Scallops are called capesante in Italian, literally holy cloaks.
The name’s origin lies in the scallops shells worn by medieval pilgrims on their way to Santiago de Campostela.
There are two theories as to the reason why (from Wikipedia):
The scallop shell, often found on the shores in Galicia, has long been the symbol of the Camino de Santiago. Over the centuries the scallop shell has taken on mythical, metaphorical and practical meanings, even if its relevance may actually derive from the desire of pilgrims to take home a souvenir.
Two versions of the most common myth about the origin of the symbol concern the death of Saint James, who was killed in Jerusalem for his convictions about his brother, John. James had spent some time preaching on the Iberian Peninsula.
Version 1: After James’ death, his disciples shipped his body to the Iberian Peninsula to be buried in what is now Santiago. Off the coast of Spain a heavy storm hit the ship, and the body was lost to the ocean. After some time, however, the body washed ashore undamaged, covered in scallops.
Version 2: After James’ death his body was mysteriously transported by a ship with no crew back to the Iberian Peninsula to be buried in what is now Santiago. As James’ ship approached land, a wedding was taking place on the shore. The young groom was on horseback, and on seeing the ship approaching, his horse got spooked, and the horse and rider plunged into the sea. Through miraculous intervention, the horse and rider emerged from the water alive, covered in seashells.