NERVOUS SYSTEM. Y 1 



The dibranchiates have the principal ganglia so closely con- 

 nected that the commissures are not readily perceived. The 

 optic nerves are well developed. The superior and inferior 

 buccal ganglia have each united in one mass, and the two are 

 united by commissures around the oesophagus. The large nerves 

 of the arms, and those of the funnel or siphon, proceed from the 

 pedal ganglia, which are placed on the posterior side of the 

 gullet ; with them are connected also the auditory nerves. From 

 the parieto-splanchnic or visceral ganglia proceed nerves along 

 the shell-muscles to the anterior wall of the mantle, where they 

 enter the large ganglia stellata ; these are connected by commis- 

 sures, and send strong cords to the fins. Branches of the 

 parieto-splanchnic ganglia, following the vena-cava, supply the 

 breathing and reproductive organs. A recurrent nerve from the 

 inferior buccal ganglion follows the oesophagus, ending in a 

 ganglion upon the stomach. 



A modification of this plan is found in the Nautili. Here a 

 thick transverse cord situated in front of the oesophagus repre- 

 sents the cerebral ganglia ; from its outer angles the optic and 

 olfactory nerves proceed, from its anterior edge those of the 

 buccal mass. The pedal ganglia placed close to the cerebral 

 ganglia, are united by a slender commissure ; from them are 

 supplied all the nerves of the foot or arms, of the funnel and of 

 the ears. The parieto-splanchnic ganglia, lengthened into a thick 

 cord, united at each end with the cerebral ganglia, form the 

 oesophageal ring. Two large ganglia are found, one on either 

 side of the stomach. 



Gast7'opoda (v, 64, 61). The typical arrangement of the 

 nervous system throughout the mollusca being the same, the 

 difi"erences of which we have occasion to speak are simply modi- 

 fications thereof. The oesophageal ring consists essentially of 

 three pairs of ganglia and a double commissure on each side, and 

 in the more highly organized prosobranchiates these ganglia are 

 approximated and the commissures shortened until the cerebral 

 mass resembles that of the cephalopods, where the ganglia can 

 be distinguished only by the origin of some of the nerves. In 

 species having a simple mouth, the oesophageal ring surrounds 

 the oesophagus immediately behind the oral mass, but in those 

 having a proboscis or snout, it is situated so far back as to remain 

 at rest Avhilst the proboscis is protruded or retracted. 



The cerebral ganglia are placed above or at the sides of the 

 oesophagus, and from them proceed the nerves to the eyes, 

 tentacles, lips and mouth ; the pedal ganglia are under the 

 oesophagus, and from them the acoustic and pedal nerves arise; 

 the visceral ganglia are mostly at the under side, somewhat 

 above the pedal ganglia, and here the nerves supplying the 

 mantle, branchiae, viscera, heart and columellar muscle take 



