NERVOUS SYSTEM OF MOLLUSCA. 351 



of o-augliouic cells, from tlie inner white medullary substance, which 

 is formed principally of fibres. 



Cheron, Keck. p. serrir a l'histoire du systeme nerveux dcs Cephalopodes 



dibranchiaux. Ann. sc. nat. V. Ser. T. V. 

 Owsjannikow uud Kowalevsky, Ueber das Centralnervensystem und das Gehor- 



organ der Cephalopoden. Minn. Acad, de St. Petersbonrg. YII. Ser. T. XL 



Visceral Nerves. 

 § 270. 



Owing to the relations between the visceral nervous system and 

 the central nerve-organs, we were obliged to consider parts of this 

 visceral system while dealing with the central one ; and we had to do 

 with an example of the difference in the significance of the central 

 oro-an, when the ganglia which belong to the peripheral parts 

 become a part of it. In addition, however, to this hinder part of 

 the visceral nervous system which is united to the nerve-centres, and 

 which is principally distributed to the circulatory and excretory 

 organs, as well as to the genitalia and gills, there is another portion 

 which innervates the digestive canal. 



In the Lamellibranchiata fine filaments arise from the cerebral 

 ganglion and surround the mouth ; these are the earliest signs of a 

 portion of the nervous system, which is still more differentiated in 

 the Gastropoda. The development of complicated mouth-organs 

 appears to be correlated with the development of this system. Two 

 nerves arise from the cerebral ganglion and pass to ganglia, which 

 are placed on the buccal mass, and are connected together by a 

 commissure. These buccal ganglia (Fig. 183, B ; Fig. 184, B) 

 supply the organs of the mouth, and give off nerves to the gut. The 

 commissures differ a good deal in character. As a rule the ganglia 

 do not fuse. The same arrangement is found in the Pteropoda ; and in 

 Nautilus, among the Cephalopoda, the two buccal ganglia are con- 

 nected at their side with pharyngeal ganglia, and are connected by 

 commissural chords with the cerebral ones. There is but one buccal 

 ganglion in the Dibranchiata, and behind it there is a large supra- 

 pharyngeal ganglion (Sepia). 



The nerves given off from the buccal ganglia have various small 

 ganglia on their enteric branches. 



Sensory Organs. 



Tactile and Olfactory Organs. 

 § 271. 



The sensory organs of the Mollusca are very similar to those of 

 the Vermes. All parts of the body, with the exception of the hard 



