An echinoderm is an organism (all marine) that has a spiny skin. The word “echinoderm” means “spiny skin.” Starfish, sea cucumbers, brittle stars, and sea urchins are examples.
Most echinoderms have pentamerous radial symmetry, meaning they can be divided externally into five equal parts along a central oral/aboral axis.
Echinoderms also have a water vascular system. Water enters the animal through an opening called a madreporite. The water is is then channelled into the tube feet, the structures echinoderms use for locomotion.
A sea urchin is an echinoderm: