If the pectoral or pelvic fin of one side is injured, the body of the fish will incline to the opposite side; and if all the paired fins are functionless the fish swims with its head inclined downwards. Observations of fishes in which the dorsal or ventral fins are injured will also show that these organs are necessary to maintain a steady motion in the water. In addition to the above facts, it may be mentioned here that the paired fins are often modified into long finger-like processes that serve as organs of touch, and even as means by which the fish can creep along the bottom. This is notably the case with gurnards and a few of the other fishes that spend their time almost exclusively on the bed of the sea. Fishes are essentially gill-breathers, the gills being generally fringe-like organs, supported on bony arches (the gill arches), numbering four on each side, the cavity containing them being covered by a gill-cover (_operculum_) that opens behind. Water is taken in at the mouth, whence it passes into the gill-chamber; and after passing between and around the gills, it escapes under the opercula. The gills themselves are richly supplied with bloodvessels that are distributed close to the surface, and an exchange of gases takes place through their exceedingly thin walls, carbonic acid gas passing from the blood to the surrounding water, and oxygen, held in solution in the water, passing from the water to the blood. When fishes are in foul water, containing but little oxygen in solution, they rise to the surface in order to make up the deficiency by taking oxygen direct from the air. This, however, is an unnatural proceeding with the majority of fishes; but there are some that are provided with accessory breathing organs specially adapted to the extraction of oxygen direct from the air, and these are so dependent on the supply from this source that they are suffocated if prevented from reaching the surface. In other fishes, such as the sharks and rays, the gills are of an entirely different character from those described above, for they are pouch-like and five in number on each side, each pouch communicating with the pharynx as well as with the exterior by a slit-like opening. Before leaving the external characters of fishes we must say a word or two about their forms and colours. As regards the former, it is well known that fishes are well adapted for rapid progression through water, but there are many exceptions to this rule. These exceptions, however, apply principally to those species that have no need to swim rapidly, and a study of their habits will show that their form is just as perfectly adapted to their mode of life. They are often species that live on the bottom, or hide in the crevices and holes of rocks, and examples will be given in our future descriptions.