Fish and whales are sea animals. Are they the same? No! The whale is s mammal（哺乳动物）. The fish is not. Mammals are air-breathing, warm-blooded animals and the female ________ the young with their milk.

The mammals of the sea must breathe air, too. We all know a man will suffer brain injury if he gets no air for as little as four or five minutes. He will probably ________ if his supply of air is shut off for any longer than that. But ________  ages, the mammals of the sea have developed the ability to hold their breath underwater for an amazingly long time. Some seals are able to stay down for 20 minutes. But whales, of course, are the champions. A big whale can hold its breath underwater for an hour, perhaps even two.

Several things help sea mammals hold their breath so long. First of all, over millions of years many of them have developed nostrils（鼻孔）which can be ________ to keep water out. Whales and dolphins breathe through “blowholes”, which have a kind of stopper that acts like a valve（阀）to keep water out. These blowholes, or nostrils, are ________ the tops of their heads.

The mammals of the sea have also developed the ability to take in large amounts of air before making a deep ________. Finally, animals like seals, when they are underwater, make special use of the air in their lunge. Most of this air goes to their brains and hearts while very little is carried to other parts of their bodies.