20 



fig. 10) are much larger than the areas of transverse 

 sections of the muscles. The posterior muscle forms a 

 thicker bundle than the anterior. In the extended condi- 

 tion of the animal, the structure of the muscle bundle is 

 rather loose, being penetrated by blood spaces usually 

 filled with corpuscles. 



The force of contraction of the adductor muscles is 

 very great. Plateau* has measured this in terms of the 

 weight required to force open the valves. Two hooks 

 were inserted under the ventral edges of the valves. The 

 hook sustaining the upper valve was fixed to a support. 

 The other, which was carried by the lower valve, supported 

 a scale pan. Weights were placed in the scale pan till 

 the valves were separated to the extent of 1 mm. As the 

 mean of eight such experiments it was found that the 

 weight required was 1134 grms. And taking the transverse 

 area of both adductors into account, this gives the force 

 necessary to overcome the contraction per sq. cm. of the 

 sectional area of the adductor muscles of Gardium as 

 equal to the weight of 2856 grms. Plateau also made the 

 converse experiment. An animal in a completely relaxed 

 condition, with the shell gaping, was supported with the 

 lower valve resting on a firm support. A loop was passed 

 round the upper valve, from the lower end of which was 

 suspended a scale pan. The mean weight required to 

 overcome the elasticity of the hinge ligament was found 

 to be 106 grms. 



(2) The extrinsic muscles of the foot take their origin 

 from the superficial muscular sheath of the viscero-pedal 

 mass. The posterior retractors of the foot {Bet. p., figs. 3 

 and ID originate in the posterior margin of the proximal 

 limb of the latter, and run backward as a short apparently 

 single median bundle (Bet. p., fig. 4), this bifurcates into 



* Pull. Acad. Roy. Sci. de Belgique. Ser, III., t. VI., pp. 226—259, 1883. 



