VARIETIES OF OYSTERS. 137 



allow the valves to gape, will not permit them to be shifted 

 or wrenched aside on one another. 



Very powerful muscles run directly from shell to shell, 

 and can, when contracted, hold them together with such 

 force that it is impossible to open them without the assis- 

 tance of an oyster-knife ; and as none of the natural 

 enemies of the molluscs, except man, possess oyster-knives, 

 they are tolerably safe from this kind of forcible entry 

 upon their fortresses. 



In most Lamellibranchs there are two muscles to 

 close the valves, one in front and the other behind ; but 

 in the oyster family there is but one, and this is near the 

 centre of the shell, and represents the hind muscle of the 

 others. 



Opposed to these muscles is the ligament which runs 

 from shell to shell on the outside of some species, and lies 

 in a pit in the hinge surface in others. These ligaments 

 have no power of active contraction as the muscles have, 

 but are passively elastic. In the case of the external liga- 

 ment, it is in a state of strain when the valves are closed, 

 and opens them when the muscles relax ; while in the case 

 of the internal ligament, it is compressed when the muscles 

 are contracted, and presses the valves apart when they 

 relax. 



The mouth is without hard teeth or jaws, but it often 

 has large flattened lips. The throat is short, and leads 

 into a roundish stomach, which receives the biliary ducts. 

 The great peculiarity of the stomach is the long blind sac 

 which is attached to it, in which is enclosed a cartilaginous 

 rod, the function of which is not known. The intestine 

 twists about in several folds, entering the foot in those 

 bivalves which have feet, and always ending at the oppo- 



