The part upon which the spire turns is called the pillar. It is sometimes solid, but sometimes it is hollow; in the latter case the perforation is named the umbilicus. When the spire is long, the shell is said to be turbinated, which is the common form; but in some genera, as Planorbis, the convolution takes place in the same plane, and the shell is flat, or even concave. Such shells are termed discoid. When the upper part of each turn or whorl envelopes or covers that which preceded it, the spire is said to be concealed.
In almost all species the convolution is towards the right side. There are a few, however, which turn to the left; these shells are termed reversed. The end of the latest whorl, where the animal protrudes, is termed the mouth or aperture. In order to close this, when the animal withdraws itself into its shell, the hinder part of the foot is usually furnished with a horny or shelly plate, called the operculum, which, when the animal contracts, is brought into such a situation, as more or less completely to close the mouth of the shell, when the animal is drawn into its cavity. It has hitherto been observed only in those Mollusca which have pectinate branchiae, and in two genera, Cyclostoma and Helicina, among the air-breathing land-shells. The form of the operculum is in general either that of a very low cone, made by successive layers, each one a little larger than its predecessor, or that of a flattened spire, and the texture is either horny or shelly.The species of Gasteropoda are very numerous, and are arranged in five orders, viz.—Pulmonifera, Nudibranchiata, Tectibranchiata, Cyclobranchiata, and Pectinibranchiata.