The golden spiral is a particular logarithmic spiral based on the proportions of the "divine constant", an irrational number that is hidden behind various natural figures, from the shape of galaxies and cyclones, to the arrangement of the stamens of flowers, up to the internal structure of shells and animal organs.
Each element of the sequence follows a certain trend: a: b = b: (a + b). A segment is divided into two parts such that the larger part is an average proportional between the entire segment and the smaller part. From the ratio of these numbers we obtain a recurring number, approximating to 1.618, an increase factor of the golden spiral.
One of the most iconic examples of the golden spiral is found in the internal section of a nautilus shell, a mollusk cephalopod with incredible archaic characteristics: all its still existing evolutionary relatives such as octopuses, cuttlefish and squid have lost their outer shells for millions of years in favor of more hydrodynamic assets, and for this reason the seven species of nautilus are considered "living fossils".