1. The snail Helix aspersa Müller, is negatively geotropic during the daytime, but positive or indifferent at night.
2. The precision of geotropic orientation is a function of the gravity component acting on the body.
3. The rate of geotropic locomotion is also determined by the gravity component (sine of the angle of inclination).
4. The rate of upward movement is increased 1.51 times at 45° inclination by loading the snail with one-half its weight. No such increase is seen in loaded snails creeping on a horizontal surface.
5. Moderate centrifugation results in orientation and locomotion towards the center of rotation.
6. A response analogous to the homostrophic reflex occurs when a backward pull to right or to left is exerted on the shell. Bilaterally equal tension applied to the shell causes locomotion along a path parallel and opposite to the direction of the pull.
7. All the observations go to show that the stimulus for geotropic orientation and locomotion is tension of the body muscles produced by the downward pull of gravity, and that the stimulus is received by the proprioreceptors of these muscles. Otolith apparatus and analogous organs, when present, may assist in the response, but they do not seem to be requisite in all cases. Since the precision of orientation and the rate of locomotion are functions of the gravity component acting on the body, the muscle tension theory of the geotropic reactions accords fully with Loeb's tropism doctrine for animals.
