196 POWERS OF HEARING 



CHAP. 



attracted from the water in the immediate neighbourhood by 

 the smell of the decaying flesh. 



IV. Hearing 



Experiments made with a view to ascertain whether the 

 Mollusca 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 otocysf)^ 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 Avion and Llmax^ about a hundred in Helix 

 pomatia, nemoralis^ hispida^ arbustorum, rotundata., Succinea 

 putris, and Limnaea stagnalis ; about fifty in Planorhis contortus 

 and Physa fontinalis^ oiilj one in Cyclostoyna elegans. The 

 number increases with age. In young specimens of Limn, stag- 

 nalis 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 Journ. Mar. Biol. Ass. N.S. i. p. 217. 



2 Moquin-Tandon, Moll, de France., i. p. 133. 



