THEIR LIMITS AND THEIR FALLACIES. 317 



Some of us have descended in the diving-bell, no doubt, 

 and then have experienced a certain unpleasant sensation in 

 the ears. It lasts for some little time, and then ceases ; the 

 period of cessation being indicated by a crushing sound, as if 

 something inside the ear had broken. The pain is referable 

 to pressure of the atmosphere upon the tympanum externally; 

 the crush results from air finding its way suddenly through 

 the Eustachian tube. On ascending, the symptoms return, 

 but from an opposite cause; the chief pressure then being 

 internal, not external. Appreciation of the cause suggests an 

 easy mode of prevention moreover, it is always effectual. 

 If, during the descent, the lips and nose being closed, an 

 effort to swallow be made, thus compressing air within the 

 cheeks, the unpleasant sensation will be absent. 



In most invertebrate animals there exist no parts which 

 can be referred to special organs of hearing, as in the higher 

 classes. Neither can the motion of a creature under the in- 

 fluence of sound be recognised as a proof of existence of the 

 faculty of hearing, as already noted. 



The lowest animals in which a special auditory apparatus 

 is recognisable are the Crustacea. In them the ears are 

 placed at the lower surface of the head, on either side, at the 

 base of the lesser antennas. They are very rudimentary, con- 

 sisting only of a membranous sac, upon which the auditory 

 nerve is expanded. In the cephalopoda, or cuttle-fish tribe, 

 the special auditory apparatus is rather more elaborate than 

 in the Crustacea ; but still very rudimentary. 



Amongst vertebrate animals, fishes are those in which the 

 special auditory apparatus is simplest. First, every trace of 

 external ear is wholly wanting ; internally there is no cochlea, 

 or convoluted tube, the part in which sounds are caused to 

 linger to the sense of higher animals. Fishes, however, are 

 provided with an auditory labyrinth ; either wholly enclosed 

 in the skull, or else between the walls of the skull and the 

 brain. The other peculiarities of the auditory organ of fishes 



