LESSON 81.] NERVOUS SYSTEM IN THE PTEROPODA. 



267 



Nervous system, 

 Clio borealis. 



FIG. 38& 



tacula. Behind these central ganglia are two larger FlG - 



lateral ganglia connected together by a transverse 

 band below the oesophagus, and which supply the 

 principal nerves to the muscular-closed mantle en- 

 veloping the trunk. Two nervous bands proceed 

 from each of this middle pair of ganglia, one of which 

 connects 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 oesophagus. 



CEPHALOPODA. 



1203. The nearest approach to the vertebrated form of the ner- 

 vous system is that presented by the Cephalopods, the highest of the 

 mollusca, and of all invertebrata. The brain is enclosed in a dis- 

 tinct cranial cavity, numerous symmetrical ganglia are developed on 

 the great nervous axis, both before and behind that organ, and sym- 

 pathetic ganglia are observed in the abdominal cavity. 



1204. The principal masses of the nervous system of the Pearly 

 Nautilus (Fig. 383) are concen- 

 trated in the head. The supra- 



cesophageal part, or brain (a), 

 presents the form of a short, thick, 

 transverse, round chord, or com- 

 missure, connected at each ex- 

 tremity with three ganglionic 

 masses. The middle and supe- 

 rior of these (b) supplies the eye 

 and the inferior hollow tentacu- 

 liform organ; the anterior and 

 inferior ganglia (c, c) meet their 

 fellow below the oesophagus ; the 

 posterior ganglion (d), in like 

 manner, joins that of the oppo- 

 site side and forms a second and 

 posterior oesophageal ring. The 

 nerves given off immediately 

 from the supra-oesophageal gan- 

 glion supply the muscular and 

 other parts of the mouth, and 

 have small pharyngeal ganglions 

 developed upon them. The an- 

 terior OeSOphageal ring gives Off 1 Nervous system of the Pearly Nautilus. 



