SHELL, in zoology, the hard calcareous substance which either protects the testa ceous mollusca externally, or supports certain species of them internally.
Though shells, properly so called, which form the habitation of testaceous animals are sometimes confounded with the shelly coverings which protect the crustacea (crabs and their numerous allies), a very obvious and striking difference exists be tween them, as well as between the kinds of animals which respectively inhabit them. The shells of testacea are com posed of carbonate of lime, combined with a small portion of gelatinous mat ter; they are, in general, permanent cov erings for their inhabitants; and the animal is of a soft substance, without bones of any kind, and attached to its domicile by a certain adhesive property. On the other hand, those animals which are defended by a crustaceous covering cast their shells and renew them annu ally; while the animals themselves are of a fibrous texture, with articulated limbs, and protected, as it were, by a coat of mail.
Shells are divided into Multivalves, Bi valves, and Univalves. The first order, Multivalve, is made up of shells consisting of more shelly parts or pieces than two. The second order, Bivalve is made up of shells having two parts or valves, gen erally connected by cartilage or hinge; as in the cockle and mussel. The third order, Univalve, is made up of shells complete in one piece—as in the peri winkle and the whelk—and they are sub divided into shells with a regular spire, and those without a spire. The shells composing this order are far more nu merous than those of the two preceding, both in genera and species. The spire is a prominent feature of the Univalve; and on its being lengthened or elevated, shortened or depressed, etc., depends much of the generic and specific definitions.
Shells increase in size by the deposition of new layers internally on those already formed. Each new layer extends more or less beyond the margin of the layer to which it is applied, so that as the ani mal becomes older its shell becomes larger and thicker. The outer surface is gen erally covered by a thin layer of mem branous or horny matter, named the epi dermis, and the inner surface is often covered with a layer of a pearly nature.
In military usage the name shell is given to a hollow vessel of metal contain ing gunpowder, or other explosive com pound, so arranged that it will explode at a certain point and spread destruction around by the forcible dispersion of its fragments. The invention of this missile, formerly called a BOMB (q. v.), cannot be accurately traced. Shells were em ployed in A. D. 1480 by the Sultan of Gujerat, and by the Turks at the siege of Rhodes in 1522. The Spaniards and Dutch both used them during the war of Dutch independence; and they appear to have been generally adopted by about 1634. Formerly every shell was a hollow sphere of cast iron, having a fusehole an inch across, through which the charge was inserted, consisting of pieces of metal and powder to burst the shell. The hole was plugged by a fuse, timed to communi cate fire to the charge after the lapse of a certain number of seconds. Since the introduction of rifled ordnance, the shell has become the commonest form of projectile. It has ceased to be spherical, and is usually of cylindrical form and pointed at the end. In the World War vast quantities of large explosive shells were used to blast the way for attacks. See PROJECTILE; ARTILLERY; AMMUNI TION; EXPLOSIVES.