Some shell colors, such as blue-green iridescent sheens, are caused not by color pigments, but by the refraction of light off various layers of calcite in the shell. A straight color line or ray is formed when the color cells remain in the same position as the shell grows out. If color pigment production continually starts and stops, a pattern of dots or dashes is drawn on the shell. If the color cells actually migrate to one side, a slanting trail of color is produced. Other kinds of behavior by color cells can produce circles, triangles and other shapes.
Shells often have complex features such as spines, ribs, cords and grooves.
For some species, these features provide protection from predators. Certain features also give the shells added strength and assist in burrowing. Shells of creatures like the nautilus have complex internal structures, such as multiple chambers containing water and gas that allow the creature to adjust its buoyancy. Most gastropods have coiled shells in the form of a logarithmic spiral, the only form of a coil that can both increase in size and retain its shape. Such an architecture is an efficient way to maintain strength while retaining the compactness of the shell.