1. Isolated pieces of test of Diadema bearing a single spine show responses to a constant shadow cast on the radial nerve which are consistent in reaction time, duration, amplitude and frequency of the contractions.
2. Variations in the intensity and duration of the lighting which precedes the shadow exert similar effects on the whole reaction, affecting all the above features.
3. Variations in the duration and intensity (% decrease in intensity of illumination) of the shadow differ in their effects. The intensity affects all the features of the whole reaction, but the duration affects only the later part.
4. The differing effects of light and shade suggest that the shadow response may be a rebound from inhibition due to light which, when re-admitted, diminishes any shadow reaction that may be in progress.