1. The integrity of the peripheral sensory nervous system, associated with the skin, muscles, and connective tissue, is not essential for the transmission of a locomotory rhythm along the body of the eel (Anguilla vulgaris). The rhythm is determined by the intrinsic activity of the spinal cord.
2. The spinal cord only expresses its inherent locomotory rhythm when conditioned by stimuli of either peripheral or central origin. In the latter case the requisite level of excitation is effected by the medulla.
3. The body of a decapitated eel can be induced to swim forward by the application of appropriate electrical stimuli to the cut end of the spinal cord; the frequency of the applied stimuli bears no direct relationship to that of the emergent muscular rhythm. If the intensity of the applied stimuli be increased the direction of the resultant muscular waves is reversed.
4. A localised unilateral tactile stimulus induces a primary contraction at the point of stimulation and a secondary contraction lying contralaterally and anteriorly to itself. If the primary stimulus is persistent and of adequate intensity the postural response is replaced by a well defined locomotory rhythm.
5. If the brain of an eel is transected in front of the medulla, the fish exhibits, when at rest, marked undulatory posture. It is suggested that there is a relationship between the mechanism maintaining this posture and that which maintains the phase difference between the successive segments of the actively moving fish.
6. The mechanisms which determine locomotion in an eel are strikingly similar to those which control a stepping rhythm in the limbs of mammals.