, from the lower shore out to the sea bed offshore. It keeps a breathing channel open from its burrow up to the surface of the sand.
What color is a heart urchin?
A living sea potato is yellow-brown, with yellowish spines. A test found on a beach has often lost its spines: one source calls it white, another brownish. Either suits a sheet that gets colored in.