1. The modes of progression of Helix, Haliotis, and Pomatias are described.

2. In all three species waves of longitudinal muscular contraction, followed by relaxation, pass over the foot in postero-anterior direction.

3. Typically, any point on the sole shows during one locomotory cycle a phase of forward movement and a phase of rest. Minor variations depend on external conditions, and on the location of such a point on the sole.

4. In all three species the longitudinally contracted areas are lifted off the ground and move forward, whilst the elongated areas remain stationary.

5. The length of a step is essentially determined by the difference between the length of the musculature comprising the contraction phase of one locomotory wave, and the length of this musculature when it is fully relaxed (Fig. 6).

6. The difference between the three species consists in a difference of wave length relative to the total length of the foot . In Haliotis and Pomatias there is a functional alternation of locomotory phases of the right and left halves of the foot.

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