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CLASS FOURTH—FISHES. 
Vertebrated animals with cold, red blood, respiring by gills, or bronchia, and 
moving in the water by the aid of fins. 
Tue form cf 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 abdominai 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 
members, 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 two, 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 course. 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 
