108 



ZOOLOGY. 



muscles, and sometimes only one. One is placed in the 

 anterior part of the shell, and one in the posterior. The 

 anterior muscle is sometimes absent. These muscles 

 leave impressions in the interior of the valves, so that it 

 can be easily ascertained, by examining a shell, whether 

 the living animal had one or two. The valves in ordi- 

 nary circumstances are kept open; but, when the animal 

 is alarmed, it exerts its adductor muscles, and closes 

 them. A dead bivalve always gapes, because, when the 

 adductor muscles cease to act, the elasticity of the liga- 

 ment opens the valves. 



FIG. 45. VENUS, showing Foot and Breathing Siphons. 



Most lamellibranchs possess a foot, which is pro- 

 truded through an opening in the mantle, on the lower 

 surface of the body. The foot is a muscular organ, 

 sometimes finger-shaped, and sometimes bent like a sickle. 

 This foot enables the animals, in some instances, to move 

 along by making a number of short leaps. In other 

 cases it is used in making burrows in the sand or mud. 

 In the mussels, &c., the foot is furnished with a gland 

 which secretes a silky substance. Of this liquid silk, 

 the creature forms a tuft of thread, by which it attaches 

 itself to a stone or some other object. The foot is some- 



