91 



CLASS IV. 



PISCES. 



The class of Fishes is composed of oviparous vertebrata 

 with a double circulation, but in which respiration is alto- 

 gether effected through the medium of water. For this pur- 

 pose, on each side of the neck, they have an apparatus called 

 branchiae, which consist of laminae suspended on arches that are 

 attached to the hyoid bone, each composed of numerous sepa- 

 rate laminsB and covered with a tissue of innumerable blood- 

 vessels. The water which the fish swallows, escapes between 

 these laminae through the branchial openings, and by means 

 of the air it contains, acts upon the blood that is continually 

 arriving in the branchiae from the heart, which only repre- 

 sents the right auricle and ventricle of warm-blooded animals. 



This blood, having received the benefit of respiration, is 

 poured into an arterial trunk situated under the spine, which, 

 exercising the functions of a left ventricle, distributes it to 

 every part of the body, whence it returns to the heart by the 

 veins. 



The entire structure of the Fish is as evidently adapted for 

 natation, as that of the Bird for flight. Suspended in a liquid 

 of nearly the same specific gravity as its own body, there was 

 no necessity for large wings to support it. In a great num- 

 ber of species, immediately under the spine there is a bladder 

 filled with air, which, by compression or dilatation, varies the 

 specific gravity of the fish and assists it to rise or descend. 

 Progression is effected by the motions of the tail, which, by 

 striking the water alternately right and left, forces them for- 

 ward ; the branchiae, by impelling tlic water backwards, may also 



