Echinodermata is a group of marine invertebrates which includes animals like sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, and feather stars. The name “echinoderm” means “spiny skin,” which refers to rough surfaces or spines commonly present on their bodies. Adult echinoderms usually have bodies organized radially with five symmetrical sections arranged around a central point, while their larvae are bilaterally symmetrical. Echinoderms have a skeleton which is built from calcium carbonate structures known as ossicles. They also have an internal system which pumps water through their bodies in order to fill small fluid-filled canals and, in some groups, mobile tube feet. Changes in fluid pressure can make these tube feet stretch out, working together to produce a sort of walking motion. Some echinoderms are fully mobile while others attach themselves to a hard surface or to the loose substrate of the ocean floor. Echinoderm fossils, particularly those of crinoids, are common in deposits from the Paleozoic era but turn up less often in deposits from the Western Interior Sea. Most extant groups of echinoderms had already appeared on the scene by the Late Cretaceous and the most common echinoderm group known from these rock layers are the echinoids, or sea urchins.