Not the least interesting feature of this localized mass of nervous matter is the fact that it exhibits the same arrangement of gray and white nerve-matter that is seen in the highest brains. An outer gray and an inner white layer are discernible in the nerve-ganglia of cephalopods, as in the cerebrum of man; and, as in the highest animals, the cuttle-fish gray matter is found to consist of nerve-cells, while the white matter is chiefly composed of nerve-fibers. Thus the laws of developmental progress affect the microscopic and intimate structure of the living form as well as the more obvious details of structure. From the main nerve-mass of the cuttle-fishes nerves arise to supply the body at large. Nerves of special sense supply eyes, ears, and olfactory organs; while the viscera and the "mantle" or general body-covering are also well provided with the means of innervation.
Cuttle-fish existence possesses, in all probability, the five "gateways of knowledge," through which the impressions of the outer world are received, and by which these impressions are modified and transmitted to the brain-masses as sensations of sight, hearing, smell, touch, and taste. There is little need to draw upon hypothesis in the assumption that the arms or tentacles are efficient organs of touch in Cephalopoda, or that the structures of the mouth may subserve taste, in so far as the latter sense may be required to satisfy the demands of cuttle-fish existence. An organ of smell is definitely situated behind or above the eyes. There two small projections, or, as frequently, two minute pits or depressions, occur. These pits are ciliated, and between the cilia "olfactory cells" are situated. These cells, in turn, represent the similar structures which occur in higher animals, and which, in man himself, form the characteristic terminations to his olfactory nerves. That the cuttle-fishes can literally scent their prey from afar off is an idea confirmed by the facts of their every-day life.
The "ears" of the cuttle-fishes present us with two sacs named "auditory sacs"—which may, as in the nautilus, either be attached to the chief nerve-mass itself, or, as in the two-gilled cuttles, be lodged in special cavities in the gristly "skull." A cuttle-fish "ear" is essentially a sac or bag, called an "otocyst," containing either one or many "otoliths" or "ear-stones," suspended in a watery fluid. This, indeed, is the primitive type of "ear" we may find even in the Medusidæ or "jelly-fishes" themselves. The ear-sacs of many cuttlefishes open on the external surface of the body by two fine canals, named "Kölliker's ducts," after their distinguished discoverer. Occasionally these ducts end blindly, and do not open on the body surface. These facts lend additional support to the opinion that in the ear of the cuttle-fish we find primitive structures proper to the ears of vertebrates, the minute canals of Kölliker corresponding with the recessus vestibuli of the vertebrate organ of hearing. Once again, therefore, we find the progressive development of cephalopods and vertebrates