Pelecypoda. 



107 



merely an uncalcified portion of the shell. When the mus- 

 cles pull the valves together, this ligament is stretched. 

 Consequently, as soon as the adductor muscles relax, the 

 elasticity of the hinge ligament opens the shell. It will be 

 observed that the shell is shut by muscular effort, whereas 

 it is opened by the mechanical action of a spring, which 

 consists of practically dead tissue. It requires no effort to 



Hinge Ligament 

 External 



Hinge Ligament 

 Internal 



FIG. 65. MECHANISM FOR OPENING AND SHUTTING A CLAM SHELL. 



keep the shell open. As the shell is partially open most 

 of the time, the economy of this arrangement is apparent. 

 Why would it not do as well to have the shell opened by 

 muscular action and closed by a spring ? In some bivalve 

 mollusks the hinge ligament is internal, so that when the 

 shell is shut the ligament is compressed, instead of 

 stretched, as in these clams. Then, when the muscles 

 relax, the shell is opened by the expansive elasticity of the 

 ligament. The general principle is the same, but is carried 

 out in a different way. See Fig. 65. 



