198 TRANSACTIONS LIVERPOUL BIOLOGICAL SOCIETY. 
the right and left muscle bundles, which are inserted, one 
into each valve, at the dorsal margin of the posterior 
adductor scar (Ret.p’., fig. 10). There is a well-marked 
decussation of the fibres forming each muscle. That is, 
the fibres forming the left retractor originate in a flattened 
band, lying on the right side of the middle line, on the 
inside of the muscular posterior wall of the viscero-pedal 
mass, and vice versa. This crossing of the fibres 1s effected 
by two or three smaller inter-digitating bundles from 
each side, and is complete, none of the fibres remaining 
uncrossed. ‘The band from which these muscles take 
the origin (Ret.p., fig. 11) can be traced round the bend of 
foot into the ventral and lateral walls of the distal limb, 
after which its further course becomes very complex. 
The anterior retractors of the foot le through almost all 
their course in the interior of the viscero-pedal mass. 
‘Together they form a flattened band of fibres on the inside 
of the anterior wall (Ret.a., fig. 11). At the upper Jevel of 
the anterior adductor they come to the surface as two 
paired bundles which are inserted into the shell on the 
dorsal margin of the anterior adductor scar (Ret.a’., fig. 10). 
As in the case of the posterior retractors, a certain amount 
of crossing takes place, though this is not so evident as in 
the case of the other. Below the bend of the foot the 
further internal course of the fibres is difficult to make 
out. : 
The protractor muscles of the foot form right and left 
short bundles, which are inserted into the shell near the 
scars of attachment of the anterior retractors. Internally 
the fibres spread out in a fan-shaped sheet on the lateral 
dorsal walls of the proximal limb of the viscero-pedal mass, 
but there are no obviously distinct bundles, as in the case 
of the retractor muscles. Many of the fibres seem to pass 
. 
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