The hard, chalky internal shell of a cuttlefish, often washed ashore and used as a calcium supplement for pet birds and reptiles.
From 'cuttle' + 'bone,' a literal compound. The internal shell is not truly bone but a gas-filled calcium carbonate structure called a cuttlebone or pen.
Cuttlebones have been used as powder for drawing on paper for centuries, and today millions are ground up as supplements for parakeets—one creature's skeleton becomes another's nutrition.
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