- a large gland in most cephalopods, as the cuttlefish, octopus, and squid, that is near the rectum and ejects ink at predators.
- a flexible structure containing ink.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
Examples from the Web for ink sac
The ink taken from the ink-sac of Sepia is an article of commerce.
They are without an ink-sac, and have the foot divided into lobes bearing sheathed tentacles.
The ink-sac, absent in Nautilus, is a rectal caecum developed from its dorsal wall.
In certain cases remains of the arms provided with hooks, and of the ink-sac, have been recognized.