Information about Animal Shell

A shell is a hard, rigid outer layer, which has evolved in a very wide variety of different animals, including mollusks, sea urchins, crustaceans, turtles and tortoises, armadillos, etc. Scientific names for this type of structure include exoskeleton, armour, test, carapace, and peltidium.

The shells that are perhaps most familiar and most commonly encountered, both in the wild and for sale as decorative objects, are seashells, or more precisely, the external shells of marine mollusks (also spelled molluscs). These are usually primarily composed of calcium carbonate, which can take different crystalline forms, one being nacre otherwise known as mother of pearl.

Other kinds of animal shells are made from chitin, bone and cartilage, or silica.

Mollusks

Main article: Gastropod shell

Main article: Bivalve shell

Marine mollusks, traditional "seashells"

Main article: Seashell

The majority of shell-forming mollusks belong to two classes: Gastropoda (univalves, or snails) and Bivalvia (bivalves or clams, oysters, scallops, etc). Smaller shell-bearing classes include 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, cuttlefish and squid, especially Spirula spirula have small internal shells.

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.

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). The word conchology is also used to describe shell collecting as a hobby.

Fresh water mollusks

In fresh water shell-bearing mollusks are represented by families from the orders Unionoida (freshwater mussels) and Veneroida (clams, cockles and zebra mussels), as well as the class Gastropoda (snails).

Land mollusks

The class Gastropoda also includes many land snails, most of which are pulmonates and breathe air. Although the great majority of land snails are small and inconspicuous, the large and highly-colored shells of some tropical species are prized by collectors. In certain tropical islands such as Cuba, or Papua New Guinea, there are almost as many species of land snails as there are of marine. Land snails cannot disperse very easily, so populations frequently become isolated from each other, resulting in situations where adjacent islands, or even adjacent valleys separated by hills or mountains, contain closely-related but clearly separate species of land snails.

Shells in other animals

A large variety of other animal taxa form exoskeletons of calcium carbonate, chiton or silica, and these shells are used for protection, locomotion, defence, structure or in ways that relate to feeding.

Other sea creatures

The brachiopods, or lamp shells, superficially resemble clams, but the phylum is completely unrelated to molluscs. Most lines of brachiopods were ended during the Permian-Triassic extinction event, and their ecological niche filled by bivalves.

The construction of the shell-like structures of corals are aided by a symbiotic relationship with a class of algae, zooxanthellae. Typically a coral polyp will harbour particular species of algae, which will photosynthesise and thereby provide energy for the coral and aid in calcification,^[1] while living in a safe environment and using the carbon dioxide and nitrogenous waste produced by the polyp. Coral bleaching is a disruption of the balance between polyps and algae, and can lead to the breakdown and death of coral reefs.

Some echinoderms (starfish, sea urchins, sand dollars) and some polychaetes (annelid worms) also have hard exoskeletons. The now-extinct ostracoderms ("shell-skins") were a type of armoured marine fish which flourished in North America and Europe during the Ordovician, Silurian and Devonian geological periods.

Arthropods

Many arthropods have sclerites, or hardened body parts, which form a stiff exoskeleton made up mostly of chitin.

• In crustaceans, especially those of the class Malacostraca (crabs, shrimp and lobsters, for instance), the plates of the exoskeleton may be fused to form a more or less rigid carapace.
• The rigid part of an insect's exoskeleton is called the procuticle; when outgrown, this construct must be shed during moulting.
• Arachnids form a hard shell called peltidium.

Mammals

A few mammals have developed hard, shell-like armour. The shell of the armadillo is formed by plates of dermal bone covered in small, overlapping epidermal scutes. The pangolin or scaly anteater has armoured plates made up of hair, similar to the horn of the rhinoceros.

Echidna, spiny anteaters, porcupines and hedgehogs use spines of hardened keratin as a form of protection.

Reptiles

Turtles, tortoises and terrapins also form a hard carapace and plastron of bone and cartilage which is developed from their ribs.

A crocodile exoskeleton is formed of bony scutes and horn-like scales.

Dinosaurs

Ankylosauria and Stegosauria are among the dinosaurs which grew thick plate-like armour on their bodies

Planktons and protists

Plant-like diatoms and animal-like radiolarians are two forms of plankton which form hard silicate shells.

Foraminifera, coccolithophores and testate amoebae create shells called "tests" of calcium carbonate.

Shells as decorative items in human culture

Throughout the history of humanity, shells of many types and from many different kinds of animals have been popular as human adornments. They are often used whole and drilled so that they can be threaded like a bead, or cut into pieces of various shapes.

The uses have including all kinds of pendants, beads, buttons, brooches, rings, and hair combs among other uses.

The Moche culture of ancient Peru worshipped animals and the sea and often depicted shells in their art.^[2] Indigenous peoples of the Americas used shells for wampum and hair pipes.^[3]

Small pieces of colored and iridescent shell have been used to create mosaics and inlays, which have been used to decorate walls, or smaller items such as boxes and furniture. Large numbers of whole seashells, arranged to form patterns, have been used to decorate man-made grottos.

See also

• The Bailey-Matthews Shell Museum
• Seashell surface, a mathematical construct

References

External links

• 110 Photos of various seashells
• Conchologists of America