14. 



shell just behind the anterior adductor rmiscle and very near the 

 bases of the teeth. Of these nuscles there are tliree pairs. 

 The anterior pair pass ventral ly and posteriorly, along the sides 

 of the foot, and are distributed to its posterior portion. The 

 middle pair pass ventral ly, between the pair just mentioned, and 

 are distributed to an intermediate portion. The posterior pair 

 pass ventral ly and anteriorly, between both of the preceding pairs 

 and are distributed to the anterior and ventral portion of the 

 foot, including the muscular flaps. 



These three pairs of muscles are undoubtedly used in 

 producing the A^aried and energetic movements of the foot. Be- 

 side these special foot muscles vdiich together form a large part 

 of its outer walls, there are transverse strands of muscle fibers 

 extending from one wall to the other. Large blood spaces are 

 hollowed out between these strands. 



Movements o^ 'th.e foot are always very rapid and are re- 

 markably diversified. The movements of burrowing consist of 

 thrusting the closed foot, -wgiich is wedge-shaped, antorio?ly deep 

 into the mud, reflecting its margins to form an anchor, and then 

 withdrawing it. These movements follow eacJi other in qiiick suc- 

 cession and enable an animal to bury itself with great rapidity. 

 "jyhen the animal is placed upon its side, the closed foot is bent 



