MECHANICS OF THE LIGAMENT IN THE BIVALVE SPISULA SOLIDISSIMA IN RELATION TO MODE OF LIFE W. RUSSELL HUNTER 1 AND DAVID C. GRANT = Department of Zoology, University of Glasgow, Glasgow, W. 2, Scotland and Osborn Zoological Laboratory, Yale University, New Haven, Conn. The bivalve shell is closed by the action of adductor muscles. These, the largest muscles in any bivalve, have no single antagonists but can be stretched by several mechanisms, which include the elasticity of the horny hinge ligament and several kinds of hydraulic systems. The relative importance of each method varies in different types of bivalves. The elastic ligament connects the shell valves dorsally and is under strain when the valves are closed. The strain is tensile in the outer part of external hinge ligaments, but corresponds to compression in internal ligaments, or resilia, and in the inner layer of others. Functional morphology and development of the liga-ment have recently received considerable study (Owen, Trueman and Yonge, 1953 ; Owen, 1953, 1958, 1959a, 1959b; Trueman, 1949, 1950, 1951, 1953, 1954; Yonge, 1953, 1955, 1957), and the detailed investigations of Trueman have included mechanical analyses of the operation of the ligament in certain genera, notably Ostrea, Pecten and Mya (Trueman, 1951, 1953, 1954). The force derived when the ligament is under strain tends to open the valves, i.e., acts against the adductor muscles, and Trueman (1953, 1954) terms this the "opening moment" of the ligament. The forces which can be grouped as "hydraulic" means of shell opening are of two kinds, derived respectively from compression of (a) body-fluids in the "haemo-coele," and (b) water in the mantle-cavity. The first category includes the pro-trusion of the foot, used to force the shell valves apart in adult Cardium edule and Anodonta cygnea, and in the young of many bivalves including Spisula solidissima, but could also include the distension of fused ventral edges of the mantle. The second, water pressure, is used particularly in those bivalves where extensive fusion of the mantle margins has occurred (Yonge, 1955, 1957). Hunter (1949) showed that the method of boring in Hiatclla gallicana depended on the shell valves being forced apart by water pressure inside the mantle cavity. The use of water pressure in the mantle cavity, for extending the siphons by contraction of the adductor muscles, was later investigated in detail in Mya arenaria (Chapman and Newell, 1956; see also Chapman, 1958). Among the larger burrowing bivalves, Mya arenaria and Spisula-solidissima offer a number of contrasts in their modes of life. The former is a sedentary "deep-burrower," almost immobile in adult life, the latter an active burrower, one of the so-called "surf-clams" living just within the substrate. The mechanics of the 1 Visiting Instructor and 2 Assistant Instructor, Department of Invertebrate Zoology, Marine Biological Laboratory, Woods Hole. 369
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