Fig. 4. (A) The classic figure-of-eight path (Pettigrew, 1873) followed by the tail through one pressure cycle or tail beat of a cod (Gadus morhua) swimming 0.3 m s^–1 in a ‘Brett-style’ respirometer at 5°C. The larger figure of eight (dotted line, ) is the path of the tip of the caudal fin, and the smaller figure of eight (solid line, ) is the path of the tail at the position where the pressure sensor was sutured to the caudal peduncle. The x and y coordinates of the tail position were measured from frame-by-frame (30 Hz) playback of 8 mm film. A video frame number is indicated for each x,y data point. (B) Tailbeat differential pressure with corresponding video frame numbers versus time of the tailbeat cycle shown in A.