1. Records of impulses from the lateral-line nerves of catfish show that the lateral-line organs are in a state of continuous activity, producing a massive discharge of impulses.
2. The discharge may be increased during the direct application of pressure on the skin over the lateral-line canal, by ripples in the water, by irregular currents of water, and by movements of the fish's trunk.
3. The asynchronously discharging lateral-line organs respond to vibratory stimuli from tuning-forks by getting into phase with each other and by beating synchronously at frequencies ranging from 20 to 70 per second. The frequency of beating for a given preparation is independent of the frequency of the tuning-fork for the fork frequencies of 100, 200, and 250 double vibrations which were used.
4. The continuous discharge of the lateral-line system is markedly changed by alteration of temperature. The frequency declines on lowering the temperature and rises on increasing it. Spinal and facial nerves in the catfish fail to yield nerve impulses in response to changes of the skin temperature between 0° and 28°C., although the intact animal is known to be sensitive to temperature differences.
5. The action of the lateral-line system of Ameiurus in inhibiting responses initiated through the skin and ears (Parker and Van Heusen, 1917) is discussed in the light of the present experiments.