The fins are organs of motion; but it is chiefly the tail and the caudal fin by which the fish impels itself forward. To execute energetic locomotion the tail and caudal fin are strongly bent, with rapidity, alternately towards the right and left; whilst a gentle motion forwards is effected by a simple undulating action of the caudal fin, the lobes of which act like the blades of a screw. Retrograde motions can be made by fish in an imperfect manner only, by forward-strokes of the pectoral fins. When the fish wants to turn towards the left, he gives a stroke of the tail towards the right, the right pectoral acting simultaneously, whilst the left remains ad-pressed to the body. Thus the pectoral fins assist in the progressive motions of the fish, but rather directing its course than acting as powerful propellers. The chief function of the paired fins is to maintain the balance of the fish in the water, which is always the most unsteady where there is no weight to sink it: when the pectoral of one side, or the pectoral and ventral of the same side are removed, the fish loses its balance and falls on the side opposite; when both pectorals are removed, the fish’s head sinks; on removal of the dorsal and anal fins the motion of the fish assumes a zig-zag course. A fish deprived of all fins, as well as a dead fish, floats with the belly upwards, the back being the heavier part of the body. In numerous groups of fishes which live in mud, or are enabled to pass a longer or shorter time in soil periodically dried and hardened during the hot season, forms occur entirely devoid of, or with only rudimentary, ventral fins (Cyprinodon, Ophiocephalidæ, Galaxiidæ, Siluridæ). The chief function of these fins being to balance the body of the fish whilst swimming, it is evident that in fishes moving during a great part of their life over swampy ground, or through more or less consistent mud, this function of the ventral fins ceases, and that nature can readily dispense with these organs altogether. [Illustration: Fig. 9.--Ventrals of _Gobius_.]