Starfish, or sea stars (a less confusing designation, since they are only very distantly related to fish), are marine invertebrates belonging to the kingdom animalia, phylum Echinodermata, and class Asteroidea, of which there are over 1800 known species. The names sea star and starfish also are (incorrectly) used for the closely related brittle stars, which make up the class Ophiuroidea. Characteristics of sea stars include that they exhibit a superficially radial symmetry, since they typically have five or more "arms" which radiate from an indistinct disk (pentaradial symmetry), and tend to be covered in spines. Like other echinoderms, starfish possess an endoskeleton, but do not rely on it for support and locomotion, instead using a hydraulic water vascular system that functions via many projections called tube feet, located on the ventral surface of the starfish's arms. Starfish are an essential part of marine food chains, being eaten as larvae, and becoming voracious predators upon reaching adulthood, with their diet including mollusks and other marine organisms. They even are able to evert their stomach in order to pull apart and digest their shelled prey. For humans, they are a source of aesthetic joy and their remarkable characteristics—movement based on a hydraulics system, ability to regenerate lost limbs and evert their stomach—add to the wonder of nature. Some starfish also are captured, dried out, and turned into popular souvenirs or decorations. While starfish can reproduce sexually, if an arm of the starfish is severed from the body and it contains a portion of the central nerve ring, then another starfish can regrow from the severed portion. Lack of knowledge of this capacity increased the role of starfish as pests to commercial clam and oyster beds when fisherman, in an effort to get rid of these predators, would cut them up and throw them back into the ocean. Starfish are composed of a central disc from which arms (usually five, but the number can vary) sprout in pentaradial symmetry. The mouth is located underneath the starfish on the oral or ventral surface, while the anus is located on the top of the animal. The spiny upper surface covering the species is called the aboral (meaning the side opposite to the mouth) or dorsal surface. On this surface there is a structure called the madreporite (or the sieve plate), a small white spot located slightly off-center on the central disc, which acts as a water filter and supplies the starfish's water vascular system with water to move. Schmedelian pin-cushion sea star on Meedhupparu house reef in the Maldives, Culcita schmedeliana Also on the aboral surface, surrounding the spines, are small white objects known as pedicellarieae, which are like pairs of claws or jaws that serve to prevent encrusting organisms from colonizing the starfish. The radial canals (a part of the internal anatomy), which extend from the center of the starfish out towards the tips of the arms, have hollow, muscular tube feet branching from them. Each tube foot has a bulb-like ampulla, as well as a suckered podium (or foot part), which are a part of the water vascular system. Starfish also have a simple photoreceptor eye spot at the end of each arm that is able to "see" differences of light and dark, enabling the starfish to detect movement. Additional parts, like cribriform organs, that are present exclusively in Porcellanasteridae, are used to generate current in the burrows made by these starfish. While starfish generally adhere to this basic body plan, there is great variety in the shape (ranging from nearly pentagonal like the Indo-pacific cushion star, Culcita novaeguineae, to gracile stars like those of the Zoroaster genus), the color, and the morphology between each species. Some species have patterns that serve as camouflage or warning coloration, which include mosaic-like tiles formed by ossicles, stripes, interconnecting net between spines, and pustules with bright colors, mottles, or spots. The tube feet can be seen on this starfish. Blood star (Henricia sanguinolenta) at the New England Aquarium, displaying its tube feet. Starfish, like other echinoderms, move using a water vascular system. Water comes into the system via the madreporite. It is then circulated from the stone canal to the ring canal and into the radial canals. The radial canals carry water to the ampullae and provide suction to the tube feet. The tube feet latch on to surfaces and move in a wave, with one body section attaching to the surfaces as another releases. Much of the adhesion of starfish to surface is chemical, with the tube foot secreting substances that either bond with surfaces or break down the bonds with surfaces, allowing the tube foot to move. This is so that extra energy is not expended by the starfish in order to stay attached by exerting its muscles. As a result of all this, most starfish cannot move quickly. However, some burrowing species like starfish from genus Astropecten and Luidia are quite capable of rapid, creeping motion—it "glides" across the ocean floor. This motion results from their pointed tube feet adapted specially for excavating local area of sand. Credits New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. 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Dissection Directions: Be the first student to find a good one and you will get extra credit! Labeled Pictures: http://wwwbio200.nsm.buffalo.edu/labs/tutor/earthworm/.
How many animals are dissected every year? A reasonable estimate is that about six million vertebrate animals are dissected yearly in U.S. high schools alone, with an.