The nerves of the arms proceed from the anterior and inferior subcesophageal ganglions (d,fig. 233), corresponding in number to the parts they supply, viz. eight in the Octopodal and ten in the Decapoda. But, according to Rathke, the Loligopsis offers an exception, the nerves of each lateral series of arms being con tinued for a short distance from the brain as a single pair. In the Poulp, the eight nerves (e, e, fig. 233) glide along the inner surface of the basis of the feet, which they penetrate re spectively, running with the great artery in their substance, and forming, as Cuvier has described, a series of closely approximated ganglions, corresponding to each pair of suck ers, and sending off radiated filaments. In the Genus Eledone, where the arms are narrower, and the suckers are arranged in a single series, the ganglia are relatively smaller.
In the peduncles of the Decapoda the nerves are continued of a simple structure as far as the acetabuliferous extremities, where they become enlarged and gangliated.
Before forming the ganglionic enlargements in the ordinary arms, each brachial nerve gives off two large chords, one to each side, which traverse the fleshy substance of the base of the feet to join the two corresponding branches of the contiguous arms ; the eight nerves are thus associated by a nervous circle (f,,f, fig. 233), which subdivides into two, and forms a small loop at each chord.
Behind the origin of the brachial nerves, the large infundibular nerves, a single pair (g, fig. 233), are given off. The small acoustic nerves (h) arise below and behind the nerves of the funnel, from the nervous sub stance that effects, as it were, the junction of the two cesophageal collars below. Next arise the large visceral nerves (14, fig.232, 233), which, after distributing filaments to the muscles of the neck, descend parallel and close to one another behind the vena cava, give off from their inner sides the small filaments which con stitute the plexus upon the vein ; they then diverge from each other towards the root of each gill, where they divide into three princi pal branches : one of these dilates into an elongated ganglion (c,fig. 232), and enters the fleshy stem of the branchia; the second de scends to the bottom of the sac ; the third passes to the middle heart. The plexus pre viously formed upon the vena cava receives additional filaments from the two latter bran ches ; and a large sympathetic ganglion is formed, which is attached to the parietes of the stomach, near the pyloric orifice.* The most important and interesting nerves are the two large ones, (1 3, 13, figs. 232, 23 3,) which arise from the posterior and lateral surface of the subcesophageal mass, and extend outwards, downwards, and backwards, perforating the shell muscles, and forming upon the inner parietes of the mantle the large stellated gan glion (d, d,fig. 232), from which the nerves of
the mantle are derived. In the Octopoda the nerve terminates in this ganglion, (v, v, fig. 2260 from which about twenty branches radiate to the mantle ; but in the Decapoda, in which lateral fins are superadded to the trunk, it pre viously divides into two large branches. Of these the external alone produces the ganglion from which the sensitive nerves are distributed in a radiated manner, as in the Poulp; the other division (e, fig. 232), after having been joined by a branch (f) from the ganglion, pierces the fleshy substance of the mantle, and ends in a diverging series of twigs appropriated to the muscles of the fin (g). In proportion as the trunk of the Cephalopod is elongated, these branches become more parallel in their course, and dorsal in their position.
The anterior part of the mantle is supplied by small nerves, having a distinct origin from the posterior subcesophageal mass, above the great moto-sensitive chords.
With respect to the parts of the central axis of the nervous system of the Vertebrata which are represented by the structures above de scribed, we may reasonably infer from the fact that the supracesophageal mass in the Dibran chiate Cephalopods, especially the posterior division, is principally in communication with, and owes its superior development chiefly in relation to the complex organs of vision, that it is analogous to the optic lobes or bigeminal bodies. For if it be regarded, as Cuvier sup poses, as the cerebellum of the vertebrate brain, we have then to reconcile the anomaly of this part being the seat of origin of the optic nerves. The constancy, again, of the optic lobes in the vertebrate series, and their pricaity of develop ment to the cerebellum, leads naturally to the expectation that these would form part of such a brain as the highest invertebrate animal is endowed with. The smaller portion of the brain of the Poulp anterior to the optic lobes appears to represent an olfactory lobe. With respect to the inferior cesophageal mass, as it gives origin to the auditory and respiratory nerves, and those two large moto-sensitive co lumns, which evidently represent, by their structure and position, the spinal cord of the Vertebrata, we consider it as fulfilling the function of the medulla oblongata, and to be the part of the nervous centre which is most intimately connected with the vital energies of the animal.*