MICROANATOMICAL1–4 and electrophysiological5–8 investigations support the hypothesis that the sensory elements of the statocysts of Crustacea and of the vertebrate vestibular endorgans are stimulated adequately by lateral displacement (shear) exerted by the statolith or statoconial membrane. Behavioural findings agree with this kind of statocyst stimulation; for example, equilibrium reactions of fish9 and Crustacea10, and counterrolling of eyes and perception of vertical in humans11–15. These reactions are influenced by both parameters which determine shear, that is to say, they depend as much on the direction (which can be changed by tilting the subject) as on the magnitude of the gravito-inertial force.