A glance at the fins of a typical fish will suffice to show that they may be divided into two groups--the paired fins, representing the two pairs of limbs in most of the higher animals, and the median fins occupying the middle line of the body. The former comprise the two _pectoral fins_ that correspond with our arms, and are attached to the bones of the pectoral or shoulder girdle; and the _pelvic fins_, corresponding with the lower extremities. The pectorals, too, are present in nearly all fishes, while the pelvic pair are rather more frequently absent than the pectorals. The medial fins comprise the _dorsal_, the _ventral_, and the _caudal_ or tail-fin, and are not to be regarded as limbs, but rather mere outgrowths of the skin. They are not directly connected with any part of the main bony framework of the body, though they are generally jointed with a series of bones (interspinal bones) that run between processes of the vertebral column. The dorsal and ventral fins are often divided into two or more parts, and the tail fin is commonly distinctly forked. Although the seven fins above mentioned differ considerably in general form, some being fanlike, while others form fringe-like expansions, yet they generally agree in that they consist of bony or cartilaginous rays, between which is a soft membrane. The rays, however, vary much in character, being sometimes developed into very hard and sharp spines, and sometimes quite soft and flexible. The fins also differ in function, as will be seen when we observe the movements of a fish as it swims. It will then be noticed that the caudal fin, which is spread in the vertical plane and moved sharply from side to side by the powerful muscles of the tail, is the chief propelling organ, while the others are concerned principally in maintaining the balance of the body. This latter point becomes much more evident when we observe the movements of a fish in which one or more of the fins have been injured or lost, as we shall see presently. It is true that the pectoral fins are sometimes used to propel, but forward movement is brought about almost entirely by the caudal fin, which acts much in the same way as the blade of the propelling ‘screw’ of a steam-vessel, the pectorals being used at the same time for steering. Occasionally, too, the latter are both spread out at right angles to the body when the fish desires to stop suddenly, and are even employed at times in swimming backwards. When a fish wants to turn to one side, it will be seen to give the tail a sharp motion to the opposite side. The pectoral of the latter side is also brought into play, while the other is kept close against the body.