SENSES HEARING. 197 



V. HEARING. 



An auditory organ has been demonstrated in so many 

 species, that it may be considered a very general possession 

 of the mollusca. Its existence in the Cephalopods was 

 known to John Hunter ; MM. Eydoux and Souleyet have 

 found it in several Heteropods, and, at least, in one Ptero- 

 pod ; and it has been seen by M. Laurent in several genera 

 of this order. Van Beneden and Krohn have likewise de- 

 scribed the hearing organ of several Heteropods ; and its 

 structure in the Nudibranches has been anatomized by 

 De Quatrefages, and more especially by Messrs. Embleton 

 and Hancock. However, we owe our knowledge of the 

 organ principally to M. Siebold, who has pursued his re- 

 searches more systematically than any other anatomist, and 

 further has proved its existence in many Gasteropod and 

 bivalve acephalous mollusca. Like the other organs of 

 sense, the ear is always paired. It is formed by two hya- 

 line, 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 

 otolites ; 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 evi- 

 dent in these otolites, and there may be seen in the centre 

 of 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 



* " En effet, cet organe se trouve chez tous les Gasteropodes a la partie 

 posterieure des deux renflemens ganglionnaires les plus volumineux ; on 

 doit les chercher toujours aupres de la paire de ganglions antericurs de 

 cette portion de systeme nerveux, ct il est plus facile de les trouver a la 

 surface inferieure qu'a la surface superieure, surtout chez les Gasteropodes 

 (Limax, Helix), dont les divers ganglions sous-oesophagiens sont confondus 

 de le maniere la plus intime." — Siebold in Ann. des Sc. Nat. (1843), xix. 

 198. — In the Bivalves the organ is placed in the foot. Siebold thus de- 

 scribes it in Cyclas cornea : " On compressing the extremity of the foot of this 

 species between two plates of glass, we bring into view a large or central 

 nervous ganglion ; and on each side of this there is a minute round reservoir, 

 composed of an elastic, opake, and tenacious mass. In the centre of this 

 there is again a perfectly transparent circular and flattened nucleus, which 

 floats disconnected from the sides of the body that contains it, and has an 

 oscillatory movement. This nucleus appears to consist of a crystalline salt." 

 — Ann. des Sc. Nnl. n.s. ix. 319. 



