The eye is a prominent stalked cone in Nautilus, with a flattened end, which is pierced by a small central hole. This hole leads into the cup-shaped interior of the eye, which is lined by the retina. The latter is therefore bathed with sea-water. The eye in Dibranchiata is sunk in the head, except in Procalistes, where it is stalked. The retinal chamber is closed; its anterior surface is occupied by a biconvex lens divisible into a smaller outer and a larger semi-globular internal part, the two separated by a membrane. The two parts are secreted by the outer and inner layers respectively of an epithelial body (= ciliary body), which surrounds their margins. In front of the closed retinal chamber is an iris, supported by a cartilage, and containing a sphincter muscle; in front of the iris a transparent cornea, which is perforated in the centre by an aperture in certain Decapoda, the Oegopsidae, e. g. Ommastrephes. The Octopoda have a sphincter-like eye-lid; Sepia and some other Decapoda a horizontal ventral eye-lid. The retina of Nautilus consists of a single layer of cells, each ending in a visual rod, and pigmented round the base of the rod, and an underlying layer of ganglion cells (?). It is regarded in Dibranchiata as composed either of a single layer of cells, each furnished with a visual rod, and an underlying layer of ganglion cells, a delicate limiting membrane separating the two (Carriere); or as composed of a single layer of retinal cells, forming an inner layer of rods with swollen bases, and an outer layer of nucleated cell-bodies, the two divided by a limiting membrane, which is derived from a layer of small cells, 'Limitans-zellen,' which form a prominent zone at the outer edge of the retina.
The visual rods in this case consist of two rhabdomeres, and two to four rhabdomeres fuse into a rhabdome (Grenacher)1. The eye of the Dibranchiata is inclosed in cartilage, which is pierced by the nerves. These come from a large optic ganglion connected by a short nerve to the cerebral ganglion. A cellular 'white' body lies at the margin of the optic ganglion anteriorly. The oto-cysts are a pair of vesicles imbedded in Dibranchiata in a cavity of the cephalic cartilage, and attached to the pedal ganglion by a nerve, which is derived, however, from the corresponding cerebral ganglion. There are a number of otoliths in Nautilus, a single otolith in Dibranchiata.
1 Patten regards the 'retinal cells' as retinophorae, as they possess double rods and an axial nerve-fibre. The 'Limitans-zellen' are ganglionic cells, and are modified retinulae homologous with the retinulae of Gastropod ommatidia, and with the inner layer especially of ganglionic cells in the eye of Pecten. The closer juxtaposition of the broad ends of four retinophoral rods, has misled Grenacher into supposing that these are rhabdomes. A unique feature in the Cephalopod eye is the presence of pigment in the retinophorae round the axial nerve-fibre. See Patten, Mitth. Zool. Stat. Naples, vi. pp. 623-25, and for explanation of terms, the note, p. 452 of this book.
The mouth is provided with two jaws like a parrot's beak, calcified in Nautilus, chitinoid in Dibranchiata, the larger ventral in position1. They are borne upon the extremity of a prominent buccal cone or mass, which is surrounded externally by an integumentary fold or buccal membrane2. The cone contains a radula. The oesophagus is wide and dilated near the apex of the visceral dome in Nautilus, narrow in Dibranchiata, and provided in Octopoda with a crop-like dilatation. The stomach is large, and its walls are muscular and like the gizzard of a bird in Nautilus and Octopus. The pyloric is near the cardiac aperture, and leads into an intestine, slightly coiled in Nautilus and Octopoda, which terminates in an anus situated in the branchial cavity, between the bases of the gills in Nautilus, more anteriorly on the posterior wall of the body in Dibranchiata. A caecum is appended to the intestine near the anus in Nautilus, close to the pylorus in Dibranchiata. In the latter it may be short and round, or longer and spirally coiled. Its internal surface is generally produced into folds or processes.
A pair of glandular (? salivary) masses lie in the buccal cavity in Nautilus. A pair of glands with a single duct lies externally to the buccal mass behind the nervous centres in all Dibranchiata, and a second pair in front of them in some Octopoda. The duct opens in front of the radula. There is a liver surrounded by a firm membrane, composed of four lobes in Nautilus, two lobes in Dibranchiata. There are two bile ducts, which unite, and open into the intestine in Nautilus, into the intestinal end of the caecum in Dibranchiata. The so-called pancreas exists in three forms: as a yellowish coloured part of the liver opening into a dilatation of the bile duct in Octopoda; as glandular masses impacted in the thickened walls of the bile duct in Loligo; or as glandular caeca appended to the bile duct in other Decapoda. A gland known as the ink-bag opens in all Dibranchiata, either near (Sepia) or into the rectum, of which it is a diverticulum. It has been found in specimens of extinct Belemnitidae. Its inky secretion - a pigment known as Sepia - serves to hide the animal when attacked or alarmed.
The heart is placed posteriorly near the summit of the visceral dome. It is square in Nautilus, and receives four branchial veins, one at each corner. It is more or less pyriform in Decapoda, and transversely oval in Octopoda, and in both it receives two branchial veins, which at their cardiac ends are dilated, muscular, and contractile, thus forming two auricles. The ventricle gives off a cephalic and abdominal aorta, at its opposite extremities in Nautilus and Decapoda, close together in Octopoda. The arterial system is very complete and much branched in Dibranchiata. It is said to lead into true capillaries in some cases, or into coelomic sinuses in others. The venous blood is collected into a large median postero-ventral vein, the vena cava, which branches into four branchial arteries in Nautilus, into two in Dibranchiata. These traverse the walls of the nephridial and viscero-pericardial sacs on their way to the ctenidia, up which they run on their ad-pallial, i.e. in Dibranchiata, fixed aspect. Each vessel is dilated in the order just named, at the base of the ctenidium, into a muscular branchial heart; and before it enters the ctenidial axis, it receives veins from other parts of the body.