The most effective point in the visible spectrum for the stimulation of Pholas is 550 mµ. On the red side, the effectiveness drops rapidly to almost zero. On the violet side, the effectiveness drops to about half, and rises again in such a way as to indicate a possible second maximum in the near ultra-violet.
On the basis of certain ideas these data are assumed to represent the properties of the absorption spectrum of the photosensitive system in Pholas. A comparison with Mya shows that the absorption spectra of the photosensitive systems in the animals are distinctly different. Nevertheless the way in which intensity and reaction time are related in the two animals are found to be identical. The conclusion is then drawn from this and from previous work, that although the fundamental properties of the photoreceptor process show an identical organization in several different animals, the materials which compose these processes are specific.