In the Dibmnchiate Cephalopods which possess instruments for varied and active loco motion, where the visual organ is of large size, and attains a complexity of structure equal to that of the Vertebrate animals, where a distinct acoustic organ is developed, and where the whole surface of the body is the seat of sensi bility, the centre of nervous impression and volition is proportionally developed, and exhi bits the highest conditions which the brain pre sents in the Invertebrate series of animals.
.r_Axcept in some OI tne smatter species, as the Sepiola, in which the surrounding sub stance still retains the consistency of a mem brane, the brain, together with the anterior . and posterior (esophageal collars, is entirely surrounded by a thick cartilage. The portion of cesophagus which is thus enclosed is sepa rated from the surrounding medullary matter by a thin layer of softer substance. The cere bral cavity is larger than the brain itself, and the intervening space is filled with a gelatinous fluid. In the Cuttle-fish the supra-cesophageal mass is transversely shortened, as compared with the Nautilus, and supports a smooth, rounded, heart-shaped medullary mass, slightly divided into two lateral lobes by a mesial lon gitudinal furrow (1, fig. 232); from the lower and lateral parts of this body proceed the broad bands of cerebral substance which afterwards dilate into the large reniform optic ganglions (2, 2); upon each of these bands is placed a small spherical medullary body (k, k). These bodies, which we first discovered in the Sepia, we have since ascertained to exist in Loligo.
From the anterior apices of the cerebral lobes small nerves are continued, which almost immediately dilate into a round flattened gan glion (a, fig. 233) ; this is closely applied to the back part 9f the fleshy mass of the mouth above the nharynx it sends off nerves to the oral appa ratus (i, i, fig. 233), and two fila ments descend and form a pair of small closely approximated ganglions (8, 8, fig. 232) below the mouth, analogous to the labial ganglions of the Nautilus.
From the inferior, lateral, and an terior parts of the brain two large chords (k, fig. 233) descend, and unite and dilate below the cesopha gus to form the anterior subceso phageal ganglion, or pes anserinus of Cuvier, from which the nerves of the feet and tentacles arise. Two still
larger bands (1, fig. 233) descend from the brain behind the preceding to form, by a similar enlargement and union, the posterior cesophageal gan glionic collar. From a comparison of these with the corresponding gan glions of the Nautilus, it will be seen that by their approximation in the t ansverse direction the distinction of the ganglions at the lower part of the collar is lost; and a corre sponding approximation in the antero posterior direction, being accompa nied by an additional accumulation of nervous substance, has produced a blending together of the four gan glions into one large continuous sub cesophageal mass. The portions of this mass corresponding to the four ganglions and double cesophageal collar of the Nautilus, are notwithstanding indicated in a manner not to be mistaken, by the origins of the nerves which it sends off, and by the chords which bring it into communication with the cerebral mass above.
We shall now briefly mention the points in which the brain in other Dibranchiata differs from what we have described, after careful examination of this part in the Cuttle-fish. In the Poulp, the brain or supra-cesophageal mass is divided, according to Cuvier, into two parts, an anterior (a, fig. 233), which is of a flatter and squarer figure and of a whiter colour, compared by Cuvier to the cerebrum, but which seems to be the pharyngeal ganglion more closely approximated to the brain than in the Sepia : and a posterior globular mass (b), of a grey- colour, which he compares to the cerebellum ; the optic nerves (c) are much smaller than in the Cuttle-fish, and do not support the small spherical bodies which exist in the Cuttlefish and Calamary.
The brain of the Argonauta does not present a rounded form above, but when seen from this aspect, is composed, as in the Octopus, of an anterior white oblong band, flattened trans versely, and of a posterior raised convex semi lunar mass, which terminates behind in a semi lunar border, the extremities of which are con tinued directly to form the posterior collar of the cesophagus.