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 l-300th or l-400th of 



Fig. 54. — A'sfaciis JiuvlatiUs. — 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, go', commissural cords connecting the ganglia 

 with those in front, and those behind them. gl.c. points to the 

 ganglionic corpuscles of the ganglia ; n, 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, wlien the nerve trunk gives 



