MUSSELS, CLAMS, OYSTERS, SNAILS, SQUIDS 105 



lAciatMre 



the scar of the protractor muscle of the foot; just above the 

 posterior adductor muscle scar is the small scar of the 

 posterior retractor foot muscle; parallel with the edge of 

 the valve and a short distance from it is a delicate streak, 

 the pallial line, that marks the hne along which the 

 mantle lobe is joined to the shell (Fig. 54). 



How the mussel opens and shuts its shell. — The anterior 

 adductor muscle and the posterior adductor muscle run 

 straight across the cavity between 

 the valves, and their ends are 

 fastened firmly to the inside walls 

 of the shell (Fig. 55). Therefore, 

 when they contract and shorten, 

 the valves are pulled together 

 and held tightly closed. On the 

 other hand, whenever the ad- 

 ductor muscles are relaxed, the 

 strong hinge ligament, which all 

 the time the valves are closed is 

 tightly stretched, throws the shell 

 open by its elasticity. Thus the 

 shell is closed with muscular 

 effort, but is opened by a me- 

 chanical, springlike action which 

 requires no effort on the part of the animal, 

 is open much more than it is shut, this is a striking adap- 

 tation to the mussel's mode of living. 



The mantle and siphons. — Lining the inside of both 

 valves and completely enveloping the body is a soft, white, 

 delicate membrane known as the mantle. It really consists 

 of two lobes corresponding to the valves. The ventral 

 edges of the mantle lobes are free and run parallel with the 



Fig. 55. — Cross section of mus- 

 sel, showing mechanism of 

 opening and closing. 



Since the shell 



