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. Parts of the shell The terminology used to describe the shells of gastropods includes: Aperture: the opening of the shell Lip: peristome: the margin of the aperture Apex: the smallest few whorls of the shell Body whorl (or last whorl): the largest whorl in which the main part of the visceral mass of the mollusk is found Columella: the "little column" at the axis of revolution of the shell Operculum: the "trapdoor" of the shell Parietal callus: a ridge on the inner lip of the aperture in certain gastropods Periostracum: a thin layer of organic "skin" which forms the outer layer of the shell of many species Peristome: the part of the shell that is right around the aperture, also known as the lip Plait: folds on the columella. Protoconch: the nuclear or embryonic whorl; the larval shell, often remains in position even on an adult shell Sculpture: ornamentation on the outer surface of a shell Lira: raised lines or ridges on the surface of the shell Siphonal canal: an extension of the aperture in certain gastropods Spire: the part of the shell above the body whorl. Suture: The junction between whorls of most gastropods Teleoconch : the entire shell without the protoconch; the postnuclear whorls. 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 Varix: on some mollusk shells, spaced raised and thickened vertical ribs mark the end of a period of rapid growth; these are varices Whorl: each one of the complete rotations of the shell spiral Shape of the shell The overall shape of the shell varies. For example, three groups can be distinguished based on the height – width ratio: oblong – the height is much bigger than the width globose or conical shell – the height and the width of the shell are approximately the same depressed – the width is much bigger than the height The following are the principal modifications of form in the gastropod shell. Regularly spiral: Bulloid: