Sharks
Sharks are fish, but they are very different from most fish. They belong to a class of fish known as Chondrichthyes (软骨鱼纲).(26)But unlike other fish, they have no other bones.
Sharks breathe through gill (鳃). While most fish have one pair of gills, sharks have five to seven. Most fish don't have eyelids, but some sharks have three for each eye. Two are like yours, an upper and lower eyelid.(27)
People are afraid of sharks, but there are fewer than 100 shark attacks reported worldwide each year. In fact, you have a better chance of being hit by lightning than of being attacked by a shark. Americans killed by sharks in the past 100 years are fewer than those killed in car accidents over a holiday weekend.(28)
Sharks have powerful jaws and rows of sharp teeth. Some sharks can bite nearly 300 times harder than a human, enough to cut through a thick piece of steel. A shark's jaw is lined with as many as twenty rows of teeth, one behind the other. When a shark breaks or loses a tooth, another one moves forward to replace it.(29)
Most sharks are cold-blooded. The temperature of a shark's body depends upon the temperature of the water around it.(30)But some live in cold seas.
A.Actually no shark counts people as its regular food.
B.Some sharks travel great distance.
C. Most sharks live in warm ocean waters.
D.Sharks have backbones and teeth.
E.A shark may go through thousands of teeth during its life.
F.The third eyelid covers the entire eye and protects it.