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Nautilus siphuncle structure. Coloured scanning electron micrograph (SEM) of part of the siphuncle structure in a nautilus shell. Here, the layered region is the shell, while the needle-like surface area is associated with a structure called the siphuncle. 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. The siphuncle connects the chambers. Magnification: x100 when printed at 10 centimetres across.
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