Sea stars, also known as starfish, are echinoderms belonging to the class Asteroidea. The names "Sea Stars" and "Starfish" are sometimes differentiated, with "Starfish" used in a broader sense to include the closely related brittle stars, which make up the class Ophiuroidea, as well as excluding sea stars which do not have five arms, such as the sun stars and cushion stars.
Sea stars are composed of a central disc from which arms sprout in pentaradial symmetry. Most sea stars have 5 arms, but some have more fewer. The mouth is located underneath the sea star, on its ventral surface. The spiny upper surfaceis called the aboral or dorsal surface. On the aboral surface there is a structure called the madreporite, a small white spot located slightly offcenter on the central disc which acts as a water filter and supplies the sea star's water vascular system with water to move.
Sea stars usually hunt for shelled animals such as oysters and clams. They have two stomachs. One stomach is used for digestion, and the other stomach can be extended outward to engulf and digest prey. This allows the sea star to hunt prey that is much loarger than its mouth would otherwise allow.
