66 



The anterior centres are situated in the cephalic region 

 and surround the alimentary canal, forming a collar 

 almost hidden by the salivary glands. The posterior 

 centres lie just behind the pallial cavity in the region 

 between that and the visceral mass. The two groups of 

 ganglia are connected by the long visceral com- 

 missure (fig. 41, vis. com.). Dissection of the nervous 

 system is a matter of some difficulty and is usually a 

 stumbling block to some students. It is quite easy to 

 find the nerve collar, but rather difficult to expose clearly 

 the component parts owing to the presence of a tenacious 

 fluffy connective tissue which obscures all. 



The nerves and commissures passing into the body 

 wall are also difficult to follow by reason of the toughness 

 of their surroundings and their resemblance in colour to 

 the tissues in which they are embedded. 



It is best to dissect the nervous system in whelks that 

 have been kept for a short time in alcohol. This hardens 

 the muscle but brings out the nerves much more clearly. 

 In addition, the action of a solution of oxalic acid is 

 decidedly useful. It brings out the otocysts and clears up 

 the muscular mass of the foot. In order to investigate 

 the centres making up the nerve collar, it is best to cut 

 through the cerebral commissure which lies above the 

 oesophagus and then to dissect the alimentary canal care- 

 fully away. The ring of ganglia can be examined from 

 the inside, and thus observed without requiring the 

 removal of so much connective tissue. 



The Anterior Centres are ten in number. They consist 

 of the cerebral, pleural, pedal and buccal ganglia, which 

 are paired and the supra- and sub-intestinal ganglia 

 which have been drawn in, in the general concentration, 

 until they have reached a position close to the pleural 

 ganglia (fig. 42, Sup. int. and Sub. int.). 



