NERVOUS SYSTEM OF MOLLUSC A. 49'> 



head, and organs of the higher senses, (such as the 

 Aplysia, of which the nervous system is exhibited 

 in Fig. 446), there is generally a special cephalic 

 ganglion (c), which probably performs the functions 

 of the brain.* In others, again, as in the Patella 

 (Fig. 447), the cephalic ganglion is scarcely dis- 

 cernible, and its place is supplied by two lateral 

 ganglia (l, l) ; and there is besides a transverse 

 ganglion (t), below the oesophagus. The cephalic 

 ganglion, on the other hand, attains a considerable 

 size in the Cephalopoda (c, Fig. 448), where it has 

 extensive connexions with all the parts of the head : 

 the optic ganglia (o, o), in particular, are of very 

 great size, each of them, singly, being larger than 

 the brain itself, t 



§ 3. Nervous System of Vertehrated Animals. 



The characteristic type of the nervous system of 

 vertebrated animals is that of an elongated cylinder 

 of nervous matter (m z. Fig. 449), extending down 

 the back, and lodged in the canal formed by the 

 grooves and arches of the vertebrae. It has re- 



* This figure also shows a ganglion (a) which is placed higher, 

 and communicates by lateral filaments with the cephalic ganglion 

 (c); two lateral ganglia (l, l), of great size ; and a large abdominal 

 ganglion (g). 



t Some peculiarities in the structure of the cephalic ganglion of 

 the Sepia have been supposed to indicate an approach to the verte- 

 brated structure; for this ganglion, together with the labyrinth of 

 the ear, is enclosed in a cartilaginous ring, perforated at the centre 

 to allow of the passage of the oesophagus, and imagined to be analo- 

 gous to a cranium. 



