=How Fishes Breathe.=--The fish breathes the air which is dissolved in water. It cannot use the oxygen which is a component part of water, nor can it, as a rule, make use of atmospheric air. The amount of oxygen required for the low vegetative processes of the fish is comparatively small. According to Dr. Günther, a man consumes 50,000 times as much oxygen as a tench. But some fishes demand more oxygen than others. Some, like the catfish or the loach, will survive long out of water, while others die almost instantly if removed from their element or if the water is allowed to become foul. In most cases the temperature of the blood of the fish is but little above that of the water in which they live, but in the mackerel and other muscular fishes the temperature of the body may be somewhat higher. Some fishes which live in mud, especially in places which become dry in summer, have special contrivances by which they can make use of atmospheric air. In a few primitive fishes (Dipnoans, Crossopterygians, Ganoids) the air-bladder retains its original function of a lung. In other cases some peculiar structure exists in connection with the gills. Such a contrivance for holding water above the gills is seen in the climbing perch of India (_Anabas scandens_) and other members of the group called Labyrinthici. In respiration, in fishes generally, the water is swallowed through the mouth and allowed to pass out through the gill-openings, thus bathing the gills. In a few of the lower types a breathing-pore takes the place of the gill-openings. The gills, or branchiæ, are primarily folds of the skin lining the branchial cavity. In most fishes they form fleshy fringes or laminæ throughout which the capillaries are distributed. In the embryos of sharks, skates, chimæras, lung-fishes, and Crossopterygians external gills are developed, but in the more specialized forms these do not appear outside the gill-cavity. In some of the sharks, and especially the rays, a spiracle or open foramen remains behind the eye. Through this spiracle, leading from the outside into the cavity of the mouth, water is drawn downwards to pass outward over the gills. The presence of this breathing-hole permits these animals to lie on the bottom without danger of inhaling sand. [Illustration: FIG. 76.--Gill-basket of Lamprey. (After Dean.)]