THE EAR OF THE CRAYFISH. 



117 



and sometimes of other foreign matter. A nerve (n n' ,) is 

 distributed to the sac, and its fibres enter the bases of 

 the hairs, and may be traced to their apices, where they 

 end in peculiar elongated rod-like bodies (fig. 27, C). 

 Here is an auditory organ of the simplest description. 



Fig. 27. — Astacus fiuviatilis. A, the auditory sac detached and seen 

 from the outside ( x 15) ; B, auditory hair ( x 100) ; C, the distal ex- 

 tremity of the same more highly magnified, a, aperture of sac ; as, 

 auditory sefae ; T>, its inner or posterior extremity ; n n', nerves ; 

 r, ridge. 



It retains', in fact, throughout life, the condition of a 

 simple sac or involution of the integument, such as is 

 that of the vertebrate ear in its earliest stage. 



