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If you've ever had the privilege of walking the beach, you've probably already noted that the starfish is one of the most popular creatures of the sea world. Perhaps one of the most unusual animals, as well, the starfish is still somewhat of a mystery.

The starfish (which is also known as a Sea Star) includes in its class, any marine invertebrate of the class Asteroidea. There are approximately 1800 different species of the common starfish, none of which can be classified as fish, despite their name.

This star-shaped carnivorous animal is usually a dull yellow or orange, but can also be brightly colored. As a natural defense mechanism, the starfish is able to change its body color to hide or escape from predators. Starfish vary greatly in size from under 1/2 inch to over 3 feet. The arms of the starfish are used for movement, catching prey and digestion. Unlike other animals, the starfish is able to grow a new arm if one is lost. Many starfish have five arms, but there are some deep-water species that carry more than fifty. The starfish's arms surround a small disc-like body, which houses a mouth in the center of the underside of the body.

Starfish bodies are highly flexible. The starfish is capable of making itself surprisingly rigid through the use of skeletal plates that are imbedded in their flesh. Blunt spines project from these plates. By making itself rigid, the starfish is able to protect himself from being eaten, handled or moved by protracting these spines. In addition, some starfish are covered with an organ called, pedicellariae. Pedicellariae make the starfish's arms light sensitive, so they are better able to manipulate themselves. Despite the added benefits of their many arms, the starfish has no eyes, ears or nose and rely solely on their legs for almost everything.

The upper body of the starfish houses tiny holes that admit water into the animal's vascular system. This system includes several canals that extend to the starfish's arms and connect to many sucker-tipped, tubed feet. The hydraulic pressure of the circulating water helps to extend the tube feet and allow the starfish to move in a multitude of directions on the ocean floor. Because of their many arms, starfish are able to wedge themselves in small nooks and under rocks as a means of protection.

Most starfish feed on slow-moving or stationary animals. Clams, oysters and snails are the diet of choice, though starfish are also known to eat fish eggs and mollusk. The starfish stomach actually extends itself through the mouth to grab and consume food. The material is then transported to the starfish's digestive glands, which are located within its arms.

Most starfish spread their eggs and sperm naturally into the water, and fertilization occurs externally. A female starfish will shed several million eggs into the water in a two-hour period. After it is matched with sperm, a hollow ball (called the blastula) develops. Much like the butterfly, the starfish changes form during early life, transforming itself from an embryo to blastula to larva to miniature starfish, a process that takes two months.

Juveniles are any starfish under the age of six months. These young starfish begin life with three or four arms and only begin to develop more as they provide themselves nutrients by consuming encrusting algae. Their rudimentary, poorly developed arms are not well used at this age, and because of their slow nature, many juvenile starfish do not survive.

By the age of six months, starfish are considered mature. Their diet begins to change from algae and eggs to coral and then to more adult like foods. The starfish becomes sexually mature at the age of two years.

After their third or fourth year of life, it's thought that starfish go through a senile phase where growth declines dramatically and reproduction is low. It is not known exactly how long the common starfish lives. Scientists have successfully kept starfish alive up to eight years, but their exact lifespan is not known.

Today, starfish live close to the sea around the world and typically stay in groups of forty or more. Most starfish live in the deepest parts of the sea and are often found hiding under rocks or in tide pools. Starfish are sometimes carried into shore during high tide. These are the starfish you may commonly see on the sand or close to shore. Because starfish can only survive two hours out of water, it's recommended that you leave starfish where you find them.