74 EVENINGS AT THE MICROSCOPE 



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

 be only that our senses are too dull to perceive the deli- 

 cate sounds which they utter, and which may be suffi- 

 ciently audible to their more sensitive organs; and, be- 

 sides, some Mollusca can certainly emit sounds audible 

 to us. Two very elegant species of Sea- slug, viz. Eolis 

 punctata and Tritonia arborescens, 1 certainly produce audi- 

 ble sounds. Professor Grant, who first observed the inter- 

 esting fact in some specimens of the latter which he was 

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

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

 a 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 heard when they are cold and without any motion; 

 in the dark I have not observed any light emitted at the 

 time of the stroke; no globule of air escapes to the surface 

 of the water, nor is any ripple produced on the surface at 

 the instant of the stroke; the sound, when in a glass ves- 

 sel, is mellow and distinct. ' ' The Professor has kept these 

 Tritoniae alive in his room for a month, and, during the 

 whcle period of their confinement, they have continued 

 to produce the sounds, with very little diminution of their 

 original intensity. In a small apartment they are audible 

 at the distance of twelve feet. "The sounds obviously pro- 

 ceed from the mouth of the animal ; and, at the instant 

 of the stroke, we observe the lips suddenly separate, as if 



1 Now called Dendronotus arborescens. 



