CLASS IV.-OF VERTEBRATA. 



FISHES. 



THE class of Fishes is composed of oviparous vertebrata with a double 

 circulation, but in which respiration is altogether effected through the me- 

 dium of water. For this purpose, 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 laminae 

 placed in a row, and covered with a tissue of innumerable blood-vessels. 

 The water which the fish swallows, penetrates between these lamime, 

 through the openings, called gills, and, by means of the air it contains, 

 acts upon the blood that is continually sent to the branchiae from the 

 heart, which only represents the right auricle and ventricle of warm- 

 blooded animals. 



This blood, having undergone respiration, is poured into an artei-ial 

 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 swimming 

 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 number of species, immediately under the spine, 

 there is a bladder filled with air, which, by compression or dilatation, va- 

 ries the specific gravity of the fish, and assists it to rise or descend. Pro- 

 gression is effected by the motions of the tail, which, by striking the 

 water alternately right and left, forces them forward; the branchiae, by 

 impelling the water backwards, may also contribute to this effect. The 

 limbs being thus of but little use, are greatly reduced ; the parts analo- 

 gous to the bones of the arms and legs are extremely short, or even com- 

 pletely concealed ; rays, more or less numerous, which support membra- 

 nous fins, form a rude representation of the fingers and toes. The fins 

 which correspond to the anterior extremities are termed pectoral, and 

 those which answer to the posterior ones, ventral. Other rays attached 

 to particular bones placed on or between the extremities of the spinous 

 apophyses support vertical fins on the back, under the tail, aud at its ex- 

 tremity, which, by being raised or lowered, increase or diminish the sur- 

 fnce that strikes against the water. The superior fins are called dorsal, 



