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Respiration
Sharks, like all other fish (with a few exceptions), oxygenize their blood in the gills. Most sharks swim with their mouth open, so that water can flow over the gills. When the water passes over the gills, the blood takes up oxygen and leave carbon dioxide, which is then transported away with the outgoing water. The flow of blood is opposite to the direction of water flow. This countercurrent flow is the best arrangement for extracting the greatest possible amount of oxygen from the water.
Some species don't have to swim to breath. Instead they keep a steady stream of water over the gills by pumping with their mouth. Many bottom dwelling species takes in water through a spiracle, a whole behind the eye, and pump it out through the gills, thus preventing bottom sediments from being inhaled.
The gills of bony fishes are covered by a lid, the operculum, which leaves a single slit on each side for the outgoing water. All now living sharks lack these operculum, instead they have a series of gill slits (one for each gill arch, altogether 5-7) out of which the water flows. Examination of fossils shows that some prehistoric sharks had as many as 10 pairs of gill arches. A high number of gill arches are thought to be a sign of primitiveness among fishes. Throughout the millions of years that fishes have existed evolution has lead to fewer and fewer gill arches. Sharks that have 6 or 7 gill arches (e.g. frilled sharks, cow sharks) are therefore believed to be more primitive then those with 5 gill arches.
Ref: Hickman CP, Roberts LS and Larson
A (2000), Animal Diversity, McGraw-Hill Higher Education, USA -- Springer
VG and Gold JP (1989), Sharks in quetion: the Smithsonian answear
book, Smithsonian Institution Press, Washington, D.C.
London -- Taylor LR (editor) (1997), Collins Sharks & Rays -The
ultimate guide to underwater predators, HarperCollins Publishers,
London
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