A retractable foot, a siphon for sucking up water, powerful muscles, and, sometimes, a pearl ... and mussels, clams are bivalves, a kind of mollusk that's encased in a shell made of two valves ...

A: The shell or clam shell has symbolic value in many cultures ... portal of the Pisa dome where the Annunciation is accompanied with the symbol of shell and pearl, titled Rore coelesti foecundor ...

Pearls are made by marine oysters and freshwater mussels as a natural defence against an irritant such as a parasite entering their shell or damage to their fragile body. The oyster or mussel slowly ...