Whales are sea-livings mammals. They therefore breathe air but cannot survive on land. Some species are very large indeed and the blue whale, which can exceed 30 meters in length, is the largest animal to have lived on earth. Superficially, the whale looks rather like a fish, but there are important differences in its external structure; its tail consists of a pair of broad, flat horizontal paddles (the tail of a fish is vertical) and it has a single nostril on top of its large, broad head. The skin is smooth and shiny and beneath it lies a layer of fat (blubber). This is can be up to 30 centimeters in thickness and serves heat and body fluids.