Rugae

In anatomy, rugae (sg.: ruga) are a series of ridges produced by folding of the wall of an organ.

In general, rugae are a biological feature found in many organisms, and serving purposes such as increasing surface-area or flexibility, or providing structural support.

Most commonly, the term rugae refers to the gastric rugae of the internal surface of the stomach.

In terrestrial gastropods, rugae often appear as fine, transverse folds or wrinkles on the mantle, back, or sides of the body. They are particularly visible when the animal extends its body or contracts, and may also be interrupted or intersected by other grooves or structures (such as dorsal grooves or keels) .