1. When Cerianthus membranaceus is illuminated upon one side, the animal turns its anterior portion toward the source of light. The number of degrees through which the animal turns is proportional to the logarithm of the intensity of the light.
2. A light intensity of between 250 m.c. and 15,000 m.c. is necessary to cause retraction of the animal.
3. The part of the spectrum which is most effective in causing heliotropic bending of Cerianthus lies between µµ 510 and µµ 570.