In these experiments, which are a continuation of work carried out in 1927 and 1928, certain aquatic animals were exposed to sunlight behind filters which transmitted different regions of the solar spectrum. The first filter transmitted both the visible and ultra-violet regions, the second the visible only and the third the ultra-violet only. Young eels, Anguilla rostrata, which are in shallow water when ascending the streams, an Amphipod, Gammarus locusta, which was taken from shallow tide-pools, and a Ctenophore, Bolinopsis infundibulum, which floats about at the surface of the sea, all proved to be very resistant to ultra-violet radiation, thus showing a marked contrast to those animals which remain at some considerable depth in the sea or which come to the surface only when the illumination is very weak, as the latter are killed in a short time by exposure to ultra-violet radiation.