Or Fishes, are defined by Cuvier to be vertebrated animals with red blood, breathing through the medium of water by means of their branchiæ or gills. To this definition may be added, that fishes have no neck, and that the body generally tapers from the head to the tail; that most of the species are furnished with air-bladders which enable them to swim; and that their bodies are generally covered with scales. The heart has only one auricle, and the blood is cold. The gills require to be kept moist to enable the fish to breathe, and as soon as they become dry, the fish dies. Thus fishes with large gill openings die almost as soon as they are taken out of the water; while those with very small openings, like the eel, live a long time. Fishes have no feet, but are furnished with fins. The scientific knowledge of Fishes is called Ichthyology. Fishes are first divided into two great series, viz. the Bony Fishes, and the Cartilaginous Fishes, and these are again subdivided into nine orders, as follows:-- OSSEOUS OR BONY FISHES. 1. _Acanthopterygii_, or fishes with hard fins. 2. _Malacopterygii abdominales_, or soft-finned fishes, with the ventral fins on the abdomen behind the pectorals. 3. _Malacopterygii sub-brachiati_, or soft-finned fishes, with the ventral fins under the gills. 4. _Malacopterygii apodes_, or soft-finned fishes, without ventral fins. 5. _Lophobranchii_, or fishes with tufted gills. 6. _Plectognathii_, or fishes with the upper jaw fixed. CHONDROPTERYGII, OR CARTILAGINOUS FISHES. 7. _Cyclostomi_, or fishes with jaws fixed in an immovable ring, and with holes for the gills. 8. _Selachii_, or fishes with movable jaws and holes for the gills. 9. _Sturiones_, with the branchiæ in the usual form. Of the bony fishes the _Acanthopterygii_, or fishes with hard spiny fins, are divided into fifteen families, the principal of which are the perch family, the mailed cheek fishes, including the gurnards, the flying fish of the Mediterranean, and the sticklebacks, or jack banticles; the mackerel family, including the tunny, bonito, and sword-fish; the pilot-fish, the dolphin of the Mediterranean, so celebrated for the beauty of its dying tints, and the John Dory. Among the _Malacopterygii abdominales_, or soft-finned fishes, that have their ventral fins suspended from the abdomen, the most interesting are the carp family, the pike family, the flying-fish of the ocean, the salmon family, and the herring family, including the sprat, pilchard, and anchovy. _The Malacopterygii sub-brachiati_ are soft-finned fishes, with the ventral fins beneath the pectorals; the principal of which are the cod family, including the haddock, whiting, and ling; the flat-fish family, including soles, turbots, plaice, and flounders; and the suckers or lump-fish. _The Malacopterygii apodes_ are confined to the eel family.