Whales
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Whales are aquatic mammals belonging to order CetaceA．A few species live in fresh water, but most species live in the seA．They have a streamlined shape and a powerful tail to drive them forwarD．With its two large horizontal fins or flukes, the tail produces the driving force by beating strongly up and down. Flippers at the front are used for steering and balance. The hind limbs of whales have completely disappeared, apart from a few small bones inside the body. Body hair has also disappeared, giving whales a smoother outline and less resistance to water.Instead of hair, whales are insulated by a thick layer of fat, or blubber, under the skin. The blubber may be as much as 61 cm thick on some parts of the body. Besides protecting the animal against the cold, the blubber is an important food reserve.
Most of the best-known whales large creatures.For example, the blue whale reaches a length of more than 30 m.However, many whales, such as dolphins and porpoises, are small.Some are only 1.5 to 1.8 m long.
Whales live entirely in water.Sometimes, whales are stranded on the shore.Although they are air-breathing animals, they soon die because their great weight keeps them from expanding (opening out) the chest cavity.They can breathe easily when afloat, because the water supports most of their weight.
The bottle-nosed whale has been known to stay under water for about two hours.The sperm whale can dive down to depths of 500 fathoms.Such long and deep dives are unusual.Most dives last between 10 and 30 minutes.Whales have special mechanisms that help them to stay under water.When they breathe, they renew about nine-tenths of the air in their lungs.When human beings breathe in, only about one-fourth of the air is renewed.Whales therefore have a fairly large supply of fresh air to start with.They also have an additional oxygen supply in the muscles, where air is loosely held in combination with a pigment called myohaemoglobin.Another thing that helps whales hold their breath for long periods of time is their low sensitivity to carbon dioxide in the blood.(It is the carbon dioxide building up in human blood that affects the brain and makes the human being take another breath.)
When a whale surfaces to renew its air supply, it needs only to push the top of its snout out of the water.This is because the nostril or blowhole is at the top of the heaD．The expelled air is forced out strongly to form. the spout or blow.
Living whales are divided into two groups—tooth whales and whalebone whales.Tooth whales, which include most species, generally have many conical teeth and eat mainly squids and fishes.The killer whale feeds on seals.One African river dolphin feeds mostly on plants.Other tooth whales include the narwhal and the sperm whale.
Whalebone whales have no teeth.Their mouths contain huge comblike fringes of baleen or whalebone.This horny substance is usually black.All whalebone whales feed by straining small animals from the water.The mouth is filled with water and the water is then forced out through the fringes by the tongue.The animals caught in the baleen are swept into the stomach by the tongue. All whalebone whales are large animals, which are usually found in cold seas.They include the blue whales, the right whales, and the rorquals.
The future of many of the larger kinds of whales is uncertain. Whalers have killed so many blue, bowhead, humpback, and right whales that those species are threatened with extinction. Overhunting has also greatly reduced the number of fin and sei whales.Also, if the human population does not stop increasing, people may have to compete with whales for food in the seA．Some nations have begun fishing for krill.Krill is the chief food of wha

A. good bodily resistance to water
B. only a few small bones inside the body
C. neither hind limbs nor body hair
D. a thick layer of fat under the skin