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 cmious auditory sacs (fig. 26, A, au) 

 which are lodged in the basal joints of the antennules. 



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 setae, 

 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 setae (as), the 

 longest of which measures about -j^th of an inch ; they 

 thus form a longitudinal band bent upon itself. These 

 avditory setm project into the fluid contents of the sac, 

 and their apices are for the most part imbedded in a 

 gelatinous mass, which contains irregular particles of sand 



