SEA-FISHERIES LABORATORY. 197 
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 
ascale pan. Weights were placed in the scale pan till 
the valves were separated to the extent of 1mm. 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 Cardiuwm 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 (Ret.p., figs. 3 
and 11) originate in the posterior margin of the proximal 
limb of the latter, and run backward as a short apparently 
single median bundle (Ret.p., fig. 4), this bifurcates into 
* Bull. Acad. Roy. Sci. de Belgique. Ser. III., t. VI., pp. 226—259, 1883. 
