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Nascent Drama

To understand why this distinction is important, let's take a look at how sharks and other fish breathe. To breathe, sharks must remove oxygen from the water around them.
 
The water enters the shark's mouth (the shark's nose is used exclusively for smell) and flows over the gills. Inside the gills are hundreds of feathery gill filaments. Each filament in turn has thousands of leaflike lamellae, or flaps, which contain blood vessels.
 
The blood absorbs the oxygen from the incoming water, and the excess water flows back out the shark's body through gill slits. Sharks have five to seven pairs of gill slits, depending on the species.
 
Scientists thought so because other fish seemed to have the equipment to actively pump the water through their mouth and over their gill slits, whereas sharks looked less developed.
 
But how to account for sharks that don't seem to swim at all, like the angel sharks and nurse sharks we might see at aquariums? It turns out that not all sharks have to stay moving to breathe. Find out which sharks are constant swimmers on the next page.
 
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