Land animals move easily through air, because air does not slow them down. Sea creatures, on the other hand, have to move through water, which is hundreds of times thicker than air. A sea animal has to push itself through water in order to move. Sea animals use many different ways to swim, creep or glide through water. Fish are able to swim by bending their bodies into waves. They have flattened fins and tails that push against the water, like, or blades converting their body waves into forward movement. The size of a fish's tail contributes to the swimming speed. Small tail fins are found in slow swimmers, like the eel. The medium sized tail of the base is linked with a medium to fast swimming speed. Long pointed tail lobes, like those on the marlin, are found only on fast swimmers. Sea mammals, like whales and dolphins, swim in a very fish like way, except for one important difference. Because their ancestors lived on the land. They developed tails that moved up and down. Whales and dolphins wave their tails up and down rather than side to side like fish do. The seahorse is a fish whose tail is not used for swimming at all. The seahorse uses its thin coil tail to attach itself to sea weed like a monkey's tail holds onto a tree branch. And then there are the creatures that live on the bottom of the sea. Sea slugs, limpets and whelk creep on a single flat piece of muscle called a foot. Ripples pass along the foot, which allows these animals to glide smoothly forward.