AUDITOBY HAIES. 91 



The interior of the sac is thus described by Farre : 

 " Along the lower surface of the vestibular sac is seen 

 running a semicircular line, broader at its upper than 

 its lower extremity (Fig. 58, h). This part is more 

 easily examined after the sand has been washed away 

 by agitation under water. It is then seen, with a power 

 of 18-linear, to consist of several rows of ciliated pro- 

 cesses, of which one row is more regular and prominent 

 than the rest, and crests the entire margin of the 

 ridge. The procesfes diminish in size and number 

 on either side, and are in some places seen in groups, 

 but always assume the general form represented in" 

 Fig. 58. ' 



In Astacus there are four rows of hairs. The first 

 are somewhat scattered, and above the otolithes ; the 

 second consists of larger hairs, arranged close together ; 

 the third and fourth are smaller again, and more scat- 

 tered. These three rows of hairs are covered by the 

 otolithes. They stand in connection with the terminal 

 fibrils of the acoustic nerve, and through their vibra- 

 tions the sense of sound is supposed to be conveyed. 

 In the lobster Hensen counted 548 auditory hairs. 

 He divides auditory hairs of Crustacea into three 

 classes : otolithe hairs ; free hairs, enclosed in the audi- 

 tory sac ; and auditory hairs on the outer body surface. 



These latter auditory hairs (Fig. 59) are situated 

 over an orifice in the chitinous integument, and stand 

 in direct communication with a fibril from the nerve ; 

 the stem of the hair does not rest directly on the 

 chitinous integument, but is supported by a delicate 

 membrane, which is sometimes dilated at the base ; the 

 edge of the chitine at one side of the hair is raised into 

 a tooth ; lastly, according to Hensen, each auditory hair 



