The starfish and sea urchins belong to the group of the echinoderms (Phylum: Echinodermata). They have a prominent calcareous internal skeleton (endoskeleton) which makes the surface of their bodies rigid and spiny. In fact, their name is derived from the Greek echinos, spiny and derma, skin, referring to this feature.
There are approximately 7,000 living species described and 13,000 fossil species, however, as they inhabit deep ocean areas, it is estimated that there are many more undiscovered species (Brusca and Brusca, 2005; Pawson, 2007). In Mexico about 500 species have been described (Llorente-Bousquets and Ocegueda 2008).
There are five groups (classes) of echinoderms. The sea lilies and feather stars (Class: Crinoidea) that are cup-shaped; the starfish (Class: Asteroidea) with a star-shaped body, the basket stars and brittle stars (Class: Ophiuroidea) also with star-shaped body but with slimmer arms; sea urchins and sand dollars (Class: Echinoidea) whose bodies are spherical or discoidal; and sea cucumbers (Class: Holothuroidea) that are soft and elongated (Brusca and Brusca, 2005).