MOLLUSCA I THEIR EARS. 67 



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 

 physiologists, 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 arlorescens,* certainly produce audible sounds. 

 Professor Grant, who first observed the interesting 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 being 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 Tritoniae are lively and moving about, and is not 

 * Now called Dendronotus arborescens. 



