The gastropods are the most diverse group of mollusks, and include snails, slugs, limpets, conchs, sea slugs, and others. They typically have a coiled body form with a twisted gut and nervous system. The coiling results in the unusual arrangement of their anus being above their head. Most have a corresponding coiled shell. Coils are added as the gastropod grows, and they tend to get larger, making a cone shape. Some shells are adorned with spines, grooves, or other textures that add strength. Many have a hard flap (operculum) on the foot that can be closed over the shell opening like a trap door. The shape of a gastropod shell often can be used to identify species. A gastropod's shell not only protects it from predation, but also keeps land species (terrestrial) from drying out. Some terrestrial gastropods that live in humid places, such as slugs, have a thin shell or no shell at all.