248 IKNERVATION. [CHAP. X. 



unsymmetrical disposition of the nervous system. The principal portion of it 

 coDsists in a ring surrounding the gullet, on which one or more ganglia are 

 placed. In the ascidia mamillata there is but a solitary ganglion, which regu- 

 lates the orifices of ingestion and egestion by nerves which it sends to their 

 respective sphincter muscles. And in all the other classes of mollusks the 

 nervous system is the more complex, as the kind and number of the vital 

 actions demand a higher degree of organization. In conchifera, for instance, 

 ingestion of the food, respiration, and locomotion have distinct organs assigned 

 to them, and accordingly the nervous system is so disposed that there is a 

 nervous centre or ganglion in immediate relation to the principal organs con- 

 nected with each of these functions. Thus, in the more perfect animals of this 

 class we find, 1. two cesophageal ganglia situate near the mouth, connected to 

 each other by nervous filaments, which form a ring round the gullet. These 

 ganglia are connected with all the rest, and probably exercise an influence upon 

 them, as the principal centre of the nervous system or the brain. 2. There is 

 a branchial ganglion, which presides over the function of respiration. When 

 there is but one respiratory organ, this ganglion is single ; but it is double 

 when there are two branchiae. From this source are supplied not only the 

 organs of respiration themselves, but also the muscles upon which the respi- 

 ratory movements depend. The posterior adductor muscle, the mantle, and 

 intestine, derive nerves from it. 3. We find & pedal ganglion, which is imme- 

 diately connected with the locomotive function. This ganglion exists only in 

 those genera in which a muscular organ called the foot is developed, and its 

 size is always in direct proportion to the muscular power of that organ : it is 

 situated at its base, and imbedded in it. We find it, therefore, in the mussel 

 (mytilus), but not in the oyster. The development of organs of sense in the 

 higher animals of this group demands an increased development of the cere- 

 bral ganglion, as is the case in the gasteropod and cephalopod mollusks. And 

 the great powers of locomotion enjoyed by the latter animals require a high 

 development of the pedal ganglion, and a multiplication of smaller ganglia in 

 connexion with the muscular apparatus of their arms or tentacles. These 

 organs are supplied with nerves from the suboesophageal ganglion ; and, where 

 suckers exist upon the arms, an additional series of ganglia is provided, which 

 seem to exert an especial influence in the exercise and the maintenance of 

 their suction power. 



3. The third, or Homogangliate group, is distinguished by the symmetrical 

 arrangement of the nervous system. The articulate classes furnish examples 

 of this type. A bilobed ganglion is situated above the oesophagus, and is con- 

 nected with the organs of sense when they are present. From this there pro- 

 ceeds on each side of the oesophagus a nervous cord to a pair of ganglia beneath 

 that canal, and therefore on the ventral surface. To these succeed, in most of 

 the articulata, and likewise on the ventral surface, a pair of ganglia for each 

 segment, from which are supplied the nerves to that segment. The ganglia 

 are connected throughout, however, by cords of communication from the 

 cephalic to the caudal segment. The number of pairs of ganglia is always in 

 accordance with the number of segments of the animal ; and, if some of these 

 segments be fused together, a similar coalescence of the ganglia takes place. 

 This is observed in insects in the change from the larva to the perfect state ; 

 and, in some genera of Crustacea, the permanent form of the nervous system 

 has obvious relation to peculiarity in the shape of the body. The annular 

 arrangement of the ganglia in the body of the common crab (cancer), and of 



