20() NERVOUS SYSTEM. 



other ganglia are observed in the abdominal cavity, ex- 

 tended, as the ganglionic columns in the succeeding class, 

 between the alimentary and respiratory cavities. 



The nervous columns of the conchifera are almost con- 

 fined to a sub-oesophageal position, and extend along the 

 inferior or ventral surface of the abdominal cavity, above 

 the respiratory sac; they are most detached from each 

 other at their anterior part, and often continue separate 

 through their whole course. The motor or simple co- 

 lums, which I have generally found more transpa- 

 rent and colourless than the ganglionic, keep in contact 

 with the sensitive chords, but sometimes they occupy a 

 lateral position, passing over the sides of the ganglia, 

 especially the last pair where the two kinds of nerves are 

 most obvious. On opening the respiratory sac of a conch- 

 iferous animal, as of the common muscle, mytilus edulis 

 (Fig. 8S) and throwing 



aside the branchiae, the FIG< 8$? 



foot, and the pectinated 

 lateral prolongations of 

 the lips, we perceive two 

 large white ganglia (88. 

 a.b.) placed on the lower 

 and lateral parts of the 

 mouth, resting on the 

 peritoneal covering of the 

 stomach or of the liver which envelopes it, and sending 

 upwards and forwards numerous large branches to the lips 

 and neighbouring parts, and to the sympathetics. Two 

 of these branches, after encompassing the short oesopha- 

 gus, often meet above that passage, and form a distinct 

 supra-cesophageal or cerebral ganglion, and two other branches 

 passing inwards from the same ganglia, beneath the 

 stomach, sometimes form another double ganglion on the 

 median plain, close to the first sub-oesophageal pair (88. a.b.) 

 The double nervous columns continue their course back- 

 wards, from these anterior lateral ganglia, running on the 

 same plain along the interior surface of the abdominal 

 cavity, to the broad expanded base of the muscular foot, 

 where the middle pair of ganglia are placed. These ab- 

 dominal or pedal ganglia (88. c. d.) , are the most variable in 



