116 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
There is no doubt, however, as to the special recipients 
of sonorous and luminous vibrations; and these are of 
particular importance, as they enable the nervous ma- 
chinery to be affected by bodies indefinitely remote 
from it, and to change the place of the organism in 
relation to such bodies. 
Sonorous vibrations are enabled to act as the stimulants 
of a special nerve (fig. 25, a’n) connected with the brain, 
by means of the very curious auditory sacs (fig. 26, A, au) 
which are lodged in the basal joints of the antenuules. 
Each of these joints is trihedral, the outer face being con- 
vex; the inner, applied to its fellow, flat ; and the upper, 
on which the eyestalk rests, concave. On this upper face 
there is a narrow elongated oval aperture, the outer lip of 
which is beset with a flat brush of long close-set sete, 
which lie horizontally over the aperture, and effectually 
close it. The aperture leads into a small sac (au) with 
delicate walls formed by a chitinous continuation of the. 
general cuticula. The inferior and posterior wall of the 
sac is raised up along a curved line into a ridge which 
projects into its interior (fig. 27, A,r). Each side of this 
ridge is beset with a series of delicate sete (as), the 
longest of which measures about ‘jth of an inch; they 
thus form a longitudinal band bent upon itself. These 
auditory sete project into the fluid contents of the sac, 
and thei apices are for the most part imbedded in a 
gelatinous mass, which contains irregular particles of sand 
