Fig. 1. Dorsal (x, z axes, viewed using a mirror, upper panels) and lateral (y, x axes, lower panels) views of the trailing (A) and leading (B) surfaces of finlets 4 and 5 and the caudal peduncle of a mackerel 22 cm in fork length (FL) during the deceleratory phase of a stroke at a steady swimming speed of 1.2FLs^-1. The dorsal and lateral views are synchronous; time is shown in the lower field only. The outer finlet margin is outlined with the filled line; the circle indicates the anterior attachment point of each finlet. The finlets oscillate from the left (A) to the right (B) side of the dorsal and ventral body midlines (dotted lines) during the stroke and reach maximum excursions (note the tips of dorsal and ventral finlets 5 touching at the lateral midline of the body in the lower panel of A) on the trailing surface of the peduncle as the peduncle decelerates (the case shown in A is the left surface of the peduncle as it decelerates to the right). Scale bars (white lines) represent 1 cm; white arrows show the direction of tail movement.