Cuttlefish bone. Coloured scanning electron micrograph (SEM) of the internal structure of cuttlefish (Sepia sp.) bone, or internal shell, mostly composed of calcium carbonate. The air filled spaces make it light but it is also strong enabling it to withstand the hydrostatic pressure it is put under. Despite their name, cuttlefish are not fish but molluscs or cephalopods, related to squid and octopus. Cuttlefish are marine animals of the order Sepiida. Magnification x174 at 10cm high.