548 GASTEKOPODA. 



scription of the Snail already given as an example of the general struc- 

 ture of the entire class. 



(1462.) The organ of hearing is now universally admitted to exist in 

 all the Gasteropod Mollusca, and, according to Siebold*, is invariably 

 situated in the immediate vicinity of the two most voluminous cerebral 

 masses. Like the other organs of sense, the organ is always double. 

 It is formed f by two hyaline ovate or orbicular capsules, situated on 

 the head or neck at the bases of the tentacula, and is supplied with its 

 specifically-endowed nerve from the cerebral ganglions. In the capsule 

 there are enclosed one or several (and sometimes they are numerous) 

 oval or round crystalline bodies, named otoliths ; and it is observable 

 that the number varies not only in neighbouring genera, but even in 

 nearly-allied species. Siebold says that a concentric depression is 

 evident in these otoliths, and there may be seen, in the greater number 

 of them, a shaded spot, or, rather, a minute aperture, which penetrates 

 through the concretion from the one flattened surface to the other. 

 Subjected to a strong pressure, the otoliths crack in radiating lines, 

 separating often into four pyramidal pieces. This separation also ensues 

 when the otoliths are immersed in diluted nitric acid ; and if we touch 

 them with the concentrated acid, they suddenly dissolve, with the dis- 

 engagement of a gas, whence Siebold concludes them to be composed of 

 carbonate of lime. The size of the otoliths is not equal ; and in the 

 same capsule there are always some which are smaller than others. 

 "Within the capsule they have, during life, a very remarkable, and in 

 some respects peculiar, lively oscillatory movement, being driven about 

 as particles of any light insoluble powder might be in boiling water. 

 The otoliths in the centre have the appearance of being pressed together, 

 so as to form a sort of solid nucleus ; and towards this centre the otoliths 

 seem ever to be violently urged, their centripetal rush being invariably 

 repulsed, and themselves driven back again in a centrifugal direction. 

 Removed from the capsule, the motions of the otoliths instantly cease. 

 The cause of these curious oscillations remains undiscovered. Siebold 

 could detect no vibratile cilia on the surfaces of the capsule J ; and the 

 cessation of the motion when the otoliths are removed proves them to 

 be unciliated themselves, and at the same time distinguishes the motion 

 from that of inorganic molecules, as described by Mr. Brown. 



(1463.) Dr. Nordmann, in an elaborate memoir on the anatomy of the 

 Tergipes Edwardsii, minutely describes the structure of the auditory 

 capsules of that species, in which they are found situated immediately 



* Ann. des Sci. Nat. 1843, xix. p. 198. 



t Introduction to Conchology ; or, Elements of the Natural History of Molluscous 

 Animals, by George Johnston, M.D., LL.D., to which the student is referred, as 

 being by far the best treatise upon the subject in the English language, for fuller 

 details concerning the habits and organization of the Mollusca. 



J Kolliker has observed that the motion of the otoliths in the Mollusca is de- 

 pendent upon cilia, with which the internal surface of the auditory cyst is covered. 



