Gastropods have coiled shells. The shell is wrapped around a central axis, termed the columella, forming whorls. Each whorl is a complete revolution of the shell around the columella. Whorls are in contact with successive whorls along a groove or indentation called a suture. The whorl ending in the aperture (opening where the snail’s mantle and foot emerged) is the body whorl, also called the last whorl. Whorls above the body whorl combine to form the spire. Spires in gastropods are quite variable. Some gastropod shells have a tall (high) spire composed of many whorls, while others, lack a prominent spire, and consist of only a large body whorl.
Most modern and fossil snail shells are dextrally coiled (right coiled), which means the aperture is on the right side of the axis of the shell when looking at the aperture. Rare left coiling is termed sinistral coiling. Most gastropods are coiled out of a single plane, termed conispiral. Coiling in the same plane is termed planispiral. Planispiral coiling also occurs in nautiloid cephalopods.
The shape of whorls, number of whorls, angle of coiling, and curvature or convexity of the outer surface of whorls are all variables that add to the diversity of gastropod shell shapes, and are all used in the description and classification of gastropods.
Content and graphics by Stephen Greb, Kentucky Geological Survey