The Mechanism of Locomotion in Gastropod Molluscs: II. Kinetics

As is the case in all terrestrial animals, the progression of a snail or other gastropod depends on the exercise by the animal of an adequate force directed posteriorly against the ground ; in an animal moving horizontally and at constant speed, this force overcomes the frictional resistance encountered by the parts which are gliding forwards. The movements of the pedal surface of some gastropods have been discussed in a previous paper (Lissmann, 1945). The experiments described below were designed to correlate these movements with the forces set up during locomotion. The salient point which appears to be established is that during progression the foot exhibits (i) areas of forward motion, coinciding with phases of longitudinal muscular contraction, followed by extension ; (ii) areas at rest, which remain essentially elongated. However, the state of the muscles is no indication of the tensions they may set up within the body, nor of the propulsive force they may exert; it is by no means obvious how the external forces are produced, or how they lead to progression.