Fishes mainly use their trunk for swimming. If fact, the trunk is the tail region of the fish. There is no viscera, but muscles and vertebral column only. The muscle in the trunk is built from myotomes. Theoretically, each body segment can have a vertebra and one myotome segment. In fishes, as the structure is simple, so, all myotomes are remained not distorted. It is very easy to observe the myotomes in the trunk, when the skin is removed.
he swimming movement of a fish is executed by the left and right flippering of the trunk. The action is simple, so, myotomes can perform very well.
When the fish moves its tail to the right hand side, an opposite direction reaction force will act on the body through its tail. This reaction force can be splitted into two vectors. One is in the direction of motion. This is the force of propelling. Another is perpendicular to the body axis. It is the lateral drag. The lateral drag will cause yawing, the instability of the body. This yawing can be counteracted by the vertical dorsal fins and the massive body (or the laterally flattened body).
The fish continues to bend its tail, the fish can get the propelling force continually.
In the action, the myotomes on both sides contract alternately. Then, the trunk (tail) will be bent to the sides alternately. By this action, a propelling force is produced