190 PISCES. 



CLASS IV. 



PISCES. 



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

 circulation, but in which respiration is altogether effected through the 

 medium of water. For this purpose, on each side of the neck, they 

 have an apparatus called branchiae, or gills, which consist of laminae 

 suspended on arches that are attached to the hyoid bone, each composed 

 of numerous separate laminae 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 represents the right auricle and ventricle of 

 warm-blooded animals. This blood, having received the benefit of respi- 

 ration, 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 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 dila- 

 tation, 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 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 analogous to the bones of the arms and legs are extremely 

 short, or even completely concealed ; rays, more or less numerous, 

 which support membranous fins, form a rude representation of the 

 fingers and toes. The fins which correspond to the anterior extre- 

 mities are termed pectorals, and those which answer to the posterior 

 ones, venlrals. 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, and at its extremity, which, by being raised 

 or lowered, increase or diminish the surface which strikes against the 

 water. The superior fins are called dorsal, the inferior anal, and that 

 at the end of the tail caudal. The rays are of two kinds; some of them 

 consist of a single bony piece, usually hard and pointed, sometimes 

 flexible and elastic, divided longitudinally these are called spinous 



