GEOTROPIC EXCITATION IN HELIX

Abstract

Rotation of an inclined surface on which Helix is creeping straight upward, such that the axis of the animal is turned at a right angle to its previous position, but in the same plane, leads to negatively geotropic orientation after a measurable latent period or reaction time. The duration of the latent period is a function of the slope of the surface. The magnitude of the standard deviation of the mean latent period is directly proportional to the mean latent period itself, so that the relative variability of response is constant. The dependence of reaction time upon extent of displacement from symmetrical orientation in the gravitational field is found also by tilting the supporting surface, without rotation in the animal's own plane. On slopes up to 55°, the relation between latent period and the sine of the slope is hyperbolic; above this inclination, the latent period sharply declines. This change in the curve is not affected by the attachment of moderate loads to the snail's shell (up to 1/3 of its own mass), and is probably a consequence of loss of passive stable equilibrium when rotated. When added loads do not too greatly extend the snail's anterior musculature, the latent period for the geotropic reaction is decreased, and, proportionately, its σ. These facts are discussed from the standpoint that geotropic excitation in these gasteropods is due to impressed muscle-tensions.