The foot of pulmonate gastropods adheres to the surface upon which the animal crawls, the adhesive being a thin layer of pedal mucus. The power for locomotion in these animals is provided by rhythmic muscular contractions (pedal waves) on the ventral surface of the foot, and the force due to these waves is coupled to the substratum by the pedal mucus. Movement of the animal is consequently a result of a balance of forces: if the reactive force provided by the adhesive beneath the stationary portions of the foot is sufficient to offset the frictional force caused by portions of the foot moving forward, the animal is able to crawl. Mechanisms previously proposed to explain this adhesive crawling require that the frictional resistance to forward movement be minimized by the foot being lifted during the passage of a pedal wave. This lifting, however, cannot be reconciled with the properties of the pedal mucus and epithelium, and the terrestrial slug, Ariolimax columbianus, is shown to not lift its foot when crawling. A new mechanism is proposed which does not require lifting of the foot and explains several difficulties inherent in previous studies. Based on this mechanism and the mechanical properties of pedal mucus, a quantitative model is constructed which predicts the frictional and reactive forces, and these predictions are tested against force measurements made under crawling slugs. The model accurately predicts the magnitude of forces under slugs crawling horizontally, but is less accurate in predicting forces under slugs crawling vertically.

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