Sharks (Chondrichthyes) possess a heterocercal tail shape,meaning that the upper lobe is larger than the lower lobe of thetail (Fig. 4). The upper and lower lobes of the tail are supported bythin cartilaginous rods, or ceratotrichia, in a thin matrix of collagen.The vertebral column extends nearly to the end of the dorsal lobeof the tail and is surrounded by epaxial and hypaxial myomeres.Ventral to the axial (epaxial and hypaxial) musculature in the tail isan additional red muscle, the m. radialis