Predatory feeding in cone snails involves the use of a complex venom system that has evolved through various morphological modifications of the foregut. This specialisation has facilitated the diversification of the group into species that first preyed on worms and more recently molluscs or fish. Remarkably, free-swimming, filter-feeding larvae become benthic, carnivorous juvenile soon after metamorphosis. While early morphogenesis of the venom apparatus has been explored (Ball, 2002; Page, 2011), the onset of venom expression and toxin expression patterns across the lifecycle have not previously been studied. Here, we investigated the ontogeny of the venom system in Conus (Pionoconus) magus to provide new insights into cone snail venom evolution through a developmental lens. Histological analyses confirmed that key features of the adult feeding system could be identified in late-stage larvae, including the prospective venom gland and the radular sac, while differentiation of the proboscis and the venom bulb were observed to occur during metamorphosis. Interestingly, young juveniles C. magus were observed to prey on small polychaete worms using a simple radular tooth, while adults are known to feed exclusively on fish using a highly modified barbed harpoon. For the first time, we demonstrate distinct feeding strategies between different life stages in a cone snail species, expressed at all behavioural, morphological and molecular levels. These results highlight the value of integrating developmental considerations when studying the evolution of venom systems, particularly in species with complex life cycles.