The position of the ink-bag varies in different families. In the octopus it is buried in the substance of the liver; and this animal does not emit its ink so readily as the cuttle or squid. I have very rarely seen it do so in captivity except when greatly exhausted or persistently irritated. It has been said that after being a few hours in captivity the octopus loses the power of secreting ink. There is no foundation at all for such a statement. When placed in a tank especially reserved for it, in which are no enemies to cause it fear, it has no need to conceal itself, and therefore does not unnecessarily eject its cloudy fluid; but I have never dissected an octopus, no matter how long it might have lived in confinement, without finding the ink-bag fairly charged, though some of its contents are sometimes emitted when the animal is at the point of death. The cuttle (_Sepia_) discharges it on the slightest provocation; and this is sometimes very troublesome and annoying when this species is exhibited in an aquarium. The quantity of water its ink will obscure is really surprising. The fluid is secreted with amazing rapidity, and the black ejection frequently occurs several times in succession. I have often seen a cuttle completely spoil in a few seconds all the water in a tank containing a thousand gallons. When first taken, the _Sepia_ is most sensitively timid. Its keen, unwinking eye watches for, and perceives the slightest movement of its captor; and if even most cautiously looked at from above, its ink is belched forth in eddying volumes, rolling over and over like the smoke which follows the discharge of a great gun from a ship’s port, and mixes with marvellous rapidity with the water, whilst the animal simultaneously recedes to the best shelter it can find. But, like all of its class, the _Sepia_ is very intelligent. It soon learns to discriminate between friend and foe, and ultimately becomes very tame, and ceases to shoot its ink, unless it be teased and excited. Professor Owen has remarked that the ejection of the ink of the cephalopods serves by its colour as a means of defence, as corresponding secretions in some of the mammalia by their odour. It is worthy of notice that the Pearly Nautilus and the allied fossil forms are without this means of concealment, which their strong external shells renders unnecessary for their protection.