Animal shell
The hard, rigid outer covering of certain
animals is called a shell. While many animals, particularly those that live in the sea, produce
exoskeletons, usually only those of
mollusks are considered to be shells. It is sometimes erroneously claimed that shells are made of
chitin, but these are unrelated materials .
The shell is usually made of
nacre, an organic mixture of outer layers of horny conchiolin , followed by an intermediate layer of
calcium carbonate as either
calcite or
aragonite in the form of platy crystals.
Nacre is secreted by the epithelial cells of the mantle tissue of certain species of mollusk.
Encyclopedia
The hard, rigid outer covering of certain
animals is called a
shell. While many animals, particularly those that live in the sea, produce
exoskeletons, usually only those of
mollusks are considered to be shells. It is sometimes erroneously claimed that shells are made of
chitin, but these are unrelated materials .
The shell is usually made of
nacre, an organic mixture of outer layers of horny conchiolin , followed by an intermediate layer of
calcium carbonate as either
calcite or
aragonite in the form of platy crystals.
Nacre is secreted by the epithelial cells of the mantle tissue of certain species of mollusk. Mollusk
blood is rich in dissolved calcium. In these mollusks the calcium is concentrated out from the blood where it can crystallize as calcium carbonate. The individual crystals of each layer differ in shape and orientation. Nacre is continually deposited onto the inner surface of the animal's shell , both as a means to smoothen the shell itself and as a defense against parasitic organisms and damaging detritus.
The shell may grow over time as the animal inside adds nacre to the leading edge near the opening. This causes the shell to become longer and wider to better accommodate the growing animal inside.
When a mollusk is invaded by a parasite or is irritated by a foreign object that the animal cannot eject, a process known as
encystation entombs the offending entity in successive, concentric layers of nacre. This process eventually forms what we call pearls and continues for as long as the mollusk lives.
Shells are very durable and outlast the otherwise soft-bodied animals that produce them by a very long time. Large amounts of shells may form sediment and become compressed into limestone. Shells that wash up on beaches are called seashells, and are
collected by some enthusiasts.
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See also
