146 



BIVALVES. 



The shells belonging to this class are com- 

 posed of two pieces united by an elastic horny 

 ligament :f the part where the valves are joined 

 together, is called the cardo, J 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 ex- 

 ternal 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 com- 

 pression 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. 



J Cardo is the Latin word for a hinge. 



Equivalve, having equal valves, from Latin, equ zts, 

 equal, and valve. 



|| Equilateral, having equal sides, from Latin, *equ its 

 equal, and latera, sides. 



