We have now to consider the large group of head-bearing molluscs (_Cephalophora_), the study of which forms a very important part of the work of the sea-side naturalist; and while we deal with the general characteristics of this group, the reader will do well to have before him a few living typical species in order that he may be able to verify as many as possible of the descriptions here given by actual observation. These types may include such creatures as the whelk, periwinkle, and limpet; or if marine species are not at hand at the time, the garden snail, fresh-water snail, and slug will serve the purpose fairly well. 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_. [Illustration: 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.