To aid the Angler in his scientific researches, as well as to add to the interest of the ordinary observer, we now proceed to a brief exposition of the principal characteristics of the _class_ of fishes, and shall, at an after period, expatiate upon the more peculiar attributes of each particular kind, when we come to treat of the _species_ in their order. We need scarcely say to the student of nature, that the form and functions of fishes are as admirably adapted for easy movement through the water, as are those of birds for that aërial motion called flight. Suspended in a liquid element of almost equal specific gravity with themselves, external organs resembling those of birds in size, would have been disproportionate and unnecessary; but the air-bladder (the functions of which, by no means entirely understood, have never been satisfactorily explained in all their bearings) is known to possess the power of contraction and dilatation, the exercise of which is followed by a corresponding descent or ascent of the animal’s body. Thus a small central and inconspicuous organ effects, in the easiest and most simple manner, the same object which even the soaring eagle or giant condor can only accomplish by great exertion of the wings, and after laborious and frequently repeated gyrations. We shall ere long, however, have occasion to remark in more detail, that the air-bladder, although essential to the economy of such species as possess it, is by no means indispensable as a general attribute of the class, as in many tribes it is entirely wanting. It is not even a generic characteristic, as it does not exist in the red mullets of the British seas, though possessed by the corresponding species of Asia and America—while of our two kinds of mackerel, the so called Spanish species (_Scomber colias_) is distinguished by a swimming bladder, and the common mackerel (_Sc. scomber_) does not possess that organ. Fishes being without a neck, and the portion called the tail being usually equal at its origin to the part of the body from which it springs, the prevailing shape is somewhat uniform and continuous, diminishing gradually towards either extremity. Of this, the most elegant and characteristic form of fishes, the salmon and mackerel exhibit familiar examples. Yet a vast variety of shape, as well as of size and colour, is naturally presented by a class which now contains some seven or eight thousand known species; and no further illustration of the subject will be deemed necessary by him who has seen and remembers the difference between an eel and a skate.