188 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
The nerve fibres (fig. 55) of the crayfish are remarkable 
for the large size which some of them attain. In the 
central nervous system a few reach as much as 1-200th of 
an inch in diameter; and fibres of 1-800th or 1-400th of 
Fig. 54.—Astacus fluviatilis —A, one of the (double) abdominal gan- 
glia, with the nerves connected with it (x 25); B,a nerve cell or 
ganglionic corpuscle (x 250). a, sheath of the nerves; c, sheath 
of the ganglion ; co, co’. commissural cords connecting the gangha 
with those in front, and those behind them. gl.c. points to the 
ganglionic corpuscles of the ganglia; », nerve fibres. 
an inch in diameter are not rare in the main branches. 
Each fibre is a tube, formed of a strong and elastic, some- 
times fibrillated, sheath, in which nuclei are imbedded 
at irregular intervals; and, when the nerve trunk gives 
