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 

 readil}' 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 thes^ 

 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 (ces), 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 ganglion. 



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 



6 



