Spatangoids typically have sunken petals, while other groups have all ambulacra flush on the surface. Tube-feet in this region of the test are responsible for constructing a respiratory tunnel in burrowing species. Enlarged pore-pairs may also be present on ambulacral plates at the rear of the test for constructing a sanitary tunnel. This zone is termed the plastron , and is often the most densely tuberculated part of the test. Spines and tubercles are generally rather fine in heart urchins, although some taxa have scattered large primary tubercles over their aboral surface which support long, sharp spines.
Aboral tubercles are always finer and more dense than oral tubercles.
Tubercles are always perforate and typically crenulate, although a great deal of specialization in both spine and tubercle structure is found in this group. Spines are long and slender, and often have spatulate tips. Burrowing heart urchins have bands of very fine spines that are termed fascioles that help them live in fine sediments. The detailed shape and position of these fascioles is important taxonomically. Heart urchins are deposit feeders using their oral tube-feet to gather detritus.