By far the large majority of Cephalopods are enclosed in a single shell, though a few have a rudimentary shell or none at all.
As is the case with the lamellibranchs, the shell is composed of both animal and mineral substance, the latter being a calcareous deposit secreted by the mantle of the animal. The shell is usually spiral in form, as in the whelk, but sometimes conical (limpet) or tubular.
Spiral shells are nearly always dextral or right-handed; that is, if we trace the direction of the spiral from the apex to the mouth, we find that its turns or whorls run in the same direction as the hands of a watch. A few, however, are sinistral, or left-handed, and occasionally we meet with left-handed varieties of those species that are normally of the right-handed type. The cavity of the shell is a single spiral chamber which winds round a central pillar, and each whorl of the shell generally overlaps the preceding one, the two being separated externally by a spiral depression called the suture.
Sometimes the coils of a shell are not close together internally, so that the central column of the spiral is hollow, and opens to the exterior at the base of the shell. In this case the shell is said to be umbilicated, and the opening referred to is the umbilicus. In others the spiral winds round a solid central pillar which is spoken of as the columella.
Fig. 156.—Section of the Shell of the Whelk, showing the Columella
The apex of the shell, sometimes called the nucleus, is the oldest part, and represents what was once the whole. It is generally directed backwards as the animal crawls, and in adult shells is often more or less worn away by constant friction. We speak of the whorls as first, second, third, &c., taking them in the order of their growth, and it will generally be found that the last whorl is much larger than the others, so much so that it contains the greater part of the body of the animal; hence this one is commonly spoken of as the body-whorl, and the others make up the spire of the shell.
The mouth of the shell is of different forms in different species, but in the herbivorous kinds it is usually simple, while in the carnivorous species it is notched or produced. The edge of the mouth (peristome) is formed by an outer lip which is usually sharp in young shells and either thickened, reflected (turned outward), or inflected (turned inward) in adults; also it may be considerably expanded, or ornamented by a fringed margin. The inner lip is that side of the peristome adjacent to the central pillar of the shell.
If we examine the external surface of several different shells, we find that they are usually more or less distinctly furrowed or sculptured, and that they are often marked by lines or bands of a colour different from that of the ground tint. These furrows, lines, or bands sometimes pass directly from the apex, across the various whorls, to the base of the shell, in which case they are said to be longitudinal. If they follow the course of the whorls, they are described as spiral; and if parallel with the peristome, so that they mark the former positions of the mouth of the shell, thus denoting the lines of growth, they are said to be transverse.