Fig. 1. (A) Experimental arrangement. The side view and view from above, seen in the mirror, of the swimming lamprey were video recorded. (B) Processing of video images. The body outlines in the frontal (top) and sagittal (bottom) planes obtained from the frame in which the animal was positioned with its back up. The position of the gills is shown in the side view. The body midline is marked by 21 circles located equidistantly along the `body length' axis (x), where anterior = 0% and posterior = 100%. The body shape was approximated by circles drawn though each of three neighboring midline points. The arcs of the circles for one of the points are shown together with their radii, Ry and Rz, for the frontal and sagittal planes, respectively. (C) The inverse of the circle radius characterizes the curvature at a given point of the body. The curvature was plotted in coordinates x (`body length') versus 1/Rz (curvature in the sagittal plane), x (`body length') versus 1/Ry (curvature in the frontal plane), and 1/Ry (curvature in the frontal plane) versus 1/Rz (curvature in the sagittal plane). The last plot provides information about coordination of flexions in the sagittal and frontal plane. In these and all following plots, the units for the x axis are percent of body length, while those for the 1/Ry and 1/Ry axes are arbitrary units inverse to the x units. The scales for 1/Ry and 1/Rz axes are always the same.