<P> The apertural end of the gastropod shell is the anterior end, nearest to the head of the animal; the apex of the spire is often the posterior end or at least is the dorsal side . Most authors figure the shells with the apex of the spire uppermost . In life, when the soft parts of these snail are retracted, in some groups the aperture of the shell is closed by using a horny or calcareous operculum, a door - like structure which is secreted by, and attached to, the upper surface of the posterior part of the foot . The operculum is of very variable form in the different groups of snails that possess one . </P> <P> The terminology used to describe the shells of gastropods includes: </P> <Ul> <Li> Aperture: the opening of the shell </Li> <Li> Lip = peristome: the margin of the aperture </Li> <Li> Apex: the smallest few whorls of the shell </Li> <Li> Body whorl (or last whorl): the largest whorl in which the main part of the visceral mass of the mollusk is found </Li> <Li> Columella: the "little column" at the axis of revolution of the shell </Li> <Li> Operculum: the "trapdoor" of the shell </Li> <Li> Parietal callus: a ridge on the inner lip of the aperture in certain gastropods </Li> <Li> Periostracum: a thin layer of organic "skin" which forms the outer layer of the shell of many species </Li> <Li> Peristome: the part of the shell that is right around the aperture, also known as the lip </Li> <Li> Plait: folds on the columella . </Li> <Li> Protoconch: the nuclear or embryonic whorl; the larval shell, often remains in position even on an adult shell </Li> <Li> Sculpture: ornamentation on the outer surface of a shell <Ul> <Li> Lira: raised lines or ridges on the surface of the shell </Li> </Ul> </Li> <Li> Siphonal canal: an extension of the aperture in certain gastropods </Li> <Li> Spire: the part of the shell above the body whorl . </Li> <Li> Suture: The junction between whorls of most gastropods </Li> <Li> Teleoconch: the entire shell without the protoconch; the postnuclear whorls . </Li> <Li> Umbilicus: in shells where the whorls move apart as they grow, on the underside of the shell there is a deep depression reaching up towards the spire; this is the umbilicus </Li> <Li> Varix: on some mollusk shells, spaced raised and thickened vertical ribs mark the end of a period of rapid growth; these are varices </Li> <Li> Whorl: each one of the complete rotations of the shell spiral </Li> </Ul> <Li> Aperture: the opening of the shell </Li>

The opening to the snail shell is known as the