If I was surprised to see my mussels moving about, I was astonished to see the clams run. I check the three larger ones every day to see if they're still alive, then replace them carefully in an open area and watch. After a few minutes, one will extend a foot about twice as long as the shell is wide. The tip curls into the sand, grabs hold, and pulls the clam along after it, one foot length at a time, which is still quite fast, considering. In a few minutes the clam will be under shelter halfway across the dishpan.
But a couple of times, that hasn't been fast enough. The clam shoots out his foot, touches ground with the tip, and pole vaults on it, landing a good distance away. Instantly, the foot angles off to a new support, and off he goes again. Flip - flip - flip - flip - squirm, and he has disappeared under the sand.
Apparently, the technique is mainly used to escape from hungry starfish. This YouTube video shows how it's done.
Clams have to breathe and eat, but being economical critters (they make do with only one foot, for example), they use the same structure for both purposes:
I caught this clam with its "mouth" open, a rare event. Usually, it closes up as soon as I disturb the water. The two white tubes are the siphons, incoming and outgoing. The water flows in, passing the gills, which absorb oxygen. That takes care of the breathing. Cilia on the gills keep the water moving, and while they're at it, catch bits of food and push it towards the stomach. The filtered water flows on by and exits through the second siphon.
By the way, those tiny, barely visible clams I found a month ago are still alive, in spite of all the sloshing and stirring involved in my manual site-cleaning process. Occasionally I see one out for a walk on the sand. A few minutes later, it's gone.