1. When foreign substance enters mollush of the shell category in sea water or freshwater and the outer membrane is stimulated, a kind of nacre will screte which will wrap up the tiny foreign matter layer by layer and gradually form into a small ball, and that is pearl.
海水或淡水中的贝类软件动物体内进入细小杂质时，外套膜受到刺激便分泌出一种珍珠质，将细小杂质层层包裹起来，逐渐成为一颗小圆珠，就是珍珠。