Clams are the burrowing bivalves. They dig themselves into ocean or river sediments by extending a foot muscle downward, expanding its end into an anchor, and then contracting the foot while squirting water and rocking the shell. In order to reach the water from their burrow to breathe and feed, they have a pair of muscular tubes or “siphons,” one for inhaling and the other for exhaling, which may be separate or else joined together into a single “neck.”