part of hindgut ink sac 2011-10-01T10:48:29Z BTO:0005251 BrendaTissueOBO With the exception of nocturnal and very deep water cephalopods, all coeloids which dwell in light conditions have an ink sac, which can be used to expel a cloud of dark ink to confuse predators. This sac is a muscular bag which originated as an extension of the hind gut. It lies beneath the gut and opens into the anus, into which its contents - almost pure melanin - can be squirted. XPMUser