 Bivalves vary greatly in overall shape . Some , such as the cockles , have shells that are nearly globular ; cockles can jump by bending and straightening their foot . Others , such as the razor clams , are burrowing specialists with elongated shells and a powerful foot adapted for rapid digging . The <unk> , in the family <unk> have greatly elongated bodies , but their shell valves are much reduced and restricted to the anterior end of the body , where they function as scraping organs that permit the animal to dig tunnels through wood . 
