The shell is usually composed of the mineral Aragonite. The shell walls can be simplified into three layers called, from outside to inside: Periostracum, Ostracum and Hypostracum.
- Periostracum: a thin organic layer which forms the outside of the Gastropod shell and protects it from dissolution
- Ostracum: layer of calcium carbonate prisms orientated vertically to the surface held together by a proteine. It forms inbetween the fresh Periostracum and the mantle.
- Hypostracum: thin, inner-most layer of horizontal calcium carbonate platelets covered by Nacre on the most inner layer.
The common features of a shell are:
- Apex, the tip of the shell.
- Columella, a spiralised cone in the centre.
- Aperture, the only opening of the shell which may have different shapes.
- Siphonal channel, an extension of the Aperture
- Suture, the junction of whorls
- Protoconch: the larval shell
- Teleconch: the adult shell
- endogastritic: the spiral points away from the head
- exogastritic: the spiral points towards the head
- Torsion: rotation of the visceral mass, mantle and shell by 180° with respect to the head that brings the anus and mantle cavitiy above the head. Detorsion in some Gastropod species towards the side or complete detorsion.
- Spirality: spiralling of the upper part of the visceral mass usually to the right
An important feature in the classification of recent Gastropods is the Radula. There are several types of Radulas. It exists a trend in the development of the Radula to reduce the number of teeth. Few teeth can be an indication of a carnivor way of life. The most extreme case maybe being Conus (Danger: highly toxic!) with only a single tooth that is used to poison the prey. There are central teeth (Rhachis-tooth), teeth on the sides (Admedian-teeth) and marginal teeth.
- rhipidogloss: most primitive type with most teeth
- docogloss: fewer teeth, no marginal teeth
- taeniogloss: few teeth per row
- ptenogloss: numerous identical teeth
- rhachigloss: mostly only central teeth and seldom marginal tooth
- toxogloss: one tooth in the centre with hooks. Toxic!