146 



BIVALVES.* 



1 The shells belonging to this class are composed 

 of two pieces united by an elastic horny ligament:^ 

 the part where the valves are joined together, is 

 called the cardo^ or hinge, and corresponds in 

 position with the back of the animal : it is either 

 plain or furnished with teeth. The ligament 

 serves not only to connect the valves, but also to 

 open them, and is either external or internal ; 

 the muscle or muscles by which the animal is 

 attached to the shell keep it closed : when these 

 are relaxed, the ligament, which was either in a 

 state of tension or compression according as it 

 was either external or internal, by its efforts to 

 recover its position, opens the valves. If the two 

 valves are quite alike, the shell is said to be 

 equivalve ; if they differ in form or size, it is 

 called inequivalve. If the sides of the valve are 

 symmetrical, the valve is said to be equilateral ; || 

 if they are not, it is said to be inequilateral. 



* For the parts of a bivalve shell, see Plate I. 



+ Ligament, a substance which serves to bind things toge- 

 ther, from the Latin, ligare, to bind. 



+ Cardo is the Latin word for a hinge. 



Equivalve, having equal valves, from Latin, equ us 

 equal, and valve. 



|| Equilateral, having equal sides, from Latin, equ us 

 equal, and latera, sides. 



