What is "ink sac"

Ink \Ink\, n. [OE. enke, inke, OF. enque, F. encre, L. encaustum the purple red ink with which the Roman emperors signed their edicts, Gr. ?, fr. ? burnt in, encaustic, fr. ? to burn in. See Encaustic, Caustic.]

1. A fluid, or a viscous material or preparation of various kinds (commonly black or colored), used in writing or printing.

   Make there a prick with ink.
   --Chaucer.

   Deformed monsters, foul and black as ink.
   --Spenser.

2. A pigment. See India ink, under India. Note: Ordinarily, black ink is made from nutgalls and a solution of some salt of iron, and consists essentially of a tannate or gallate of iron; sometimes indigo sulphate, or other coloring matter, is added. Other black inks contain potassium chromate, and extract of logwood, salts of vanadium, etc. Blue ink is usually a solution of Prussian blue. Red ink was formerly made from carmine (cochineal), Brazil wood, etc., but potassium eosin is now used. Also red, blue, violet, and yellow inks are largely made from aniline dyes. Indelible ink is usually a weak solution of silver nitrate, but carbon in the form of lampblack or India ink, salts of molybdenum, vanadium, etc., are also used. Sympathetic inks may be made of milk, salts of cobalt, etc. See Sympathetic ink (below). Copying ink, a peculiar ink used for writings of which copies by impression are to be taken. Ink bag (Zo["o]l.), an ink sac. Ink berry. (Bot.)

   1. A shrub of the Holly family ( Ilex glabra), found in sandy grounds along the coast from New England to Florida, and producing a small black berry.

   2. The West Indian indigo berry. See Indigo.

      Ink plant (Bot.), a New Zealand shrub ( Coriaria thymifolia), the berries of which yield a juice which forms an ink.

      Ink powder, a powder from which ink is made by solution.

      Ink sac (Zo["o]l.), an organ, found in most cephalopods, containing an inky fluid which can be ejected from a duct opening at the base of the siphon. The fluid serves to cloud the water, and enable these animals to escape from their enemies. See Illust. of Dibranchiata.

      Printer's ink, or Printing ink. See under Printing.

      Sympathetic ink, a writing fluid of such a nature that what is written remains invisible till the action of a reagent on the characters makes it visible.

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.