1. Dendrocoelum lacteum can orientate itself to a lateral stimulation intensity of less than 7 ergs/cm.2/sec. when not stimulated by other light sources.
2. This orientation is produced equally well by "diffuse" or by point sources of light.
3. The threshold value of lateral stimulation, to which orientation is possible, is a function of the total stimulation to which the animal is subjected.
4. Graber (1884) and workers after him found that if planarian worms were put into a Petri dish one-half of which had been darkened with black paper, they congregated in the darker half. This was described as a phobotactic or non-orientated response.
5. It has been shown that if a random movement brings an animal into the darker half of the dish, it immediately moves in an orientated fashion away from the lighter half, on account of its extreme sensitivity to lateral illumination. In reality tropotaxis and not phobotaxis is responsible for the behaviour observed.
6. A reinvestigation of photophobotaxis is necessary.