The typical shark is one of the most beautifully streamlined of all fishes. It is graceful as it cruises through the water. Most shark’s mouths are crescent shaped and located below their shovel shaped snout. Inside the mouth of most sharks are row after row of teeth designed for seizing, tearing, piercing or crunching their prey. When a shark’s tooth falls out or is torn out it is replaced by a tooth that moves forward to take the lost tooth’s place. These teeth are actually specialized versions of the thousands of teeth with which a shark’s body is covered. These tiny razor sharp external teeth covering the shark’s skin are called denticles and can inflict a scrape wound to a person when brushed up against. The shark’s eyes are set far apart on both sides of the head.. The paired fins on the forward part of a shark’s body are called “pectoral” fins and are mainly used for steering. The smaller fins near the shark’s tail are called the “pelvic” fins. The tail fin is known as the “caudal ” fin and provides the shark’s propulsion. The shark’s skeleton is made up almost entirely of cartilage rather than bone. The gill slits on the shark are very primitive and may consist of 5 to 7 slits found on each side just behind the head. On some sharks is found a diminished gill opening called a “ spiracle “. The spiracle is fitted with a small valve that open and closes as the shark breathes. Water is drawn in through the spiracle and expelled through the gill slits.

Another feature of the shark is the so-called “ spiral valve “ in the intestine. The name is somewhat misleading as it is actually a device to increase the absorptive surface of the intestines which are much shorter than those of most invertebrates. For example a 10 foot long shark may only have about 9 feet of intestines where a 6 foot man has about 25 feet of intestines. Though it varies in shape in different species of sharks, the spiral valve is generally built on the same principle as a circular ramp or when more tightly coiled, a series of spiraling scrolls, one inside the other with food passing through the spirals in the process of digestion. Since it may have as many as 45 turns, the absorptive surface provided within a limited space is very impressive.

An outstanding feature of sharks is their extraordinary sense of smell. A large portion of their brain is given to the sense of smelling. The forward part of the brain has two forks that extend toward the nostrils on each side of the snout and their perception is so delicate that the shark can actually steer itself up a scent trail zeroing in on the scent source. When a shark first picks up a scent trail it may veer back and fourth to establish the direction of the scent source. A shark can detect one drop of blood in a million drops of water!

The eyesight of a shark is far less acute than it’s sense of smell. Whether or not a shark can see colors is still up for debate. While it was once thought that several species of sharks could distinguish one color from another, recent experiments to prove this capability have proved negative.

A shark’s ears are mainly balance organs containing semicircular canals that inform the shark of change in direction, acceleration, deceleration and whether it is right side up in the water. There are no main channels to the outside of the body of the shark from the ears. Concentrated on the head and face of sharks are small sense organs sunk at the bottom of tiny pits in the shark’s skin. Each pit with it’s sense organ transmits vibrations and changes in the flow of water. Corresponding to the lateral line organs in the higher bony fishes, these continue along a line back to the tail in a fine tube beneath the skin which opens at intervals to the outside through minute canals. On the head, these organs are supplemented by what are called the “Ampullae of Larenzini, very deep canals filled with a jelly like substance which registers changes in temperature of the water and water pressure.

Coloration of sharks varies for it depends on what type of shark it is. For instance a Great White shark is darker on the top and lighter on the bottom. This is to make it hard to spot from above as the darker color on it’s upper body tends to blend in with the bottom. Conversely the lighter underside of the Great White shark makes it more difficult to see from below as the lighter color will blend in with the lighter depths toward the surface.