As it grows, the mollusc adds extra calcareous material onto the outside edge of the shell. Univalve mollusc shells usually grow as an enlarging spiral . The suture divides one whorl from another. Shell sculpture often consists of ribs, bumps or other ornate markings. Ribs may be axial or longitudinal , running parallel to the axis of the body from the anterior to the posterior .
Spiral ribs run around the whorls.
Other forms of sculpture may be denticles , which are bumps that look like teeth, or pustules that look like pimples. Some shells may have long spines, and other extensions, which may be a camouflage, or assist the mollusc in some other way. During their larval life stages, all gastropods go through a body-change process called torsion , or twisting. The mantle cavity moves from the rear position and comes to rest over the head, facing forward towards incoming water. So inside the mantle cavity are the gills , anus and reproductive glands , which lie directly over the head.