In order to determine the effect of the ultra-violet and other components of sunlight upon certain marine organisms an apparatus was constructed with three different light filters. Seven marine organisms, varying widely in their habits with reference to light, were then exposed in the apparatus to various degrees of sunlight. Through one filter they received 91 per cent of the total sunlight, comprising both the visible range and the ultra-violet; through another, 82 per cent of the total, including the visible spectrum but no ultra-violet; through the third, 14 per cent of the total radiation, at the short-wave end of the spectrum. A fairly close relation was observed between the depth at which the organisms live and their susceptibility to short-wave radiation. In several cases ultra-violet light was found to be lethal in its effect. (F. E. L.)