Echinoderms are a phylum of marine animals. The adults are recognized easily by their symmetry, and include well known animals such as sea stars, sea urchins, and sea cucumbers. they are found at every ocean depth, because their solid skeletal system allows for the pressures of the deep water. The phylum contains over 70,000 living species. Echinoderms are also the largest phylum that has no freshwater or terrestrial organisms in it.