Pearls inside mussels form because of invasion by foreign bodies or pathological stimulation of the outer epidermis of the outer coat membrane of the mussel.

The outer coat membrane of the mussel is stimulated by the invasion of foreign matter (sand, parasites), the epidermal cells at the stimulated place take the foreign matter as the nucleus, and the pearl capsule cells secrete perlage, and the layer after layer encapsulates the nucleus into pearls, and this kind of pearl is called "nucleated pearl". Mussels of the outer coat membrane epidermis by pathological stimuli, part of the cell division occurs after the separation, encapsulated in their own secretion of organic matter, while gradually plunged into the connective tissue of the outer coat membrane, the formation of the pearl capsule after the formation of the pearl, known as the "non-nuclear pearl".

Some areas with freshwater pearl-breeding mussels, used as pearl-breeding mussels and folded crown mussels mainly triangular sail mussels. Mussels is the mollusc family mussel family of a class of animals collectively, in some places known as mussels, crooked children, living in freshwater lakes, ponds, rivers and other underwater, half-buried in the sediment.

Morphological Characteristics

Shell shape varies, two shells are equal, the top of the shell engraving is often concentric circles or folded lines, but more or less degraded. Hinged part highly variable, sometimes with proposed primary teeth. With 1 external ligament. Gill interlobular septa intact and arranged parallel to gill filaments, posterior part of outer lobe of outer gill fused to outer coat membrane, with gill water tubes. The gill and anal openings are completely distinguished by septa.

There are ligaments on its outer side, which, relying on their elasticity, open the bivalve shell. The inner surface of the shell has muscle scars for muscle attachment. Coat scar parallel to the ventral margin of the shell; three muscle scars on the anterior upper part of the shell, the largest one oval, the anterior closed-shell muscle scar; the slightly triangular anterior retractile peduncle muscle scar at a small angle on its posterior upper margin; and the extensor peduncle muscle scar on its posterior lower margin. At the posterior end of the shell, near the dorsal margin, there are two muscle scars, the large one is the posterior closed-shell muscle scar, oval in shape, and its anterior upper margin is a small one is the posterior retractor muscle scar.

Refer to Baidu Encyclopedia-Mussels