1. The response of the hag to light consists of one or more local movements followed after a further interval by general locomotory activity. The first local movement has been used as a measure of the reaction time.
2. The reaction time is inversely proportional to the intensity of the stimulus at illuminations less than about 10 e.f.c. At higher levels of illumination it attains a constant minimum value. Hags respond to intensities at least as low as 0.1 e.f.c. but only after several minutes illumination.
3. Estimates of the penetration of light through sea water suggest that the hag's light sense is of functional value.
4. The spectral sensitivity maximum lies between 500 and 520 mµ. Hags are virtually insensitive to wave-lengths longer than about 600 mµ.
5. The significance of the spectral sensitivity is discussed in relation to the spectral transmission of sea water and the evolution of photosensitive systems.