Reactions to Shading in the Sea Urchin, Psammechinus miliaris (Gmelin)

Abstract

IN this urchin, podia withdraw in response to sudden shading. When the aboral hemisphere was subjected to an illumination of 250 metre-candles from a tungsten filament lamp, according to the individual, a 3–20 per cent decrease in intensity proved a minimal stimulus. Most of the podia responded simultaneously when the light was relatively bright; but when dim, or when the shadow was faint, many fewer responded and then at different times.