CEPHALOPODA. 



349 



mass {fig. 135. «) consists prin- 

 cipally of a cordiform body, 

 superficially divided into two 

 lateral lobes by a median lon- 

 gitudinal furrow. From the 

 lower and lateral parts of this 

 body proceed the short and 

 broad optic nerves, w^hich con- 

 stitute the peduncles of the 

 large reniform optic ganglions 

 (6), and upon each peduncle 

 there is placed a small spherical 

 medullary tubercle : these tu- 

 bercles exist also in the Cala- 

 maries, but appear not to be 

 present in the Octopods. 



From the inferior and ante- 

 rior parts of the supra> oeso- 

 phageal mass, a thick cord de- 

 scends on each side of the 

 oesophagus, unites with its fel- 

 low, and dilates below that tube 

 to form the anterior sub-oeso- 

 phageal ganglion, from which 

 the nerves of the feet and ten- 

 tacles arise. Two broader bands 

 descend from the supra-oeso- 

 phageal mass behind the pre- 

 cedingj and form, by a like enlargement and union, the posterior 

 oesophageal body, which blends laterally with the anterior one, and 

 forms with it a large mass with a central perforation. Four 

 short and slender chords, two of which (e) are continued from the 

 anterior apices of the optic lobes, and two from the anterior sub- 

 cesophageal lobes, converge forwards and unite to form a round 

 flattened ganglion (^), which is closely applied to the back part of 

 the fleshy mass of the mouth above the pharynx, from which are sent 

 off" the nerves to the different parts of the mouth. Two filaments 

 from the pharyngeal ganglion descend to join a pair of ganglions 

 below the mouth, analogous to the labial ganglions of the Nautilus. 

 The nerves of the arm (/,/) proceed from the anterior and inferior 

 sub-cesophageal ganglion, and correspond in number to the organs 

 which they supply, being eight in the Octopoda, and ten in the De- 

 capoda : the corresponding nerves are much more numerous in the 



A'- 



Nervous system. Sepia officinalis. 



