The so-called mantle of soft tissue which covers the animal inside the shell is what secretes the shell and produces the pattern on it. Some species deposit material in a highly regular way every day; others seem to do it intermittently over periods of months or years. In many species the outer surface of the shell is covered by a kind of skin known as the periostracum, and in most cases this skin is opaque, thereby obscuring the patterns underneath until long after the animal has died. Note that if one makes a hole in a shell, the pattern is usually quite unaffected, suggesting that the pattern is primarily a consequence of features of the underlying mantle. In addition, patterns are often divided into three or four large bands, presumably in correspondence with features of the anatomy of the mantle. Sometimes physical ridges exist on shells in correspondence with their pigmentation patterns. It is not clear whether multiple kinds of shell patterns can occur within one species, or whether they are always associated with genetically different species.