1883.] on Oysters and the Oyster Question. 337 



exterior. At the bases of the umbones, the valves are joined together, 

 along a short transverse line, in a sort of hinge by a band of dark- 

 brown elastic substance, the ligament (I.), which, in the middle of the 

 hinge, forms a thicker cushion. About the centre of the inner face 

 of each valve there is a large well-defined rounded depression, like a 

 scar, which marks the place of attachment of a strong and important 

 muscle. 



Each valve is sometimes solid throughout, but in old oysters, and 

 especially in those that live in deep water, the substance of the valves, 

 and more particularly of the thick convex left valve, contains wide 

 cavities, separated only by thin layers of nacreous substance, which 

 are full of sea-water. 



In structure, the nacre, or mother-of-pearl, is very dense, hard, 

 and finely laminated ; but the superficial outer layer is made up of 

 small polygonal prisms, and is somewhat friable. Each of these sub- 

 stances, the nacreous and the prismatic, consists of layers of organic 

 matter impregnated with salts of lime. 



If the oyster has been left at peace for some time in its native sea- 

 water, the edges of the valves, beyond the hinge, will be seen to be 

 separated by a chink which is wider opposite the umbones. But, 

 upon the least disturbance, the chink is closed and the shut valves 

 cannot be thrust asunder, without the expenditure of an amount of 

 force which usually breaks them. 



An expert oyster-opener, however, mindful of the maxim, arte non 

 vi, gets them apart with the utmost ease. A strong, flat-bladed knife 

 is introduced between the margins of the valves, and the knife being 

 kept close to the inner face of one of them, is swept round the region 

 of the muscular impression. If the operation is properly performed, 

 the shell at once gapes widely ; and it will now be found that, if the 

 valve which has sprung up is pressed down, it immediately returns to 

 its former position. The shell that, before, could hardly be forced 

 open, now will not keep shut. The reason of this becomes apparent 

 if the soft body, the edible part of the oyster, which lies within the 

 shell is carefully cleaned out so that the interior of the valves can be 

 seen. On looking towards the hinge, the thick elastic cushion 

 formed by the middle of the ligament, will be found to be compressed 

 when the valves are brought together; and, when the external 

 pressure is removed, its elastic reaction suffices to thrust them apart. 

 In fact, it is like the spring of a door, arranged in such a manner as 

 to keep the door ajar. While the oyster is alive, the great muscle 

 already mentioned, which is called the adductor (Fig. 1, add.), the ends 

 of which are attached to the two scars on the inner face of the valves, 

 is always ready to overcome the elasticity of the ligament and close the 

 valves, when need arises. And what the judicious oyster-opener does 

 is to cut this muscle close to one or other of its attachments. Thus 

 the force by which the valves are made to gape is elasticity of a purely 

 mechanical character, and is as active in the dead as in the living 

 oyster ; while that by which the valves are closed is the contractility 



z 2 



