The scallop (vieira, pectinidae) is a mollusk that has turned into the symbol of the pilgrims in St. James Way. There are several theories as to why: some claim that, since it’s a species typical of Galicia, pilgrims carried them back home as proof of having reached Santiago. (Curiously, the species most often associated with the Camino, the pecten jacobaeus, is actually most common in the Mediterranean, while the species found in Galicia is another one, called pecten maximus).
Other sources, however, trace its origin to a well known medieval legend, according to which, while carrying St. James’ body to Santiago, one of the horses fell to the water and emerged covered in shells (a variant of this legend claims that, while disembarking the saint’s body, they found a knight on the beach whose horse got scared and ran to the sea, from which both emerged covered in shells).
Aside of this symbolic meaning, the shell also had a practical purpose. According to these sources, during the Middle Ages, pilgrims would carry a scallop with them and present themselves at abbeys, farms… where they would be given as much food and drink as they could pick up in one scoop.