Shells are derived from the shells of mollusks

I always thought the shell was this:

A shell is an animal’s shell, what kind of animal’s shell is it?

But after study and practice:

Shells are shells belonging to mollusks, second only to arthropods.

And mollusks: snails, clams, shellfish, snails, octopus, squid

Arthropods: Shrimp, Crab

the following:

The creatures that live inside the shells are mollusks:

The types and habits of mollusks vary greatly. The largest mollusk, the king squid, has a wrist spread of up to 12 meters, while the smallest snail is only 1 cm long. But they have common basic features. The body has no endoskeleton and is soft, and most of them are not segmented. The body structure can be divided into four parts: head, foot, visceral mass and mantle. The mantle of some mollusks secretes a hard shell of calcium to protect the body. The shape of the mantle varies from species to species. With the exception of adult gastropods, the shells of mollusks are bilaterally symmetrical.

Most mollusks have shells, such as snails, clams and other shellfish; a few on land include snails and slugs; the shells of octopuses, squids, and sea slugs have disappeared; most mollusks rely on a fleshy foot to slide forward to move themselves The body, many have a coiled shell to protect the soft body snails inside.

The most important feature of mollusks is that their bodies are soft and have no endoskeleton. Most of them are symmetrical and unsegmented. Their outer skin folds from the back to form a mantle that surrounds the whole body and can secrete a protective calcareous shell. However, some mollusks The shell has degenerated (eg slugs, octopuses) or hidden inside the body (eg squid).

The shell is: calcareous (calcium carbonate) shell.

Calcium carbonate (English: Calcium carbonate), commonly known as limestone, limestone, and stone powder, is a compound with the chemical formula CaCO3, which is alkaline, almost insoluble in water (Ksp = 4.8×10-9), and can react with acids.

The composition of the shell is calcium carbonate:

Calcium carbonate is abundant on earth and exists in many forms in rocks, minerals and organisms, such as: aragonite, calcite, chalk, limestone, marble, travertine. It is also the main component of animal bones or shells (such as reptiles and bivalves). Calcium carbonate is also the main component of scale, which is usually formed by the combination of calcium ions in water and carbonate ions, and often occurs in areas with high water hardness. Calcium carbonate is also one of the active ingredients in agricultural lime, which can be used to neutralize soil acidity, but excessive use also has the risk of soil salinization. In medical applications, calcium carbonate is also a commonly used source of calcium, and is often used as one of the active ingredients in acid suppressants.