io8 Descriptive Zoology. 



Why the Clam opens and shuts its Shell. It is only by 

 opening the shell that the clam is able to place itself in 

 communication with the outside world. The opening allows 

 the foot and siphons to be protruded. When disturbed, the 

 clam withdraws the foot and the siphons, and completely 

 closes the shell, and usually remains in this condition until 

 the disturbance ceases. The muscles are of the slow-acting, 

 non-striped kind, and can remain shortened a long time ; 

 but they evidently get tired, and after a while they relax, 

 and the shell gapes open. 



The Location of the Siphons. Since the clam pulls itself 

 forward by the foot, which it imbeds in the mud, the foot 

 naturally extends forward and downward. As a good share 

 of the ventral part of the shell is below the level of the sand 

 or mud, the only available place to take in clear, fresh water 

 is at the upper and posterior border. And here we find 

 the siphons. 



How the Clam Progresses. The foot is slowly extended 

 forward and downward into the mud. When it has become 

 well imbedded in the mud, if the clam wishes to move for- 

 ward, it shortens the muscles of the foot and body, and thus 

 pulls forward the body, shell and all. Then another inter- 

 val must elapse until the foot is again anchored before an- 

 other move can be made. In this way the clam slowly 

 plows its way along, leaving a distinct furrow by which 

 it may be traced in clear water. The protruding of the foot 

 is a slow process, while the act of pulling forward is of com- 

 paratively short duration. It is stated that the extension of 

 the foot is mainly due to an inflow of blood which is kept 

 from returning by a tightening of the sphincter muscles 

 around the veins. At any rate, the foot is often found 

 dilated toward the extremity, which plainly increases its 

 efficiency as an anchor. 



