The echinoderms represent a biological puzzle for zoologists. Biologists generally agree that bilateral symmetry is adaptive for free-moving animals and radial symmetry is adaptive for sedentary animals, but the echinoderms are freemoving yet exhibit radial symmetry. Since the echinoderm larval stages are bilaterally symmetrical, they obviously evolved frombilateral ancestry; however, the larvae metamorphose into radially symmetrical adults. The echinoderms live in marine environments and include a varied group that is considerably different from all other members of the animal kingdom. Echinoderms include the starfishes, sea urchins, brittle stars, sea cucumbers, and sea lilies. A calcareous skeleton is present in all members of the phylum, and their name is derived from the presence of external spines or protuberances.
Thanks for description - Animal life club