These animals are bilaterally symmetrical, have spiny skins, and an internal skeleton of porous calcite. This group includes crinoids, sand-dollars, sea stars, and sea cucumbers. The list of classes below are the most well known fossil groups.

Classes:

Blastoidea

Holothuroidea

Asterozoa

Ophiuroidea

Crinoidea

Echinoidea

For more information:

http://www.ucmp.berkeley.edu/echinodermata/echinodermata.html

http://www.palaeos.com/Invertebrates/Echinoderms/echinodermata.htm