(a) Overlayed outlines of representative control (left) and spinal transect (right) lampreys during a swimming cycle viewed from a lab-fixed frame. Horizontal lines indicate 2 cm scale. (b) Same overlayed outlines as in panel (a) but in a head-fixed frame. (c,d) Comparison of normalized body surface rotation (BSR) and body surface vorticity (BSV) for two phases of a control lamprey swimming cycle. Colour scale indicates local surface angular velocity (for BSR) or fluid vorticity (for BSV) divided by the maximum values on the body. Red and blue icons next to each color bar indicate direction of rotation corresponding to each colour. (e,f) Same data as in panels (c,d) but for a spinal transect lamprey.