What's the Significance of the Scallop Shell on the Camino?

Scallop Shells Backpack & Pillar

When walking the Camino you will see countless scallop shells, as this is the most iconic symbol associated with The Way. Painted on buildings, on official Camino signs, in store windows and on travelers’ backpacks, the simple shell becomes a friendly image that shows pilgrims they are on the right path. But how exactly did this symbol become so popular and representative of The Road of St. James?