SEA-FISHERIES LABORATORY. 199 
round and meet on the posterior margin, and the bundles 
may be regarded as being constituted by a break in the 
continuity of the circular muscular sheath of the proximal 
limb, the free ends being gathered up into two short 
bundles and attached to the shell. Judging from the 
direction of the fibres, the only effect of the contraction of 
these muscles will be to slightly rotate the whole viscero- 
pedal mass about its dorsal attachment on the shell, so 
that the term “‘ protractor’’ 1s rather a misnomer. 
The superior retractors of the foot (elevatores pedis, 
Pelseneer) form two paired muscle bundles, which are 
inserted one into each valve in the most dorsal part of the 
umbo. The scars cannot be easily seen without breaking 
the shell. Each bundle consists of fibres gathered up 
from the transverse and oblique musculature of the dorsal 
body-wall. 
(3) The intrinsic muscles of the foot include all the pedal 
muscles, which are not inserted into the shell, but have 
their attachments within the viscero-pedal mass itself. 
There is a thick hypodermal muscular sheath (fig. 25) in 
which typically four muscle layers can be distinguished. 
Beneath the epidermis is a thin layer of fibrous connective 
tissue within which is a layer of muscle fibres running trans- 
versely round the foot. This is succeeded by a thick layer 
of obliquely running fibres, which passes into another layer 
of transverse fibres, and internal to all is an irregular 
sheath of longitudinal fibres. The precise arrangement 1s 
variable at different levels, and all the layers may not be 
present. The oblique and circular layers are always 
represented. Here and there in a transverse section 
through the proximal limb of the viscero-pedal mass, 
strong muscle bundles (A/.p.v., fig. 11) may be seen passing 
across between the lateral walls in a transverse direction. 
In the extended condition the structure of these is rather 
