ANODONTA — MUSCULAR SYSTEM. 
181 
the alimeiitaiy canal, the heart, the arteries and other organs are all 
more or less closely invested with or composed of muscnlar fibres. 
The i)laces of attachment of the extrinsic muscles to the internal 
surface of the valves is indicated by distinct and permanent scars, 
owing to the fibres composing the muscles being connected to the 
epithelial cellules, whose calcified cuticle forms the nacreous lining 
of the shell. 
The Pallial muscles are chiefly developed towards the mantle 
margin and are composed of variously directed and interlaced 
muscular fibres, which, however, mainly run in three directions, viz. : 
parallel to the mantle margin, at right angles to it, or perpendicnlarly 
to the two first-named and joining the inner and outer pallial surfaces ; 
those fibres running at right angles to the margin are the most strongly 
and numerously developed, they are proximally attached to the 
Fig. 352. — Muscular system of Anodonta JluviatiliSy illustrating the distribution and functional 
area of the extrinsic muscles. The right mantle lobe and right branchiaj removed (after Simpson). 
a. ad. anterior adductor; a./>. anterior protractor, showing the radiation of its muscular fibres 
and their comparatively superficial position ; a.r. anterior retractor, showing its deeply seated 
ramifications ; br. left branchiaj ; o.tn. orbicular muscles, showing their sub-marginal insertion in 
the shell to form the pallial line, their posterior modification form the siphonal retractors ; p.ad. 
posterior adductor ; pc. pericardium ; p.r. posterior retractor, the fibres of which are not so super- 
ficially distributed generally, as those of the protractor, but less deeply imbedded than those of the 
anterior retractor ; u.r. umbonal retentors or pedal levators, distributing their fibres over the 
region of the stomach, pericardium, etc. 
inner lining of the shell and form the retractor, s of the mantle margin, 
being di.stinguished as the Orbicular muscles, their line of attachment 
to the shell constituting the linear sub-marginal muscular scar, known 
as the pallial line, which runs parallel with the ventral margin of the 
shell and is also continuous with the external margin of the adductors. 
At the posterior margin of the mantle, these complex and variously 
divergent muscles become specialized to form the siphonal retractors, 
the constituent muscular fibres taking a circular, longitudinal or 
radial direction. 
