90 USE OF GRAINS OF SAND AS OTOLITHES. 



the open sacs are simply grains of sand, and are so 

 numerous as sometimes to occupy one-fourth, or even 

 one-third, of the sac. 



Farre stated that the otolithes in the auditory sacs 

 of Crustacea were simply grains of sand, selected by 

 the Crustacea, and put into their own sacs to serve as 

 otolithes. It seemed, however, so improbable that 

 Crustacea should pick up suitable particles of sand 

 and place them in their ears, that the statement was 

 not unnaturally received with incredulity. The obser- 

 vation of Hensen appears, however, to leave no doubt 

 on the subject. The sac, whether open or closed, is an 

 extension of the outer skin, and is cast with it at each 

 moult. Hensen examined them shortly after moultiDg, 

 and found that the sacs contained no stones ; he saw 

 the shrimps carefully selecting particles of sand, but 

 could never detect one in the very act of placing 

 one in the auditory sac. He therefore placed some 

 shrimps in a vessel of filtered sea- water, and streued 

 over the bottom some crystals of uric acid. Soon 

 afterwards one of the shrimps moulted, and the auditory 

 sac was found on examination to contain a few grains 

 of sand, but no crystals of uric acid. Three hours 

 later, however, Hensen found that the new sac con- 

 tained numerous crystals of uric acid, but none re- 

 sembling common sand. Evidently, therefore, the 

 Crustacea pick up grains of sand, and actually intro- 

 duce them into their own ears to serve as otolithes. 



Otolithes are not, however, universally present. In 

 the true crabs (Brachyura) they appear to be always 

 wanting, so that the auditory hairs (which present very 

 nearly the same character as those of the lobsters, etc.) 

 are capable of being thrown into vibrations without the 

 mediation of otolitbes. 



