Clams seem to squirt water through their siphons. What purpose does the siphon serve?

Three main purposes: breathing, obtaining food, and eliminating waste products. Since clams are relatively immobile and movement is usually limited to burrowing in the sand, their double-tubed siphon--which operates much like a snorkel--is their lifeline. Inflowing water is pumped through the siphon, passed over the gills, and strained to remove food particles. After receiving carbon dioxide from the gills and other waste products from the digestive tract, the water is expelled through the outgoing siphon. Constant circulation of the water is maintained by the beating of a multitude of microscopic hairs (called cilia) located inside the tube and in the gill chamber.