Sea stars are among the most conspicuous of all the invertebrate animals, having a distinctive flattened body basically in the form of a star. Found only in marine environments, these free-living creatures exhibit radial symmetry, generally based on a five-pointed star. Of course there are many exceptions: certain sea stars have six or even more arms. Heliaster kubiniji, found in the Panamic region, may have up to 50 arms, believed to be the most of any sea star.
The body of a sea star is generally flattened and at least somewhat flexible. The internal skeleton is comprised of very numerous separate calcareous pieces bound together by connective tissue and often bearing spines, plates and tubercles. The arrangement and structure of these calcareous plates is the principal diagnostic tool in the identification of a sea star. Although most sea stars appear quite rigid, specialized connective tissue enables them to become surprisingly supple when necessary, especially when attacking prey, fleeing predators or when righting themselves after having been flipped upside down.
Situated on the upper (aboral) side of a sea star are the central anus and the madreporite, a circular calcareous sieve located just off centre. The mouth is located in the centre of the under (oral) side of the body at the centre of open furrows called ambulacral grooves. These structures run the length of each arm and are where the tube-feet are located.
Although sea stars lack developed eyes, they do have "eye spots" complete with a simple lens located at the tip of every arm. These eye-spots are light-sensitive and it has been shown that certain species are drawn toward while others shun light. Spread over the skins of sea stars are neurosensory cells which are highly sensitive to both touch and chemical tastes. These are especially numerous in the suckers of the tube-feet, where they are incredibly dense, up to 70,000 per square millimetre!
With the aid of a magnifying glass, many tiny but exquisite details can be seen on the skin of sea stars. Many species have tiny calcareous pincers called pedicellariae, usually situated in clusters or in wreaths surrounding spines. Variously shaped, pedicellariae may look like forceps, bird beaks, or even bivalve clams. These mini-pincers are extremely effective at deterring predators and keeping the surface free of parasites and fouling organisms. Also scattered over the surface are clusters of tiny finger-like protrusions of the body wall. These thin-walled gills protrude between the skeletal plates, serving to exchange respiratory gases and excrete liquid wastes.
Inside the skeleton of a sea star lie the internal organs, including the water vascular system, digestive tract, reproductive organs and nervous system. Controlled by an internal plumbing mechanism called the water vascular system, the tube-feet can be employed in a coordinated fashion, enabling the animals to move about and grasp prey tightly. Open to the sea via the madreporite, the water vascular system uses muscles and hydraulic pressure to operate the tube-feet. When fluid is withdrawn the tip of a tube-foot creates suction, enabling the sea star to cling to a rock, climb a piling or securely grip prey. Individually these suckers are not especially strong, but when used in concert they can apply a surprisingly powerful force. While most species possess tube-feet that terminate in suckers, the sand star, found on soft substrates, has pointed ones.
The digestive system occupies much of the space inside a sea star. The mouth opens into two stomachs that are connected to paired lobed organs called pyloric caeca that extend into each arm. These organs secrete digestive juices and also serve for dissolved food storage, becoming swollen when prey is abundant. Wastes are passed out through the anus, located in the centre of the aboral side. Reproductive organs called gonads lie beneath the paired pyloric caeca in each arm. These open into the sea via pores located in the "armpits" of the stars.
Sea stars have no brain or central nervous system. Instead a nerve ring in the central disc connects to radial nerves running down the length of each arm. These nerves join a diffuse network of nerve cells scattered throughout the skin.
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