240 INTRODUCTION. 
verse band, above or in front of tbe 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-ocsophageal ganglia forward with the buccal body, 
