Ink sac - Wikipedia Ink sac Ventral view of the viscera of Chtenopteryx sicula showing the position of the ink sac An ink sac is an anatomical feature that is found in many cephalopod mollusks used to produce the defensive cephalopod ink. With the exception of nocturnal and very deep water cephalopods, all coeloids ( squid, octopus and cuttlefish ) which dwell in light conditions have an ink sac, which can be used to expel a cloud of dark ink in order to confuse predators. The ink sac is a muscular bag which originated as an extension of the hind gut ; it is a modified hypobranchial gland. It lies beneath the gut and opens into the anus, into which its contents -- almost pure melanin -- can be squirted ; its proximity to the base of the funnel means that the ink can be distributed by ejected water as the cephalopod uses its jet propulsion. The ejected cloud of melanin is bound by mucus particles, so it forms a lump approximately the size and shape of the cephalopod, fixing the predator 's attention while the mollusc itself makes a hasty escape. A general level of provocation is necessary to trigger an octopus to release its ink, as it is biologically costly to produce. Some species can even use their ink to stun or numb their predators. References ^ Jump up to : Boyle, Peter ; Rodhouse, Paul ( 2004 ). Cephalopods : ecology and fisheries. Ames, Iowa : Blackwell. doi : 10.1002 / 9780470995310. ch2. ISBN 0 - 632 - 06048 - 4. Jump up ^ Nair, J.R., D. Pillai, S.M. Joseph, P. Gomathi, P.V. Senan & P.M. Sherief ( 2011 ). `` Cephalopod research and bioactive substances '' ( PDF ). Indian Journal of Geo - Marine Sciences 40 ( 1 ) : 13 -- 27. Cephalopod anatomy Shell Shell types External shells : Ammonoid shell Argonautid eggcase Nautiloid shell Orthocone Internal shells : Belemnoid guard Cirrate shell Cuttlebone Gladius ( pen ) Spirula shell Stylet External features Aperture Apex Callus Lirae Periostracum Sculpture Spire Sutures Umbilicus Internal features Body chamber Camerae Nacre Phragmocone Septa Siphuncle Whorls Mantle External anatomy Funnel Dermal structures Fins ( wings ) Mantle locking - apparatus Internal anatomy Branchial hearts Ctenidia ( gills ) Hepatopancreas ( digestive gland ) Ink sac and ink Needham 's sac Nephridia ( `` kidneys '' ) Nidamental glands Osphradium Pericardial glands Head and limbs Brachial crown Arms Hectocotylus Tentacles Buccal region Aptychus Beak Odontophore Radula Spadix and antispadix Other parts Eyes Statocysts General Chromatophores Photophores Nervous system Squid giant axon Squid giant synapse Developmental stages : Spawn → Paralarva ( Doratopsis stage ) → Juvenile → Subadult → Adult Egg fossils Protoconch ( embryonic shell ) This cephalopod - related article is a stub. You can help Wikipedia by expanding it. Retrieved from `` https://en.wikipedia.org/w/index.php?title=Ink_sac&oldid=749996824 '' Categories : Cephalopod stubs Cephalopod zootomy Hidden categories : All articles with unsourced statements Articles with unsourced statements from February 2007 All stub articles Talk About Wikipedia Deutsch Français 日本 語 Plattdüütsch Polski Português Русский Українська Edit links This page was last edited on 17 November 2016, at 07 : 42. Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: ink glands in cephalopods are used mainly for
Answer: to confuse predators