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 fliiviatilis. 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 setae ; b, its inner or posterior extremity ; n n', nerves ; 

 ?, 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. 



