The shell of the Gasteropods is composed either of a single piece (univalve), or of a number of plates succeeding one another from before backwards (mul-tivalve). The univalve shell is to be regarded as essentially a cone, the apex of which is more or less oblique. In the simplest form of the shell, the conical shape is retained without any alteration, as is seen in the common Limpet (Patella). In the great majority of cases, however, the cone is considerably elongated, so as to form a tube, which may retain this shape (as in Dentalium), but is usually coiled up into a spiral. The "spiral univalve" (figs. 216, 217) may, in fact, be looked upon as the typical form of the shell in the Gasteropoda. In some cases the coils of the shell - termed technically the "whorls" - are hardly in contact with one another (as in Vermetus). More commonly the whorls are in contact, and are so amalgamated that the inner side of each convolution is formed by the pre-existing whorl. In some cases the whorls of the shell are coiled round a central axis in the same plane, when the shell is said to be "discoidal" (as in the common freshwater shell Planorbis). In most cases, however, the whorls are wound round an axis in an oblique manner, a true spiral being formed, and the shell becoming "turreted," "trochoid," "turbinated," etc. This last form is the one which may be looked upon as most characteristic of the Gasteropods, the shell being composed of a number of whorls passing obliquely round a central axis or "columella," having the embryonic shell or "nucleus" at its apex, and having the mouth or "aperture" of the shell placed at the extremity of the last and largest of the whorls, termed the "body-whorl" (fig. 216). The lines or grooves formed by the junction of the whorls are termed the "sutures," and the whorls above the body-whorl constitute the "spire" of the shell. The axis of the shell (columella) round which the whorls are coiled is usually solid, when the shell is said to be "imperforate;" but it is sometimes hollow, when the shell is said to be "perforated," and the aperture of the axis near the mouth of the shell is called the "umbilicus." The margin of the "aperture" of the shell is termed the "peristome," or "peritreme," and is composed of an outer and inner lip, of which the former is often expanded or fringed with spines. When these expansions or fringes are periodically formed, the place of the mouth of the shell at different stages of its growth is marked by ridges or rows of spines, which cross the whorls, and are called "varices." In most of the phytophagous Gasteropods (Holostomatd) the aperture of the shell (fig. 217) is unbrokenly round or "entire," but in the carnivorous forms (Siphonostomatd) it is notched, or produced into a canal (fig. 218). Often there are two of these canals, an anterior and a posterior, but they do not necessarily indicate the nature of the food, as their function is to protect the respiratory siphons. The animal withdraws into its shell by a retractor muscle, which passes into the foot, or is attached to the operculum; its scar or impression being placed, in the spiral univalves, upon the columella.
Fig. 215. - A, Young of AEolis, a water-breathing Gasteropod, showing the provisional buccal lobes or "velum." B, Adult Pteropod (Limacina antarctica). (After Woodward).
Fig. 216. - Anterior and posterior views of Cassis cancellata, a spiral Gasteropod. a " Spire," placed at the posterior end of the shell; b " Mouth," placed at the anterior end of the shell; c Inner or columellar lip; d Outer lip ; e Notch for the passage of a respiratory siphon.
In the multivalve Gasteropods, the shell is composed of eight transverse imbricated plates, which succeed one another from before backwards, and are embedded in the leathery or fibrous border of the mantle, which may be plain, or may be beset with bristles, spines or scales.