Mammals of two orders Pinnipeds (walruses, seals, seals) and Cetaceans (whales, dolphins, sperm whales) have adapted to life in the aquatic environment. They have a streamlined body, short limbs, turned into flippers (in pinnipeds). In cetaceans, only the forelimbs have become fins, the hind ones are absent; a powerful tail fin serves them for movement. Animals of both orders have a thick layer of subcutaneous fatty tissue that conserves body heat. They breathe atmospheric air, periodically floating to the surface of the water. Only pinnipeds go to land for breeding. Cetaceans spend their entire life in water.
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