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in zoology, hard outer covering secreted by an animal for protection. It is also called the test, crust, or carapace. The term usually refers to the calcareous shells of the many species of mollusk but is also applied to the exoskeleton of the crab and other crustaceans, to the bony covering of the turtle, and to the hard exterior of a bird's egg. People have made use of mollusk shells since early times as receptacles for food and water, as currency (see shell moneyshell money,
medium of exchange consisting of shells, the most widely distributed type of ancient currency. Shells are particularly useful as money because they may be strung in long strips of proportionate value or they may be used to provide a single unit value in exchange.
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), and for ornament. The scientific study of shells is called conchology.


See P. A. Morris, A Field Guide to the Shells (of the Atlantic coast, 1973; the Pacific, 1974); J. M. Eisenberg, A Collector's Guide to Seashells of the World (1980); The Audubon Society Field Guide to North American Shells (1981); S. D. Romashko, The Shell Book (1984); K. R. Wye, The Simon & Schuster Pocket Guide to Shells of the World (1989); M. G. Harasewych and F. Moretzsohn, The Book of Shells (2010).


The basic structure and enclosure of a building, exclusive of any interior finishes and mechanical, plumbing, and electrical systems; often describes a deteriorated structure that has lost much of its original fabric.



a cylindrical or conical drum with open ends that is fabricated for the manufacture of steam boilers, tanks, reservoirs, and similar items made from sheet metal. Shells are obtained by rolling (for sheet thicknesses up to 40 mm) and by bending and rolling (for greater thicknesses). The joint of the shell and the bottom is closed for welding or riveting by means of shrink rings or jigs.



in engineering and elasticity theory, a solid bounded by two curvilinear surfaces the distance between which is small in comparison with the other two dimensions. The surface that bisects the shell thickness is called the middle surface. Depending on the shape of the middle surface, distinctions are made between various types of shells, such as conical or toroidal shells and cylindrical shells with circular, elliptical, or other cross sections. Shells are also classified according to the total, or Gaussian, curvature of the surface. Thus spherical, ellipsoidal, and certain other shells have positive curvature; cylindrical and conical shells have zero curvature; and hyperbolic paraboloids have negative curvature. Shells may be of constant or variable thickness. They are subdivided into single-layer, double-layer, and multilayer types. Depending on the material from which they are constructed, shells may be isotropic or anisotropic. Shells are made of reinforced concrete, steel, wood, light alloys, plastics, and other building materials.

The action of external loads on shells induces internal forces that are uniformly distributed over the thickness of the shell and are called membrane stresses, or stresses in the middle surface. External loads also induce flexural forces that form transverse forces, bending moments, and torsional moments in cross sections of the shell. Because of the presence of membrane forces, shells combine considerable rigidity and strength with comparatively low weight, differing from plates in this regard. If bending stresses can be ignored in design, the shell is said to be a zero-torque shell. The presence of torsional stresses is characteristic of regions of a shell that border on the edges; this tendency is known as the edge effect.

If the stresses are within the limits of proportionality for the shell material, the methods used to design the shell are based on functions provided by the theory of elasticity. According to the Kirchhoff-Love hypothesis, which is used most often for thin shells, any straight fiber normal to the middle surface prior to deformation remains straight and normal to the surface after deformation, and the length of the fiber remains unchanged. Moreover, it is assumed that normal stresses in a direction perpendicular to the middle surface are negligibly small in comparison with the primary stresses.

Under these conditions, the general three-dimensional problem of the theory of elasticity becomes a two-dimensional problem. This problem is solved basically by integration of a system of higher-order partial differential equations under boundary conditions determined by the character of the contact between the shell and other parts of the structure. Stresses, deformations, and displacements of various points on the shell in relation to a given load must be determined in the static design of shells for strength and rigidity. In calculations of strength, deflections of the shell along the normal to the middle surface generally may be assumed to be small in comparison with the thickness of the shell. Given this assumption, the relations between these deflections, or displacements, and deformations are linear. Hence, with regard to the theory of elasticity, the main differential equations will be linear also.

Shells must often be reinforced with ribs, mainly to ensure stability. Examples include airplane fuselages and wings and some types of thin-walled coverings.

Stability is an important consideration in shell design. A peculiarity of thin-walled shells is that the loss of stability upon a denting impact is expressed as an abrupt transition from one stable state of equilibrium to another. The critical loads that effect this transition depend on such factors as initial imperfections in the shape of the shell and initial stresses. In the case of a jolting impact, the deflections turn out to be commensurate with shell thickness; analysis of shell behavior must in this case be based on nonlinear equations.

Periodic vibrations and transient processes associated with sudden or shock loads are considered under the heading of shell dynamics. When a stream of liquid or gas flows around a shell, an unstable self-oscillating condition may occur. (The determination of this condition is the object of hydroelasticity or aero-elasticity theory.) The study of nonlinear vibrations of shells is a branch of the theory of vibrations with important applications. When considering dynamic processes of shells, relationships based on the Kirchhoff-Love hypothesis do not always turn out to be applicable; in such cases, more complex differential equations are employed.

Shell construction is widely used for building-enclosures, aircraft, ships, all-metal railroad cars, television towers, and machine parts.


Ambartsumian, S. A. Teoriia anizotropnykh obolochek. Moscow, 1961.
Bolotin, V. V. Dinamicheskaia ustoichivost’ uprugikh sistem. Moscow, 1956.
Vlasov, V. Z. Obshchaia teoriia obolochek i ee primeneniia v tekhnike. Moscow-Leningrad, 1949.
Vol’mir, A. S. Gibkie plastinki i obolochki. Moscow, 1956.
Vol’mir, A. S. Nelineinaia dinamika plastinok i obolochek. Moscow, 1972.
Gol’denveizer, A. L. Teoriia uprugikh tonkikh obolochek. Moscow, 1953.
Lur’e, A. I. Statika tonkostennykh uprugikh obolochek. Moscow-Leningrad, 1947.
Mushtari, Kh. M., and K. Z. Galimov. Nelineinaia teoriia uprugikh obolochek. Kazan’, 1957.
Novozhilov, V. V. Teoriia tonkikh obolochek. Leningrad, 1951.
Chernykh, K. F. Lineinaia teoriia obolochek, parts 1–2. Leningrad, 1962–1964.




a hard protective formation that completely or partially covers the bodies of certain animals. In invertebrates, the shell originates from a thickened chitinous covering or from calcareous plates.

A chitinous shell is characteristic of certain insects (for example, Coleoptera) and lower crustaceans (for example, Notos-traca). In higher crustaceans, such as crabs, the chitinous shell is impregnated with limestone salts, which give it great toughness. Echinoderms have a shell of calcareous plates; in sea urchins the plates of the shell are closely joined. Among vertebrates, representatives of all classes—except birds—have shells. The bony shell of fossil Agnatha was especially well developed. Among extant fishes, an armor of rhomboid ganoid scales covers the bodies of the Lepidosteiformes (Holostei) and the Polypterus. An armor of bony plates protects the Loricariidae, the Ostraciontoidei, the great pipefish, and seahorses.

Among amphibians, a ventral shell was developed only in fossil Stegocephalus. Representatives of a number of extinct reptile groups had bony armor. Turtles have well-developed shells consisting of two concresced bony plates—a dorsal and a ventral —covered on the outside with horny scales. The shells of crocodiles, which are made of large bony plates covered on the outside with horny plates, are also well developed. Among mammals, the shell of the extinct glyptodonts consisted of a bony dorsal plate. A shell is characteristic of armadillos and African pangolins. In the former, it consists of separate movable and articulated bony plates, and in the latter of large, horny scales placed one on top of the other.



an external protective skeletal formation that covers the body of many invertebrates. The shell usually is loosely attached to the body and has an opening through which the animal can partially protrude to the outside. It consists of organic matter, often with an admixture of calcium carbonate, encrusted sand grains, diatomaceous carapaces, and sponge needles.

Shells characterize a number of protozoans, most mollusks, and some arthropods and brachiopods. The shells of Thecamoebina consist of a chitinous or gelatinous substance and are often attached by grains of sand and other particles previously swallowed by the animal. In most testaceous flagellates, the shell is formed from several cellulosic plates. Foraminiferan shells are most often impregnated with calcium carbonate and sometimes are encrusted with sand grains; they are rarely formed totally from organic matter. The shells, which are one-or multi-chambered, range in length from 50 microns to several centimeters.

In mollusks, the shell is secreted by a special skin fold—the mantle—and usually consists of three layers. The outer layer, the periostracum, contains the organic substance conchiolin. The innermost, or nacreous, layer, is made up of small prisms of arragonite, joined together by conchiolin, that are located angularly to the surface of the shell. The middle, or prismatic, layer consists of thin plates of arragonite that are also attached each to the other by conchiolin.

Mollusk shells are extremely diverse in size and shape. The shell of the marine bivalve mollusk Tridacna weighs up to 25 kg and reaches a length of 1.7 m. The shell of loricate mollusks consists of eight dorsal overlapping plates. In gastropods the shell resembles a conical tube and is usually wound in a spiral. Bivalve shells consist of two valves connected to each other on the dorsal side by an elastic cord (ligament) and a hinge (cardo). In some cephalopods the shell is spirally coiled and has many chambers (nautilus, fossil ammonites). The shell of some extant cephalopods is internal, since it lies under the skin of the back (cuttlefish and squid). In octopuses and some representatives of other classes of mollusks, the shell is reduced.

The shell of brachiopods consists of two valves—a dorsal one and a ventral one (unlike the right and left valves of mollusks). Ostracod shells have two lateral valves. The valves of cirripede crustaceans resemble truncated cones and are formed of several scales secreted by the mantle.

Cutting tools, scrapers, mattocks, fishhooks, musical instruments, and various ornaments have been crafted from shells. Shells have also been used as vessels, and in some countries they have served as coins (for example, the cowrie’s shell) and amulets. Mother-of-pearl, which is used in the manufacture of buttons, inlays, and other products, is obtained from shells.

Accumulations of shells have formed many sedimentary rocks. For example, the shells of crustaceans form fusulinid and nummulitic limestones, and mollusk shells form coquina and pteropod ooze.




a racing boat used in rowing competition and featuring extremely lightweight construction. The lines are rounded, with a length-to-width ratio of 25:1 to 35:1. The skin is made of polished laminated materials, such as veneers of expensive woods, plastics, and other materials. Shells are equipped with outriggers, sliding seats, and footrests.

What does it mean when you dream about a shell?

Shells may represent the womb and the desire to be once again sheltered, nourished, and protected from life’s problems.


A reinforced concrete arched or domed roof used over unpartitioned areas.
(building construction)
A building without internal partitions or furnishings.
(computer science)
A program that provides an interface between a user and the computer's operating system by reading commands and sending them to the operating system for execution.
(design engineering)
The case of a pulley block.
A thin hollow cylinder.
A hollow hemispherical structure.
The outer wall of a vessel or tank.
The crust of the earth.
A thin hard layer of rock.
(graphic arts)
An engraved roller made of copper and used in calico printing.
The outer wall of a metal mold.
The hard layer of sand and thermosetting plastic formed over a pattern and used as a mold wall in shell molding.
The metal sleeve remaining when a billet is extruded with a dummy block at smaller diameter.
A tubular casting used in preparing seamless drawn tubes.
A pierced forging.
A hollow metal projectile designed to be projected from a gun, containing or intended to contain a high-explosive, chemical, atomic, or other charge.
A shotgun cartridge or a cartridge for artillery of small arms.
A set of energy levels with approximately the same energy in an atom or nucleus.
A hard, usually calcareous, outer covering on an animal body, as of bivalves and turtles.
The hard covering of an egg.
Chitinous exoskeleton of certain arthropods.


1. A hollow structure in the form of a thin, curved slab or plate whose thickness is small compared with its other dimensions and with its radii of curvature.
2. Any framework or exterior structure which is regarded as not completed or filled in.
3. An ornament similar in design to a seashell.


1. the protective calcareous or membranous outer layer of an egg, esp a bird's egg
2. the hard outer covering of many molluscs that is secreted by the mantle
3. any other hard outer layer, such as the exoskeleton of many arthropods
4. the hard outer layer of some fruits, esp of nuts
5. Rowing a very light narrow racing boat
6. the basic structural case of something, such as a machine, vehicle, etc.
7. Physics
a. a class of electron orbits in an atom in which the electrons have the same principal quantum number and orbital angular momentum quantum number and differences in their energy are small compared with differences in energy between shells
b. an analogous energy state of nucleons in certain theories (shell models) of the structure of the atomic nucleus
8. Brit (in some schools) a class or form


An early system on the Datatron 200 series.

[Listed in CACM 2(5):16 (May 1959)].


(operating system)
(Originally from Multics, widely propagated via Unix) The command interpreter used to pass commands to an operating system; so called because it is the part of the operating system that interfaces with the outside world.

The commonest Unix shells are the c shell (csh) and the Bourne shell (sh).


(Or "wrapper") Any interface program that mediates access to a special resource or server for convenience, efficiency, or security reasons; for this meaning, the usage is usually "a shell around" whatever.


The outer layer of an operating system, otherwise known as the user interface. The term originally referred to the software that processed the commands typed into the Unix operating system. For example, the Bourne shell was the original Unix command line processor, and C shell and Korn shell were developed later. The default command line interface in DOS was provided by the COMMAND.COM module, which, starting with Windows 95 was superseded by CMD.EXE. See command line and user interface.

Later, the term was applied to graphical user interfaces (GUIs). For example, the default shell in Windows is Explorer, which provides the Start menu, taskbar and desktop, but third-party choices are also available (see skin). DOS also had an alternative to the command line (see DOS Shell). See Explorer, PowerShell, Bourne shell, C shell and Korn shell.