1. Electrophysiological recordings have been obtained from tactile receptors which occur in shallow pits on the crayfish carapace.
2. Controlled mechanical stimulation has established that each receptor is innervated by a pair of sensory neurons. One of these neurons responds when the receptor is moved anteriorly, and the partner cell responds to displacement in the opposite direction.
3. The receptors are sensitive to motion of the fluid medium in which they are immersed, and they may provide the animal with a means for determining the speed and direction of water currents.
4. Vital staining procedures have confirmed the presence of two bipolar sensory cells associated with each receptor. An abrupt increase in the diameter of the sensory axons occurs several hundred microns proximal to the cell bodies of the neurons, and it is suggested that this is the locus of spike initiation.