Whales are sea living mammals. They therefore breathe air but cannot survive on land. Some species are very large in deed 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 asingle nostril on top of its large, broad head. The skin is smooth and shiny and beneath it lies a layer of fat (blubber). It can be up to 30 meters in thicknees and serves to conserve heat and body fluids.