Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.3.5. lnk Sac
Like the Octopus and the Cuttle-fish, the squid has an ink-sac for defence. It lies dorsal to the rectum and its duct open into the siphon near the anus.
When in danger, it discharges a black ink which, when spread in water, forms a cloud-like 'smoke screen' which obscures the vision and perhaps numbs the olfactory organs of the enemy, thus giving the squid an opportunity to escape. The black colouring matter probably serves to distract the attention of the enemy, while the squid escapes. Recent investigations have shown that squid does not eject a large cloud cover of ink as has been supposed. But it discharges just enough ink to colour a volume of water of its own size. The predator often mistakes the ink for the squid, which gets a chance to escape. Thus, the squid possesses a defensive device which man has only recently used in warfare.