22 



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 " is 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 is 

 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 (M.p.i., 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 



