1. Stimulation by non-directional light produces in the animal an increase in random movements, that is to say an increase in the rate of change of direction (r.c.d.).

2. This r.c.d. can be expressed as a function of the stimulating intensity.

3. In total darkness there is a basal r.c.d.

4. In constant stimulation the r.c.d. decreases as the animal becomes adapted.

5. The "shock reaction" is not an essential part of the mechanism of phobotactic behaviour. Photophobotaxis occurs without it.

6. The r.c.d. phenomena and the adaptation associated with them are sufficient to explain the mechanism of photophobotaxis. Alternate adaptation and stimulation occur when the animal is moving down or up a non-directional gradient, as a result of its random movements.

7. This alternate stimulation and adaptation has an effect on the r.c.d. of such a kind that the animal is led automatically to the place of minimal intensity.

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