Plate from 18th century encyclopedia showing an illustration of echinoderms and crustaceans. Echinoderms are a phylum of marine animals. The adults are recognizable by their (usually five-point) radial symmetry, and include such well-known animals as starfish, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives. Crustaceans (Crustacea) form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles.