MODULATION OF HYDRA-ATTENUATA RHYTHMIC ACTIVITY .6. COMBINED EFFECTS OF BACKGROUND AND PULSE LIGHT WAVELENGTH

Hydra is a photosensitive organism, although lacking identified photoreceptive organs. The behaviour of Hydra in undisturbed conditions is characterized by a repetitive body contraction-elongation activity. A transient alteration of the period length of such natural rhythmic activity is evoked by a light pulse stimulus and results in an immediate phase shift of the activity cycle. The sign and amplitude of such a photoresponse is a function of the light pulse intensity, application phase and wavelength. A persistent alteration in the period length of the rhythmic activity can be produced by a change in the background illumination wavelength.