1. An attempt has been made to analyse the kinetic effects as observed on the sole of a gastropod in locomotion, and to illustrate these effects by analogy with mechanical models.

2. The internal mechanism involves during locomotion an internal force of longitudinal contraction, and an internal force of extension: the former is considered to be represented by the contracting longitudinal muscles, the latter is probably produced by hydrostatic pressure.

The external forces acting on the animal at any one moment represent the balance of the two antagonistic internal forces.

3. During normal ambulation in Pomatias elegans the posterior margin of the foot is exclusively protracted by the longitudinal contraction of the musculature of the foot; the anterior margin is propelled forward exclusively by the force of extension.

4. There is no evidence in favour of an antagonism of longitudinal and transverse fibres in Pomatias elegans; both sets of muscles appear to contract and relax synchronously in one half of the foot and to act antagonistically with both sets of muscles in the other half.

5. The external forces in Pomatias are set up between one area of fixation and an area of dynamic friction.

6. An experimental analysis of the snail showed the existence of an external force of extension (longitudinal thrust) acting between the anterior and the central region of the sole. A similar force of longitudinal contraction (tension) acts between the central and the posterior end. Both forces are of the order of 2.5 g. in Helix pomatia, and about 1 g. in H. aspersa.

7. A static reaction from the ground has been demonstrated to exist under the relaxed parts of the sole. This force reaches a maximum at a point near the central region of the foot.

8. Dynamic friction has been recorded under the forward gliding zones of contraction.

9. Static thrusts are developed between successive areas of fixation in the anterior region, while similar tensions can be observed posteriorly.

10. The foot of the snail as a whole must be considered as a mechanical unit; the individual locomotory waves do not represent mechanically balanced systems.

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