Members of the order cetacean have undergone a number of changes or adaptations needed to fare well in their watery home: Their bodies have become streamlined for efficient movement through the water; their forelimbs have been modified into flippers which aid them in steering; their hind limbs have disappeared almost completely; their tail has become broadened horizontally and consists of two large flukes which propel them powerfully through the water by moving up and down, rather than side to side like a fish; in place of hair they have developed a thick layer of fat called blubber under their skin that insulates them from the cold and provides buoyancy ; and the position of their nostrils has shifted to the top of their head creating a blowhole that allows them to effectively come to the surface for air. A whale's blowhole generally reaches the surface before the rest of its body.
