How does a clam shell work?

For different groups of mollusks, the shells may be arranged differently. There are two groups in which the shell species differ: Conchifera and Polyplacophora. These groups have different shell structures, but common layers: outer, middle and inner layers.
In mollusks of the Conchifera species, the following layers are distinguished:
Periostracum is the thinnest outer layer of the shell, which consists of protein.
Ostrakum is a middle layer consisting of calcium carbonate crystals and a protein shell.
Hypostracum is the inner layer of the mollusk shell; it also consists of calcium carbonate crystals, but already in the form of plates wrapped in protein.
Many highly organized gastropods lack the third layer, hypostracum, and the second layer, ostracum, may contain various sublayers.
The molluscs Polyplacophora have three layers that make up their shell:
Periostracum is the outer layer of the shell of a mollusc, which is made up of protein.
Tegmentum – the middle layer, which consists mainly of protein, with an admixture of calcium carbonate.
Articulomentum is the inner layer of the mollusk shell, which consists only of calcium carbonate, with minor impurities. The group of mollusks Polyplacophora has one important difference from the other group, between the second and third layers of the shell, they have strands of living tissue.