Clam is a word which can be used for all or only a few species of bivalve mollusks; the word is a common name which has no real taxonomic significance in biology. It is however quite widely used as part of the common names of bivalves, and also has significance in fisheries and cuisine. A clam's shell consists of two (usually equal) valves, which are connected by a hinge joint and a ligament which can be external or internal. In most clams, two adductor muscles contract to close the shells. The clam has no head, and usually has no eyes, (scallops are a notable exception), but a clam does have kidneys, a heart, a mouth, and an anus. For more information see bivalve and pseudofeces. 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 by filter feeding, and they themselves are eaten by small sharks and squid.