What is an ink sac?

An ink sac is a sac or gland found in some species of cephalopods (such as squids, cuttlefish, and octopuses). The sac contains a dark liquid called ink that is expelled as a defense mechanism against predators.

When threatened or attacked, the cephalopod can contract the ink sac muscles, forcing the ink through the funnel and into the water creating a cloud of dark ink that serves to obscure the predator's vision and allow the cephalopod to make a quick escape.

Ink sacs have been used since ancient times for various purposes, such as writing and dyeing fabrics, and they are still used today in the production of various products, including ink and pigments.