Animal shell Totally Explained

Everything about Animal Shell totally explained

A shell is a hard, rigid outer layer developed by a wide variety of different animal species, including mollusks, crustaceans, turtles and tortoises, armadillos, and microscopic organisms. Shells are used for protection, locomotion, defence, structure or in ways that relate to feeding. Scientific names for shell types include exoskeleton, armour, test, carapace, cuticle and peltidium.
The shells that are perhaps most familiar and most commonly encountered, both in the wild and as decorative objects, are seashells. 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

Mollusks (also spelled molluscs) can thrive in the sea, in fresh water, or on land.

Marine mollusks, traditional "seashells"

The majority of shell-forming marine mollusks belong to two classes: Gastropoda (univalves, or snails) and Bivalvia (bivalves, including clams, oysters, and scallops). Smaller shell-bearing classes include Scaphopoda (tusk shells), Polyplacophora (chitons, which have eight articulating shelly plates), and Monoplacophora (single-shelled chiton-like animals). Nautiluses are the only extant cephalopods which have an external shell, although octopuses, cuttlefish and squid 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. 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 sometimes used to describe the hobby of seashell collecting.

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 (which includes freshwater snails).

Terrestrial mollusks

The class Gastropoda also includes many land snails, most of which are of the order Pulmonata 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 can't 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, chitin or silica.

Other sea creatures

The brachiopods, or lamp shells, superficially resemble clams, but the phylum is completely unrelated to mollusks. Most lines of brachiopods were ended during the Permian-Triassic extinction event, and their ecological niche filled by bivalves. Corals are small anemone-like polyps which secrete aragonite (a form of calcium carbonate) to form a hard skeleton. The many skeletons in a colony aggregate to form coral reefs. The construction of the shell-like structures are aided by a symbiotic relationship with a class of algae, zooxanthellae.
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 a cuticle made up of sclerites, or hardened body parts, which form a stiff exoskeleton formed mostly of chitin. For mechanical strength, some crustaceans, myriapods and trilobites impregnate the cuticle with mineral salts, especially calcium carbonate, which can make up up to 40% of the cuticle. 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 (spiders, scorpions, harvestmen, ticks, and mites) have a peltidium made up of several plates which may or may not be fused. Myriapods (centipedes and millipedes) have overlapping scales of chitin, which are quite hard in some species.

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 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.
Protists such as foraminifera, coccolithophores and testate amoebae create shells called "tests" of calcium carbonate.

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