Echinodermata is a phylum of marine animals. The adults are recognizable by their radial symmetry, and include
animals such as sea stars, sea urchins, sand dollars, and sea cucumbers, as well as the Crinoids or "sea lilies". Echinoderms are found at every ocean depth, from tide pools to deep ocean trenches.. The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes, after the chordates. Echinoderms are also the largest phylum that has no freshwater or terrestrial representatives.
The echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the deep sea, as well as shallower oceans. Most echinoderms are able to regenerate tissue, organs, limbs, and reproduce asexually; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is mainly in their ossified skeletons, which are major contributors to many limestone formations, and can provide clues as to the geological environment.