The cuttle-fish, a family of naked molluscs, with an oval body and with eight short arms surrounding the head, also two long arms or tentacles. It has an internal shell or plate along the back, known as cuttle-fish bone. It also possesses a bladder containing a black liquid, which it ejects when pursued, staining the water black so that it can escape. This secretion, obtained from the ink-bag and dried, is used as a pigment called India ink. This, when dissolved in potash, boiled and filtered, precipitated by an acid, and dried, becomes the pigment sepia.