Fish moving in any direction Because the function of the muscles attached on either side of the spine. The contraction of the muscles on one side from head to tail. And a wag of the tail to come. Making body of the fish is a curve alternates. Heads and tails to one. Rumped and pectoral fins, twin fins, which are comparable to the forelegs and hind legs of the animal, respectively. This fin has helped the fish to move up and down in a straight line. The movement of the fish is S-shaped toggle switch into a flat shape slender fish with mucus, making the fish move in the water. It reduces the friction of moving water. The streamlined shape of the body may make the fish a chance to tip over or easily overturned. So fin fish requires help to maintain balance in movement and balance.