A clam is a type of mollusk. Clams are invertebrates with shells divided into two pieces called valves. While the term "clam" has no taxonomic significance in biology, it usually refers to either a bivalve (a mollusk whose body is protected by two symmetrical shells) that is not an oyster, mussel, or scallop, and that has a more-or-less oval shape, or to a freshwater mussel (Merriam-Webster Dictionary.). An exception to the oval shape is the razor clam, which has an elongated shell whose shape suggests a straight razor. The clam's two shells are connected by a hinge joint. Two adductor muscles open and close the shells. The clam has no brain or eyes, but it does have kidneys, a heart, a mouth, and an anus. Clams, like most mollusks, also have open circulatory systems, which means that their organs are surrounded by watery blood that contains nutrients and oxygen. Clams eat plankton, and they themselves are eaten by small sharks and squid. In culinary use, the term "clam" most often refers to the hard clam (Taxonomically, Mercenaria mercenaria) but may refer to other species such as the soft-shell clam Mya arenaria. They are eaten raw, steamed, boiled, baked or fried; the method of preparation depends partly on size and species. Clam chowder is a popular soup in the U.S. and Canada. In Italy, they are often an ingredient of mixed seafood dishes, or are eaten together with pasta. Saltwater Maxima clams are also somewhat popular in the hobby of fish-keeping. Clams travel by pushing off with a muscular foot-like structure. They also contain little toes that are used in reproduction.