THE CHAIN OF GANGLIA. 105 
and often give off one or more processes. These pro- 
cesses, under favourable circumstances, can be traced 
into continuity with nerve fibres. 
The chief ganglia of the crayfish are disposed in a 
longitudinal series in the middle line of the ventral 
aspect of the body close to the integument (fig. 25). 
In the abdomen, for example, six ganglionic masses are 
readily observed, one lying over the sternum of each 
somite, connected by longitudinal bands of nerve fibres, 
and giving off branches to the muscles. On careful ex- 
amination, the longitudinal connecting bands, or com- 
missures (fig. 24, co), are seen to be double, and each 
mass appears slightly bilobed. In the thorax, there are 
six, larger, double ganglionic masses, likewise connected 
by double commissures ; and the most anterior of these, 
which is the largest (fig. 25, gn. 2), is marked at the 
sides by notches, as if it were made up of several pairs 
of ganglia, run together into one continuous whole. 
In front of this, two commissures (c) pass forwards, 
separating widely, to give room for the gullet (ws), which 
passes between them ; while in front of the gullet, just 
behind the eyes, they unite with a transversely elongated 
mass of ganglionic substance (gn. 1), termed the brain, or 
cerebral gdnglion. 
All the motor nerves, as has been said, are traceable, 
directly or indirectly, to one or other of these thirteen 
sets of ganglia; but other nerves are given off from the 
ganglia, which cannot be followed into any muscle. In 
