132 OILMAN A. DREW. 



the mantle posterior to the ventral opening are especially mus- 

 cular and seem to replace the posterior adductor in function to a 

 marked extent. 



The foot, when retracted into the shell, is nearly cylindrical 

 and together with other organs completely fills the part anterior 

 to the ventral opening in the mantle. The dorsal portion of its 

 extremity is pointed, and a slight ridge marks the boundary of 

 what may be called the sole (Fig. 2). The foot is very power- 

 ful and remarkably active, its movements being very unlike the 

 slow movements of the foot in most lamellibranchs. It may be 

 thrust from the anterior end of the shell to a distance exceeding 

 one half the length of the shell, and in this position the end may 

 be swelled into a knob or bulb that considerably exceeds the 

 diameter of the shell (Fig. i). The knob is not cylindrical but 

 is extended dorso-ventrally and laterally and the free extremity 

 or sole is comparatively flattened. In this swollen condition the 

 end of the foot forms a very efficient anchor, as will be found by 

 grasping a shell and trying to withdraw it from the mud. The 

 resistance of the expanded foot is so great that the foot is fre- 

 quently torn away from the shell when the shell is jerked 

 quickly (7). The foot is attached to the shell by two pairs of 

 foot muscles, both of which are strong and aid in withdrawing the 

 foot into the shell. With the end of the foot anchored, the ob- 

 vious result of the contraction of these muscles is to pull the 

 shell into the mud up to the position of the bulbous portion of 

 the foot. 



For our present purpose it is not necessary to give more atten- 

 tion to the anatomy of the animal, and we will proceed at once to 

 the study of the movements. 



Burrowing.- -The movements of burrowing may be best 

 studied either in specimens placed in shallow dishes of sea water, 

 which are very likely to execute the movements soon after they 

 are placed in the water, or in specimens held with the anterior 

 end pointing downward and stimulated to activity by stroking 

 the sense tentacles around the ventral opening in the mantle and 

 around the siphons. Specimens in the water are more normal in 

 their movements than the specimens held and stimulated as de- 

 scribed. Apparently the action of gravity may cause the held 

 specimen to protrude its foot and to partially expand the end of 



