Sea-urchins with a regular, round, spherical or flattened test. The anal opening is in the middle of the apical area. The peristome with one pair or a series of plates in each radius, while the interambulacral plates never form regular series over the peristome. Gills and, usually, gill-cuts are present at the peristomial edge of the test.
Ambulacral plates and tubercles
The adambulacral plates are mostly compound, more rarely simple. Usually both ambulacral and interambulacral plates have a primary tubercle; often some of the secondary tubercles grow to the same size as the primary and form regular vertical or horizontal series. Primary tubercles are perforate or imperforate, crenulate or smooth.
Spines and pedicellariae
The primary spines are only exceptionally with a special outer layer, never with fine,
anastomosing hairs. The secondary spines are usually of the same shape as the primaries. Teeth with or without keel. Mostly four sorts of pedicellariae present (globiferous, tridentate, ophicephalous and triphyllous). Sphaeridia are present. Spicules of tube-feet are irregular or C-shaped.
- Family Echinidae
- Family Strongylocentrotidae