hard, brittle internal structure found in all members of the family Sepiidae

Cuttlebone is a hard brittle structure found inside cuttlefish. It is made mainly of calcium carbonate. The cuttlefish use it to control floating. Cuttlebones have been used to make polishing powder. This was then added to toothpaste. Today, cuttlebone is mainly used for caged birds and turtles in their habitats, so they get their calcium easily and fast.

A cuttlebone from a sepia
Common cuttlefish Sepia officinalis