The Phylum Echinodermata contains over 6,000 species in 6 classes having 35 orders. The word echinoderm means "spiny skin" and this phylum includes urchins, brittle stars, starfish, cucumbers, and feather stars. These creatures all have an internal calcium skeleton and mostly travel using tube feet. Their vascular system pumps water, not blood. By pumping water into or out of their tube feet, the animal is capable of securing itself to most surfaces. Their mouth area usually contacts the substrate, except for cucumbers. They are scavengers and/or filter/suspension feeders.
Urchins are part of the Class Echinoidea (Greek for 'like a hedgehog') and its sphere-shaped, multi-spine nocturnal members are an excellent herbivore. There are probably over 900 described species coming from shallow to areas several miles deep. They feed on detritus, dead animals or are simply filter feeders. Those preferred by marine hobbyists often feed on algae, including the preferred coralline algae. In fact, urchins have a desire for calcium carbonate, and therefore can be given pieces of cuttlebone to graze upon, which may help satisfy this need and possibly save some of your valuable coralline algae.
Between their spines are tube feet and small pincer-like organs called "pedicellariae" that is used to clean and defend themselves, some of which contain poison glands. Both spines and tube feet help move them, and their mouths always face the substrate. They are also rather clumsy and may knock over anything smaller and possibly slightly larger, which isn’t secured. Beware, even though some are great alga consumers, they can be bulldozers!