While several kinds of sea animals have exoskeletons which may after
death be found in beach drift and picked up by beachcombers, usually
only those of molluscs (also spelled "mollusks") are known as seashells. The majority of shell-forming molluscs belong to two classes: Gastropoda (univalves, or snails) and Bivalvia (bivalves or clams, oysters, scallops, etc).
There are three other classes of mollusks which routinely create a shell, and those are: Scaphopoda (tusk shells), Polyplacophora (chitons, which have eight articulating shelly plates), and Monoplacophora (single-shelled chiton-like animals which live in very deep water, and which superficially resemble minute limpets.
Nautiluses are the only extant cephalopods which have an external shell, although octopuses, squid, cuttlefish, and Spirula spirula have small internal shells. Females of the octopus genus Argonauta
secrete a specialised paper-thin eggcase in which they reside, and this
is popularly regarded as a shell, although it is not attached to the
body of the animal.
Malacology, the scientific study of molluscs as living organisms, has a branch devoted to shells, called conchology
- although it should be noted that these terms used to be, and to a
minor extent still are, used interchangeably, even by scientists (this
is more common in Europe).
In those mollusks which have a shell, the shell grows gradually over
the lifetime of the mollusc by the addition of calcium carbonate to the
leading edge or opening, and thus the shell gradually becomes longer
and wider, in an increasing spiral shape, to better accommodate the
growing animal inside. The animal also thickens the shell as it grows,
so that the shell stays proportionately strong for its size.
A mollusk shell is formed, repaired and maintained by a part of the anatomy called the mantle.
Any injuries to or abnormal conditions of the mantle are usually
reflected in the shape and form and even color of the shell. When the
animal encounters harsh conditions which limit its food supply or
otherwise cause it to become dormant for a while, the mantle often
ceases to produce the shell substance. When conditions improve again
and the mantle resumes its task, a "growth line" which extends the
entire length of the shell is produced, and the pattern and even the
colors on the shell after these dormant periods are sometimes quite
different from previous colors and patterns.
Interestingly, within some species of mollusk there is often a
surprising degree of variation in the exact shape, pattern,
ornamentation, and color of the shell.
Shells are composite materials of calcium carbonate, found either as calcite or aragonite
and organic macromolecules, mainly proteins and polysaccharides. Shells
can have numerous ultrastructural motifs, the most common being
crossed-lamellar (aragonite), prismatic (aragonite or calcite),
homogeneous (aragonite), foliated (aragonite) and nacre (aragonite). Although not the most common, the nacre is the most studied layer. Shells of the class Polyplacophora are made of aragonite
also known as mother of pearl, is an important part of the shell
structure in many gastropod and bivalve mollusks especially the more
ancient families such as top shells and pearl oysters. Like the other
calcareous layers of the shell, the nacre is created by the epithelial cells (formed by the germ layer ectoderm) of the mantle tissue. Mollusk blood is rich in dissolved calcium,
and during shell deposition, the calcium is concentrated out from the
blood and crystallized as calcium carbonate. Nacre is continually
deposited onto the inner surface of the animal's shell (the iridescent nacreous layer or mother of pearl).
This is done both as a means to thicken, strengthen and smooth the
inner surface of the shell itself and as a defense against parasitic organisms and damaging detritus.
When a mollusc 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. Almost any species of
bivalve or gastropod is capable of producing pearls, even mollusks
which have no inner nacreous layer. However, only a few species, such
as the famous pearl oysters, can create pearls which are highly prized.
Mollusc shells (especially those formed by marine species) are very
durable and outlast the otherwise soft-bodied animals that produce them
by a very long time (sometimes thousands of years). They fossilise
easily, and fossil mollusc shells date all the way back to the Cambrian period. Large amounts of shells sometimes form sediment and become compressed into shelly limestone deposits.
Shells of marine molluscs (some of which wash up on beaches or live
in the intertidal or sub-tidal zones and are therefore easily found
without specialized equipment) are called "seashells", and are
collected by a large number of enthusiasts. Many shell collectors find
their own material or are interested in "specimen shells" - shells
which come with full scientific collecting data: information including
how, when, where and in what habitat and by whom they were collected.
In the tropical and sub-tropical areas of our planet, there are more
far more species of colorful, large and intertidal seashells than there
are in regions closer to the poles.
Above information from Wikepedia - http://en.wikipedia.org/wiki/Animal_shell