The vertebral column, composed of numerous articulations, united by cartilages which permit of certain movements, curves with great facility from side to side; but the vertical motion is much more restricted, chiefly in consequence of the projection of the upper and under spiny processes of the vertebræ. The great organ of movement in all fishes is the tail. The muscles, by which it is brought into play, extend in lengthened masses on either side of the vertebral column. The body, being supported chiefly by the swimming bladder, (which, however, is absent in several species), is propelled forward by the rapid flexure of the extremity acting laterally upon the resistance offered by the water. Generally speaking, neither the pectoral nor the ventral fins are of any material use during swift progressive motion; they rather serve to balance the body, or to aid its gentler movements while in a state of comparative repose. In _flying fishes_, as they are called, the pectoral fins are of such great length and expansion as to support these creatures in the air; and the strength of muscular action might probably suffice even for a longer flight, but for the necessity of constant moisture for the purposes of respiration. The drying of the gills in an individual of this class is attended by results analogous to those produced in the case of a land animal; and a flying fish is obliged to descend to respire, in like manner as a swimming quadruped, or disguised mammiferous animal, as we may term a whale, is under the necessity of ascending for the same purpose. The heads of fishes exercise but a slight movement independent of the rest of the body, but the jaws, opercular bones, branchial arches, and other parts, are very free in their motions. The muscles, like those of other vertebrated animals, are composed of fleshy fibres more or less colored, and of tendonous fibres of a white or silvery aspect. With the exception, however, of certain spinal muscles, which are sometimes of a deep red, the flesh of fishes is much paler than that of quadrupeds, and still more so than that of birds. In several species it is even entirely white. THE NERVOUS SYSTEM AND SENSES OF FISHES.