216 NERVOUS SYSTEM. 



each of this middle pair of ganglia, one of which con- 

 nects them with the cerebral, and another proceeding 

 backwards connects them with a posterior pair of ganglia, 

 which are united by a transverse chord above the oeso- 

 phagus. The ganglionic portion of this cerebral ring, 

 or perforated brain, is more developed below than above 

 the oesophagus in the pneumodermon, where there is only 

 a narrow transverse band above that passage, and three 

 pairs of ganglia disposed symmetrically below. Four of 

 these inferior ganglia are almost in contact on the me- 

 dian plain, and two are more lateral and separate. But 

 in the hyalea the nervous matter is chiefly concentrated 

 into a large supra-oesophageal broad ganglion of a quadran- 

 gular form, which gives off branches from its four angles. 

 Two of these nerves passing round the oesophagus enter 

 a double ganglion placed below that passage. 



The nearest approach to the vertebrated form of the 

 nervous sytem is that presented by the cephalopods, the 

 highest of the mollusca and of all the invertebrata. The 

 oesophagus still perforates the brain, as in all the infe- 

 rior classes, but the greatest portion of that organ and 

 the symmetrical columns prolonged from it are here placed 

 above the alimentary canal. The brain is enclosed in a 

 distinct organized cranial cavity, numerous symmetrical 

 ganglia are developed on the great nervous axis both 

 before and behind that organ, and sympathetic ganglia 

 are observed in the abdominal cavity. The supra-oeso- 

 phageal portion of the brain in the nautilus forms a 

 thick transversely-elongated band, imperfectly surrounded 

 by the cranial cartilage, and enclosed in a tough mem- 

 brane, as in many of the gasteropods. It is extended 

 laterally into the small optic ganglia of this animal, and 

 is connected laterally with an anterior and a posterior 

 sub-oesophageal ganglionic ring, as in many of the inferior 

 mollusca. Each of the sub-oesophageal bands exhibits 

 two lateral ganglionic enlargements from which numerous 

 branches are ramified forwards and backwards, and the 

 two columns are prolonged backwards from the lateral 

 parts of the brain to the palleal ganglia, as in other 

 cephalopods. In the loligopsis (Fig. 93. A.) the brain is 

 enlarged both above and below the oesophagus, and its 



