The Bivalvia (AKA Pelecypoda or Lamellibranchiata) – more familiarly known as clams, scallops, mussels, oysters, and their relatives – are bilateral mollusks (Mollusca) that lack a distinct head (also jaws, radular teeth, and sense organs well-known from other groups such as the snails or Gastropoda) and are usually surrounded by two shelly valves connected at one end or side by a horny ligament. The valves are closed by contraction of muscles attached to the inner surface of the valves, are opened by the ligament when the muscles relax, and are held in position by a series of hinge teeth near the ligament and sometimes denticles along the rim of each valve. The dorsal side of a bivalve is recognized by the location of the hinge, ligament, and umbones (or “beak,” which represents the earliest growth stages of the bivalve). Internally, the mouth and the tip of the foot are usually anterior, whereas the rectum and siphons (if present) are posterior. Anterior and posterior (along with left and right) are more difficult to determine from empty shells. Posterior can be easily determined in species that have a pallial sinus on the inner surface or elongated rostrum at one end of the shell, both of which are associated with siphons. In bivalves without these features, determining anterior and posterior often requires knowledge of the animal.
Several features of bivalve anatomy have been broadly categorized across the families, and some have been used to define taxonomic groups. The best known of these are the hinge teeth, ligaments, gills, labial palps, siphons, statoliths, and stomachs (e.g., Morton, 1985; Pelseneer, 1891; Purchon, 1987; Ridewood, 1903; Stasek, 1963; Yonge, 1957 – see Literature). Most of these groups, based on single-character systems, are no longer used in favor of phylogenetic classifications that use multiple organ systems to define clades or groups of closely related taxa. Details of these classification systems and other technical terms can be found in the Glossary.