SHELL 



385 





the calcified areolar tissue. In the Mollnsca, in- 

 cluding the Brachiopoda, a shell, which is closely 

 associated with the breathing organ, is usually 

 present, and appears in the embryo as a growth 

 from a special gland. Its further development 

 depends on the mantle. It grows in superficial 

 extent at the edge of the mantle, and increases in 

 thickness by growth from the surface of the mantle 

 already covered. It is thus essentially a cuticular 

 structure. It may be univalve or bivalve : in only 

 one genus (Chiton) is it composed of several pieces. 

 A section of the shell of the fresh-water mussel 

 (fig. 2) shows (a) the thin, horny, uncalcified outer 

 layer which is formed by the thickened edge of 



Fig. 2. Shell of Fresh-water Mussel, magnified : 

 A, section at right angles to the long axis of the shell ; a, outer : 

 6, middle ; c, inner layers. B, end view of the prisms of the 

 middle layer. 



the mantle, and is the first to be deposited ; (6) 

 the middle layer, consisting of numerous calcified 

 polygonal prisms placed side by side, slightly 

 obliquely to the surface of the shell ; ( c ) the 

 nacreous or pearly inner layer, finely granulated 

 and traverseu by delicate strise. The calcium car- 

 bonate, to which the hardness of the shell is due, 

 may be dissolved away, leaving the organic basis. 

 In the Mollusca shells are distinguished according 

 to their texture as porcellanous, nacreous, and 

 fibrous. In composition they usually consist of car- 

 bonate of lime (in the form of calcite, less com- 

 monly of aragonite). Some Cephalopods develop 

 an internal calcareous shell. In Arthropoda ( Crus- 

 taceans, Insects, &c.) the shell is composed of a 

 firm, coloured, lamellated, nearly structureless 

 layer of chitin, with or without an interstitial 

 calcareous deposit, pierced 

 by many pores and by pro- 

 cesses dt the skin, and sned 

 and renewed periodically. 

 A section of the hard shell 

 of a crayfish (fig. 3) shows 

 (<t) an outer structureless 

 resistant cuticle ; ( 6 ) a 

 pigmented layer consisting 

 of fine plates parallel to 

 the surface, alternately 

 more or less retractile, and 

 traversed by small vertical 

 pores; (c) a thick non- 

 pigmented calcified layer, 

 whose pores are continuous 

 with those of the second 

 layer, and whose lamellre 

 gradually merge into (d) a 

 thin layer of delicate non- 

 calcified lamellae with a 

 The organic substratum is 

 calcareous matter is chiefly 

 and to a much less extent 



Fig. 3. 



A, part of transverse section 

 of an appendage of a cray- 

 fish, magnified ; a, 6, c, d, 

 the various layers. B, 

 tangential section of the 

 same, showing the pore 

 canals represented by 

 small dots. 



few vertical pores, 

 chitinoid, and the 

 calcium carbonate, 

 calcium phosphate. 

 14] 



For an account of the structure of shells and the 

 literature on the subject, see Rolleston's Forms of Ani- 

 mal Life (2d ed.) ; Carpenter's Microscope and its 

 Revelation*; and Quekett's Lectures on Histology 

 (voL ii.), in which there are many excellent drawings. 

 See BIVALVES, CONCHOLOGY, EGG, FOHAMINIFEBA, IBI- 

 DESCENCE, MOLLUSCA, MOTHEK OP PEAKL, las. 



Shell, a hollow projectile containing a burst- 

 ing charge of gunpowder or other explosive ignited 

 at the required instant by means of either time or 

 percussion fuzes (see FUZE). Originally shells 

 were made of cast-iron, spherical in form, and fired 

 only from mortars (see MORTAR) or Howitzers (q.v.) 

 with time fuzes. They were called bombs at first 

 (see BOMB), and after\vards common shells. A 

 smaller kind were thrown by hand and called 

 hand-grenades. Shells appear to have been first 

 used by the sultan of Gujarat in 1480 ; they were 

 in general use about the middle of the 17th century. 



But the introduction of Shrapnel shell (so called 

 from the inventor, Colonel Henry Shrapnel, It. A. ; 

 died 1842) during the Peninsular 

 war led to the employment of this 

 projectile first with specially manu- 

 factured shell-guns, and then, as at 

 present, with all guns of whatever 

 construction. Shrapnel shellt have 

 thinner walls than common, and, 

 instead of powder, are filled with 

 bullets and a small bursting charge 

 just strong enough to open them 

 without disturbing the flight of the 

 bullets. The latter then spread 

 over a considerable area with the 

 velocity which the shell had at the 

 moment of bursting. These projec- 

 tiles are generally burst by time 

 fuzes at least 100 yards in front of 

 and some 50 feet alx>ve the target, 

 so that what is called the cone of 

 dispersion of the bullets may be as Fig. 1. 

 favourably placed as possible. They Shrapnel Shell, 

 are essentially man-Killing projec- 

 tiles, in contradistinction to common shells, which 

 are chiefly useful for destroying materiel. 



The original Shrapnel shell was of course spher- 

 ical for use with smooth-bore guns. It was im- 

 proved by the introduction or a diaphragm to 

 separate the bursting charge from 

 the bullets. When rifled guns and 

 elongated projectiles came into 

 vogue it was found that the ricochet 

 of a solid shot was so erratic that 

 it could not, like the old round 

 shot, have any useful effect, and 

 solid shot gradually ceased to be 

 used, Shrapnel and common shell 

 and case-snot being the artillery 

 projectiles retained. 



The Armstrong Segment shell (a 

 sort of Shrapnel) is tilled with iron 

 segments built up inside it instead 

 of bullets. It was found to give 

 poor results owing to the spin of 

 the shell due to the rifling dispers- 

 ing the segments on bursting. The 

 German and Swiss Ring shell is 

 somewhat simitar, having rings of 

 iron built up inside it round the 

 bursting charge ; but it is a sub- 

 stitute for common shell, not for 

 Shrapnel. 



Until quite recently Shrapnel Fig. 2. 

 shell found no favour with foreign Common Shell, 

 nations, but by 1892 they had all 

 followed the example set by Great Britain in 

 adopting it. Fig. 1 is a section of modern British 

 steel Shrapnel, with soft metal driving-baud at 



