Irregular Echinoids or Spatangoids
Sea-urchins with a more or less elongated by not greatly flattened test. The mouth is anterior and the anus posterior. There no no gills or gill-cuts at the peristomal edge of the test. There is no dental apparatus.
The ambulacra are often petaloid on the upper surface. The posterior interambulacrum is different from the other interambulacra on the oral side, and is usually more or less modified so as to form a sternum or plastron. The first plate of the posterior interambulacrum is often specially developed into a prominent labrum.
Tubercles and spines
The tubercles are perforate. The spines are of various length, those on the plastron are often widened into a spade shape. There are often distinct bands of very small, close-set spines called fascioles, which function to set up water currents.
Tube-feet and pedicellariae
The tube-feet are confined to the ambulacra, usually very diversified, those around the peristome are penicillate. The pedicellariae are often very conspicuous; besides the usual four types a fifth form, the rostrate pedicellariae, also occurs. Sphaeridia are present, but not in the grooves.
- Family Spatangidae