64 INNERVATION. [CHAP. XVIII. 



ment in the various classes of animals, than in the ear. We see it existing as 

 a simple sac in the cephalopod and gasteropod mollusks, and in Crustacea. In 

 the cuttle-fish, it consists of a small sac filled with fluid, lodged in a chamber 

 excavated in the cranial cartilage. The chamber is closed everywhere except 

 at the entrance of the auditory nerve, which passes in to expand upon the sac. 

 From its inner surface there project several obtuse processes, of a soft, elastic 

 nature, which support the sac. The sac contains a calcareous body or otolithe. 

 Even at this early stage of development the organ is double, and the two ca- 

 vities are separated from each other by a very thin septum. It is obvious 

 that these cavities are strictly analogous to the vestibule in the higher classes. 



In Gasteropoda, the organ consists of a sac, to which the nerve is distributed, 

 and which contains fluid and several small otolithes, which according to 

 Siebold, exhibit remarkable movements. 



In Crustacea, the organ still exists as a simple sac. This, as Dr. Arthur Farre 

 has shown, is situate, in the lobster, in the base or first joint of the lesser an- 

 tenna. Its place is indicated by a tough membrane which covers an oval 

 aperture in the upper surface of this joint ; the membrane being a continuation 

 of the same structure which forms the shell, but in which the earthy matter 

 is wanting. Towards the inner and anterior margin of this membrane, there 

 is a small round aperture, through which a bristle may be passed. " On re- 

 moving this oval membrane together with a portion of the surrounding shell, 

 the internal organ is brought into view, completely imbedded in the soft in- 

 tegument and muscular structure of the antenna." It consists of a sac, in 

 shape like an auricle, and of a horny structure, like soft quill, suspended in the 

 centre of the joint, free on all sides, and having only a single attachment near 

 the aperture in the oval membrane already described ; it nearly fills the cavity 

 of the joint. The sac contains particles of siliceous sand, which find their way 

 into it through the aperture already described, and probably fulfil the office of 

 the otolithes which exist in other classes of animals. Numerous very remark- 

 able ciliated processes are attached to the lower surface of the vrstibulaf sac : 

 they are arranged in a semicircular line. In the neighbourhood of this line 

 the auditory nerve attaches itself to the sac, and forms a plexus, which covers 

 the whole under surface of the sac, extending also towards its upper surface. 

 The nerve is derived from the lesser and greater antennal nerves. 



Dr. A. Farre has shown that the cavity situate at the base of the greater an- 

 tenna is not, as has been hitherto supposed, suited to act as an organ of hear- 

 ing. It is a conical papilla, abruptly truncated, and having stretched over it 

 a membrane, which is pierced in its centre by an aperture capable of admit- 

 ting a small bristle. On making a section of this part, nothing more is seen 

 than a narrow canal in the fleshy substance leading perpendicularly from the 

 external orifices, and terminating abruptly at the depth of two lines. A nerve 

 is sent off to this organ from the supra-cesophageal ganglion. Such an organ 

 is very ill-adapted for hearing. Dr. Farre has ascertained that this is the most 

 sensitive part of the body of the lobster ; " since, while the mechanical irrita- 

 tion of any other parts excited only a slight movement in the limbs of the 

 animal, when out of water, and somewhat feeble, the touching of this part 

 was immediately followed by a violent and almost spasmodic flapping of the 

 tail."* 



* Farre on the Organ of Hearing in Crustacea. Phil. Tr. for 1843, p. 223. 



