S56 



ON THE RESPIRATION OF FISIIESr. 



readily appear on comparing the structure of the os hyoides 

 and bronchiostegous arches, which represent its horns, or 

 elongations. 



The motion that produces the expulsion of the water is 

 altogether independent of that of the mouth and throat; 

 though it is its common consequence, and occurs alternate- 

 ly, and is nearly isochronous with it. The powers that 

 occasion the expulsion of the water are equally situate about 

 the head; and it is chiefly this apparatus of bones and nu- 

 merous muscles, that renders the heads of fishes so bulky, 

 for they are frequently more than a third of the whole 

 length of the animal. The bronchiostegous arches, in- 

 cluded in the substance of the membrane, take the place of 

 the true ribs : and in those species that want these bones, 

 muscles in the shape of pouches fulfil the same office; 

 acting on the water nearly in the same manner as the au- 

 ricles of the heart on the blood, which they propel into the 

 ventricles; or as the muscular bags found in the spiracles of 

 whales. 



The theory I have laid down seems to be confirmed eve» 

 by the anomalies, that occur in some fishes, in which re- 

 spiration appears to be effected in a diflferent way, and 

 which consequently exhibit some slight modifications in its 

 organs. 



The common phenomena of the respiration of fishes have 

 already been described ; but there are some, as the piagio- 

 stomes and cyclostoraes among the cartilaginous fishes, 

 which do not constantly take in water at the mouth; and 

 others, in which respiration may be termed arbitrary in its 

 movements. These two peculiarities, which I shall exa- 

 mine somewhat at large, seem to show great analogy with 

 the reptiles already noticed. 



The rays, sawfish, and many of the squali, when they 

 that frequently make no effort to sustain themselves in the water, naturally 

 torn of the"*^ *'"^ *^ ^^^ bottom, and lie on the sand: and as their mouths 

 water. ate in the under surface of the body, if they opened thenj 



like other fishes to draw in water, they would necessarily 

 suck up gravel and other foreign matters, by which their 

 gills might be wounded. On this account they seem to be 

 able to breathe iu the manner of frogs and adult salaman. 

 ^^ ' •• ■ ders. 



This theory 

 confirmed by 

 apparent de- 

 viations. 



Two pecu- 

 liarities. 



lit, in fishes,' 



