The fins of fishes constitute interesting features in their structure. Almost all fishes have two sets of fins--those which exist in pairs and those which are unpaired, and which are developed in the middle line of the body. To the former class belong the two pectoral or “breast-fins” and the two ventral or “belly-fins.” The “breast-fins” correspond to the forelegs of other animals or to the arms of man; while the ventral fins correspond to the hindlegs or to man’s lower limbs; and these latter fins may be placed, as hind-limbs should be, to the rear of the body (as in sturgeons); or they may be found (as in the cod) placed beneath the breast-fins on the throat. It may be asked, How do we know that these two pairs of fins represent the limbs of other animals? We reply, because when we investigate their structure we find them to be supported by a bony skeleton, the various portions of which correspond to those existing in the skeleton of the limbs of man or other vertebrates. And it is only through this important principle of tracing out what are known as the homologies or resemblances between parts, and by looking at and comparing their structure, that we are enabled to find out the real nature of many organs in animals; similar organs frequently existing under very different and varied guises. The other fins of fishes do not exist in pairs, but are placed in the middle line of the body. Hence they are named the median or unpaired fins. Thus we find the back or dorsal fins to represent the unpaired fins, as also do the tail and anal fins; the latter being placed on the lower surface of the body. These unpaired fins, if they correspond to any other structures in the fishes, are simply to be regarded as special developments of the skin, and therefore bear no true relationship to the limbs of other animals. We may find one or more dorsal and one or more anal fins; but the tail-fin, by the action of which, as every one knows, the fish chiefly swims, is always single, but may be divided into halves. Most of our common fishes have the halves of the tail-fin of equal size; others, such as the sharks, sturgeons, etc., having the upper half greatly exceeding the lower half of the tail-fin in size. In one species of shark, named the Thresher or Fox-shark, the upper half of the tail-fin appears enormously developed as compared with the lower half; and the names of this species have been derived from the use the fish makes of its tail in lashing the water, and from the long-tailed appearance suggesting a resemblance to the familiar Reynard of the land. In fishes the tail-fin is always placed vertically, or in the same line as the body, and moves from side to side; while in the whales--which are not fishes, but Mammalia or quadrupeds possessing fish-like bodies--the tail-fin is placed across the body. Some fishes may want arms or legs--that is, the pectoral or ventral fins; the eels, for example, possessing no ventral fins. The flying-fishes, on the