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A shell is the most universally identifiable part of a creature known as a mollusk. Mollusks are invertebrate animals (think of a snail) with an unsegmented, basically symmetrical body, generally consisting of head, foot, visceral hump and mantle. Mollusks are descendants from primitive wormlike creatures inched around in the ooze of primeval seas millions of years ago. As dissolving land masses began to feed salts and chemicals into oceans, the first mollusks digested them and eventually used them to build durable shelters.
Today, mollusks not only exist in the ocean, but in fresh water as well as on land. Marine mollusks generally use gills to breath while some terrestrial mollusks have lungs. A main characteristic of mollusks is that they have no internal skeleton. The shell serves as an exo-skeleton to protect the soft bodied against the outside world. The shell is actually only loosely attached and does not provide support or rigidity to the animal. In fact, some mollusks including octopus and squid do not utilize a shell at all.
SHELL FORM & COMPOSITION
The shell is generally composed of calcium carbonate, which is secreted by the outer surface of the mantle of the shell as the creature ages. The composition can be thickened, enlarged and repaired, but does not play a part in the metabolism of the animal. When the mollusk dies, its shell is the one part that typically remains intact. Chances are, you have come across thousands of shells that have wash up on the seashore during your last trip to the beach.
SHELL GROWTH
The shell of a mollusk grows periodically, not continuously, like that of a human child. The upper surface of the shell is formed by tissue at the mantle edge, while inner layers are secreted by all parts of the mantle surface. Therefore, growth takes place in two directions: Parallel to the edge– growth in size, and at right angles of the surface-growth in thickness. This varying growth process assures that no two species are exactly the same. A shell formation can produce new structural elements (folds or tubercles) and as growth continues, these irregularities can produce what is known as an ornament or sculpture on the upper surface of the shell.
COLOR & PATTERN
Mankind has always been astonished by the intricate design and varying color patterns of shells. The color and design of the shell is mainly dependent on the diet of the animal. In warmer waters where a variety of food sources are abundant, you will find thousand of species in all shapes, sizes and especially colors. In colder waters where food sources are typically the same, you will find mostly darker shells lacking in any type of vibrant color.
The food consumed by the mollusk causes pigments to be produced within the mantle epithelium of the mollusk. These pigments are deposited in the calcareous layer lying directly beneath the periostracum. As with shell growth process, which can produce varying shell ornamentation, the color pattern may also be the result of periodic activity. If the pigment secretion is continuous, then spiral or radial lines or bands will be laid down. However if pigment secretion is periodic, then spots or flecks will appear on the shell. If the whole mantle is secreting pigment at the same rate, the shell will have uniform color, but if the process is interrupted, then axial or concentric lines appear. If the pigment is secreted in zones, then wavy bands or angular markings occur.
FUNCTION
Depending on the species the shell is used for a variety of functions. Most marine mollusks use the shell for protection from predators and from their hostile environment. Generally speaking, mollusks can withdraw completely or at least partly into the shell. Terrestrial mollusks use the shell to prevent water loss that can lead to desiccation. Some mollusks use the shell as a tool to move or open objects. Still others use the shell for locomotion and buoyancy whether it is crawling, swimming or floating.
© Copyright 2007 by SeashellWorld.com
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