shell

shell, 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 money), and for ornament. The scientific study of shells is called conchology.

Bibliography

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).

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shell

Artillery projectile, cartridge case, or shotgun cartridge. It originated in the 15th century as a container for metal or stone shot, dispersed when the container burst after leaving the gun. Explosive shells, in use by the 16th century, were hollow cast-iron balls filled with gunpowder and lit by a fuse. Until the 18th century, such shells were used only in high-angle fire (including mortars). In the 19th century, shells were adopted for direct-fire artillery, notably in the form of shrapnel. Modern artillery shells consist of a casing (usually steel), a propelling charge, and a bursting charge; the propelling charge is ignited by a primer at the base of the shell and the bursting charge by a fuse in the nose. In rifle, pistol, and machine-gun ammunition, the word usually signifies the brass casing that contains the propulsive charge. In shotgun ammunition, the shell is the entire cartridge, including shot, powder, primer, and case.

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The outer layer of a program that provides the user interface, or way of commanding the computer. It typically applies to operating systems. In Unix, the Bourne shell was the original command processor, which executes typed-in commands. C shell and Korn shell were developed later. In DOS, the default shell was COMMAND.COM, which also required commands to be typed in. The term often refers only to command line processors in contrast to a graphical interface.

In Windows, the default shell is Explorer, which provides the Start menu, Taskbar and desktop. Alternative Windows shells are also available. See Unix, Explorer and DOS Shell.

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1.	(language)	SHELL - An early system on the Datatron 200 series. [Listed in CACM 2(5):16 (May 1959)].
2.	(operating system)	shell - (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).
3.		shell - (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.