If pearls are broken open, a centre, or nucleus, will be found, consisting of some particle of dirt or sand, or some substance which had found its way by accident between the mantle and the shell, and around which the pearly matter has been formed in successive layers.
In shells having a brilliant, pearly lining, or nacre, the pearls obtained are oftentimes very beautiful, and from certain Oriental species living in the sea, called Avicula, the most brilliant pearls of commerce are obtained. If, on the other hand, the nacre lining the shell is dull white, as in the common oyster, the pearls are dull colored. These kinds of pearls are often found in oysters.
The Chinese have long been familiar with the art of making artificial pearls. By partly opening the shells of certain fresh-water mussels, and inserting little lead images, or other objects, between the mantle and the shell, the objects soon become covered with a natural layer of pearl.
Let us now study the markings on the inner surface of the shells of river-mussels. The shells of these creatures are called valves, and are spoken of as right or left valves, according to whether they are on the right or left side of the animal.
Certain ridges and prominences will be seen at the hinge, and, when the valves are carefully joined, the ridges in one valve will correspond to grooves in the other valve. These ridges are called teeth. The short ones, near the beak, are called cardinal teeth, and the long ones lateral teeth. The margin upon which they occur is called the hinge-margin, for it is upon this margin that the valves turn. (See Fig. 11.) Certain scars, or impressions, will be found marking the inside of the valves, and these indicate the point of the attachment of certain muscles to move the valves, and to enable the animal to protrude its foot, and crawl along. These marks are hence called muscular marks, or muscular impressions, and will be found to correspond in the right and left valves.
An irregular, round impression will be found at each end of the valve, near the hinge-margin. These show where the muscles are attached to move and close the valves, and hold them firmly together. The muscles run directly across from one valve to the other; and, to open a live mussel, it is necessary to pass a sharp blade between the