The adductor muscle has to perform two very different kinds of work. One is to close the shell quickly to expel sediment, accumulated wastes, or eggs, or to slam the door on predators. The other is to keep the shell tightly closed for hours, sometimes even days, until the danger passes. These two jobs are performed by adjoining parts of the muscle. The fast-contracting “quick” portion is quite similar to the fast muscles of fish and crustaceans; it’s white, translucent, and relatively tender. But the slow, tension-maintaining “catch” portion is among the strongest muscles known, and can maintain its contraction with very little expenditure of energy, thanks to biochemical tricks that lock the muscle fibers in place once they’ve shortened, and reinforcement with large amounts of connective-tissue collagen. Catch muscles have an opalescent appearance, much like the tough tendons in a chicken leg or leg of lamb, and they are tough to eat as well unless cooked for a long time. In the scallop, the small catch portion would detract from the large quick portion’s tenderness, and so is usually cut away.