the valves tightly closed, varies greatly in oysters 



of the same size and environment. Prolonged 



exposure to air so weakens the adductor that 



oysters left out of water for several days can be 



easily opened. 



In attempts to measure the power of the 



adductor of various bivalves Plateau (1S84), 



Marceau (1905a, 1905b), and Tamura (1929, 1931) 



drilled holes near the edge of the shells and inserted 



rods or hooks to wliich they attached weights. 



The opposite valve was immobilized. Assuming 



that the adductor muscle is an elastic body, the 



amount of work (W) done by the adductor 



against the loaded weight (G) was calculated by 



ac 

 using a simple formula W=— jG where ac is a 



ad 



distance in centimeters from the ligament to the 



attachment of the weights; ad is the distance in 



centimeters from the ligament to the center of the 



adductor muscle ; and G is the weight in grams 



applied to the valve. Under a known pulling 



force the shell movements were traced on a 



kymograph and a record was made of the time 



and load under which the muscle fibers were torn 



off. Continuous irritation of the adductor by the 



foreign body (hook or rod) inside the sliell near 



the mantle makes this technique objectionable. 

 Furthermore, the end point of tlie experiment, 

 the tearing off of the muscle, is of no biological 

 significance compared to a determination of the 

 tensile force of the muscle fibers. 



The method used in the Bureau's shellfish 

 lalxiratory eliminates these objections. Tlie left 

 valve of tlie oyster is mounted on a heavj^ cement 

 block, using a very strong mixture of portland 

 cement and sand to which a small amount of 

 plaster of paris is added (fig. 159). The base is 

 bolted to the frame D which may be placed in the 

 a(|uarium tank B supplied with running sea water. 

 A galvanized iron screw (a) about 1 inch in length 

 is inserted into the valve at the center of the 

 attachment of the adductor muscle. Its tip 

 should not penetrate the valve. Enough portland 

 cement or other highly adhesive mixture is applied 

 to the shell surface around the screw to make a 

 cone of about 1 incli in diameter; the top of the 

 screw (a) should protrude above the cement. A 

 metal stirrup (E) consisting of a pair of iron bars 

 (b) with pronged arms at the lower end and a 

 hook (d) mounted at the upper end connect the 

 valve and the pan (e) of the laboratory balance 



Figure 159. — Method of determining the resistance of the adductor muscle of C. virginica to a pulling force. A — cement 

 base, bolted to wooden frame D and placed in tank B; a — galvanized iron screw; b — bars of the stirrup E; c — adjust- 

 ing nut; d — hook for connecting the stirrup to the balance; e — left pan of the balance: F — seawater intake; H — 

 overflow; K — kymograph; L — writing lever; M — signal magnet and pen; R — Telechron timer; T — transformer. 



176 



FISH AND WILDLIFE SERVICE 



