Body Structure Scientists classify sharks as Chondrichthyes. Species in this class have cartilage instead of bone, jaws, paired fins, and paired nostrils. Sharks' skeletons are made up of cartilage, not bone. Cartilage is a firm flexible tissue of great strength made up of collagen fibers. Lacking the calcium phosphate salt of bone, which makes bone rigid, cartilage is lighter and provides sharks with strong but flexible support. (Johnson, 95) A shark has five kinds of fins, supported by cartilaginous rods: a pair of pectoral fins that lift a shark as it swims, a pair of pelvic fins that help stabilize the shark, one or two dorsal fins that also aid in stabilization, an anal fin on those species that need extra stabilization, and a caudal fin of varying size and structure that propels the shark. (Allen, 13)

Unlike most fish, sharks lack a swim bladder that gives bony fish their buoyancy. Instead, sharks have large livers, that in some sharks make up as much as seventy-five percent of their body weight, giving them a state close to neutral buoyancy. (Allen, 13) Instead of typical fish scales, dermal denticles cover the shark's body. Denticles range from microscopic to visible to the naked eye. These denticles, like tiny teeth, give the tough hides of most sharks a sandpaper-like roughness that can scratch or even tear a swimmer's flesh. Shark hide, known as shagreen, has been used by cabinetmakers instead of sandpaper. Some denticles in sharks, so large and closely spaced, form a shield that can ward off a harpoon. Denticles also help sharks swim faster. By secreting a slime through pores strategically located between their denticles, sharks cut down on drag. (Allen, 13-15)

The denticles themselves also aid in the reduction of drag in what engineers call the "boundary layer" between the moving body and the sea. Ridges on the denticles, like the keel ridge that runs longitudinally along the shark's body, further aid in drag reduction and also in the smoothing of boundary turbulence. This results in speed. Scientists have clocked sharks at bursts of speed of about twenty-five miles per hour. Usually, however, a shark typically cruises at about five or six miles per hour. Scientists have studied sharks intensely with the purpose of applying sharks' fast-swimming techniques to submarines and torpedoes. Borrowing from the sharks' strategy, yachtmen apply a kind of slime, in the form of strips of sticky polymer, to racing boats. (Allen, 13-5)

Shark Teeth Sharks may shed as many as 30,000 teeth during their lifetime. The teeth of sharks are lined up in several orderly rows, adding up to a thousand or more. Their sizes and shapes vary in the different shark species, from the stiletto-shaped teeth of the sand shark to the blunt teeth of most skates and rays. Some sharks may call into action as many as five rows of teeth, which erect or depress when needed. And behind these actively used ones are row upon row of reserves lying in deep grooves inside the jaw. (Allen, p 15)

Life Span
Sharks mature slowly and have relatively long life spans. Sharks rarely have infections, do not develop carcinogen tumors, and do not suffer any circulatory diseases. They recover rapidly from severe injuries. These facts have given scientists a reason to study sharks more intensively and have inspired much quackery about the alleged cure-all properties of shark products. (Allen, p 17) Scientists have proven, for instance, that shark liver has anticancer properties. Shark and other marine organisms provide chemicals used in cosmetics and pharmaceuticals worth $400 million per year. (Miller, 631)

Reproduction
Unlike most other fishes, sharks reproduce slowly and have few offspring. While most fish may produce thousands of young each season, most shark species produce between two and ten young once every year or two, take ten to twenty-four years to reach sexual maturity, and have gestation periods (pregnancies) that last as long as two years. (Miller, 185)

Senses: Smell
The olfactory lobes, which control the sense of smell, constitute the largest part of the shark's brain. Because of that, the shark's brain has often been called a "brain of smell." It can detect the smell of prey one quarter of a mile away. Scientists have observed sharks following swimmers who had a mere scratch on their body. As zoologist A. D. Hasler remarked, "We are concerned here with a sense of such refined acuity that it defies comparable attainment by the most sensitive instruments of modern chemical analysis." (Allen, 23)

Senses: Vision
Sharks have a well-developed eyesight and some can also see colors. Their eyes vary widely depending on their species from the enormous eyes bulging from the heads of some deep-sea sharks to the relatively small eyes of the whale sharks. Behind the retina of the eyes of some sharks are located light-reflecting tissues similar to those in a cat's eye. These natural mirrors enhance the feeble underwater light. If the shark is swimming in water that is too bright, it will drop a curtain of non-reflecting cells over the mirror-like tissue. (Allen, 24)

Experiments have indicated that sharks' iris muscle responds directly to sunlight and does not act through nerve impulses from the brain. Thus, this muscle will continue to expand and contract even when it is removed from the head. (Allen, p 24)

Extra Senses
Sharks can detect vibrations of other animals via a sense called the lateral line, a network of fluid-filled channels that run the length of the shark's body. Most sharks, skates, and rays also have a sense system, which appears as a number of pores scattered about the head, and is called Lorenzini's ampullae. These ampullae may be used by some shark species to communicate using weak electrical signals. Some scientist believe that they can also detect changes in water temperatures, pressure, and salinity. (Allen, p 26-7)

Bibliography
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