Echinoderms Echinoderms are a group of animals with external spines, such as the starfish, sea urchin, sand dollar, and sea lily. There are over 6,000 species of echinoderms. Their spines can be used for many purposes, but are mainly used for protection. Echinoderms are radially symmetrical animals that are only found in the sea. You can't find any of them on land or in freshwater.
Echinoderms usually have five appendages, but there are some exceptions.
A lot of them have suckers on the ends of their feet that are used to capture and hold their prey, and also to hold onto rocks when there is a swift current. The differences between the two subtypes lies in how the arms connect to the central disk. Ophiuroids have arms that don't connect with each other. There is a distinct boundary between the arm and central disk. Asteroids have arms that are connected to each other.