NERVOUS SYSTEM OF MOLLUSCA. 551 



Gasteropoda, which are furnished with a distinct 

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

 the Aplysia, of which the nervous system is exhi- 

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

 cephalic ganglion (c), which may be supposed to 

 serve the office of brain.* In others, again, as 

 in the Patella (Fig. 447), the cephalic ganglion 

 is scarcely discernible, and its place is supplied 

 by two lateral ganglia (l, l) ; and there is be- 

 sides a transverse ganglion (t), below the oeso- 

 phagus. The cephalic ganglion, on the other 

 hand, attains a considerable size in the Cepha- 

 lopoda (c. Fig. 448), where it has extensive con- 

 nexions 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 



* 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 abdo- 

 minal ganglion (g). 



f Some peculiarities in the structure of the cephalic ganglion 

 of the Sepia have been supposed to indicate an approach to the 

 vertebrated structure ; for this ganglion, together with the laby- 

 rinth 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 analogous to a cranium. 



