THE FOURTH CLASS 



OF 



VERTEBRATED ANIMALS. 



THE FISH 



Is composed of oviparous vertebrata, with a double 

 circulation, whose respiration nevertheless is effected 

 exclusively through the medium of water. For this 

 purpose they have on each side the neck an apparatus 

 called gills, which consists of leaflets suspended to 

 certain arches attached to the os hyoides, and com- 

 posed each of a great number of successive laminae, 

 covered with a tissue of innumerable blood vessels. 

 The water swallowed by the fish escapes between the 

 openings of these lamina?, and acts by means of the 

 air which it contains on the blood, which is inces- 

 santly impelled to the gills by the heart. The heart 

 has only the right auricle and ventricle of the warm- 

 blooded animals. 



The blood after respiration passes into an arterial 

 trunk situated under the spine, which performing the 

 office of the left ventricle, disperses it throughout the 

 body, whence it returns to the heart by the veins. 



The entire structure of fish is as evidently adapted 



vol. x. B 



