1. The input-output properties of the eyestalk responses of Palmurus vulgaris to substrate tilt are described in both the time and frequency domains.
2. Trapezoidal substrate movements elicit phaso-tonic eyestalk movements which decay from the phasic peak to the tonic plateau with time constants of 9.2 s (tilt) and 6.1 s (return) (Fig. 1). Average eye excursions for tilt and return are equivalent (Fig. 2).
3. The relationship to tilt amplitude is non-linear (Fig. 3), and with stepwise tilts in opposite directions a marked hysteresis is evident (Fig. 4).
4. Sinusoidal stimuli produce consistent eyestalk responses (Fig. 5), and a frequency analysis reveals a high gain, phase-leading region below 0.25 Hz (Fig. 6). Around this frequency the eyestalk response is effectively a compensation with respect to both amplitude and phase position.
5. The results are considered in relation both to the experimental arrangement with fixed body and vision excluded, and to the normal biological situation.