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Figure 66. — Trueman's nipthod for measuring the mo- 

 ment of thrust of bivalve ligament. Redrawn from 

 Quarterly Journal of Microscopical Science, 19.51, 

 .series 3, vol. 92, part 2, p. 137. 



valve is so adjusted that the weight at the point 

 of application to the centroid is twice that placed 

 on the pan. Weights are gradually added until 

 the valves just close so that the opening moment 

 M is exactly counterbalanced. The ratio M' 

 between the opening moment M and the surface 

 area of the valve A is determined by the formula: 



M' = d-^ , where d is the straight line 



A 



distance from the point of weight application on 



the shell to its pivotal axis; W is the weight 



applied; and V is the weight of the upper valve. 



There are two objections to this metliod. The 

 central point of the valve can be accurately 

 determined only for round, symmetrical shells; 

 for tlie irregularly curved shells of C. virgifnca, 

 C. anyulata, or C. (jigas, its position can only be 

 guessed. Another more serious objection refers to 

 the determination of the weight under which the 

 valves "just closed." Experimenting with C. 

 rirginica, I found that visual observation, even 

 with a magnifying glass, is not sufficient to deter- 

 mine when the valves are completely closed. 

 Freciuently a tiny slit between the valves cannot 

 be seen but becomes apparent on a magnified 

 kymograph record of shell movement. Trueman's 

 method with modifications was used by Hunter 

 and Grant (1962) to study the mechanical ciiarac- 

 teristics of the ligament of the surf clam, Spmda 

 solidissima. They found that the ligament of the 

 clam is about 3.5 times stronger (in terms of 

 opening moments) than that of Alya arenaria. 

 The mechanical differences, according to their 

 opinion, reflect the modes of life of the two clams. 



The moment of thrust measured by Trueman's 

 method is of no particular significance to the 

 physiology of the oyster because it does not repre- 

 sent the pulling force which the adductor nuiscle 

 must exert to close tlie valves or to I^cep them 

 partially open. This force differs from Trueman's 



moment of thrust because the site of the attachment 

 of the adductor muscle is located not in the center 

 but in the ventroposterior quadrant of the valves. 

 The following method overcomes these difficulties: 

 the body of the oyster is removed without injuring 

 the ligament; tlie gaping shell is placed with the 

 left valve resting on concave cement support (fig. 

 67) and immobilized by small lead wedges. Tite 

 right valve is connected to writing lever N of 

 kj-mograph K. A glass hypodermic sj-ringe of 

 10 ml. capacity, mounted on wooden frame G, is 

 placed so that its plunger F touches the valve 

 over the center of the muscle attachment area. 

 The flattened end of the plunger is cut off, and its 

 stem is sharpened to a point. A three-way stop- 

 cock L is attached by hard rubber tubing to the 

 upper end of the syringe; one of its arms is con- 

 nected to a hand pump D (automobile or bicycle 

 tire type) ; the other arm leads to an open mercury 

 manometer C. Two dry cell batteries E activate 

 the recording electro-magnet M which makes a 

 mark on the drum only when the key switch 5* 

 is pushed down. As the pump is worked the 

 pressure created in the system forces the plunger 

 down, gradually closing the shell. Each time the 

 mercury column rises 2 mm. the operator pushes 

 the signal key down. Pumping is continued after 

 the valves are closed until the horizontal line on 

 the drum record indicates that increase in pressure 

 produces no further change in the position of the 

 upper valve. The point corresponding to the 

 complete closure of the valves is easily determined 

 by placing a ruler against the horizontal portion 

 of the kymograph curve and noting the point at 

 which the line begins to curve down (fig. 68). 

 The number of signal marks from the beginning 

 of the recording to the end of the curved line 

 multiplied by two gives the height of the mercury 

 column in millimeters. The manometer must he 

 calibrated to correct for tlie error resulting from 

 slight irregularities in the diameter of the glass 

 tubing in its two arms. 



To minimize friction between the walls of the 

 syringe and its piston, several lubricants were 

 tried until it was finally discovered that a minute 

 quantity of high-speed centrifuge oil permits free 

 movement of the piston under its own weight. 

 The weight of the piston in the operating position, 

 determined by placing the balance pan under the 

 point of the piston, was recorded at 17. Og.; weight 

 of the same piston taken out of the syringe was 



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