Page:Popular Science Monthly Volume 21.djvu/777

Rh defense, and for enabling them to escape from their enemies. Discharging the inky fluid through the "funnel," into which the duct of the ink-sac opens, it rapidly diffuses itself through the water, and enables the animal to escape under a literal cloak of darkness. The exact nature and relationship of this ink-sac to the other organs of the cuttle-fish have long been disputed. According to one authority, the ink-bag represented the gall-bladder, because in the octopus it is imbedded in the liver. From another point of view, it was declared to represent an intestinal gland; while a third opinion maintained its entirely special nature. The ink-sac is now known to be developed as an offshoot from the digestive tube; and, taking development as the one infallible criterion and test of the nature of living structures, we may conclude that it represents at once a highly specialized part of the digestive tract, and an organ which, unrepresented entirely in the oldest cuttle-fishes, has been developed in obedience to the demands and exigencies of the later growths of the race. It is this ink-sac which is frequently found fossilized in certain extinct cuttle-fish shells. Its secretion forms the original sepia color, a term derived from the name of a cuttle-fish genus. The fossilized sepia has been used with good effect when ground down. The late Dean Buckland gave some of this fossil ink to Sir Francis Chantrey, who made with it a drawing of the specimen from which it had been taken; and Cuvier is said to have used this fossilized ink in the preparation of the plates wherewith he illustrated his "Mollusca." At the present time, recent cuttle-fish ink is said to be utilized in the manufacture of ordinary artists* "sepia."

The due regulation of cuttle-fish existence is determined by the action of its nervous apparatus. The ordinary type of molluscan nervous system undergoes in the cuttle-fishes a decided change of form. In a snail or whelk, for example, the nervous system exhibits an arrangement of three chief nerve-masses or "ganglia," connected by nervous cords. Of these three nerve-centers, one is situated in the head, a second in the "foot" or organ of movement, and a third in the neighborhood of heart and gills, or amid the viscera generally. Increased concentration of this type of nerve-arrangement awaits us in cuttle-fish organization. Just as the spider possesses a more concentrated and localized nerve-axis than the insect, or as the gangliated chain of the latter becomes the fused nerve-mass of the spider, so in the cuttle-fish, the molluscan nerve-system, scattered and diffused in the snail, whelk, or mussel, becomes localized in adaptation to the increased nerve-control and to the wider instincts of cuttle-fish existence. This process of nerve-localization and concentration is accompanied by certain important modifications affecting other regions and structures of cuttle-fish economy. Thus the nerve-centers are found to be protected and inclosed within a gristly or cartilaginous case, that foreshadows the functions of the vertebrate skull, though in no sense connected with that structure.