196 POWERS OF HEARING chap. 



water in the immediate neighbourhood by the smell of the 

 decaying flesh. 



lY. Hearing 



Experiments made with a view to ascertain whether the 

 MoUusca are sensitive to noises have usually led to the con- 

 clusion that they are deaf to very loud sounds. This is the 

 more curious, because an undoubted auditory apparatus has been 

 discovered in a large number of genera. In the case of an 

 experiment, it is not easy to be sure that the animal is not 

 affected, at least in part, by the shock or jar, rather than by the 

 actual sound. In some experiments, however, conducted at the 

 Plymouth Marine Biological Laboratory, Mr. Bateson found ^ 

 that Anomia could be made to shut its shell by smearing the 

 glass of the tank with the finger in such a way as to make a 

 creaking sound. It was evident that the cause of alarm was not 

 the jarring of the solid framework of the tank, for the same 

 result occurred when the object on which the Anomia were 

 fixed was suspended in the water by a thread. It was found 

 that the sound had to be of a particular pitch to excite the 

 attention of the mollusc. 



As a rule the organ of hearing is nothing more than a small 

 vesicle or sac (the otocyst), filled with a fluid secretion, in which 

 are suspended one or usually more calcareous concretions known as 

 otoliths. By means of cilia, which connect with sense-cells, these 

 otoliths are given a peculiar movement or oscillation in the 

 medium in which they are suspended. The number of the 

 otoliths varies in different genera and species ; there are several 

 hundreds in Arion and Limax, about a hundred in Helix 

 jpomatia, nemoralis, hispida, arhustorum, rotundata, Succinea 

 putris, and Zimnaea stagnalis ; about fifty in Flanorhis contortus 

 and Physa fontinalis, only one in Cyclostoma elegans. The number 

 increases with age. In young specimens of Limn, stagnalis 

 as few as ten, nine, and seven have been noticed.'-^ 



The otocysts are always paired, and, in Gasteropoda, are 

 placed close to the pedal ganglia. The acoustic nerve, however, 

 has been shown by Lacaze-Duthiers to connect with the cerebral 

 ganglia in certain cases. The otocysts are never on the surface 



1 Joimi. Mar. Biol Ass. N.S. i. p. 217. 

 2 Moquin-Tandon, 3IoIl. de France, i. p. 133. 



