Starfish are the best-known members of the invertebrate phylum Echinodermata. Commonly 5, but up to 20 or more short or long rays, radiate from a central disk. Starfish demonstrate a term called radial symmetry. The rays of the starfish are vital organs to the starfish an are an extension of the starfish's body cavity. Starfish can be found around the waters of the world. They can be found in the ranges from along the shore down to depths of 6 km (3.7 mi). Most starfish are anywhere from 10 to 30 cm (4-12 in.) in diameter. The smallest species are known to be as small as 1 cm (0.4 in.). The largest known species is the Midgardia xandaros. This species grows as big as 1m (3.3 ft) in diameter.
The starfish's body is covered by flexible armor, skeletal pieces, or ossicles. From the mouth of the starfish channels with two or four rows of tube fet jut outward along the arms. The central anus is found on the upper side of the starfish where it opens into the water vascular system. Each arm that the starfish has a sensory tentacle that responds to a plethora of stimuli, and a red photosensitive eyespot located at the tip of the starfish's arm. Different starfish have many different colors. Bilaterally symmetrical,swimming larvae are a result of external fertilization.The larvae morph into free-living adult forms of starfish.
Most starfish are carnivors or scavengers. They feed on a variety of plant or animal material. Carnivor starfish prefer to eat bivalve mollusks; they can open them slightly by the ulling action of the sucking tube feet and then digest the soft parts externally. Most of the times, the starfish devastate oyster and clam beds, and some extensive damage to coral reefs.