Fig. 2. (A) Tracing of eye movements using `Object Image'. The horizontal eye position was measured as an angle from the centre of the pupil to the anterior—posterior axis. (B—D) Dorsal viewing angle of the three species of fish used in this study: the sandlance (B), the pipefish (C) and the butterflyfish (D). In all images the animals' nose points towards the top of the photograph. Scale bars, 2.5 mm.