SEA-FISHERIES LABORATORY. 363 
fibrils being thrown into folds by contraction, producing 
an apparent striation only. Transverse striation has also 
been observed in /Pecten opercularis, on the ctenidial 
muscles (fig. 45, Br. m.), the appearance here being 
exactly as in the mantle. Both cases are probably due to 
contraction. | 
The Circular Muscles run parallel to the margin of 
the mantle and are very well developed in the Velum 
(fig. 4, V. M. c.), which is made up almost wholly of these 
muscle fibres. When Pecten closes its valves rapidly, 
whilst swimming, the water between the valves must 
endeavour to escape at the ventral margin by forcing the 
two vela apart. One can see, then, the use of this develop- 
ment of circular muscles, because if the vela are kept im 
a rigid condition by their action, the water will be com- 
pelled to pass out at each side dorsally, near the hinge 
line, as previously described. These circular muscles are 
inserted into the shell in conspicuous bundles anteriorly 
and posteriorly (fig. 5, V. JZ. a.) at the same level as the 
fusion of the mantle lobes. 
The Retractor Muscle of the foot is the posterior 
retractor of the left side, and is the sole representative of 
the four retractor muscles which attach the foot and con- 
tained viscera to the shell in the majority of lamelli- 
branchs. In monomyarian forms, the two anterior 
retractors are usually absent, but Pecten has gone further, 
and, moreover, the single retractor which is obvious in 
P. opercularis has become even more vestigial in P. 
MALUMUS. 
In both species the attachment to the shell is in the 
same position, along the dorsal margin of the adductor 
muscle, near the junction of its two parts, and the 
retractor impression on the shell cannot be distinguished 
from that of the adductor. 
