GILL SLITS:

  1. Sharks, like other fish, live in the water and use their gills to filter oxygen from the water. When water passes over the gills, a system of very fine blood vessels (capillaries) in the gills takes up (absorbs) oxygen from the water.
  2. Sharks have 5-7 pairs of gill slits located on the sides of their heads. Unlike bony fish, they do not have gill covers. Water must continually flow across these slits in order for the shark to breathe. This can be accomplished by the shark's swimming, by it standing still in a current, or by it fanning water across the gills with its fins (this is done by the nurse shark).
  3. Batoids (rays) have 5 pairs of ventral gill slits.

SPIRACLES:

  1. Some sharks have spiracles, which are special gill slits located just behind the eyes. They supply oxygen directly to the eyes and brain of the shark. Bottom dwelling sharks like the angelsharks use this extra respiratory organ to breathe while at rest on the sea floor. It is also used for respiration when the shark's mouth is used for eating. Spiracles originate from the rudimentary first gill slits and are reduced or absent in active, fast-swimming sharks.

SCALES:

  1. Sharks and batoids have placoid scales, also called dermal denticles (dermal = skin and denticles = teeth). Placoid scales have the same structure as the tooth, consisting of three layers: an outer layer of vitro-dentine (an enamel), dentine, and a pulp cavity. Placoid scales are arranged in a regular pattern in sharks and an irregular pattern in batoids.

  2. Unlike other types of scales, placoids scales do not get larger as the fish grows. Instead, the fish grows more scales.
  3. Like teeth, the shape of the scales vary among apecies and can be used to identify the species.
  4. Placoid scales gave rise to teeth, stingrays' spines, and the dorsal spines of dogfish and horn sharks.

  5. As a shark or batoid (like the stingray) swims, placoid scales may create a series of vortices or whirlpools behind each scale. This enables a shark to swim efficiently.
  6. European cabinet makers used the rough skin of a shark as sandpaper, called shagreen. With the denticles removed, shark skin is also used for leather.
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