Before revision: Some coleoid cephalopods (squids and cuttlefish) have an internalized vestigial chitin shell known as gladius or cuttlebone, which can serve as muscle attachments but the main function is often to maintain buoyancy rather than structural support.
After revision: Some coleoid cephalopods (squids and cuttlefish) have an internalized vestigial aragonite/calcite-chitin shell known as gladius or cuttlebone, which can serve as muscle attachments but the main function is often to maintain buoyancy rather than structural support.