13. 



I? T. 

 Figures 3, 5 and 9, f. 



The foot, figure 3, f, is very large and powerful. Dur- 

 ing development its sides grow out into two muscular f Isps, that 

 lie side by side, and are capable of being extended laterallj^ 

 30 - that tth. -e4x— iaa^g— s44»s-^^a— aar— alfne-st fla t o r— archgdr'suTfaco, 

 figure 9. It is this part of the foot that is called the sole. 



Notwithstanding its being a rigid muscular organ, known 

 to bo adapted for burrowing C* ) and executing movements with re- 

 markable rapidity ^1 ^^^ 21)^ .^ is gtill referred to as a 

 "creeping sole" (10-12-15 and 21). 



The free margins of the flaps are thin, and are fringed 

 witli flattened papillae that are very sensitive to touch. There 

 are four pairs of foot muscles, one posterior arj.d tliree anterior. 

 The posterior foot muscles, figure 3, pfra, are inserted on the 

 shell, just in front of the posterior adductor muscle and very 

 near the bases of the teeth. They are very large and send all 

 of their fibers forward, along the sides of tlie foot, to be dis- 

 tributed to its anterior and ventral portions. Their function 

 is to retract the foot. 



The anterior foot-muscles, a fm, are inserted on the 



