NERVOUS SYSTEM 93 



by nerve -connectives, and giving off nerves which supply the 

 various organs. 



The ganglia are about the size of large pin-heads, and 

 are of a bright orange colour which renders them easy to see. 



1. The cerebral ganglia are placed one on each side of 



the hinder border of the mouth, just above the line 

 of attachment of the mantle-lobe, and below and in 

 front of the protractor muscle. The ganglion lies 

 very close to the surface, just beneath the skin, and 

 is very readily dissected if its exact position be de- 

 termined first. 



The two cerebral ganglia are connected together 

 by a commissure which runs round the front of the 

 mouth. Each ganglion also gives nerves to the palps, 

 the adductor muscle, and other adjacent parts, and 

 is united by nerve-connectives with the other ganglia 

 of its side. 



The cerebral ganglia of the mussel probably corre- 

 spond to the cerebral (supra-oesophageal), visceral, and 

 pleural ganglia of other molluscs, such as the snail. 



2. The pedal ganglia are a pair of closely apposed 



ganglia about the same size as the cerebral. They 

 are placed in the foot, along the line of junction of 

 the visceral portion with the foot proper, and about a 

 third of the length of the foot from its anterior end. 



To find the pedal ganglia split the anterior part of the 

 foot with a scalpel in the median plane, and dissect the two 

 halves apart until the ganglia are met with. The junction 

 of the muscular and visceral portions is very clearly defined, 

 and the ganglia are placed close to it. 



Each pedal ganglion gives nerves to the foot, one of 

 these supplying the auditory organ or otocyst, which 

 is placed a little behind and below the ganglion. 



3. The cerebro-pedal connectives are a pair of nerve-cords 



connecting the cerebral ganglia with the pedal 

 ganglia. They run in an almost straight course 

 between the two ganglia, and are easily dissected. 



