The function of the labial spine and the feeding behavior of the predatory gastropod Acanthina angelica were observed under controlled conditions. Long- and short-spined snails were presented three size classes of barnacle prey. The mode of attack was related to the length of the labial spine and the prey size. The spine was observed to function as a wedge to force apart the opercular valves of the prey (here termed wedging); drilling through the test or valves was an alternative mode of attack. As prey size increased, snails switched from wedging to drilling, with the short-spined snails switching at a smaller prey size than long-spined snails. The long-spined snails consumed medium sized prey significantly sooner than short-spined snails. Short-spined snails are usually found in association with small barnacles, while long-spined snails predominate among larger barnacles. However, spine length is not fixed, and available evidence indicates that prey size controls spine length.