CEPHALOPODA. 



551 



nerve terminates in this ganglion, (v, v, Jig. 

 226,) 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 suboesophageal 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 supraoesophageal mass in the Dibran- 

 chiate Cephalopod s, 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 priority 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 oesophageal 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.* 



ORGANS OF SENSE. The Cephalopodous 

 class is the only one in the Invertebrate series 

 in which distinct organs of sight, hearing, smell, 

 and taste, have been detected, although the en- 

 joyment of these senses is evidently by no means 

 limited to this class. Considerable differences, 

 however, present themselves in the relative 

 complexity, and even as to the existence of 

 the different Organs of Sense in the two orders 

 of Cephalopods : thus, of the senses which 

 relate to distant objects, the Organ of Hearing 

 appears to be wanting in the Nautilus, and 

 the Organ of Vision is comparatively imperfect, 



* See vol. iii. pt. 1, p. 187. Physiological Cata- 

 logue of the Museum of the Royal College of Sur- 

 geons, 4to. 1835. 



while those which take cognizance of proximate 

 objects are more distinctly and extensively 

 developed. 



Organ of Sight. In the Nautilus the eyes 

 are supported on short pedicles which project 

 outwardly from the sides of the head. They 

 are of a spherical form, slightly flattened ante- 

 riorly ; are large as compared with the pe- 

 dunculated eyes of Gasteropods, but are of 

 small size as compared with the complex visual 

 organs of the Dibranchiates. They presented, 

 in Mr. Bennett's specimen, the simplest con- 

 dition of an organ of vision, consisting only 

 of a darkened globular cavity or camera ob- 

 scura, into which light was admitted by a single 

 orifice, and a nerve expanded at the opposite 

 side to receive the impression ; the mechanism 

 for regulating the admission of the impinging 

 rays was wanting, and every trace of that 

 which modifies their direction had disappeared. 

 The form of the eye was maintained by a tough 

 unyielding sclerotic coat (k, jig. 231), which 

 became thinner towards the anterior part of the 

 eye, where it was perforated by a circular aper- 

 ture less than a line in diameter (rn). The nerves 

 continued from the small oval optic ganglion (2) 

 expand, and immediately line the sclerotic as far 

 as the middle of the globe, forming a strong re- 

 ticulate retina (o), which, together with the rest 

 of the cavity, is lined by a black pigment (n). 

 There was no appearance of vitreous humour 

 or crystalline lens; but both parts would no 

 doubt be found to exist in the recent state. 



In the Dibranchiata the eyes are sessile, 

 but in some species project beyond the sur- 

 face of the head more than in others; their 

 complicated structure is truly one of the most 

 remarkable features of the organization of this 

 singular class. 



The eyeball in the Cuttle-fish is inclosed in a 

 capsule consisting posteriorly of a thick carti- 

 lage (a, a, Jig. 2 34), in its lateral circumference 



Fig. 234. 



m. 



Section of the Eye of the Cuttle-fish. 



of a strong white fibrous membrane (b, b), and 

 anteriorly of the cornea (o). 



The whole of the inner surface of the cap- 

 sule is lined by a thin serous membrane, as far 



