MOLLUSCA : THEIR EARS. 57 



violently urged, their centripetal rush being invariably 

 repulsed, and as often driven again into a centrifugal direc- 

 tion. Removed from the capsule, the motions of the 

 otolithes instantly cease. The cause of these curious 

 oscillations remains undiscovered. Siebold could detect 

 no vibratile cilia on the surfaces of the capsule, and the 

 cessation of the motion when the otolithes are removed, 

 proves them to be unciliated themselves, and, at the same 

 time, distinguishes the motion from that of inorganic 

 molecules. 



It has been, however, more recently ascertained that 

 the movements of the otolithes are due to very minute 

 cilia with which the interior surface of the capsule is 

 covered. This had been long suspected, and some eminent 

 23hysiologists, as Wagner and Kolliker, have distinctly 

 seen the cilia themselves. 



If you ask what can be the use of ears to a class of 

 animals which are invariably dumb, I answer that though 

 this is true with respect to the great majority, yet it may 

 be only that our senses are too dull to perceive the delicate 

 sounds which they utter, and which may be sufficiently 

 audible to their more sensitive organs; and besides, some 

 Mollusca can certainly emit sounds audible by us. Two 

 very elegant species of Sea-slug, viz., Eolis punctata, 

 and Tritonia arborescens,* certainly produce audible 

 sounds. Professor Grant, who first observed the interest- 

 ing fact in some specimens of the latter which he was 

 keeping in an aquarium, says of the sounds, that " they 

 resemble very much the clink of a steel wire on the side 

 of the jar, one stroke only been given at a time, and 

 repeated at intervals of a minute or two; when placed 

 in a large basin of water the sound is much obscured, 

 and is like that of a watch, one stroke being repeated, as 

 before, at intervals. The sound is longest and oftenest 

 repeated when the Tritonia) are lively and moving about, 

 * Now called Dendronotus arlorescens. 



