NERVOUS SYSTEM. 217 



superior portion, which forms an oval encephalic mass, is 

 more completely surrounded with a cartilaginous cranium. 

 From the lateral parts of this encephalic ring (,) come 

 off the optic nerves, and the two large longitudinal sym- 

 metrical columns (a. b. d.) which run parallel and near to 

 each other along the dorsal surface of the abdominal ca- 

 vity to the caudal extremity of the sac, having the pal- 

 leal ganglia (b,) in their course. The cerebral ganglion of 

 the octopus, (Fig. 93 C. ,) forms a more globular concentrated 



mass enclosed in a thick cartilaginous cranium, covered with a 

 gelatinous cellular arachnoid and a dura mater giving off laterally 

 optic nerves (C. b.) to very large optic ganglia (C.c) contained 

 within the sclerotic coat of the eyes. The optic ganglia are sur- 

 rounded with lobed masses of adipose substance (e. e.) and 

 send out a large radiated pencil of detached optic fila- 

 ments (d,) to penetrate the choroid. The brain is sepa- 

 rated from the oesophagus (o,) and the aorta (n,) by the 

 membranous floor of the cranium, the whole periphery of 

 this perforated cerebral mass, encompassing the oesophagus, 

 not being yet surrounded with the consolidated portion 

 of the cranium. On each side of the cranium is a small 

 vestibular cavity occupied by a sac on which the audi- 

 tory nerve is distributed and containing a limpid fluid, 

 and a calcareous concretion. In the sepia the brain is 

 more distinctly bilobate, of a yellowish white colour, and 

 pulpy consistence, smooth on the surface and contained 



