an organ in most cephalopods, for example, octopuses, cuttlefish, and squids, in which a fluid is formed containing granules of a black pigment of the melanin group.

The ink sac, which serves a protective function, consists of a glandular portion and a reservoir, which opens into the rectum through a duct. Old cells of the glandular portion are gradually destroyed, dissolve in the secretions of the gland, and accumulate in the reservoir. When threatened, the mollusk ejects the contents of the reservoir, creating a dark cloud in the water that serves as a smokescreen to hide the mollusk. The coloring capacity of the inky liquid is unusually high; for example, the cuttlefish can color the water of a tank holding up to 5,500 liters in 5 seconds. A dye, natural sepia, has been obtained from the dried contents of the ink sac treated with potassium hydroxide.