The head and tail of a fish are smaller than the middle portion of the body and the body tapers at both ends. This shape is called streamlined. The skeleton of the fish is covered with strong muscles. During swimming these muscles make the front part of the body curve to one side and the tail part swings towards the opposite side. First the fish forms a curve, then quickly, the body and the tail curve to the other side. This makes a jerk and pushes the body forward. A series of such jerks make the fish swim ahead. This is helped by the fins of the tail. Fish also have other fins on their body which mainly help to keep the balance of the body and to maintain direction, while swimming.