8 JOHNS HOPKINS UNIVERSITY MOEPHOLOGICAL MONOGRAPHS. 



sides of the foot, to be distributed to its anterior and ventral portions. 

 Their function is to retract the foot. 



The anterior foot-muscles, afm., are inserted on the shell just 

 behind the anterior adductor muscle and very near the bases of 

 the teeth. Of these muscles there are three pairs. The anterior pair 

 pass ventrally and posteriorly along the sides of the foot, and are 

 distributed to its posterior portion. The middle pair pass ventrally, 

 between the pair just mentioned, and are distributed to an intermediate 

 portion. The posterior pair pass ventrally 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 varied and energetic 

 movements of the foot. 



Besides these special foot-muscles, which together form a large part 

 of the outer walls of the foot, there are transverse strands of muscle 

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

 hollowed out between these strands. 



Movements of the foot are always very rapid and are remarkably 

 diversified. The movements of burrowing consist of thrusting the 

 closed foot, which is wedge-shaped, deep into the mud, reflecting its 

 margins to form an anchor, and then withdrawing it. These move- 

 ments follow each other in quick succession and enable an animal to 

 bury itself with great rapidity. When a specimen is placed upon its 

 side, the closed foot is bent back under the shell, its margins are 

 reflected, and it is then withdrawn. If these movements are executed 

 on mud, the lower reflected margin of the foot buries itself, and gives 

 the necessary purchase for the animal to right itself. If instead, they 

 are executed on a smooth, hard surface, where no purchase can be 

 obtained, these movements are likely to be replaced by leaping 

 movements. 



One leaping movement consists of bending the foot back under the 

 shell, turning the shell nearly on its dorsal margin, planting the surface 

 of the expanded "sole" on the bottom, and giving a quick downward 

 movement. If the foot does not slip, the shell is generally turned end 

 for end and thrown some inches. If the foot slips, as more frequently 

 happens in smooth-bottomed aquaria, it shoots forward with wonderful 

 rapidity and causes a posterior movement. Other movements are 

 common but will not be described. It seems very hard to imagine that 

 the foot could possibly be used as a creeping organ, and its present 



