Fact: 
Scallops are the family Pectinidae of bivalve mollusks. Like the true oysters, they have a central adductor muscle, and thus their shells have a characteristic central scar marking its point of attachment. However, the adductor muscle of scallops is larger and more developed than that of oysters because they are active swimmers and the sole migratory bivalve. Their shell shape tends to be highly regular and like the standard image of a shell. Scallops may be attached to a substrate by a structure called a byssus, or cemented to their substrate. They can also be free living. A scallop can swim by rapidly opening and closing its shell. This method of rapidly opening and closing its shell is also a defense technique, protecting it from any threats.
Source: Fact