240 INTRODUCTION. 



verse band, above or in front of the buccal body, usually 

 at its anterior part, but varying in the latter position, to 

 some degree depending upon the movements of the buc- 

 cal body. When the latter is protruded, the band of 

 ganglia is thrown back to the commencement of the 

 oesophagus ; when retracted it is placed just behind the 

 upper lip. It consists of two symmetrical halves united 

 by a short, transverse commissure. Each half is com- 

 posed of several ganglia, aggregated to a greater or less 

 degree in different genera and species. Sometimes they 

 are so aggregated, or are so covered by enveloping tis- 

 sue, as to appear a single mass ; in others five or six 

 distinct masses may be readily counted. 



The second set, or sub-oesophageal ganglia, form a cir- 

 cular mass, placed infero-posteriorly to the buccal body, 

 in the excavation of the foot. It is asymmetrical, and is 

 composed of several ganglia, more or less aggregated 

 together ; from four to seven masses can generally be 

 counted. It is usually more developed upon the right 

 than the left side, and passes through all the shades of 

 color, in different genera and species, from white, yellow, 

 to orange. Through the opening formed by the arrange- 

 ment of the ganglia into a circle, passes the cephalic 

 branch of the aorta. The supra-oesophageal and sub- 

 oesophageal ganglia are connected together on each side 

 of the buccal body by a double commissure, which varies 

 in length in different genera ; thus, in Helices, etc., it is 

 generally so long as to allow of much movement of the 

 supra-oesophageal ganglia forward with the buccal body, 



