I. The waves of muscular contraction which pass along the body of a swimming eel occur also in other fish. The waves vary greatly in speed of propagation, amplitude and frequency. The speed of propagation of the waves is too low to be controlled by the rate of conduction of a simple nervous impulse.

2. The movements executed by a localised area on the surface of the body are such that each area moves in a direction transvene to the line of forward movement. During these movements the leading surface of the body is inclined backwards towards the tail and at an angle to the path of motion of the area concerned. The angle of inclination and the angle made with the path of motion vary with (a) different regions of the body, and (b) with different phases in the motion of each region.

3. Each point on the body travels in a horizontal figure of 8 relative to a transvene axis which is moving forward at the same average velocity as the whole fish. A segment of the body at the mid-point of its transverse motion is travelling forwards at a rate slightly less than that of a segment at the extreme position of its transverse movements. These movements are the mechanical result of the inextensibility of the body, and they effect significant changes in the angle between the surface of the body and its direction of movement.

4. The movements of each part of the body are shown to be such as to generate a forward thrust which drives the fish forwards against the resistance of the water. The magnitude of the forward thrust depends among other things on (a) the angle which the surface of the fish makes with its own path of motion, and (b) on the angle between the surface of the fish and the axis of forward movement of the whole fish, (c) on the velocity of transverse movement of the body.

5. The propulsive properties of each segment of the body are greatest as the segment is crossing the axis of forward movement.

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