Experiments made with a view to ascertain whether the Mollusca are sensitive to noises have usually led to the conclusion 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[306] 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 pomatia_, _nemoralis_, _hispida_, _arbustorum_, _rotundata_, _Succinea putris_, and _Limnaea stagnalis_; about fifty in _Planorbis 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.[307] 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 of the body and are rarely connected with it by any passage or tube; it is probable therefore that sound reaches them simply through the medium of the tissues. In the _Pelecypoda_ the otocyst is similarly situated near the pedal ganglion, and is probably (though this has not yet been proved) similarly connected with the cerebral. There is only a single otolith. Pelseneer finds[308] in Nuculidae alone a free communication between the otocyst and the exterior. _Anodonta_ has been observed[309] to withdraw its foot into the shell at the noise of an opening door, a loud voice, or a shrill whistle, whether in a basin of water or lying on a study table. [Illustration: FIG. 96.--Illustrating the otocyst in =A=, _Anodonta_, =B=, _Cyclas_; _ot_, otolith; _a_, _b_, _c_, _c´_, cellular layers surrounding the chamber; _ci_, cilia on interior walls of chamber: =C=, an otolith crushed. (After Simroth.)]