Sharks are commonly defined as a marine fish of the order Squaliformes or Selechii. They make up the Chondrichthyes, or "cartilaginous fish" along with skates, rays, and a few other fish. Members of the Chondrichthyes lack true bone and have a skeleton made of cartilage. Only their teeth, and sometimes their vertebrae, are calcified, but this is still not true bone. The bone structure is one of the main differences that separates sharks from other fish and marine animals. Sharks also differ in that they have a skull without sutures and reserve rows of teeth attached to the skin of the jaws. They lack an air bladder forcing all but shallow-water forms to swim constantly in order to keep from sinking.
             When looking at sharks there are eight orders and about 350 species so it is difficult to discuss each one in detail. All sharks are Selachians and are characterized by a fusiform body with five to seven pairs of lateral gill slits. Sharks are easily identified by distinctive characteristics such as a ventrally positioned mouth, an asymmetrical tail with the vertebral column extended into the upper lobe, and a body covered with tiny, sharply pointed placoid scales. Sharks are found at all depths of all the oceans except the Antarctic. They are the most abundant in tropical and subtropical waters. Few inhabit freshwater lakes and rivers, though some migrate regularly from salt to freshwater. The swimming of these animals is constant but leisurely, propelled by the sculling motion of the body and tail, but many are capable of sudden bursts of speed, such as the blue shark that has recorded speeds of more than 64 km/h (40 mph).
             Sharks also have highly developed senses. The eyes, which are lateral, are highly sensitive to contrasts of light and shadow and are especially adapted to see in dim light. Some species have an eyelid-like structure called a nicititating membrane. This protects the eye from b
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