Pelecypoda. 109 



Extent of the Clam's Locomotion. The clam does not 

 travel far. Since it brings its food in by the currents of 

 water which it creates, it does not have to move about for 

 food. When the water gets low, as in most creeks in the 

 summer time, the clam apparently seeks the deeper water. 

 The question naturally arises, "How does the clam become 

 aware of this change, and how does it know in what direc- 

 tion to go ? " 



The Clam's Muscles and their Functions. There are five 

 chief muscles : 



I. Anterior adductor. > ... See Figs. 63 



> Close the shell. 

 - 2. Posterior adductor. > and 65. 



3. Protractor pulls the foot and body forward and 

 downward. 



4. Anterior retractor pulls foot and body upward and 

 backward. 



5. Posterior retractor pulls foot and body upward and 

 backward. 



When the foot is imbedded in the sand or mud, the 

 shortening of the retractors, whose fibers spread over the 

 body and foot, pull the shell forward instead of retracting 

 the foot. 



Structure of the Clam Shell. If a shell is roasted thor- 

 oughly, its structure may more easily be learned. The first 

 fact to be noted is that the shell consists of layers ; the 

 next, that these layers are in two sets, the dividing plane 

 between which starts from the mantle line and extends 

 toward the umbo. The shell is an outgrowth of the outer 

 layer, or epidermis, of the mantle. But the layers of the 

 shell made by the part of the mantle outside of the mantle 

 line are not directly continuous with the layers formed by 

 that part of the mantle which is dorsal to the mantle line. 

 The mantle line is really a row of small muscle scars where 



