717 



CLASS FOURTH— FISHES. 



Vertebrated animals ivith cold, red blood, resfiring by gills, or bronchia., and 

 moving in the water by the aid of fins. 



The form of fishes seems as admirably adapted for motion in the water, as 

 that of birds for flight in the air. Suspended in a liquid of nearly the same 

 specific gravity as their own bodies, they do not require extended members 

 for their support. Their general form is elongated, but thicker in the middle 

 than at their extremities ; and the tail, the principal instrument in their 

 progressive motion, is always long, and terminated by a vertical fin, capable 

 of expansion and attraction like a fan. The organs of locomotion assume, 

 in this class, the form of fins, of which those named thoracic or pectoral, from 

 their situation on the body, have been considered as analogous to the fore 

 feet of quadrupeds ; and those placed further backwards, called abdominal or 

 ventral fins, have been conceived to represent the hind feet of the first class 

 of vertebrated animals. The vertical fins on the back are termed dorsal 

 fins, and those on the under surface of the body, anal fins ; the fin by which 

 the tail is terminated, being termed the caudal fin. The membranes of these 

 fins are supported by rays or bands, more or less numerous ; and those of 

 the pectoral and ventral fins, according to the supposed analogy between the 

 organs of fishes and quadrupeds, have been supposed to represent the toes 

 of the feet. Other rays placed at the extremity of the spinous processes, 

 support the vertical fins of the back, those under the tail, and at its extremity. 

 These radii, or rays, are of two kinds; those of a solid, bony, and pointed 

 form, sometimes flexible and elastic, are called spinous rays; and those 

 composed of a number of small joints, generally divided into branches at 

 their extremity, are called soft, or articulated rays. In the number of the 

 m.embers, as many varieties are observed among the fishes as among the 

 reptiles; for, though generally speaking, the fishes have four, yet in some 

 families, there are but tvv'o, and others want them altogether. 



The greater part of fishes make their progressive motion in the water, by 

 means of their expanded tail, which, striking the water alternately from 

 right to left, impels them forward; and they change their direction by 

 striking more rapidly, or with greater force on one side than on another. 

 The fins, which are in pairs, appear to be intended, besides aiding in pro-' 

 gressions, to maintain their equilibrium in the water, and to regulate the direc- 

 tion of their coarse. Some families, however, as the rays, swim chiefly by 

 means of these fins ; but the particular form of the body must naturally 

 influence their medium of locomotion. The greater part of fishes, besides 



