21 



the right and left muscle bundles, which are inserted, one 

 into each valve, at the dorsal margin of the posterior 

 adductor scar (Bet.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 is 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 (Bet.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 auterior retractors of the foot lie 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 (Bet. a., fig. 11). At the upper level 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 (Bet. 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 



