14 Our Food Mollusks 



close. An attempt to pry them open will show that they 

 are held together with great force. The closing mecha- 

 nism consists of two cylindrical bundles of muscle fibers, 

 known as the anterior and posterior adductors, running 

 directly across from one valve to the other. The ends 

 of these muscles, severed from the left valve, are shown 

 in the figure (a a and p a). 



The functional relations of ligament, hinge, and 

 muscle, may be understood by referring to the text figure 



(Figure 2), which repre- 

 sents a transverse section of 

 the shell in the region of 

 hinge and ligament. The 

 rubbery ligament (/) occu- 

 pies such a position and is of 

 such a width, that when the 

 adductors (a m) contract, 

 the hinge (h) acts as a ful- 

 crum, and the ligament is 

 stretched. On the relaxation 

 of the muscles, the mechani- 

 cal contraction of the liga- 

 ment, acting on the hinge, 

 causes the lower edges of 

 the shell to separate. 



Examining the outer sur- 

 face of a valve, there is to be noticed, far dorsalward and 

 forward, in Venus, a rounded prominence, the umbo, so 

 called on account of its fancied resemblance to the boss of 

 a shield. Its position on the shell varies in different 

 bivalves, and in some it is very inconspicuous or absent 

 altogether. 



From the umbo as a center, concentric lines of growth 



FIG. 2. Transverse section of 

 the shell of Venus to show 

 relations of ligament (/), 

 hinge (/*), and adductor 

 muscles (a m). 



