Axial undulation- or caudal-fin-based swimming,
which involves lateral bending of the body and oscillatory movement of the tail,
is the primitive swimming mode of fish. Amongst extant forms,
axial swimming is used at least some of the time by fish as diverse as tunas, tarpons, sturgeons, sharks and butterflyfish. Axial swimming is powered by the myotomal musculature-the segmented myomeres that line each side of the animal’s body. Bending of the body and oscillation of the tail are achieved through alternating waves of contraction moving down each side of the animal.