Although whales have fishlike bodies, with the same sleek curves and tails you can find on tunas and sharks, they have a number of distinguishing characters. Whales do not have gills, so they cannot extract dissolved oxygen from the water in which they live. Whales must rise to the surface of the ocean in order to breathe. Whales and dolphins have long muscles that run the length of their bodies - much like the long muscles running down your back - whereas tuna have muscles that form vertical blocks from head to tail. Whales lift and lower their tails to generate thrust. Sharks and tunas move their tails from side to side. And whales give birth to live young that cannot get their own food; instead, the young must drink milk produced by their mothers. Only some species of sharks and fish give birth to live young, but those offspring can feed themselves; sharks and tuna do not produce milk. So despite their fishlike appearance, whales are very different from sharks and tunas.