BIVALVES.* 



The shells belonging to this class are com- 

 posed of two pieces united by an elastic horny 

 ligament :| this part where the valves are joined 

 together, is called the cardo^ or hinge, and cor- 

 responds 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 ac- 

 cording 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 equilate- 

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



The animals belonging to the Bivalve shells 

 are acephalous mollusca having no distinct 

 head ; they have no eyes, and the mouth, which 

 is hidden under the mantle, is only a simple 



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



t Ligament, a substance which serves to bind things together, 

 from the Latin, lig are, to bind. 



| Cardo is the Latin word for a hinge. 



Equivalve, having equal valves, from Latin, equ us, equa^ 

 and valve. 



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

 ana latera, sides. 



