The Shadow Reaction of Diadema Antillarum Philippi

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Abstract
1. Light is shown to suppress the shadow reactions of Diadema spines to a degree which varies with its intensity. 2. Inhibition can occur when there is spatial separation between the areas shaded and illuminated in the radial nerve and skin. 3. The degree of inhibition is affected by the position and size of the area lighted. In both skin and radial nerves, uniform meridional gradients of inhibition are found, inhibition being maximal when the areas lighted and shaded are near or coincide, decreasing as these areas are moved apart. The effect of light may be reversed when it is projected at more than a critical distance from the shadow. Gradients in the skin which run parallel to the ambitus, show maximal inhibition at the ambulacral margins, so that the inhibitory gradient corresponds with that of sensitivity to shadows. 4. Interaction between excitation and inhibition may occur in the radial nerve or at the periphery and there are several pathways for excitation and inhibition. 5. The findings are discussed in relation to ‘off’ effects and receptor fields in retinae.