They have a water vascular system, and many members move via tube feet.
Cryptasterina hystera is a rare example of a viviparous organism in this phylum. The skeletal plates of creatures in this phylum are made up of fine networks of calcium carbonate known as stereom. One member of this phylum, the Xyloplax [ZY-lo-plax], may represent a sixth class, but it may just be an unusual member of the Asteroidea class. Nearly all of its members are benthic, including sea lilies and sea cucumbers. They have a water vascular system, and many members move via tube feet. For 10 points, name this phylum of "spiny-skinned" creatures, including sand dollars, sea urchins, and starfish. 
Cryptasterina hystera is a rare example of a viviparous organism in this phylum.
Echinodermata or echinoderm 