Mytilus, Mytilus, Pinctada, Mytilidae, Mytilus and Shellfish. Distributed in Asia, Europe, North America and North Africa. Most of them can naturally form pearls in the body. The shape is oval or oval. The shell is thin and fragile. The two shells spread out, sometimes with hind wings on the back. The top of the shell is broad and slightly convex, located in the middle or front of the trailing edge. The shell surface is smooth, with concentric growth lines or green radiation from the top of the shell to the abdominal edge. The gluing part is narrow and toothless. The shaft and feet are developed. Dioecious. Meat is edible, and it is also a natural bait and feed for fish and poultry. Some varieties can be used as freshwater pearls.
External cause
The mantle of mussels is stimulated by the invasion of foreign bodies (sand and parasites). The epidermal cells in the stimulation area take foreign bodies as the core and fall into the connective tissue of the mantle. Some of the trapped epidermal cells divide themselves to form nacre, which secretes nacre and the nucleus is wrapped layer by layer to form pearl. Pearls with foreign bodies as the core are called "nucleated pearls".
2. Internal factors
After pathological stimulation, a part of the outer skin of the mantle undergoes cell division, and then it is separated, and then coated with organic substances secreted by itself, and gradually falls into the connective tissue of the mantle, forming pearl sacs, and then forming pearls. Because there is no foreign body as the core, it is called "seedless pearl". At present, cultured pearls are cut from the mantle of pearl-breeding mussels by artificial methods according to the above principles, and implanted into the connective tissue of the mantle together with the artificial nucleus prepared by mussel shells. The implanted cell fragments proliferate rapidly around the artificial nucleus by the nutrition provided by connective tissue, forming pearl sacs and secreting nacre, thus producing artificial nucleated pearls. Artificial seedless pearls are pearls produced by implanting only small pieces of cells into the mantle, forming pearl sacs through cell proliferation, and secreting mother-of-pearl into the sacs.