OP HEAEING. 79 



salts are not found in the labyrinth of cyclostomes, although 

 such bodies exist in the cuttle-fish, among the invertebrata. 

 No vestige of an auditory organ has been detected in the am- 

 phioxus, which forms, in this respect, an exception to the law 

 which prevails in all other vertebrata. T. W.] 



159. In crabs, the organ of hearing is found at the lower 

 surface of the head, at the base of the large antennae. It is a 

 bony chamber, closed by a membrane, in the interior of which 

 is suspended a membranous sac, filled with fluid. On this sac 

 the auditory nerve is expanded. In the cuttle-fish, the vesti- 

 bule is a simple excavation of the cartilage of the head, contain- 

 ing a little membranous sac [and otolith], in which the auditory 

 nerve terminates. 



160. Finally, some insects, as, for instance, the grass- 

 hopper, have an auditory apparatus, no longer situated in the 

 head, as with other animals, but in the legs ; and from this fact 

 we may be allowed to suppose, that if no organ of hearing has 

 yet been found in most insects, it is because it has been sought 

 for in the head only. 



[Much doubt exists as to the true seat of the organ of 

 hearing in insects. Treviranus thought it was situated in 

 Blatta orientalis, at the base of the antennae. Ramdohr con- 

 sidered a vesicle placed at the base of the jaws of the bee as an 

 organ of hearing. Straus-Durckheim thinks the seat of this 

 sense in the cockchaffer is in the plates of the antennae. 

 D'Blainville thought that certain vesicles situate in the sides of 

 the body, and covered by a membrane, were organs of audition. 

 These differences of opinion about a matter of fact, is a proof 

 that we as yet possess no certain knowledge of the true seat of 

 this sense, although there can be no doubt that insects hear. 

 T.W.] 



161. It appears from these examples, that the part of the 

 organ of hearing uniformly present, is that in which the audi- 

 tory nerve ends ; this, therefore, is the essential part of the 

 organ. The other parts of the apparatus, the tympanum, 

 auditory passage, and the semicircular canals, have for their 

 object merely to aid, with more precision and accuracy, the 

 perception of sound. Hence we may conclude, that the sense 

 of hearing is dull in animals where the organ is reduced to 

 its most simple form ; and that animals which have merely 

 a simple membranous sac, without a tympanum and audi- 



