Nautilus shell fragment. Coloured scanning electron micrograph (SEM) of a fragment from a nautilus shell. The centre of the shell is at upper left. Parts of the shell chambers and their dividing walls are at right. The interior shell layer in a nautilus consists of a composite material called nacre (also known as mother-of-pearl). This material is mostly a form of aragonite, a calcium carbonate mineral. Nacre has a very high tensile strength due to a combination of soft organic layers and hard inorganic platelets of aragonite. The nautilus is a marine mollusc related to the octopus and squid (all are examples of cephalopods). It has a spiral shell divided into many chambers, of which the animal inhabits only the outermost. Magnification: x33 when printed at 10 centimetres across.