Adductors contract, the shell closes and the water forced out loosens the substrate around the shell so it no longer is an anchor. The pedal anchor persists. Pedal retractor muscles contract, pulling the animal down toward the foot. A rocking motion helps the shell move down through the sediment. If you've ever tried to catch one of those, you understand.
Shipworm burrows into wood substrates.
The juvenile uses the shell to carve into the substrate. Rocks the shell back and forth to cut into the wood, which is then eaten. Adult lives in the substrate with siphons extending out to the surrounding water . Individual shipworms have been known to be up to 4 ft. long and an inch in diameter. They are able to tolerate anaerobic conditions a while and so wood carrying shipworms is sometimes shipped and then later discovered to be infested.