1. In 1884, Schiemenz published an essentially correct interpretation of the mechanism of pedal expansion in naticid snails. A historical review discusses how this was ignored or denied in the literature for eighty years, and summarizes the need for unequivocal evidence on the problem. 2. Larger naticids, including Polinices duplicatus and Lunatia heros, have an extensive pedal water-sinus system, and considerable intake of sea water is required for expansion. On dry surfaces, in air, they cannot expand. 3. Naticids can be habituated to handling, and then weighed in air at all degrees of expansion. Extensive weighing experiments show that the sea water expelled on contraction is always equivalent to the weight difference between the expanded and contracted states. An index of expansion can be calculated relating expanded to contracted weight and this correlates with visual assessments of degree of expansion. Taking contracted weight as 100, half-expanded specimens of Polinices duplicatus have index values around 195 and fully expanded ones around 350. For example, this means that a Polinices weighing 46 g. contracted, takes in 124 ml. of sea water when it expands fully to a weight of 170 g. Shell weights and capacities can be incorporated in computing predicted curves which fit results from weighing live snails. 4. In healthy medium-sized specimens of Polinices duplicatus, retraction takes 2.5-4.0 seconds and expansion takes 3-8 minutes. Retraction is brought about by a sequence of muscle contractions; expansion is largely based on a "recoil" elasticity augmented by local hemal dilation. The larger naticids often remain continuously expanded for many days. 5. The capacity for disproportionate pedal expansion conferred by the water-sinus system is of great adaptive significance to naticid snails—particularly for locomotion in sand and for prey capture. This is discussed in relation to the hemal hydraulic skeleton and the systems of distant antagonists more usual in molluscs. The hypothesis is advanced that the pedal water-sinus system of naticids may have evolved from epidermal invaginations resembling the pedal mucous glands of other mesogastropods.