Gastropod shells are usually formed in a spirally coiled shape and are mostly made of a mineral called calcium carbonate. The shell is structured with a series of whorls that wind around a central column called the columella. The final and largest whorl, the body whorl, holds the internal organs and most of the gastropod’s soft tissues. The smaller whorls above this large spiral together form the spire.  At the tip of the shell’s spire there is a point called the apex which includes the protoconch, the earliest shell formed during the larval stage. The opening where the gastropod is able to come out of its shell is called an aperture and the shape of this opening can vary markedly between gastropod groups. The aperture is surrounded by an outer lip and a second inner lip which goes along the edge of the columella.The outside of the shells of gastropods can be covered in a lot of different types of ornamental and protective structures including ridges, grooves and even sharp spines. They also usually show a pattern of growth lines which mark the changes in the shape of the shell over time and give the snail an extra bit of structural strength. In many species the aperture of the shell is covered in a piece of hardened tissue called an operculum.