CEPHALOPODA. 



540 



which they perforate, but there are no columns 

 prolonged backwards from the lateral parts of 

 the brain to form pal Hal ganglia as in the 

 higher Cephalopods ; the structure and func- 

 tions of the cloak to which these ganglia are 

 subservient, not being enjoyed by the shell- 

 clad Nautilus. The nerves corresponding to 

 the large visceral nerves of the Dibranchiates 

 are, however, proportionally developed; for in 

 the organs of plastic life the Nautilus is upon 

 an equality with its naked congeners. These 

 nerves, which combine the functions of the 

 sympathetic and par vagum, consist of a large 

 pair derived from the lower part of the pos- 

 terior cesophageal collar, and extending back- 

 wards on each side of the vena cava ; and of 

 smaller twigs (17) coming off between the 

 origins of the preceding nerves, and forming a 

 plexus upon the parietes of the vein. The 

 larger chords swell into ganglions at the termi- 

 nation of the vena cava, (16, 16,) and send off 

 ramifications to the branchiae, (15, 15,) the 

 contents of the pericardium, and the viscera of 

 digestion and generation. 



In the Dibranchiate 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- 



Fig. 232. 



Nervous system of the Cuttle-fish. 



bility, the centre of nervous iinpression and 

 volition is proportionally developed, and exhi- 

 bits the highest conditions which the brain pre- 

 sents in the Invertebrate series of animals. 



Except in some of the smaller 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 oasophageal collars, is entirely 

 surrounded by a thick cartilage. The portion 

 of oesophagus 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-resophageal 

 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, Jig. 233) ; this is closely 

 applied to the back part of the fleshy 

 mass of the mouth above the pharynx ; 

 it sends off nerves to the oral appa- 

 ratus (i, i, Jig. 233), and two fila- 

 ments descend and form a pair of 

 small closely approximated ganglions 

 (8, 8, Jig. 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, Jig. 233) descend, and 

 unite and dilate below the oesopha- 

 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 (I, 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 

 transverse 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 



