CKUSTACEA. 89 



garded it as an olfactory organ, as did also Treviranus, 

 Fabricius, Scarpa, Brandt, Milne Edwards, and, in fact, 

 the older naturalists generally. The discovery of its 

 true nature is due to Farre,* was confirmed, by Huxley f 

 and Leuckart, and is now generally admitted. It is 

 a sac situated in the base, or first segment, of the 

 lesser pair of antenna?, which is slightly dilated. In 

 some species the sac communicates freely with the 



Fig. 59.— Part of wall of auditory sac of lobster (^Astacus marinus) ; after Hensen. 

 a. Thickened bars in the membrane of the sac ; >), first row of auditory hairs; jj', 

 second row of auditory hairs ; »", third row of auditory hairs; >/", fourth row of 

 auditory hairs ; e, grains of sand, serving as otolithes. 



water by means of an orifice situated towards the inner 

 and anterior margin, and guarded by rows of fine 

 hairs. In others the orifice is closed, but its position 

 is always marked, as the auditory sac is at this point 

 connected with the skin. 



Both contain otolithes. Those of the closed sacs are 

 generally rounded ; while, on the contrary, those of 



* Pliilosophical Transactions, 1843. 



t Ann. and Mag. of Natural History, 1851. 



