What term is used to mention a distinctive two piece wall of silica in diatoms?

A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed of silica made from silicic acid, and is coated with a layer of organic substance. This layer is actually composed of several types of polysaccharides. The frustule is usually composed of two overlapping sections known as thecae. The join between two thecae is supported by bands of silica that hold them together. This overlapping allows for some internal expansion room and is essential during the reproduction process. The frustule also contains many pores and slits that provide the diatoms access to the external environment for processes such as waste removal and mucilage secretion.