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The chemical composition and mechanical properties of the hinge ligament in bivalve molluscs.

Members of the molluscan class Bivalvia have the shell as two calcareous valves which articulate dorsally in the cardinal region to form a hinge. In the area of the hinge is a secreted proteinaceous structure, the ligament, which is common in most members of the class. The ligament acts in opposition to the adductor musculature and functions in opening the two portions of the shell. The form of the ligament varies throughout the Bivalvia ; however, in this study only two funda mental mechanical arrangements are considered. When the ligament is dorsal to the pivotal axis, the ligament is subject to tensile stress upon contraction of the adductor musculature. Alternatively, the ligament is positioned between two func tionally different areas of the hinge. The part of the ligament ventral to the pivotal axis undergoes compression when the adductor musculature contracts, and it is this part of the system which is responsible for the mechanical characteristics of this type of ligament (Trueman, 1949, 1953 ; Alexander, 1966) . The portion of the ligament dorsal to the pivotal axis is a rigid structure which maintains the juxta position of the valves. This latter condition is thought to be the most recent (Dall,1895),andit is themostcommon formin extant bivalves.In suchforms as Mytilus edulis, however, there may be considerable extension and contraction of the outer ligament during movement of the valves (Trueman, 1951). Contraction of the adductor musculature stores energy in the ligament and relaxation of this musculature results in the release of the stored energy and opening of the valves. Some energy is probably lost as heat ; however, the major increment is used in opening the shell. The animals chosen for this study represent three different modes of life among bivalve molluscs. The swimming forms are represented by Aequipecte@ irradians and Placopecten magellanicus. Both species swim by rapidly opening and closing the valves. During swimming the rapid closing of the valves by the large, single adductor muscle, expulses water dorsally with a resultant ventrally directed move ment of the animal. The â€œ?�recovery strokeâ€ or opening of the valves is executed by the inner ligament which is compressed during adduction. These animals are capable of about three opening and closing cycles per second (Alexander, 1966). The inner ligament is reminiscent of a small piece of rubber ; it is black, apparently amorphous and without growth lamellae (Fig. la) . This ligament is mounted on an extension of the shell, the chrondrophore. The outer ligament is minute in trans