Fish have gills that allow them to “breathe” oxygen in water. Water enters the mouth, passes over the gills, and exits the body through a special opening. Gills absorb oxygen from the water as it passes over them.
Fish have a stream-lined body. They are typically long and narrow, which reduces water resistance when they swim.
Most fish have several fins for swimming. They use some of their fins to propel themselves through the water and others to steer the body as they swim.
Fish have a system of muscles for movement. Muscle contractions ripple through the body in waves from head to tail. The contractions whip the tail fin against the water to propel the fish through the water.
Most fish have a swim bladder. This is a balloon-like internal organ that contains gas. By changing the amount of gas in the bladder, a fish can move up or down through the water column.