Starfish

All information is from the Microsoft© Encarta 99© Encyclopedia

The Common Starfish's Usual Size is up to 50 cm, tip of one arm to another including the central disc.

Starfish, or sea star, is the common name for 5 orders and about 1500 living species of marine invertebrate animals characterized by radially arranged arms with tube feet that allow them to move. Some starfish are only 0.4 to 0.8 inches wide as adults, but others are as much as 26 inches wide. The group is abundant at all depths of the ocean.

Generally starfish are rather stiff bodied and crawl slowly. The mouth is directed downward and is surrounded by a number of arms radiating away from it, each arm bearing rows of tube feet used in crawling, attachment, and feeding. The arm length varies, and four, six, or more arms instead of the usual five are not uncommon. The anus is often lacking; if present, it is on the body's upper surface. The skin, rough and leathery, usually bears spines. The animal has a large gut, a complex system of body cavities, and a simple, brainless nervous system.

Starfish have moderately well developed senses of touch, smell, and taste and also respond to light. They feed on a wide range of items, including ocean-bottom deposits. They can eat small animals whole; they deal with larger prey by extruding their stomachs and digesting the prey outside their bodies. Some starfish eat bivalves by prying open the shells with their arms and then slipping their stomachs between the two shell valves.

Most starfish have separate sexes and spawn both sperm and eggs into the water, where fertilization and early development occur. Sometimes the mother retains the eggs on the bottom and protects them. Many starfish have the power to regenerate body parts, and in some starfish this becomes a regular means of asexual reproduction, new animals being produced from each fragment.

Starfish play an important role in some animal communities, because they eat a variety of materials and are often numerous. Under unusual conditions they can do some harm: a few are pests of oyster beds, and the crown-of-thorns starfish sometimes has population explosions that damage coral reefs through overfeeding on the coral. Few animals consume adult starfish, which are neither palatable nor nutritious.

The starfish exhibits the internal hydraulic system particular to echinoderms. A central ring connects canals that run the length of each arm and branch into rows of tube feet. The hollow network, a modified body cavity, fills with water, creating a "skeleton" firm enough to walk on. By forcing water out through the tube feet, the starfish can extend and attach its suckers to a surface; by contracting the muscles at the base of the tube feet, it can move itself forward.