• Radial Symmetry
• Internal Skeleton
• Water-vascular System

They have radial symmetry as adults unlike other phylums that have bilateral symmetry. (Biology of Echinoderms)

Echinoderms have an internal skeleton made up of calcium carbonate or bony plates. They deposit these materials after they dissolve the calcium and carbonate ions, which they collect from sea water. Echinoderms skeleton allows them to flex and bend their "arms" unlike other phylums, which makes their skeleton jointed. They also have spines in their skeleton. Thats where they get their name, Echinodermata which means "spiny skinned." (Biology of Echinoderms)

The water vascular system is the other characterisitic. This functions during movement, but has oxygen exchange, and molecular nutrient uptake are also used. (Biology of Echinoderms)