An ink sac is an anatomical feature that is found in many

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is es ...

s used to produce the defensive

cephalopod ink Cephalopod ink is a dark-coloured or luminous ink released into water by most species of cephalopod, usually as an escape mechanism. All cephalopods, with the exception of the Nautilidae and the Cirrina (deep-sea octopuses), are able to releas ...

. With the exception of nocturnal and very deep water

s, all Coleoidea (squid, octopus and cuttlefish) which dwell in light conditions have an ink sac, which can be used to expel a cloud of dark

ink Ink is a gel, sol, or solution that contains at least one colorant, such as a dye or pigment, and is used to color a surface to produce an image, text, or design. Ink is used for drawing or writing with a pen, brush, reed pen, or quill. ...

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 The hypobranchial gland is a glandular structure which is part of the anatomy of many mollusks, including several different families of gastropods, and also many protobranch bivalves. This gland produces mucus as well as biologically active compou ...

.Nair, J.R., D. Pillai, S.M. Joseph, P. Gomathi, P.V. Senan & P.M. Sherief (2011). ''Indian Journal of Geo-Marine Sciences'' 40(1): 13–27. It lies beneath the gut and opens into the anus, into which its contents – almost pure

melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amin ...

– 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 Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

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.

References

Cephalopod zootomy {{cephalopod-stub