Like most echinoderms, the skeleton of asterozoans is composed of numerous small calcite plates called ossicles which readily disarticulate upon the animal's death. Asteroids are asterozoans whose internal body parts extend into the arms. Unlike the ophiuroids, there is no sharp distinction between the arm structure and the central body structure. Most asteroids are predatory, preying on clams, corals, other echinoderms, or anything else unlucky enough to cross their path. Some asteroids can extrude their stomach through their mouth, killing and partially digesting their prey outside their body.
Asteroids are among the most active echinoderms.
The arms of ophiuroids are muscular extensions and contain only the ambulacra. Many ophiuroids are scavengers, and will eat just about anything. Along with crinoids and holothurians, ophiuroids essentially rule the floor of the deep oceans, although they are also found in shallower depths. Echinozoans include the familiar sea urchins, sea biscuits, and sand dollars, as well as several obscure Paleozoic groups, and the unusual sea cucumbers. Except for sea cucumbers, echinozoans have a globose or discoidal test with 5 ambulacral regions.