32O VERTEBRATES : FISHES. 



Fishes come through the hard covering of the head ; 

 hence they hear scarcely more than the loudest sounds.* 

 The taste is generally regarded as feeble ; f nor are they 

 highly endowed with the sense of touch. The vegeta- 

 tive functions of Fishes follow the same order as those 

 of higher vertebrates. Respiration, as stated above, is 

 performed by means of branchiae or gills, an apparatus 

 on each side of the neck consisting of fringes suspended 

 on arches attached to the hyoid bone, and traversed by 

 innumerable bloodvessels. In most species, the great 

 opening of the gills is closed by means of the bran- 

 chial membrane, which is supported by rays attached 

 to the hyoid bone, together with a sort of lid composed 

 of three pieces, plainly or obscurely indicated, and called 



the operculum, suboperculum, and 

 interoperculnm. This lid or gill- 

 cover is articulated with the tym- 

 panal bone, and plays on one called 

 the preoperculum. The general 

 relative positions of these parts 

 is shown in Fig. 174. In some 

 groups, however, the gill-covers 



i. preoperculum ; a, operculum ; T- /* t 



s, subopercuium; i, inter- are wanting. By motion of their 



jaws and the opercular and hy- 

 oidean apparatus, Fishes keep currents of water flowing 

 through their gills, where the blood which is continually 

 sent from the heart is purified. The blood, having under- 

 gone respiration, is poured into an arterial trunk situated 

 under the spine, whence it is distributed to every part of 

 the body, and in due time returns by the veins to the 

 heart. The heart of Fishes contains but two cavities, 



* Some naturalists believe that the sense of hearing is much less obtuse 

 than it is here represented to be. 



t It is well known that some fishes show a decided preference for one 

 kind of bait over another, not only in taking it, but in clinging to it. 



