(_f_) Plectognathi--comprising the file, porcupine, and sun fishes--are distinguished by their maxillaries and premaxillaries being joined immovably to each other, so as to render the upper jaw incapable of protrusion. ] Most fishes possess a remarkable accessory organ of locomotion in the air-bladder or swim-bladder which extends to a greater or smaller distance along the ventral surface of the spine, and enables them voluntarily to increase or diminish the specific gravity of their body. When they contract this remarkable gas-reservoir, or press out the included air by means of the abdominal muscles, the bulk of the body is diminished, its weight in proportion to the water is increased, and the fish swims easily at a greater depth. The contrary takes place on relaxing the tension of the abdominal muscles; and thus we see fishes rise and fall in their denser element by the application of the same physical law which is made use of by our aëronauts, to scale the heavens or to descend again upon the earth. Those fishes which are destined to live at the bottom of the sea or to conceal themselves in the mud, such as eels and skates, have either no air-bladder or a very small one--for economical Nature gives none of her creatures any organ that would be useless to them. Even the slimy glutinous matter which is secreted from the pores of most fishes, and lubricates their bodies, assists them in gliding through the waters, so that no means have been neglected to promote the rapidity of their movements. The skin of fishes is but seldom naked; in most species it is covered with scales, that sometimes appear in the form of osseous plates, as in the ostracions, or project into formidable prickles, as in the porcupine-fish, but generally offer the aspect of thin laminæ, overlapping each other like the tiles of a roof, and embedded, like our nails, in furrows of the skin. In nearly all the existing fishes, the scales are flexible and generally either of a more or less circular form (_cycloid_), as in the salmon, herring, roach, &c., or provided with comb-like teeth projecting from the posterior margin (_ctenoid_), as in the sole, perch, pike, &c.; while the majority of fossil fishes were decked with hard bony scales, either rhomboidal in their form, of a highly polished surface, as in our sturgeons (_ganoid_), and arranged in regular rows, the posterior edges of each slightly overlapping the anterior ones of the next, so as to form a very complete defensive armour to the body; or irregular in their shape and separately imbedded in the skin (_placoid_), as in the sharks and rays of the present day. [Illustration: Portion of Skin of Sole highly magnified.] The scales of almost any fish afford admirable subjects for microscopic observation, but more particularly those of the ctenoid kind, which exhibit a brilliancy of reflected light, and a regularity of structure, such as no human mosaic could ever equal.